Zeke Hausfather and colleagues recently wrote a Nature comment with suggestions about how to deal with what is called the ‘hot model’ problem. The issue is that some of the latest group of climate models have somewhat higher than expected climate sensitivities. To account for this, in the latest IPCC report the models were weighted to give more weight to those that better represented historical temperature observations.
This weighting reduced the range of projected warming for the various scenarios and reduced the likely climate sensitivity ranges. Additionally, in previous IPCC reports what has often been presented is what will happen in 2100 along different possible emission scenarios. However, if there are some potentially anomalous “hot models”, then the range of warming in 2100 for each scenario would be quite broad and the possible 2100 climate could vary quite widely for a given scenario.
To account for this, what the recent IPCC report did was to use Global Warming Levels (GWLs) rather than presenting the output at 2100 for each set of scenarios. In other words, rather than reporting how the climate will change in 2100 for each scenario set, they highlighted the changes that will probably occur if we were to warm by 1.5oC, 2oC, 3oC and 4oC. The advantage of this is that you can use all the models and may not need to weight them; even a “hot model” may give a reasonable representation of the climate in a, for example, 2oC world, even if it gets there sooner and for a lower level of cumulative emissions.
However a lot of the current research that assesses the impact of climate change still use the scenarios, rather than using GWLs, and doesn’t account for the possibility that some models are “too hot”. The Hausfather et al. article was basically suggesting that researchers who use climate model output to assess the impact of climate change should aim to follow a similar practice to what was presented in the most recent IPCC report. Use GWLs, rather than simply focussing on scenarios going to 2100, weight the models if the warming trajectory is relevant, and try to consider which models may be best suited to the problem that is being considered.
The reason I thought I would write about this is partly that it’s interesting and I haven’t had much to write about recently, and partly because Mike Hulme wrote a critical response. In the original article, the authors claim that “despite some differences related to the rate of warming and aerosol forcing, the world largely looks the same at 2 °C, no matter how we get there.” Mike Hulme’s response suggests that “[a]gainst criteria that matter, not all future 2 °C worlds would be the same — even though the climate might be.“
Of course, Mike Hulme is correct that there isn’t a single 2oC world. It will depend on the various socio-economic factors that Mike Hulme mentions in his response. However, I’m pretty sure that the authors of the article were not implying otherwise, even if it is unfortunate that they chose to say “the world looks largely the same“. Given that the context was the output from climate models, I’m pretty sure that what they meant was that the world’s climate will look largely the same, which even Mike Hulme acknowledges in his response.
I find this kind of thing rather frustrating, especially as the original article was addressing a problem and making constructive suggestions about how to use climate model outputs to assess the impact of climate change. The point of the article was not to convince people that the future world will depend only on the level of global warming. The article was presenting a set of suggestions for how those who assess what the world might look like should use the output from climate models. In a sense, the article was trying to help researchers do the very thing that Mike Hulme’s criticism seems to imply that the authors don’t understand.