I wanted to just quickly write a post about something Dana highlighted in his recent Guardian article, global warming is unpaused and stuck on fast-forward. The article discusses the recent paper by Trenberth & Fasullo (2013) that considers whether we really have had a global warming pause. Although I had already read bits of the paper, until I read Dana’s article, I hadn’t appreciated that the paper had re-estimated the equilibrium cimate sensitivity (ECS), using some updated estimates of the system heat uptake.
In case anyone who reads this doesn’t know, the ECS is a crucial number in climate science in that it is the increase in global surface temperature resulting from a doubling of atmospheric CO2 [Addendum 12/12/2013 : See Steve Bloom’s comment below – I should have made clear that what I’m talking about here is the equilibrium temperature determined only by fast feedbacks. There are likely slower feedbacks that will result in an equilibrium temperature even higher than that determined only from fast feedbacks.]. It essentially tells us how much warming we should expect. There are a number of ways to estimate the ECS. One is to use past climate history, and another is to use detailed climate models. Recently, however, there have been some estimates based on recent observations (Otto et al. 2013, for example). What was interesting was that these observationally constrained estimates produced values that were quite a bit lower than those from other methods. Past climate history and climate modelling suggests an ECS around 3oC. The observationally constrained values are closer to 2oC.
In truth, the uncertainties on the different estimates are quite large, and so there is a large overlap in the possible ranges, but the difference between the methods is likely why the IPCC reduced the lower limit of its ECS range and didn’t produce a best estimate. The way one can estimate the ECS, observationally, is to use
where ΔQ2x = 3.7Wm-2 is the adjusted anthropogenic forcing after CO2 has doubled, ΔT is the change in global surface temperature over the time interval considered, ΔQ is the change in adjusted forcing over the time interval considered, and H is the current rate at which the system is gaining energy (i.e., it is the current total energy imbalance). Essentially, the equation is telling us that if H = 0, then the ECS = ΔT and the system is already in equilibrium. If H is positive, then the system is not yet in balance, is still accruing energy, and to reach equilibrium the temperature will have to continue rising (i.e., the ECS is greater than ΔT).
Otto et al. (2013) did all their calculations relative to 1860-1879 and for the period 2001-2010, their values were ΔT = 0.75oC, ΔQ = 1.95 Wm-2, and H = 0.65 Wm-2. If you put these numbers into the above equation, you get 2.1 Wm-2. It’s reasonably well accepted (Karsten could likely put me right) that anthropogenic aerosols are providing a not insignificant negative forcing. When Otto et al. considered the impact of aerosols, they reduced ΔQ to 1.73 Wm-2, giving an ECS of 2.6oC, so still lower than other estimates (I think I’m using 3.7 Wm-2 for ΔQ2x, instead of 3.44 Wm-2, but it doesn’t change things much).
What’s interesting about the Trenberth & Fasullo (2013) paper is that they suggest that the system heat uptake rate, H, should be 0.91 Wm-2, quite a bit bigger than the 0.65 Wm-2 used by Otto et al. Using this value gives an ECS of 2.7oC (no aerosols), and 3.4oC (with aerosols). With this value, this estimate is much more in line with the other estimates. So, why am I writing this? Well, I suspect most climate scientists suspected that the observationally constrained estimates were on the low side. They don’t cover the full time interval, they’re sensitive to short-term variations, and the uncertainty of the current aerosol forcing likely influences their robustness. So, it’s no great surprise that a more recent paper is bringing the observationally constrained estimate more in-line with other estimates.
To be clear, it doesn’t mean this new paper is right and I suspect this won’t be the last word. However, this is kind of how you expect science to work. Someone comes up with a new way to determine something which maybe initially disagrees with other methods. That could imply that the other methods are wrong, but could also imply that the new method needs refinement. As time goes on, either people discover that results from the new method are superior, or they discover that as the new method is further tested, its results come into line with the other methods. We still don’t know that that the case yet, but this is how the process is meant to work.