John Lanchbery, Principal Climate Advisor
In the few days since the Intergovernmental Panel on Climate Change (IPCC) published its report on climate science, there have been some liberal interpretations of what it says. One major source of confusion has been the IPCCs’ projections of future climate change, especially in the longer term through to 2100.
Usually, projections are made by assuming that things carry on much as they are, with current trends continuing and perhaps the possible effects of a few policies thrown in for good measure. Yet this is sort of approach is not very helpful for tackling climate change because all that it basically tells you is that the more greenhouse gases you put into the atmosphere the more warming you get, as has been known for ages (Svante Arrhenius, On the Influence of Carbonic Acid in the Air upon the Temperature of the Ground, London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science (fifth series), April 1896. vol 41, pages 237–275.)
Moreover, the governments of the World have already agreed that the average temperature increase should be kept below two degrees. So what would be really helpful would be for the IPCC to develop a range of ‘scenarios’ which include not only a business-as-usual projection but also visions of how to limit warming, including to below two degrees.
This is what the IPCC has done. Together with the research community, it has developed a set of four ‘Representative Concentration Pathways’ (RCPs) of greenhouse gases which lead to different degrees of ‘radiative forcing’, which is what leads to temperature rise. The highest (RCP8.5) is really a business-as-usual pathway leading to a forcing of more than 8.5 W/m2, and rising, and a temperature of between 2.6 and 4.8OC by 2100, and rising. The lowest is RCP2.6 in which forcing peaks at 3 W/m2 and then falls to 2.6, and temperature rises between 0.3 and 1.7OC by 2100. The other two RCPs are in between: RCP4.5 in which temperature would rise to between 1.1 and 2.6OC by 2100, and RCP6.0 with a temperature range of 1.4 to 3.1OC. Temperature would continue to rise in both cases after 2100. (In all cases, the base period that the temperature rise is from is 1986-2005 - so you can add around 0.8OC to these numbers).
All a bit techy perhaps, but the important thing to realise is that the IPCC is not simply projecting that the temperature will rise by between 0.3 and 4.8OC by 2100 come what may. It is saying that if we do nothing to tackle climate change then the temperature may rise by as much as 4.8OC from present. On the other hand, if we take a lot of action it is still possible to stay well below two degrees, a good chance on RCP2.6.
Yet RCP2.6 requires a lot of global action, a peak and decline in global emissions by 2020, and sharp emission reductions of about 3% per year after that.
And the RCP8.5 pathway looks pretty grim.