@eem: Vaughan, a climate sensitivity of 1.73C per doubling of CO2 looks believable, about 50% more than what would be expected with CO2 without any feedback.
Quite right, Erik. Bear in mind however that what we’re currently experiencing is neither equilibrium climate sensitivity (ECS) nor transient climate response (TCR) but the empirical or observed climate sensitivity (OCS, I suppose).
ECS is the eventual temperature rise following a doubling of CO2, which may take centuries to reach that temperature, necessarily long after the doubled CO2 has stopped changing. As the temperature approaches its equilibrium value its slope decreases until in the limit it’s flat, the equilibrium condition.
TCR is the rise in 20-year climate during 70 years of a CO2 CAGR of 1%. It is measured at the start and end of the 70 years, and the rise is the difference. A CAGR of 1% over 70 years corresponds to a doubling in CO2 over that period. Unlike ECS, there is no requirement that either CO2 or temperature cease increasing at any point. Currently TCR is only meaningful for models, but RCP8.5 (“business as usual’) CO2 for 2010-2080 should be quite similar on average since it will hit the 1% CAGR mark shortly before mid-century and then rise above it.
Empirical climate sensitivity is like TCR but with CO2 doing its own thing instead of obeying some rule like a fixed CAGR. In the 1960s CO2 CAGR was around a quarter of a percent, by 1980 it had climbed to 0.36%, by 2000 0.5%, and today it’s around 0.65%.
The advantage of plotting climate against forcing instead of time is that it tends to factor out any changes in CAGR because CAGR is defined in terms of time while forcing is not. This is what makes it possible to observe a straight line after removing the known oscillations and TSI. The longer the observation, the more confident one can be about whether the trend line has any real meaning.
I say “tends to” because in practice if forcing were to slow down it would give the ocean more time to absorb heat and rise in temperature, making the situation slightly more like the equilibrium case. While this might sound hard to model, it turns out to be much simpler than you’d expect if you judged it by the enormous complexity of global ocean-land circulation models (GCMs) like the CMIP5 suite.
So far forcing has not slowed down, and so we have had no opportunity to date to observe what would happen if it did. We can only theorize, but that’s not necessarily a tall order, the complexity of the CMIP5 models notwithstanding.
An empirical climate sensitivity of 1.7 or 1.8 °C is entirely consistent with an equilibrium climate sensitivity of 3 °C. For the purposes of 21st century forecasts, ECS is entirely irrelevant. Moreover if CO2 stops following RCP8.5 then even observed climate sensitivity will become less relevant. Theory (good theory anyway) then becomes the more reliable basis for projection.
If CO2 continues to follow RCP8.5 it will reach 936 ppm in 2100. If in doing so climate continues along that straight line with slope 1.73 °C per doubling of CO2, climate in 2100 will be 1.73*log2(936/400) = 2 degrees hotter than today (so 3 degrees hotter than in 1910).
Naively using the 3 °C/doubling figure for equilibrium climate sensitivity might make you think that the rise would be 3*log2(936/400 = 3.7 °C hotter than today in 2100. However that’s not what equilibrium climate sensitivity means. It means, very roughly, that if CO2 stops rising after 2100 then a few centuries later (depending on how long the climate takes to reach equilibrium) the climate will have risen 3.7 °C over today.
Actually even that is not quite right. More precisely the definition of ECS assumes that the starting temperature is measured while in equilibrium, whereas today we’re well out of equilibrium.
