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The Two Degree Dilemma

Credit: PhotoSG/Adobe

Credit: PhotoSG/Adobe

By Evan Gray

We’ve agreed to limit the rise in global temperature to 2°C, but how will we do it?

At the United Nations Climate Conference in December 2015, world leaders agreed to limit the global average temperature rise to 2°C, and to work towards an even lower rise of 1.5°C. The big question is: how?

Can we keep burning fossil fuels but capture the carbon dioxide generated? Is a world powered by renewable energy feasible? Should we build more nuclear power stations?

Pundits on all sides of the energy debate insist on these and other solutions, but we hear little about whatever evidence exists for their impact on a global scale. Let’s look at the evidence and how it relates to the 2°C goal.

The 2° Scenario

The International Energy Agency’s (IEA) 2°C scenario envisions a transformation of the global energy supply to cut CO2 emissions by almost 60% by 2050 compared with 2013. This has at least a 50% chance of limiting the temperature increase to 2°C.

Realising the 2°C scenario requires first that emissions of CO2 fall very swiftly. Continuing to emit CO2 at the present rate and recapturing it from the atmosphere is not an option because no existing technology can be scaled up fast enough.

The immediate and urgent challenge is therefore to greatly diminish CO2 emissions. This implies rapid deployment of energy generators that emit little or no CO2, either because they don’t generate any during operation or because their CO2 emissions are captured and sequestered before entering the atmosphere.

The Fossil Fuel Problem

According to the IEA’s Key World Energy Statistics 2016 (, 80.1% of the world’s primary energy supply in 2015 was from fossil fuels – coal, oil and natural gas – whose combustion to liberate useable energy for electricity generation, transport and industry emits CO2. Coal is the worst offender, liberating about 0.9 kg of CO2 per kWh of electricity produced in a typical black coal-fired power station. The figures for oil and natural gas are somewhat less.

Fossil fuels are ancient stores of chemical energy that are accessible, easy to use and cheap. The process of forming fossil fuels from dead plants is, on the other hand, geologically slow. We are burning fossil fuels much faster than they are replenished through natural capture of CO2 from the atmosphere – hence the build-up of atmospheric CO2.

What Are Our Options?

Does all this mean that we have to use less energy on a global scale? Is there enough to support the global economy in the long term, especially as poorer countries industrialise? Can we instead use energy from a different source? For instance, if we all drove battery- or fuel-cell electric cars, wouldn’t that help?

It certainly wouldn’t help if the electricity or hydrogen came from fossil fuels, because the CO2 emissions would be displaced to the point of energy production, not avoided. Electrical energy accessed without generating CO2 would solve this problem in principle, although we should carefully examine solutions that sound simple.

The Scale of the Energy Problem

According to the Key World Energy Statistics 2016, the amount of energy used on Earth in its final useful form is about 110,000 TWh/year. About 22% of the final energy used (about 24,000 TWh/year is provided as electricity.

Now for a key point. Electricity generation accounts for much more of the total energy used than 22% because the thermal efficiency of coal-fired power stations is about 37%. Therefore about 64,000 TWh of thermal energy would be required to generate 24,000 TWh of electricity by this means. Furthermore, the fraction of total energy demanded as electricity will increase as the transport sector moves away from oil-derived fuels.

An enormous and growing amount of energy is directed to electricity generation, which explains why the IEA’s 2°C scenario has such a strong focus on electricity supply. Clearly, we must use much less fossil-fuel energy or succeed in sequestering the CO2 on a grand scale. Is that feasible?

Carbon Sequestration

Carbon capture and storage (CCS) has been demonstrated at pilot scale. According to the Global CCS Institute’s 2015 Summary Report (, 15 projects are currently in operation with a combined storage capacity of 28 Mt of CO2 per year. Seven projects are under construction and another 23 projects are in planning, taking the total present and immediately projected capacity to about 80 Mt per annum.

At 0.9 kg CO2 per kWh of electricity produced, 80 Mt of annual storage capacity corresponds to approximately 90 TWh of electricity from black coal. However, our annual electricity use is about 23,000 TWh, or about 250 times more.

The IEA’s 2°C scenario demands less: a cumulative emissions reduction of nearly 50 Gt from 2016 to 2050, or an average of about 1.5 Gt per annum. This is nearly 20 times the currently planned capacity of CCS.

A 20-fold expansion in carbon sequestration capacity within just several decades is scarcely feasible, and coal/CCS as the major clean electricity supply technology is certainly not feasible.

Nuclear Energy

Nuclear power provides about 11% of global electricity ( According to the World Nuclear Association (, nuclear can supply at least 200 years of energy with current reactor technology at the present consumption rate. Using fast breeder reactor technology would extend this time to many thousands of years.

But there are three practical problems with contemplating nuclear power for supplying the majority of the current global electricity demand. First, the “reasonably assured reserves” would only last a few decades at this higher rate of consumption if used in conventional reactors. Second, the lead time for nuclear power stations presently averages around 10 years. Third, breeder reactor technology is not commercial at any significant scale. Therefore a rapid scale-up to provide most of the world’s electricity seems no more feasible than grand-scale carbon sequestration.

Nuclear fusion has for decades been said to be decades away, and appears still to be so as a commercial proposition despite amazing feats of technology in achieving ever-higher plasma temperatures and longer fusion times.

Sustainable Energy Sources

Instead of assuming that we irreversibly consume resources found on Earth, suppose we tap into energy that arrives anyway. The surface of the Earth receives energy from two main sources – the Sun and geothermal energy. Geothermal heating corresponds to about 44 TW of power. This is a big resource, with a contribution to electricity production expected to grow to 104 TWh (electric) by 2020 according to the IEA.

Yet this is dwarfed by the solar power the Earth receives. The power received from the Sun is on average 1366 W/m2 of area perpendicular to the line between the Earth and the Sun. Multiplying this by the area of the Earth seen from the Sun gives a total intercepted power of 174,000 TW, or 1,520,000,000 TWh per annum. This is 14,500 times our current global energy consumption! About 1000 W/m2 reaches the surface of the Earth near the Equator at midday in clear conditions.

Around 5% of the incoming annual solar radiation ends up as atmospheric wind, and around 10% of that ends up as waves. Not all of the available solar radiation or wind or wave energy could be intercepted, of course, but our present energy usage is so tiny by comparison that an area about 600 km × 600 km near the Equator, with a dry climate, covered with common solar photovoltaic panels could supply the world’s entire present energy requirement as electricity. To supply just the present global demand for electricity (22% of all energy used), less than 300 km × 300 km and around 2.5 TW average power would suffice.

This thought experiment isn’t a proposal to build such a generator. It establishes the principle that our energy consumption is able to be satisfied by an area of solar collection that is tiny compared with the size of any of the continents.

The installed cost of photovoltaic systems is crashing. Even small residential systems are approaching USD0.50 per peak Watt. A further fall in cost by a factor of two for large systems is reasonable in the near future. At USD0.25 per peak Watt, our hypothetical 2.5 TW average generator would require a peak power capacity of about 8 TW and therefore cost about USD$2 trillion. This enormous sum is about 3 years of military spending by the USA and a small fraction of its gross debt of about USD$23 trillion.

Is there capacity in global industry to undertake this mighty feat? A 2015 study by the Fraunhofer Institute for Solar Energy Systems ( concluded that global installed photovoltaic capacity could exceed 30 TW peak power by 2050, well above the IEA’s prediction and enough to supply our current electricity requirement several times over.

Already the distributed photovoltaic resource in Queensland represents the second-largest power station in the state, with a peak capacity of some 1.5 GW. In South Australia the installed wind power capacity is similar, supplying around 40% of the electricity used there in 2014.

The take-away message is that solar power can supply our conceivable energy needs many times over. In response to the obvious retort from sceptics – “if it’s so good, why aren’t we doing it?” – we have to acknowledge that solar power is inconveniently low in energy density compared with coal or uranium mines and conventional or nuclear power stations, and its availability is not constant. On the other hand, we can now generate solar electricity more cheaply than we can buy coal-fired electricity, and that’s before factoring in real societal costs in health care associated with burning fossil fuels.

The biggest problem with solar energy is its intermittency: we need to solve the puzzle of storing enough solar energy to bridge the gaps in availability. For this reason a vast increase in the capacity to store energy in a form that’s compatible with our electricity supply must go hand-in-hand with the accelerated roll-out of solar electricity capacity.

The beginning of this transformation is already apparent in the demand for residential battery storage systems to go with solar photovoltaics. Once again, the required feat at the grand scale seems mighty, but in contrast to GW-scale centralised CCS schemes or nuclear power plants, technologies for the distributed storage of sustainably captured energy in homes, businesses and vehicles are with us now.

Where Does That Leave Us?

In developing energy policy we have to choose a strategy somewhere between two extremes. At one extreme is obtaining energy from convenient and cheap resources that are themselves repositories of anciently stored energy, such as coal and uranium. The ancient resources are irreversibly consumed and their stored energy is added to the total current energy budget of the Earth. This is how we got to where we are with climate change.

The alternative extreme is to intercept just a little of the vast amount of energy that arrives at the Earth’s surface in the form of solar radiation and geothermal heating.

The reality will be somewhere in between. It really does seem that we can choose to sustainably power our society and make the 2°C scenario happen, as long as the roll-out of energy storage keeps pace with the displacement of fossil fuel technologies by renewables.

Evan Gray is Professor of Physics in the School of Natural Sciences at Griffith University. This is a modified version of a technical article published in Australian Physics.