Professor Peter Stott:

One of the graphs I showed to the committee addresses the question about the 4° Celsius increase. It shows a scenario, which in the jargon is called RCP8.5, which is the business as usual scenario of continued emissions. One of the graphs with a red band shows the different climate models and is a portrayal of the scientific uncertainty.

In answer to the Deputy's question about 4° Celsius, and remembering that there is a slight technicality here because this is being referenced relative to the 1981 to 2010 average, so one must read it off quite carefully in terms of where that is, but looking at the median of that towards the latter part of this century, maybe about the 2080s or so, it can be seen from that that there is some uncertainty about when that 4° Celsius will be crossed. There could be a range of times. If climate is warming up faster, it could be earlier, perhaps as soon as the 2060s, or it could be considerably later than that, depending on whether we are lucky or unlucky in terms of the exact climate sensitivity.

In terms of the impact of that, I refer back to my earlier comments that once we reach 4° Celsius or more, then to some extent we will be in a scenario where there are these risks that we know exist and that we know have very significant impacts, for example, in terms of food security, agriculture, a sea level rise from melting ice from Greenland and the west Antarctic, and significant increases in flooding events, heatwaves and droughts. At such levels of warming, it is difficult to be very precise about the probabilities of such impacts and what that will mean. The last graph I showed members illustrated the level at 5° Celsius when we would see impacts on huge numbers of the global population. These graphs show that there are not many parts of the world that will escape this, and this raises the question of how we will adapt. It is a difficult question to answer. It is hard to know how the global population would be able to cope with such impacts. It is hard to be precise. It is much easier to paint the picture of the graph I displayed, with the heatwaves, the cropland decline, the flooding and the water stress, which summarises briefly from an exercise that we did in the Met Office. It is easier to paint a general picture of an extremely challenging situation that would be very difficult for people to cope with.