Climate sensitivity is the amount of warming that would arise from doubling the concentration of carbon dioxide in the atmosphere. The IPCC estimates that it is between 2.0˚C and 4.5˚C, with 3.0˚C as the most likely value. They also warn that, because of feedback effects, “values substantially higher than 4.5˚C cannot be excluded.” Basically, this number refers to how many degrees of warming would arise from raising the concentration of CO2 in the atmosphere from the pre-Industrial level of 290 parts per million (ppm) to 580 ppm: 50% above today’s level of 385 ppm. The higher the number, the worse the consequences from any particular level of emissions.
We can combine that figure with the maximum amount of warming we are willing to tolerate and come up with a figure for how much more carbon dioxide humanity can release, all told. Using the 2˚C ceiling adopted by the European Union and endorsed by 200 of the world’s top climate scientists, Andrew Weaver worked out what those limits would be for different climatic sensitivities:
- 2.0˚C sensitivity – 1314 billion tonnes (gigatonnes) of carbon – 4822 gigatonnes of CO2
- 3.6˚C sensitivity – 661 billion tonnes (gigatonnes) of carbon – 2426 gigatonnes of CO2
- 4.5˚C sensitivity – 484 billion tonnes (gigatonnes) of carbon – 1776 gigatonnes of CO2
- 8.0˚C sensitivity – 163 billion tonnes (gigatonnes) of carbon – 598 gigatonnes of CO2
The 2˚C and 4.5C figures are the top and bottom of the IPCC’s probable range. The 3.6˚C figure is the one considered most probable by those running the University of Victoria climate model. The 8˚C figure illustrates the impact of much higher sensitivities on how much can be emitted.
Current annual carbon dioxide emissions about ten gigatonnes of carbon (36.7 gigatonnes of CO2) per year. Note that the figures above are how much CO2 can be emitted in total before the whole world becomes carbon neutral – certainly not about how much can be emitted before we need to begin cutting. Those totals need to include all future emissions from developed and developing states alike, between this year and whichever year the world achieves carbon neutrality.
The 4.5˚C scenario implies a peak concentration of 445 ppm – slightly lower than a commonly cited ‘safe’ level. Some people – notably James Hansen – have argued that an even lower stabilization concentration is necessary to avoid runaway climate change.