Since I’ve written about climate sensitivity before, and since I have a few free moments, I thought I would briefly highlight a new paper by Knutti, Rugenstein, and Hegerl called Beyond Equilibrium Climate Sensitivity. It’s really a review of a large number of estimates for the Transient Climate Response (TCR) and the Equilibrium Climate Sensitivity (ECS).
In case you don’t know, the TCR is essentially how much we will have warmed when we’ve doubled atmospheric CO2, and the ECS is how much we will eventually warm once the system has returned to equilibrium after a doubling of atmospheric CO2 (technically, these are model metrics and consider fast feedbacks only, but let’s ignore those details for now). There are a large number of different estimates for the TCR and ECS, and the paper doesn’t really try to reconcile them; it simply discusses the various methods.
Essentially, there is reasonably agreement between the various estimates for the TCR; most are consistent with the likely range of 1 to 2.5oC and suggest that it is extremely unlikely above 3oC. There is some disagreement amongst the estimates for ECS, but this is mostly due to those that use the observed warming. The method that uses the observed warming essentially assume that the feedback response will remain constant as we warm to equilibrium; there now seems to be a reasonable amount of agreement that this is unlikely to be the case and that we will likely warm more as we approach equilibrium than we did initially.This is illustrated quite nicely by the figure on the right, which shows the surface temperature anomaly, on the x-axis, and top-of-the atmosphere radiative imbalance, on the y-axis. The black line is the case in which we assume feedbacks remain constant; this produces what is typically referred to as the Effective Climate Sensitivity. We expect, however, that temperature dependent feedbacks and the pattern of the warming could lead to more warming in future than we would expect based on an assumption of constant feedbacks; this will eventually lead to the Equilibrium Climate Sensivity being larger than the Effective Climate Sensitivity. There are also other factors, like internal variability, the base state climate, the magnitude of the forcing, and what produces the change in forcing, that could also influence the overall warming. On top of this, there are slower feedbacks that will ultimately further amplify the warming, producing the Earth System Sensitivity.
Okay, I’m running out of time, as I have to head out to dinner with some colleagues, so will just wrap up, by quoting some of the conclusions of the paper
Our overall assessment of ECS and TCR is broadly consistent with the IPCC’s, but concerns arise about estimates of ECS from the historical period that assume constant feedbacks, raising serious questions to what extent ECS values less than 2oC are consistent with current physical understanding of climate feedbacks. A value of around 3 °C is most likely given the combined evidence and the recognition that feedbacks change over time.
The argument that an ECS value less than 2oC is inconsistent with our physical understanding of climate feedbacks is presented nicely in a video by Andrew Dessler, that I included in this post. I also wanted to quote from the paper’s abstract, which ends with
Newer metrics relating global warming directly to the total emitted CO2 show that in order to keep warming to within 2oC, future CO2 emissions have to remain strongly limited, irrespective of climate sensitivity being at the high or low end.
What this is referring to is the Transient Climate Response to Emissions (TCRE) which I discuss in this post. This attempts to include both radiative feedbacks and carbon cycle feedbacks and suggests that our warming depends linearly on emissions. Essentially, this suggests that if we want to keep warming below 2oC then – approximately – the total amount we can emit in future, is less than we’ve emitted so far. I think this is quite an important metric that probably isn’t discussed enough, but since I need to rush out, I won’t say any more about it now.