Climate scientists have bad news for governments, energy companies, motorists, passengers and citizens everywhere in the world: to contain global warming to the limits agreed by 195 nations in Paris last December, they will have to cut fossil fuel combustion at an even faster rate than anybody had predicted.
Joeri Rogelj, research scholar at the International Institute for Applied Systems Analysis in Austria, and European and Canadian colleagues propose in Nature Climate Change that all previous estimates of the quantities of carbon dioxide that can be released into the atmosphere before the thermometer rises to potentially catastrophic levels are too generous.
Instead of a range of permissible emissions estimates that ranged up to 2,390 billion tons from 2015 onwards, the very most humans could release would be 1,240 billion tons.
In effect, that halves the levels of diesel and petrol available for petrol tanks, coal for power stations, and natural gas for central heating and cooking available to humankind before the global average temperature – already 1°C higher than it was at the start of the Industrial Revolution – reaches the notional 2°C mark long agreed internationally as being the point of no return for the planet.
In fact, the UN Framework Convention on Climate Change summit in Paris agreed a target “well below” 2°C, in recognition of ominous projections − one of which was that, at such planetary temperatures, sea levels would rise high enough to submerge several small island states.
The Nature Climate Change paper is a restatement of a problem that has been clear for decades. Carbon dioxide proportions in the atmosphere are linked to planetary surface temperatures and, as they rise, so does average temperature. For most of human history, these proportions oscillated around 280 parts per million.
The global exploitation, on a massive scale, of fossil fuels drove the expansion of agriculture, the growth of economies, a sevenfold growth in human population, a sea level rise of 14cms, and a temperature rise of, so far, 1°C.
To stop temperatures increasing another 3°C or more and sea levels rising by more than a metre, humans have to reduce fossil fuel emissions. By how much these must be reduced is difficult to calculate.
The global carbon budget is really the balance between what animals emit – in this context, the word animals includes humans with cars and aeroplanes and factories – and what plants and algae can absorb. So the calculations are bedevilled by uncertainties about forests, grasslands and oceans.
To make things simpler, climate scientists translate the target into the billions of tons of carbon dioxide that, ideally, may be released into the atmosphere from 2015 onwards. Even these, however, are estimates.
There is general agreement that a limit of 590 billion tons would safely keep the world from overheating in ways that would impose ever greater strains on human society. The argument is about the upper limit of such estimates.
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