Isaac Held has a recent post called addicted to global mean temperature, that I wanted to highlight. It’s quite relevant to the energy balance models that I’ve dicussed before. You should probably just read Isaac Held’s post, but I thought I would highlight a few things that I found quite illuminating.
One point that the post makes relates to Cowtan & Way (2014) and discusses the implications of undersampling of the temperatures in some regions of the globe. If you try to infill in the undersampled region, you might find that this increases the global temperatures and, consequently, increases estimates of the transient climate response. However, this doesn’t mean that the response to CO2 has increased everywhere, it’s really telling us that
the response to CO2 has a different pattern than what we had thought, not that the response to CO2 is everywhere larger than previously estimated.
I will add, however, that it would seem that improving the sampling would then make it easier to do comparisons with models that do not suffer from this kind of undersampling.
What I found particularly useful was the explanation for why we might expect the feedback response to be non-linear, or – more correctly – not globally constant as we warm.
In models, the effective strength of the radiative restoring is stronger for perturbations in tropical temperatures than for perturbations in high latitude temperatures. In addition, temperature responses are less polar amplified in the initial as compared to the final stages of the approach to a new equilibrium with elevated CO2.
However, as pointed out above, this really means that if we consider the initial period only, we would mostly be underestimating the response in the polar regions.
I’ll just add one more thing that I found interesting and that relates to what I’ve been stressing recently; climate change is essentially irreversible on human timescales. Even if we were to entirely halt emissions, the decay in atmospheric CO2 would largely balance the rise in temperatures, resulting in global temperatures remaining fixed. However, even this is subtler than I had realised
As another example, consider the accumulated emission perspective on long-term climate change after emissions cease, in which slow carbon uptake over centuries compensates approximately for the slow equilibration of the climate to the evolving CO2 levels. The southern ocean plays a leading role for both carbon and heat uptake. And from a global perspective these are competing to change the same global mean temperature. But CO2 is well mixed in the atmosphere on time scales longer than a year or two, so any uptake of carbon affects both hemispheres with roughly equal radiative forcing. But uptake of heat in the Southern Oceans affects the southern more strongly than the northern hemisphere. This distinction can get lost when discussing this accumulated emission perspective.
If I understand this properly (and I may not) what I think it is pointing out is that the northern hemispheres – with more land – equilibrates more rapidly than the Southern Hemisphere. Therefore, what will probably happen is that the NH will rapidly warm towards a higher temperature than the SH, which will maintain an energy imbalance, with most of the energy going into the oceans. The global temperature may be largely fixed, but that doesn’t mean there will be no regional variability, or that there won’t be regional warming/cooling (as the NH and SH equilibrate). At least, I think that is right.
Anyway, those are a few things that I found interesting. You should probably read Isaac Held’s post, as there is more than just this and I may not have quite captured all the subtleties. I think it is quite easy to simply think in terms of globally averaged quantities, while forgetting that there are significant regional variations (both in time and space) and that these can influence what we might infer if we consider globally average quantities only.