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IPCC report sets a conservative carbon budget

By Will Steffen

The evidence for the continued warming of the planet is overwhelming, according to the IPCC's latest report.

With the release of its Fifth Assessment Report, the Intergovernmental Panel on Climate Change (IPCC) has emphatically set the record straight on what the most recent scientific research says about climate change.

Based on a careful assessment and synthesis of the vast body of climate-related research, especially the most recent research over the past seven years, the IPCC report – the full version of which has been released this week – strengthens our knowledge of how and why the climate is changing. It points towards the risks that an increasingly destabilised climate system poses for us.

The evidence for the continued warming of the planet is overwhelming. The world’s oceans, which absorb over 90% of the additional heat trapped by the rising concentration of greenhouse gases in the atmosphere, have warmed strongly since 1970 to the present. Although most of that additional heat is found in the upper water layers, there is now evidence of warming in the deep ocean in the North Atlantic and Southern Ocean.

The surface air temperature has also increased since the middle of the last century, at a rate of 0.12C per decade, and has increased by 0.85C since 1901.

The ice cover over the Arctic Ocean is decreasing rapidly, at a rate of about 4% per decade since 1979. Such rapid ice loss is unprecedented in the last 1,450 years.

Sea level has risen by 19cm over the 1900-2010 period. Again, this observed rate of rise over the past century is unusually high in the context of the last 2,000 years.

Glaciers and ice sheets around the world are shrinking and losing mass. The combined rate of mass loss from the large polar ice sheets on Greenland and Antarctica has risen to about 350 billion tonnes per year for the period 2002-2011 and is now making a significant contribution to sea-level rise.

The rates of sea-level rise, decrease in area of Arctic sea ice, and loss of ice mass from Greenland and Antarctica have all increased in the period from the 1990s to the present, compared to earlier periods.

All of this evidence points to the continued strong warming of Earth and the increasing destabilisation of the climate system.

We know the reason for the warming of Earth very well. The evidence is even more robust and compelling than it was seven years ago. The primary cause is the human emissions of greenhouse gases, the most important being carbon dioxide (CO2) generated by the burning of fossil fuels. The atmospheric concentration of CO2 has increased by 40% since the beginning of the industrial revolution, and CO2 levels are now higher than at any other time in human history.

A destabilised climate system creates risks for human well-being, the economy and the environment, often through changes to the frequency and severity of extreme weather events. The IPCC Fifth Assessment Report has confirmed and strengthened the key findings of last year’s IPCC special report on extremes.

Hot days and nights have increased over most land areas around the world, and many regions, including Australia, are experiencing longer and more intense heatwaves. It is likely that more land areas have experienced increases in heavy precipitation than have experienced decreases. The incidence of coastal flooding has likely increased since 1970, exacerbated by rising sea levels.

The IPCC made it clear that to limit these risks and stabilise the climate system, substantial and sustained reductions of greenhouse gases, particularly of CO2, are required.

Nearly all countries around the world have agreed to limit the rise in global temperature to 2C or less compared to the pre-industrial level. Beyond a rise of 2C, the risks to human societies are judged to be unacceptably high. We are now approaching the halfway point to that limit.

For the first time, the IPCC has adopted a carbon budget approach – looking at how much carbon we can burn before we exceed the 2C limit – as the framework for assessing the level of mitigation required. Previously they used rates of reduction over time. To meet the 2C target, the total post-industrial emissions of carbon from all sources should be limited to 1,000 billion tonnes. About half of the budget has already been consumed.

This budget may, in fact, be rather generous. Accounting for non-CO2 greenhouse gases, including the possible release of methane from melting permafrost and ocean sediments, or increasing the probability of meeting the 2C target all imply a substantially lower carbon budget.

The inescapable conclusion is that most of the world’s fossil fuel reserves will have to be left in the ground if we want to stabilise the climate.

As we have often said in the Climate Commission, and now in the Climate Council, this is the critical decade to get on with the transformation to a clean energy future.

Will Steffen is Adjunct Professor, Fenner School of Environment and Society at Australian National University. This article was originally published at The Conversation.