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Experts respond to the IPCC's 5th report on climate change

Experts from Australia, New Zealand and the UK respond to the IPCC's report “Climate Change 2013: The Physical Science Basis”.

"There are human and natural factors acting in the climate system, and understanding all the factors influencing our climate is an ongoing scientific goal. But the current increase in CO2 and its effects on our planet are attributed to human activity because they have a pattern, in space and in time, which is what we expect from increased greenhouse gases, and which cannot be explained by other forces, such as changes in solar output or volcanic ash.

This IPCC report strengthens the building evidence for the human impact on the climate system. In particular it documents multiple lines of evidence for emerging changes in climate in the atmosphere, ocean, ice, and land.

My own work has focused on ocean acidification, also discussed in AR5. Ocean acidification is a pervasive and persistent effect on the ocean, whose biological impacts have only begun to be detected in nature in the last few years

The report also documents changes in parts of the climate system –acidification and heat accumulation in the ocean, the major ice sheet melting – with very long response times. This point needs to be understood by policymakers and the community: if we intend to mitigate climate change impacts on these systems we have to start now. The very physics and chemistry of those systems places a premium on early action and a penalty on delay."

Dr Will Howard is a Visiting Fellow, Australian National University Research School of Earth Sciences. He is an Expert Reviewer of IPCC AR4 and AR5 WG1 reports and an expert on climate change in the ocean and ocean acidification.

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"All the major features of warming identified in the 2007 IPCC Fourth Assessment Report (AR4) have been confirmed and most have been re-emphasised in this report (AR5). Our understanding has improved, and we have longer observational records, both of which add to our confidence in the findings. Evidence of warming is particularly clear in the ice and snow covered regions of Earth (the cryosphere), including the polar regions. These respond clearly to warming and are a natural "climate meter" for planet Earth."

"In the northern hemisphere, snow cover, sea ice, and glaciers have all retreated rapidly in recent years. Greenland is losing ice at an accelerating rate. Together, these give a strong picture of the impact of Arctic warming. Observations of permafrost are less extensive, but most records show NH permafrost is degrading (warming and thinning), although the global picture is not clear".

"Glaciers are losing mass in almost every glaciated region on the planet and contributing to sea level rise, and the rate of this loss has increased since the 1960s. The largest portion of the total ice loss is coming from the Arctic, including glaciers on the periphery of the Greenland Ice Sheet; Alaska; the High Asian Mountains and the Andes."

"It is now very clear that in the past decade both the ice sheets of Antarctica and Greenland have lost ice and also contributed to sea level rise. Four independent methods used to determine the ice loss from the ice sheets show the same trend and the same regional focuses of loss, although with some differences in the absolute amount of loss. These differences have now been reconciled in comparative studies and, for the first time, ice loss from both ice sheets can be identified season-by-season, and region-by-region. This has allowed us to determine rather precisely where the observed rise in sea level is coming from."

"The satellite records used to measure sea-ice extent now extend over more than 30 years (33 years in total), providing us with a good long-term record of change. Loss of sea ice in the Arctic has continued rapidly with new record summer lows set in both 2007 and 2012. The average thickness and the volume of Arctic sea ice are also deceasing. This is indicative of change in the climate system, with real and direct impacts on human activities on the Arctic."

"However Antarctic sea ice extent is, on-average, increasing, although there are regional patterns of increase and decrease. Records of the extent of seasonal snow cover and permafrost in the Southern Hemisphere are not yet sufficient to identify any trends."

Professor Ian Allison is an Honorary Research Professor, Antarctic Climate and Ecosystems CRC and a lead author of chapter 4 of the report - Observations: Cryosphere.

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“The IPCC report highlights the need for all governments, particularly wealthy high polluting countries like Australia, to cut greenhouse emissions. We have an ethical responsibility to act on climate change; this is also in our long-term self-interest. Civilisation is already at risk due to resource depletion, ecological harm, and rapid population growth in developing countries; further climate change worsens these pressures. It also reflects global inequality.
Essentially, rich countries are indulging in “environmental brinkmanship”. Australians should not imagine that our wealth and power immunises us from the cumulative climatic damage we are still adding to. Global life support systems are being eroded; it is shameful to think that because others will bear most of the harm from our actions Australians therefore need do almost nothing.
But such ideas are misguided; we already experience harm from climate change (eg heatwaves, and worsening fires). Yet these harms will seem trivial compared to those which civilisation will face this later century if we continue our current trajectory. We must lessen our carbon intensity, conserve resources and contribute to regional solutions. The imperative for a low carbon transformation for our global economy is writ large in this report.”

Professor Colin Butler is an Australian Research Council Future Fellow in the Faculty of Health, University of Canberra and a contributing author to the health chapter of the IPCC Working Group 2.

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“The ethical implications of the 5th IPCC Report are clear. The conclusion of the IPCC summary statement contains a powerful re-statement of the sustainability principle of intergenerational equity. It states: ’Cumulative emissions of CO2 largely determine global mean surface warming by the late 21st century and beyond. Most aspects of climate change will persist for many centuries even if emissions of CO2 are stopped. This represents a substantial multi-century climate change commitment created by past, present and future emissions of CO2’.

Only those humans who have absolutely no empathy for the very young and the yet-to-be-born could reach a conclusion that given the information now available to decision-makers, that either we should ignore that information or that we should wilfully deny its importance. The negative impacts of climate change have already been significant to both human and non-human beings and to deliberately continue to impose negative change on life on Earth is now in the realm of the morally wicked. We know from the science the physics of climate change, we know that it is humans that cause enhanced climate change, we know what to do to prevent further climate change as we have carbon-free renewable energy technologies readily available; yet some humans think that we should do nothing about anthropogenic climate change. It is time to call those people out … their position is ethically evil.

Professor Glenn Albrecht is Professor of Sustainability and Director Institute for Social Sustainability at Murdoch University.

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“The IPCC-AR5, authored by 209 Lead Authors and 50 Review Editors from 39 countries, and by more than 600 Contributing Authors from 32 countries, constitutes an authoritative summary of the scientific peer reviewed literature related to climate science, global warming and future trends in the atmosphere-ocean system. It is a document humanity can only ignore at its own peril. Consequent on emission and land clearing releasing more than 560 billion tons of carbon to the atmosphere, at 400 ppm CO2 the level of radiative energy in the atmosphere is the highest since about 3 million years ago and the rate at which greenhouse gases are rising the fastest since 55 million years ago. The AR5 report exposes the myth as if global warming has halted over the last 20 years or so. Warming of the atmosphere/ocean system continues, with some 90 percent of the energy absorbed by the oceans, whereas the continents are in part shielded by the rise in sulphur aerosols released by industry, increasing since about 2001. The AR5 climate projections may convey an impression as if future temperature rises may follow gradual trends. Not enough emphasis is placed in the AR5 on feedbacks from fires and methane release from permafrost, as well as from drying and deforested land and warming oceans. These feedbacks threaten synergy of warming processes, with potential irreversible tipping points in the climate system. 20 – 21st centuries climate change constitutes an unprecedented event horizon in the history of the terrestrial climate. As stated in the latest paper by a large group of US researchers (Hansen et al., 2013) burning of all fossil fuels would render much of the planet inhabitable by humans”

Dr Andrew Glikson is Visiting Fellow at the Climate Change Institute at the Australian National university. He isan Earth and Paleo-climate scientist and an AR5 reviewer.

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“The warming of the earth is unequivocal. There is so much evidence now of the continued warming, melt of Greenland and Antarctica, retreat of glaciers and the warming of the oceans, rising sea level and changing ocean salinity. All of these things makes an unequivocal picture of a changed and warmed earth.

Human influence on the climate system is clear. Humans are actually the cause of all of the observed warming since 1951. Human influence on climate is found everywhere, in every continent and in every ocean basin . All this evidence really does make human influence on the climate system clear.”

Professor Nathan Bindoff is Project Leader, Oceans Program at the Antarctic Climate and Ecosystems CRC and CSIRO-UTAS Professor of Physical Oceanography and a Drafting Author of the IPCC AR5 WG1 Summary for Policymakers.

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“The really big item that quality press outlets will be reporting in the morning is the 95% (that’s ninety-five percent) confidence that most global warming is caused by humans. That much certainty is astonishing for a report like this, from a body that is renowned for its restraint.

Another resolute but sobering item that the IPCC summary communicates is that ‘climate change will persist for many centuries even if emissions of CO2 are stopped.’ In other words, what climate scientists call ‘committed warming’ is now being talked about not just as fixed global average temperature, but a commitment to temperature rise that will continue to increase even if mitigation of C02 dropped to zero. This is because the residence time of current CO2 concentrations in the atmosphere is potentially hundreds of years. This is an aspect of the science that has not been communicated adequately in the past.”

David Holmes is a Senior Lecturer in Communications and Media at Monash University.

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"The IPCC confirms once again, as if we needed confirmation, that the world is warming and defrosting. Arctic summer sea ice extent and volume, spring snow cover, alpine glaciers, and the ice sheets on Greenland and Antarctica, are all shrinking. Cold days and cold nights are becoming less frequent. Sea level continues to rise with no sign of a slowdown. We have just seen the hottest twelve months for Australia on record. As well, over the last decade or so, thousands of people have died in unprecedented heat waves and bushfires around the world. And the best tools we have for projecting climate tell us to expect more warming, more defrosting, more sea level rise, and more heat waves in the future. We can hope that these projections are wrong. But planning for a warmer future seems the safer, more conservative option."

Professor Neville Nicholls is a Professorial Fellow, School of Geography and Environmental Science at Monash University

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“Human-generated aerosols (particles in the atmosphere) exert a cooling effect on global climate. This cooling effect has “masked” some of the warming effect of increasing greenhouse gas concentrations. In other words, without aerosols, the earth would have warmed more than it already has.

“Current climate projections for the 21st century assume that aerosols will decline, as air pollution emissions are cleaned up due to concerns about human health. The rate of this decline is uncertain, but if the assumptions are broadly correct then this “unmasking” will accelerate global warming over the next few decades.”

Dr Leon Rotstayn is a Senior Principal Research Scientist at CSIRO Marine and Atmospheric Research

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“The latest update to the science is in. There is no doubt that our planet is warming, and it is clearer than ever that we are responsible.

This comprehensive new assessment presents the strongest evidence yet of human influence on the climate system. The atmosphere and ocean are warming, snow and ice is melting, and seas are rising. These changes are happening all over our planet. Nothing like this has happened for hundreds, or even thousands, of years. The footprint of human activity is clear. Our emissions have caused greenhouse gases to rise to their highest levels for at least 800,000 years. Increasing greenhouse gases are almost certainly responsible for the warming that our planet has seen since the mid-20th century.

The climate models that we use to predict future changes are more accurate and reliable than ever. They tell us that we can expect warming of at least 1.5 degrees by the end of this century, and more than 2 degrees under more extreme scenarios. Our climate will become more extreme and the oceans will continue to rise. Only through large reductions in our emissions of greenhouse gases can we avoid these outcomes.”

Dr Steven J Phipps is from CSIRO Wealth from Oceans Flagship and is part of the ARC Centre of Excellence for Climate System Science Climate Change Research Centre at University of New South Wales. Steven was not involved in writing the report, however he has contributed climate model simulations towards the chapter on past climate change. The report also cites some of his publications. Steve was a reviewer of the chapter on past climate change.

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“There are two major take home messages in the AR5 Summary for Policy Makers (SPM). The full justification for these messages is not given in the SPM, but is given in the full report. Unfortunately, the full report will not be released until Sept. 30.

The first message is that scientists are now more certain than ever that the main contribution to climate change since the mid 20th century comes from human activities. This represents a steady progression in confidence that began with the first IPCC report in 1990. As more and better observational data has accrued, and climate models have improved, the match between models and observations has become better … both in terms of global-mean temperature change and in the patterns of change of many different climate variables. Recent studies have not only shown these correspondences between the expected changes due to human influences and observations, but have also shown that the observed changes are unlike changes that could occur due to other external influences, such as changes in the output of the Sun, or to internal factors associated with interactions between the atmosphere and ocean.

Related to this, the recent slowdown in the rate of global warming, although not yet fully understood, is not an indication of flaws in models or our understanding of the greenhouse effect. Rather, it is most likely to be due to an unusual amount of heat penetration into the deeper ocean. In simple terms, the human-caused heating that models have suggested should have appeared in the atmosphere (the “signal”) has been accumulating in the deeper ocean. This is an aspect of natural climatic variability, or “noise”, that models cannot predict. Model projections tell us what the expected signal should be, but cannot predict the background noise in which this signal is embedded. All we are seeing today is an unusual manifestation of the noise that has significantly masked the underlying signal.

The second message relates to the climate sensitivity … the amount of global-mean warming that would eventually occur if we doubled the concentration of carbon dioxide in the atmosphere. In the previous report, 6 years ago (AR4), the best estimate for the climate sensitivity was 3 degC. The AR5 does not give a best estimate value this time. Footnote 16 in the SPM states that “no best estimate for equilibrium climate sensitivity can now be given”, but it is possible to derive an estimate for this quantity from the information that *is* given in the SPM. The implied value is 2.3 degC. This, in turn, if correct, would lead to slightly lower projections of future climate change than in previous reports. However, IPCC still notes that there are large uncertainties in quantifying the climate sensitivity. Ironically, although the implied best estimate for the climate sensitivity is lower than previously, at the upper end of the range of possibilities, the probability of large changes is actually increased.”

Professor Tom Wigley is an ARC Discovery Outstanding Researcher Awards Fellow at the University of Adelaide, formerly a Senior Scientist at the National Center for Atmospheric Research in Boulder, Colorado, and, before that, the Director of the Climatic Research Unit at the University of East Anglia. His speciality is the science of climate change, primarily data analysis, climate modeling, and issues related to what is referred to as “detection and attribution”. He has no involvement in the latest IPCC report. However, has contributed on many occasions to previous IPCC reports.

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“This is a bad news, and a good news story. The bad news is that the 2013 IPCC report finally puts to rest the role humans play in causing global warming. The good news is that it highlights we can still avoid 2 degrees of warming if we deeply and rapidly cut emissions of greenhouse gases. The future scale of climate change is therefore still within human control provided the global community does deeply and rapidly cut greenhouse gas emissions.”

Professor Andy Pitman is Director of the ARC Centre of Excellence for Climate System Science at The University of New South Wales. Andy is a review editor and was previously a lead author on AR4.

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“The IPCC assessments are an extraordinary exercise. I don’t know of any other area of science where scientists attempt to assess what we know, and how well we know it, in such a comprehensive way.

After 4 years, multiple drafts, assessment of more than 10,000 published studies, and preparing written responses to more than 50,0000 reviewer comments, it is a little hard to believe the assessment is now complete.

The IPCC 5th Assessment Report shows that we have even greater confidence that climate is changing, humans are largely responsible for the warming observed over the last 50 years, and that substantial and sustained reductions in greenhouse gas emissions will be needed to avoid the worst consequences of climate change. Our hope is that this scientific knowledge helps guide effective decisions about how to respond to the challenge of climate change.

Our chapter on the Oceans shows that warming of the oceans accounts for more than 90% of the energy stored by the climate system over the last 50 years. Sea levels have risen by 0.19 m since 1901, with more rapid rise in recent decades. Changes in ocean salinity show that precipitation and evaporation over the oceans has changed. The oceans have slowed climate change by absorbing about 30% of the carbon dioxide emitted by human activities, but this has come at the cost of ocean acidification."

Dr Steve Rintoul is a CSIRO Fellow at the Antarctic Climate and Ecosystems CRC Oceans Program and a Coordinating Lead Author of Chapter 3 of the report, Observations: Oceans.

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“The world can be more certain than ever that human-induced climate change is a real and serious threat to our planet. Overwhelmingly, the scientific evidence suggests the world should take action to limit the dangers posed by climate change for societies and ecosystems, and to adapt to the changes that are already inevitable. This intensive review of the past five years' scientific evidence was undertaken by hundreds of eminent scientists worldwide and confirms our growing understanding of climate science.”

Professor Suzanne Cory is President of the Australian Academy of Science.

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“Australia’s Climate Policy in a Carbon-Neutral World

The release of the IPCC Report provides an opportune time to consider five important questions related to greenhouse gases.

1. How much of the various greenhouse gases are we pumping into the atmosphere?
2. What impact do these gases have on climate change?
3. As a consequence of questions 1 and 2, how urgent is the climate change problem?
4. When should Australia be making the shift from oil, coal and gas to renewables like solar and wind power?
5. What types of policy are needed to facilitate the shift?

The IPCC Report gives greater precision to the first two questions, and hence enables us to understand better the urgency of the climate change problem. The answer is that it is continuing to be urgent, and certainly much more urgent than the current Australian target of a 5% reduction in emissions by 2020. “The atmospheric concentrations of carbon dioxide (CO2), methane, and nitrous oxide have increased to levels unprecedented in at least the last 800,000 years.” These increases are largely human caused and “most aspects of climate change will persist for many centuries even if emissions of CO2 are stopped.”

Noting that it “is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century”, the IPCC Report maintains emphasis on a global target of restricting temperature increases to below 2oC. Australia’s climate policy will need to be tightened considerably.

The answer to Question 4 is that Australia should probably be moving towards renewable energy sources more quickly than we have been. This will not only reduce our greenhouse gas impact, but also place us at a competitive advantage in the new technologies relative to our trading partners.
In answer to Question 5, almost certainly a mix of policies is the best way to encourage the required shift in technology. A carbon price, through an emissions trading scheme (ETS), has been shown to be economically efficient. Moreover it is becoming the international standard. Direct action and providing loan finance through the Clean Energy Finance Corporation can encourage fledgling renewable energy firms. Moreover, towards the end of the decade, if the greenhouse gas problem continues to build-up, direct regulation and a smaller emissions cap in the ETS may be required to restrict emissions.”

Professor Kevin A Parton of the Institute for Land, Water and Society at Charles Sturt University.

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“Confidence in the science surrounding climate change is now unequivocal.

It is clear that we must take a strategic long-term approach to tackling this long-term problem. Without concerted global efforts to reduce greenhouse gas emissions, of the order of ten per cent every year, the world will face severe climate consequences, including potential temperature rises in Australia as high as six degrees.

Implementing carbon capture and storage (CCS) is now more urgent than ever.

CCS is proven technology that can be installed right now to cut emissions from major sources such as power stations by up to 90 per cent, preventing millions of tonnes per year from entering the atmosphere.

For electricity generation, no low carbon alternative will be cheap but CCS is competitive with solar, nuclear and wind, while having the advantage of using current energy infrastructure. Further, costs are dropping every year as the technology develops.

To build the CCS systems of tomorrow we need to start today. Governments around the world, including Australia, the US, China and the UK, are investing in CCS R&D and demonstration projects but it is vital this momentum is accelerated, not left to drift.

The Fifth Assessment Report makes it clear that climate change will not go away, and that without concerted, strategic efforts to address emissions there will be major consequences to the environment and humanity, particularly the poorest and most vulnerable.

Technologies to tackle emissions are at hand – the will to implement them is what is needed now.”

Dr Richard Aldous is Chief Executive of the Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC).
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“The IPCC assessment reports are like a snapshot of our scientific understanding of planet earth. With the release of each new report, it is like we are viewing the planet with a new and better camera: the resolution is higher, the images sharper, and we can resolve increasing amounts of detail. But the basic picture hasn’t changed: the planet is still warming, that warming is still mostly caused by human emissions of greenhouse gases, and it will cause tremendous adverse impacts to humans and the natural environment.”

“As a hydrologist, we need to know whether extreme rainfall is intensifying, so that we can be prepared for any increases in the risk of natural disasters such as floods. Since the previous IPCC report, the evidence that extreme rainfall has intensified in many regions around the world has strengthened and this is reflected by stronger wording in the current document. If these trends continue - and the climate models indicate they will - then we really need to have a hard think about how to adapt to the increasing risk of natural hazards that might be expected over the coming decades.”

“The revised projections of sea level rise are particularly interesting. Australians are very vulnerable to increases in sea level, with all of Australia’s major cities except Canberra being coastal cities. In fact, a 2009 Australian parliamentary enquiry noted that approximately 711,000 addresses are less than 6 m above sea level. Of course not all of those houses will be affected by a metre of sea level rise such as is at the upper range of the IPCC’s projections, but many of them will be. And how we deal with that as a society will pose some very complex questions.”

Dr Seth Westra is a hydroclimatologist in the School of Civil, Environmental and Mining Engineering at the University of Adelaide. Seth is an IPCC expert reviewer.

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“"Anyone who thought climate change was going to diminish as a threat to Australia's future should be quickly disabused of that idea by today's IPCC report. For much of regional Australia, the challenges of a hotter drier climate are likely to escalate reinforcing the need for sustainable water, soils and land use policies. For low lying coastal Australia, urban planning must take account of the IPCC expectation of faster sea rises this century. Co-ordinated international action to reduce emissions might still be a dream, but its imperative is made no less compelling by today's IPCC report."

Professor John Cole is Executive Director of the Institute for Resilient Regions at the University of Southern Queensland.

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“This report shows that we have never been more certain that greenhouse gas emissions from human activity are causing the climate to warm. As it stands, eight years during the past decade are in the top 10 of the warmest on record since 1880: 2012 (9th), 2011 (10th), 2010 (1st), 2009 (8th), 2007 (3rd), 2006 (7th), 2005 (2nd), 2003 (6th). This is remarkable given that we really haven’t had any strong El Niño events in this time. El Niños are usually associated with very warm years globally – expect warming to ramp-up when the next El Niño is upon us.

I am concerned about the unabated increase in upper ocean heat content reported by the IPCC. This is of particular relevance for eastern Australia where heat stored in the upper ocean has contributed to the major extreme rainfall and Queensland flood events during the summers of 2011 and 2012.

With a large population living close to the coast in Australia it is a real worry that sea level estimates have been revised upwards in the current IPCC Report – and their estimate is on the conservative side. This means we need a major rethink of how we manage, use, and develop our coastal communities and cities.”

Dr Helen McGregor is AINSE Senior Research Fellow in the School of Earth and Environmental Sciences at the University of Wollongong.

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“The IPCC report is perhaps the most heavily scrutinised document in the history of science, requiring several years of work. The dedicated scientists that volunteer their time to be part of this process include some of the best climate scientists in the world. There are some 40 Australian authors involved in the development of the IPCC’s Fifth Assessment Report, including nine CSIRO scientists in their capacity as Coordinating Lead Authors and Lead Authors. The scientific evidence that is described in the assessment continues to demonstrate that the climate is changing, the earth is warming, and human actions are the cause of much of this warming.”

Dr Helen Cleugh is Deputy Chief CSIRO Marine and Atmospheric Research.

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“The release of the IPCC 5th Assessment Report Working Group 1 today has confirmed without any doubt that we are living in a warming world. Nevertheless, poor understanding of clouds, and the impact of dust in the atmosphere, has led to uncertainty about changes in global mean temperatures. Both of these have a cooling effect, so terrestrial temperatures may not have not have risen as much as previously expected (although the oceans have).

This 5th Assessment report also differs from previous ones, is that expectations about changes in global rainfall are reduced. While this means there may be little change in the quantity of rainfall expected globally, it is highly likely that the geographical distribution of this will change. This will be a result of the increased number of extreme weather events which can be expected in the foreseeable future. This will have a significant effect on humans.

All of this scientific knowledge is commendable in its thoroughness, but what does it really mean to us as citizens, dependent on the Earth System? One clear message that comes through is that as a society, we need to continue to prepare for change. Our whole approach to food production, transportation systems and energy generation must be reassessed. Across the world, we need to build stronger, more resilient communities. We need to address equity in the distributional impact of the climate shifts we can expect in the future.

What this report really tells us is that while we may not know everything about what is happening in the Earth’s climate, we do need to act now to secure the integrity of our life support system. It is now more than ever important for us to determine common core values about human survival, and build them into all aspects of human endeavour.”

Dr Caroline Sullivan is Associate Professsor of Environmental Economics and Policy, at Southern Cross University, NSW. Caroline has not contributed directly to the IPCC report but it is possible some of her work may have been cited therein. She is an ecological and environmental economist who has worked internationally on climate change projects since 2001.

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Comments from New Zealand experts

"The latest assessment forms the most comprehensive evaluation of climate change yet undertaken. The new report consolidates and expands upon scientific understanding from previous reports.

"Improved observational networks and advances in climate modelling have helped scientists continue to improve understandings of the uncertainties on the rate of warming. In terms of the global mean climate response, the report finds that:

1) It is extremely likely that human activities caused more than half of the observed increase in global mean surface temperature since 1950;

2) It is virtually certain that natural variability alone cannot account for the observed global warming since 1950;

3) Global mean temperatures will continue to rise over the 21st century if greenhouse gas emissions continue unabated;

4) The principal driver of long term warming is the total cumulative emission of CO2 over time;

5) To limit warming caused by CO2 emissions alone to be likely less than 2°C, total CO2 emissions from all anthropogenic sources would need to be limited to a cumulative budget of about one trillion tonnes of carbon, emitted as CO2, over the entire industrial era, about half of which have been emitted by 2011.

"In other words, we're increasingly confident that human influence, primarily via the use of fossil CO2, is changing the climate. We expect this to continue in line with long-held scientific expectations - as it has over the past few decades - and we think that achieving a 2°C target requires limiting CO2 emissions to around half a trillion tonnes."

Professor Dave Frame, Director NZ Climate Change Research Institute, Victoria University of Wellington

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"This report provides policymakers and the public with a very thorough assessment of the current state of knowledge. It describes the changes which have been observed around the globe, examines their causes, and outlines expected climate changes under four future greenhouse 'concentration pathway' scenarios, which between them span a broad range of possible future greenhouse gas emissions.

"These scenarios provide policy-relevant information for governments.

"Under the highest emission scenario, there is at least a 50% chance that the global surface temperature increase by the end of this century will exceed 4°C above pre-industrial times. But under the lowest scenario, global surface temperature increase is unlikely (less than 33% chance) to exceed 2°C.

"This lowest-emissions scenario includes substantial reductions in anthropogenic greenhouse gas emissions as the century progresses and possibly sustained removal of carbon dioxide from the atmosphere by the end of the 21st Century. We'll now be doing further work at NIWA on the way New Zealand climate is likely to change under these various scenarios".

Dr David Wratt, Director - New Zealand Climate Change Centre at the National Institute of Water and Atmospheric Research (NIWA)

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"Longer observational records, improved models and better understanding tell us that climate change will be on-going this century and beyond and will bring significant changes to New Zealand and to the Pacific. By the end of the century, extreme heavy rainfalls are likely to become more intense and more frequent in many places while at the same time the risk of drought is set to increase substantially, notably in the east and north of New Zealand. An increased frequency of high temperature extremes, and fewer cold extremes, is virtually certain almost everywhere.

"In the tropics, monsoon rainfall amounts are likely to increase, and while the El Niño-Southern Oscillation cycle will continue to be a major feature of year-to-variability, the associated rainfall variability is likely to increase. Tropical cyclone number are unlikely to increase, but the average strength (intensity of winds and rainfall) of tropical cyclones is likely to increase. The South Pacific Convergence Zone, a major feature of rainfall variability in the tropical Southwest Pacific, may become more variable in its movement and rainfall intensity, which would be associated with increased risk of both floods and droughts for many of our Pacific neighbours."

Dr James Renwick, Associate Professor, School of Geography, Environment and Earth Sciences, Victoria University of Wellington

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"The report shows carbon dioxide are now at levels unprecedented in at least the last 800,000 years and it is very likely that the Arctic sea ice cover will continue to shrink and thin as the region is expected to warm more rapidly than other areas of the world.

"In addition, the volume of the polar ice sheets and glaciers globally will continue to decrease contributing to a global mean sea-level rise between 26-82 cm by the end of the century, depending on the greenhouse gas concentration pathway we end up following.

"A significant effort has been made by the scientific community since the last assessment report to better understand the contribution from the melting of the polar ice sheets to future sea-level rise, and while uncertainties still remain, dynamic ice sheet loss is incorporated in the range of sea-level estimates for 2100 in this latest report.

'However, the report cautions, that collapse of marine-based sectors of the Antarctic Ice Sheet, if initiated, could cause sea-level to rise substantially above these "likely" ranges during the 21st Century

'The report also tackles the issue of climate change commitment as a consequence of the stock of anthropogenic carbon dioxide already in the atmosphere, and suggests many aspects of climate change, including polar ice sheet melt and sea-level rise will continue for centuries, even if carbon dioxide emissions were stopped."

Professor Tim Naish, Director of Victoria's Antarctic Research Centre

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Comments from UK experts

Prof Corinne Le Quéré, Director of the Tyndall Centre for Climate Change Research and Lead Author on Chapter 6 (Carbon and Other Biogeochemical Cycles) said:

"This is not just another report, this is the scientific consensus reached by hundreds of scientists after careful consideration of all the available evidence. The human influence on climate change is clear and dominant. The atmosphere and oceans are warming, the snow cover is shrinking, the Arctic sea ice is melting, sea level is rising, the oceans are acidifying, and some extreme events have increased. CO2 emissions from burning fossil fuels need to substantial decrease to limit climate change."

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Dr Tim Osborn, Reader in Climate Change at UEA’s School of Environmental Sciences and Lead Author on Chapter 5 (Information from Paleoclimate Archives) said:

"Through this exhaustive – and, at times, exhausting – process we have produced the independent and comprehensive assessment of climate science that governments and the public need to understand climate change. We are now more certain than ever that many aspects of the climate have been influenced by human activity. Looking back at past climatic changes – which has been my main contribution to this report – adds rich detail to our understanding of the climate system. For example, we now know that carbon dioxide levels, which have increased by 40% and are the largest driver of the warming we have observed over the past century, substantially exceed the levels of the last eight hundred thousand years."

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Dr Tim Johns, Met Office Hadley Centre and Lead Author on Chapter 12 (Long-term Climate Change: Projections, Commitments and Irreversibility) said:

“As the IPCC Working Group I contribution to the Fifth Assessment Report (AR5) - Climate Change 2013: The Physical Science Basis - is released, my overriding impression is of the massive worldwide scientific effort, expertise and rigour woven into the production of this assessment, underpinned by a rapidly developing science base. The report presents a robust picture of a progressively warming world, reducing Arctic sea ice extent, melting ice sheets and glaciers, rising sea level, ocean acidification, and large-scale hydrological cycle changes under the influence of anthropogenic emissions of greenhouse gases. Despite some inevitable scientific limitations to understanding of the physical climate system, the central conclusions in the Summary for Policymakers are more sharply drawn in many respects than in the IPCC's 25-year history.

“Climate models play a central role in the assessment of attributed historical climate change and projected climate change through the 21st century and beyond. An unprecedented worldwide modelling effort known as CMIP5 - described as "the moon-shot of climate modelling" by eminent US climate scientist Gerald Meehl - was undertaken to feed the most comprehensive model-based evidence ever about past and future changes and their uncertainties into AR5. Common experiments using different models were run by teams in several countries in Europe, as well as the USA, Canada, Russia, China, Japan, Australia and South Korea. Climate models have undoubtedly improved since the last report (AR4), and the new generation of "Earth System Models" increasingly incorporate biogeochemical cycles that reflect important additional climate change feedbacks, providing the means to quantify the policy-relevant issue of how much carbon-dioxide emission is compatible with a given climate stabilisation pathway.

“Taking results from the latest generation of models for a range of Representative Concentration Pathways (RCPs; future emissions scenarios), AR5 concludes that cumulative anthropogenic carbon-dioxide emissions would need to be limited to around 1000 petagrams (10 to the power 18 grams) of carbon to be likely to limit global warming to no more than 2 degrees Centigrade, relative to the early industrial era (1861-1880). However, half or more of this anthropogenic carbon budget has already been 'spent', and accounting for climate forcing agents other than carbon-dioxide tends to reduce the future carbon budget available to be likely to achieve a given warming target.

“The science has spoken and the potential for dangerous climate change in this century is increasingly clear.”

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Prof Bob Watson, Tyndall Centre for Climate Change Research and the University of East Anglia, said:

“The latest IPCC report strengthens its earlier conclusions that most of the observed warming since 1950 has been caused by human activities, and future changes are inevitable. Also, many of the other changes observed in the climate system, such as the rate of loss of Arctic sea Ice, melting of mountain glaciers and the Greenland Ice sheet are unprecedented. Without immediate reductions in global emissions of greenhouse gases, the world will not be able to achieve the political target of limiting the increase in global mean surface temperatures to 2 degrees C, but rather we are likely to see an increase of 3-5 degrees C. Time to act is running out if we are to take the threat of human-induced climate change seriously.”

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Prof Kevin Anderson, Deputy Director of the Tyndall Centre for Climate Change Research, Professor of Energy and Climate Change at the University of Manchester, said:

“What has changed significantly since the last report is that we have pumped an additional 200 billion tonnes of CO2 into the atmosphere. Annual emissions are now 60% higher than at the time of the first report in 1990 and atmospheric CO2 levels are the highest they have been for over 2 million years.

“So what are we doing in the UK to help reverse this reckless growth in emissions? Record levels of investment in North Sea oil, tax breaks for shale gas, investment in oil from tar sands and companies preparing to drill beneath the Arctic. Against this backdrop, the UK Treasury is pushing for over 30 new gas power stations, whilst the government supports further airport expansion and has dropped its 2030 decarbonisation target – all this alongside beleaguered plans for a few wind farms and weak energy efficiency measures.

“Governments, businesses and high-emitting individuals around the world now face a stark choice: to reduce emissions in line with the clear message of the IPCC report, or continue with their carbon-profligate behaviour at the expense of both climate-vulnerable communities and future generations.”

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Dr Alice Bows-Larkin, Reader in Energy and Climate Change, Tyndall Centre at Manchester University

“Six years on from the last IPCC report, and little has changed. Every year we burn more fossil fuels producing more CO2, the laws of physics still hold that the rising concentration of CO2 warms the atmosphere, and as this warming continues, the risk of disruptive physical and social impacts increases. The big unknown is if, or when, we will manage to break our addiction to fossil fuels, and where that will leave us in terms of future climate impacts. Personally, if investing in my future and that of my family, I would look beyond fossil fuels being mindful of the risk of stranded assets left in a future that whichever path we choose, will certainly be very different from today.”

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Prof Jim Hall, Chair in Climate and Environmental Risks, Director of Oxford University’s Environmental Change Institute, said:

“The scientific case for implicating human activity in climate change was made long before this Fifth Assessment of the IPCC. This new report painstakingly documents the scientific evidence that has emerged in recent years. I respect the scientists who wrote it and admire them for the work they have done for the IPCC. Some of the evidence has moved on, and future projections have apparently changed compared to the Fourth Assessment Report, but nobody with experience of complex computer models and uncertain observations would be surprised by that. The underlying trend of rising average temperatures and sea levels is clear; I have to question the motivation of anyone who disputes these facts.”

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Prof Richard Dawson, Chair of Earth System Engineering, School of Civil Engineering and Geosciences, Newcastle University and the Tyndall Centre, said:

"More than half of the world’s population now lives in urban areas, many of them located in low-lying coastal or delta areas. Urban areas concentrate people, infrastructure and economic activity, making them disproportionately vulnerable to weather extremes like heat waves or flooding. Furthermore, they are major consumers of resource and producers of pollutants both within and outside their boundaries. The latest IPCC findings highlight that in the face of continued global change it remains an international priority to adapt urban areas and infrastructure to be more resilient to a wider range of environmental conditions, and to reduce their contribution towards emissions through more efficient use of resources and reduced greenhouse gas emissions. "

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Prof Andy Jordan, Chair in Environmental Sciences, Tyndall Centre and the University of East Anglia, said:

“The latest IPCC report confirming the science of climate change comes at a pivotal moment, when EU policy makers are battling to resuscitate the emissions trading system, reform internal policies on biofuel and enthuse other countries to agree a successor to the international Kyoto protocol by 2015.”

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Dr Charlie Wilson, Lecturer in Energy and Climate Change, Tyndall Centre and the University of East Anglia
“Mitigating climate change requires both widescale diffusion and accelerated innovation of low carbon energy supply technologies and efficient energy end-use technologies. Dramatic improvements in the efficiency with which energy is used are critical in the near term to allow more flexibility in decarbonising the energy supply.”

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Prof Keith Shine, Professor of Physical Meteorology at the University of Reading and Review Editor of Chapter 8 (Anthropogenic and Natural Radiative Forcing) said:

"Even in the IPCC's first assessment report, back in 1990, it was clearly understood that natural climate variability could slow global warming in some decades and speed it up in others. We simply don't expect each year, or even each decade, to necessarily be warmer than the previous year or decade, but we do expect the longer term tendency to be for continued warming.

“So, there is no surprise that hiatus periods occur, even when the longer-term trend in temperatures is upwards. The observed temperature change over the past 60 years is consistent with expectations from our best understanding. Even accounting for the ‘hiatus’, the decade of 2000-2009 was clearly warmer than any recent decade, and the years 2005 and 2010 were two of the warmest years in the 150 year temperature record."

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Dr Chris Huntingford, Climate Modeller at the Centre for Ecology and Hydrology (CEH), said:

"Continuing uncertainty fuels the argument of those sceptical of global warming. Given that massive decarbonisation could have major economic implications, this argument can seem compelling. But to 'wait and see' could trigger hugely dangerous impacts if temperatures increase to the higher end of predicted ranges. This remains true even as recent studies suggest that the upper bounds may be lower than previously predicted. Thermal lags could create false optimism, as a CO2 concentration unrecognized as dangerous may be reached a few decades before the full temperature implications are realized. Then, even if mitigation measures somehow reduced net CO2 emissions to near zero, the planet would take centuries to reset itself. Forewarned is forearmed when preparing for climate change. Concerted effort is essential to improve the certainty of climate forecasts.”

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Dr Colin Summerhayes, Scott Polar Research Institute and reviewer of Chapter 5 (Palaeoclimates) said:

"We are warming when we should be cold. The new report from the IPCC confirms in significantly more detail than before what we understand about climate change from the geological record. This is an aspect of climate studies that the public rarely hears anything about. The new report shows yet again that the global warming we are seeing today cuts right across what we would expect from our knowledge of climate change over the past 11,000 years.

“Earth's climate is driven ultimately by energy from the sun, and the amount of solar radiation we get (our insolation) changes with (i) the eccentricity of the Earth's orbit, (ii) the changing position of the Earth relative to the sun around the orbit (Earth is now closest to the sun in December, so we have 'mild' winters, and in 11000 years time will be closest to the sun in July, which will give us colder winters), and (iii) the tilt of the Earth's axis (more tilt gives more seasonality). These three factors combine to drive the Earth in and out of glacial periods. They made our insolation highest 11000 years ago, which melted the great northern hemisphere ice sheets. Since then, insolation has declined steadily to the present, making our climate progressively cooler, such that we ended up in a Little Ice Age between about AD 1450 and 1850. Calculations of our relationship to the sun tell us that insolation should stay low for another 1000 years, so we should be experiencing a continuation of the Little Ice Age and having Frost Fairs on the Thames. But we aren't. Since about 1900, everywhere you look, past climate data suddenly shoot upwards quite quickly, indicating warming against the trend. Two things are likely to explain this. One is that sunspot activity increased slightly from about 1820 up to about 1950. That would have caused minor warming. The other is that CO2 increased from about 1760 to the present. That would also have caused minor warming. The combined sunspot activity and CO2 increase explain the warming of the first part of the 20th century. But the sunspots and other measures of solar output have not increased since about 1950, while CO2 has gone on increasing. Not surprisingly, since CO2 is a greenhouse gas, our global temperature has gone on increasing way beyond 1950. We are recreating a situation that occurred on a large scale back 55 million years ago at the boundary between the Palaeocene and Eocene geological periods when there was a massive injection of CO2 into the atmosphere, and temperatures rose about 6 degrees C, though that happened about 100 times slower than what is happening now. Never mind computer models, the record of our geological past tells us a lot about what is now going on and what to expect in future.

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Prof David MacKay FRS, Chief Scientific Advisor to the Department of Energy and Climate Change, said:

“I’m not a climate scientist, but I often read the climate science literature and attend meetings where climate scientists discuss their work.

“The climate science community are doing excellent, open science: climate scientists are healthily critical of their own work, and well aware of the questions that have yet to be resolved.

“The climate system is astonishingly complex, and I admire the steady progress that climate scientists are making to improve our understanding of this remarkable, dynamic world in which humanity is sustained. The IPCC’s fifth assessment report has been produced by the generous work of hundreds of scientific experts drawn from universities and research institutes around the world. There is no equivalent of the IPCC in any other area of science.

“This latest report is the most authoritative and comprehensive report to date of our understanding of climate change. The scientific consensus is that the world has warmed and will warm more, owing to human activities. There is robust evidence that human greenhouse gas emissions are already changing our world; global temperatures have risen every decade for the last three decades, oceans are acidifying, rainfall patterns are changing, sea levels are rising, arctic sea ice is declining, and some extreme weather events are becoming more frequent and intense.

“It is predicted, from simple physics, that the more humanity increases the quantities of greenhouse gases in the atmosphere, the warmer the Earth will become. The far-reaching consequences of this warming are becoming understood, although some uncertainties remain. The most significant uncertainty, however, is how much carbon humanity will choose to put into the atmosphere in the future. It is the total sum of all our carbon emissions that will determine the impacts. We need to take action now, to maximise our chances of being faced with impacts that we, and our children, can deal with.

“One important message of this new report is that, while there remains some uncertainty about the precise sensitivity of the climate to greenhouse gas emissions, the impact on climate is largely determined by the cumulative total of humanity’s carbon emissions. This means that waiting a decade or two before taking climate change action will certainly lead to greater harm than acting now.”

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Dr Emily Shuckburgh, British Antarctic Survey, said:

"The science as outlined in the new report is clear: our collective actions have generated a climate problem that threatens our future and our children’s future. Increasing levels of greenhouse gases are disrupting our climate. And because it is the cumulative amounts of greenhouse gases that determine the severity of the impact, any delay in reducing emissions will lead to greater risks and a need to deploy more difficult and expensive means to adapt to the impacts.

“One of the key developments since the last report has been an increased understanding of changes in the polar regions and their global effects. Arctic sea ice has declined significantly and new research is starting to shed light how this affects our weather in the UK. The Antarctic Peninsula has seen significant warming and the breakup of a number of its ice shelves. The Southern Ocean around Antarctica has warmed throughout its depth. And it has now been possible to estimate the contribution of melting of the polar ice sheets to sea level rise. But important open questions remain. For example, the Southern Ocean currently soaks up approximately about 10% of our carbon dioxide emissions, thus limiting their accumulation in the atmosphere. However, we are presently uncertain whether this ‘carbon sink’ might start to fail in future as a consequence of climate change."

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Prof Joanna Haigh, Professor of Atmospheric Physics at Imperial College London, said:

“The new IPCC report confirms, with even greater confidence than in previous reports, that global warming continues and that this is largely a result of greenhouse gases produced by human activity. The concentration of carbon dioxide in the Earth’s atmosphere now exceeds anything it has experienced in the past 3 million years and its continuing upward trend is almost certain to result in further global warming. Changes in solar activity alone cannot explain the global surface temperature variations of the past 150 years and, even if the Sun were to enter a new ‘grand minimum’ state within the next century, would be very unlikely to provide more than a small, temporary, partial compensation for likely anthropogenic warming.”

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Prof Peter Wadhams, Professor of Ocean Physics at the University of Cambridge and Review Editor for Chapter 1 (Introduction) said:

“There are serious deficiencies in the modelling. The assessment makes extensive use of the UK Met Office model which does not take account at all of the feedback due to emissions from thawing permafrost. Nor is the possibility of a major methane emission from melting offshore permafrost on Arctic shelves mentioned, even though this has been projected as capable of adding 0.6 C to global warming.

“These are examples of an instinctively cautious approach which in other circumstances might be praiseworthy but in the circumstances of a global climate emergency is not. The world does deserve to have the full range of risks and possibilities explored with, if possible, some probability factors attached to them, instead of consistent under-estimate of effects or simply ignoring a phenomenon where the magnitude is difficult to compute (as in the case of sea level rise from glacial runoff in AR4, which did endless harm).”

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Prof Jonathan Bamber, Director of the Bristol Glaciology Centre at the University of Bristol and Review Editor of Chapter 4 (Observations: Cryosphere) said:

"The evidence of persistent and continued changes to all frozen parts of the planet is clearer than ever. Glaciers around the world have been declining over at least the last 5 decades, mass loss from the great ice sheets covering Antarctica and Greenland has been increasing over the last two decades and the reduction in late-summer sea ice in the Arctic appears to be unprecedented over the last 1500 years. Permafrost temperatures have been increasing and seasonal snow cover is arriving later and melting sooner. The evidence that humans are at least partly to blame for climate change is stronger than ever."

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Prof Mike Hulme, Professor of Climate and Culture at King’s College London, said:

“This latest assessment of climate science from the IPCC threatens to distract from resolving the core issue of climate change - the political challenge of finding policy interventions that are effective and plausible. The difficulties in implementing policies that reduce the dangers of a changing climate don’t result from a deficiency of scientific knowledge. Raising the confidence that humans are a major influence on climate from ‘very likely’ to ‘extremely likely’ doesn’t change the politics of climate change. The difficulties arise because of different interests, values and attitudes to risk. These can only be worked through using political strategies that are less constrained by the need to reach global agreements.

“We need a more pragmatic politics of climate change, not more weighty science about climate change.”

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Prof Ted Shepherd, Grantham Professor of Climate Science, University of Reading and Review Editor of Chapter 11 (Near-term Climate Change: Projections and Predictability) said:

“This report is the most comprehensive assessment of climate change ever performed, involving hundreds of scientists and exhaustive peer review. It reaffirms what we have known for some time — that human activities are changing climate, and that strong action on carbon dioxide emissions will need to be taken to avoid dangerous interference with the climate system. However we now have a much better understanding of how the different pieces fit together, which is to say a more self-consistent understanding at a quantitative level. There has also been an enormous effect made to attempt to quantify the “known unknowns”, rather than just relying on the “known knowns” represented by the climate models, by comparing the climate models with both modern observations and reconstructions of past climate. This is what has led to some refinements of earlier estimates, but we now have more confidence in those estimates — especially on the upper and lower bounds of climate sensitivity (i.e. the worst case and best case scenarios), which is what the policy makers need to know.

“We need to distinguish cycles in the climate system, which can occur for natural reasons, from the long-term changes. Climate change has to do with the energy budget of the climate system, and this is best measured from the “slower” parts of the system. Surface temperature is the manifestation of climate change, but just like the difference between a person’s current account and their net wealth, it can vary much more rapidly that the total heat content of the climate system, which is mainly contained in the ocean. Thus we know that climate is continuing to change because measurements tell us that the climate system is continue to emit less energy than it receives from the Sun (the heat trapping or greenhouse effect), the ocean is continuing to warm, and sea level is continuing to rise.

“Both observations and models tell us that the sort of pause in the increase in surface temperature that we have seen over the last 15 years is neither unusual nor unexpected. It would be far more puzzling if surface temperature was continuing to rise and these other metrics were not changing!
Climate change is an especially dangerous threat because it is like a large ship: you can turn off the engines but it will continue to move forward for a long time. This is called the “commitment” problem. It means that if we wait until climate change is unmanageable before we act, it will be far, far too late because the changes will keep happening for centuries. So in that respect it is a very different environmental problem than, e.g., pollution.

“A new aspect of this report is a whole chapter devoted to assessing climate change in the near-term, i.e. over the next few decades. This is a far more challenging problem because of the influence of natural variability, so one cannot expect statements of high confidence. Instead, we need to think in terms of the risks we face from outcomes that are perhaps only “likely” (e.g. two out of three). This is a rapidly growing area of research because of its policy relevance in terms of impacts.

“The “greenhouse effect” is not just a theory, it is a fact. Life exists on earth because of the greenhouse effect, which warms the planet to habitable levels. Nobody disputes this. Climate change is an enhanced greenhouse effect due to higher levels of atmospheric carbon dioxide produced by human activities. The scientific debate concerns how much of the emitted carbon dioxide will remain in the atmosphere, how much additional warming will result from the additional carbon dioxide, and how quickly this additional warming will occur.”

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Prof Mark Maslin, University College London, said:

“A hundred year from now future generation will look back at the huge amount of evidence in the IPCC reports and wonder why people failed to act. School children will learn in their history classes that the world’s environment was devastated due to the political myth that huge profits can only be created by extracting and consuming fossil fuels.

“The IPCC have synthesised six years of detailed climate science and has found results entirely consistent with all the previous IPCC reports dating back to 1990. The message from over 23 years of detailed science is that climate change has already occurred and if we do nothing to curb global carbon emissions it will accelerate in the future.

“The clear scientific message of the IPCC 5th report is that the current level of carbon emissions will lead to dangerous climate change with major implications for the global economy and human health. With no international climate agreement currently likely before 2020, zero carbon emissions for Developed countries may not be enough to avert disaster. We may now have to phase out fossil fuels completely and start planning to take carbon dioxide out of the atmosphere.”

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Prof Andrew Shepherd, Professor of Earth Observation at the University of Leeds, said:

"It's clear, from the past 20 years of satellite measurements, that there have been dramatic losses of ice from both Antarctica and Greenland. Back in the 1990's, the polar ice sheets were responsible for just a tenth of all sea level rise, but today they are contributing three times as much. The problem for climate science is building models that can capture the changes in Earth's ice that we can see from space today.

“Until we can do that with confidence, we really can't be sure how much sea level rise to expect over the next century. If, for example, we were to discard climate models and make predictions based on satellite measurements alone, we would expect the ice sheets to contribute another 30 cm of sea level rise by the year 2100. This figure represents the upper limit of the IPCC's latest predictions in AR5, and underlines the importance of building the very best climate models."

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Prof Sir Brian Hoskins, Director of the Grantham Institute for Climate Change at Imperial College London, said:

On IPCC and robustness of the science:
“This Summary for Policy Makers provides further strong confirmation that human activity is having a significant and growing impact on the climate.

“It is based on a comprehensive review and rigorous assessment of the state of climate science by some 850 scientists, who reviewed over 9000- scientific articles, and includes voices from all sides of the issue. This report significantly strengthens the consistent message from the four previous assessment reports; we are conducting a dangerous experiment with our planet.”

“The evidence of changes in many different aspects of the climate system, from the ice sheets to the deep ocean, shows that climate change is happening. To reduce the serious risks posed by increasing changes in the climate, we need to redouble our efforts globally to limit carbon dioxide and other greenhouse gas emissions.”

On continuing warming:
“Global mean surface temperature increased by about 0.89°C since the start of the 20th century. Despite the recent slowdown in the rate of temperature rise, each of the last three decades has been warmer than all previous decades in the instrumental record. The decade that began in 2000 has been the warmest.

“Many observations of the climate system over the past 15 years paint a picture of increasing climate change. The ocean absorbs over 90 percent of the additional energy flowing into the climate system due to greenhouse gas emissions. In the past decade or so the upper ocean, like the atmosphere, has not warmed much. However with recent observations it is becoming evident that the deep ocean is taking much of the excess heat during this period. Sea level is continuing to rise at around 3mm per year partly because the warmer water in the oceans expands. The rest of the rise is due to the mountain glaciers that are continuing to melt rapidly and the ice loss from the major Greenland and Antarctic Ice Sheets that has accelerated dramatically.

“We are making long-term alterations to the climate system which will have major impacts for generations to come unless we accelerate efforts to reduce emissions.”

On equilibrium climate sensitivity:
“The IPCC’s new estimate of Equilibrium Climate Sensitivity is consistent with estimates first made in 1979. It is very difficult to estimate climate sensitivity precisely, because of the uncertainty over key aspects of the climate system’s response to human emissions of greenhouse gases.

“The reduction in the lower bound of the estimated likely range to 1.5 degrees of warming for a doubling of atmospheric carbon dioxide should not distract us from the concern that we may well be on track to exceed three degrees warming by the end of the century on current emissions trends.

“If climate sensitivity is at the lower end of the range, it may make it a bit easier to limit warming to two degrees through large-scale emissions reductions. The case for action on climate change does not rest on hoping for the best, but on the potential scale of the climate risks and reducing these risks.

“The risks depend on the trajectory of global emissions over the next few decades and the whole range of estimates of the climate sensitivity. If we continue at current rates of emissions and if the upper limit of the IPCC’s estimate of equilibrium climate sensitivity is right – which is unchanged at a 4.5 degree warming for a doubling of atmospheric carbon dioxide levels – then we really are entering extremely dangerous territory. So the message to governments is clear: we need to accelerate efforts to reduce emissions, whatever the real value of the climate sensitivity is. “

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Prof Piers Forster, Professor of Climate Change at the University of Leeds and Lead Author on AR5 Chapter 7 (Clouds and Aerosols) said:

On the whole report:

“We have been causing 40% more warming than we estimated in our 2007 report. Greenhouse gases have continued to rise giving stronger warming and the cooling effect of particles formed by pollution is weaker. Further, this report has really firmed up understanding of rainfall and sea-level change. Over much of the world extreme rainfall will be heavier and occur more often and unless we begin to dramatically change our ways, we could have up to 1m and growing sea-level rise by 2100.”

On the slowdown:

“I think Chapter 9 authors and the TS do a great job of placing the slowdown in the context of centennial-scale past and committed manmade climate warming. We have caused past climate warming and the world will continue to warm. We recognise that the slowdown is an important event to understand, especially as it is not obviously reproduced in climate models.The slowdown since 1998 itself is likely due to a combination of natural (solar and volcanic effects) and extra heat from greenhouse warming being sucked into the deep ocean. Climate models can capture such slowdown events but there is the possibility that some models are over responsive . Any over-response would only be a small effect though and the slowdown does not significantly affect our 2100 projections. However, we do take it into account for our near term projections. Provided there is not a large volcanic eruption, we can expect warming between 0.3 and 0.7 C over the next 20 years.”

On the process:

“Nevertheless, after over 4 years of work with massively bright colleagues, careful reviews and then
all-night plenary scrutiny, I believe we have produced a document of unprecedented quality, robustness and usefulness for policy makers. A document of such quality is rare in any discipline.”

Prof Rowan Sutton, and Lead Author on Chapter 11 (Near-term Climate Change) said:
“All the evidence in this new report, together with our long standing understanding that climate takes decades to respond to changes in greenhouse gas emissions, makes it clear that leaving the issue of climate change for future generations to deal with is a phenomenally high-risk option.”
“The report shows that the evidence of human activities affecting climate is increasingly widespread and stronger than ever.”

“The report shows there is a very substantial risk of exceeding 2oC warming, relative to pre-industrial climate, by the end of the century. Only under a very ambitious scenario for reducing greenhouse gas emissions is a warming of more than 2oC considered unlikely.” [This one in particular is dependent on the final version of the SPM]

“This report provides the evidence that governments need to take tough decisions on climate change policy. I hope they will read it carefully and take seriously all its findings.”

“An increasing frequency of hot days, and intense rainfall events, are some of the changes that could affect Britain. Working out more of the details is a major focus of our research.”

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Dr Ed Hawkins of the University of Reading said:

"Scientists have been asked to provide clear answers on climate change. This report does this. It represents the most comprehensive assessment ever assembled of what we know about our changing climate. The report provides more thorough evidence than ever before, strengthening and focussing the advice provided in previous IPCC assessments.

"A novel aspect to the report is a whole new chapter on how the climate is projected to change over the next 30 years, rather than concentrating on changes by the end of the 21st century. This will be extremely relevant for decisions on how to adapt to a changing climate. It helps people to realise that climate change, and its impacts, is happening now in our own lifetimes and those of our children.”

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Prof Bill Collins of the University of Reading and Lead Author on Chapter 8 (Anthropogenic and Natural Radiative Forcing) said:

“This report shows that we are now more certain than ever about the human contribution to climate change. Greenhouse gas concentrations are observed to be rising rapidly, and unabated emissions are very likely to lead to changes in climate which are unprecedented in human history.

“Climate models, which are often criticised, have improved significantly in their detail and complexity since the last report was produced in 2007, as a result of improvements in computing power and more sophisticated ways of representing the climate system. Along with more accurate observations of current and past climate, these models show the crucial benefits of reducing our emissions of CO2 and other pollutants in the future to prevent the worst-case scenarios in the parts of the globe most vulnerable to climate shocks.”

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Dr Richard Allan of the University of Reading said:

On what is new:

"Since the last IPCC report, a step change in the volume and quality of observations (particularly from satellite) has been vital in allowing scientists to verify and improve physical understanding of the processes determining climate variability and change."

On the slowdown:

"Although global surface warming has slowed over the last 15 years, observations show that heat has continued to accumulate within the oceans since 2000 - at a rate equivalent to over 250 billion 1kiloWatt electric heaters spread across the planet - consistent with rising atmospheric concentrations of greenhouse gases."

"Recent research indicates that natural fluctuations in the ocean have caused this heating to build up at deeper levels below the sea surface rather than the upper layers which influence surface temperatures. This is important since it provides evidence that the slowdown in surface warming is temporary and we can expect a return to substantial surface warming over the coming decades."

On climate sensitivity:

"Previous IPCC reports have noted that climate sensitivity (the amount of
global warming far in the future in response to a doubling of CO2
concentrations) is uncertain, ranging from around 1.5 to 4.5oC. Observations,
basic physics and detailed simulations show that atmospheric water and surface
ice are powerful amplifying agents of climate change, explaining the mid-range
values of around 3oC. The lowest values in the range of climate sensitivity
would require equally powerful counteracting effects which, while possible,
have not been observed. Furthermore, arguing over a few tenths of a degree in
climate sensitivity at the bottom of the range masks the real issue which is
the damaging climate change expected in response to continued emissions of
greenhouse gases, which are at present following the worst case emissions
scenarios."

On arctic ice:

"Recent media statements about recovery in the Arctic sea ice since 2012 ignore
the difference between weather and climate. The long term trend over recent
decades is downward and the fact of the matter is that in 2013, ice volume has
recovered from a tiny amount, around 3,500 cubic km in late August 2012
compared with the late August average for 1979-2012 of around 12,000 cubic km."

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Dr Dave Reay, Reader in Carbon Management at the University of Edinburgh, said:

"It is six years since the Intergovernmental Panel on Climate Change (IPCC) last published its voluminous assessment of climate change science. Today, in the release of its fifth assessment report (AR5) the science of climate change is brought up to date and the key findings of thousands of climate research studies are distilled. There are no great game-changers here. This latest assessment reinforces the messages of its predecessors, attributing the bulk of post-war climate change to human-induced greenhouse gas emissions and projecting far more rapid changes in climate as the 21st century progresses.

“Its in-depth look at climate sensitivity – just how much warming we’ll see from a doubling of CO2 – is probably one of the hottest topics this time around. Much recent debate has revolved around the reason for the so-called ‘hiatus’ in global warming since 1998, with deep-ocean warming being the leading contender. What is not debated in this new report or elsewhere is the fact that greenhouse gas emissions continue to increase apace, and with them our enhancement of the greenhouse effect.

“We can guess at exactly when global temperatures might start ticking upwards again. We can argue all we like about whether climate sensitivity will be closer to 2C or 4C. But the core message in this latest report is a clear one: if greenhouse gas emissions continue on their current trajectory our grandchildren will find out exactly what Earth’s climate sensitivity is."

Australian Science Media Centre