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 1C higher than it was at the start of the Industrial Revolution – reaches the notional 2C 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” 2C, 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, 1C.
To stop temperatures increasing another 3C 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.