Before I go out I wanted to briefly mention a recent paper by Patrick Brown and Ken Caldeira called [g]reater future global warming inferred from Earth’s recent energy budget. Patrick Brown already has a nice blog post about this with a couple of really informative videos. I encourage you to read Patrick’s post and watch the videos.
The basic idea, though, was to try and constrain the models on the basis of how well they match recent observations. In particular, how well do they simulate top-of-the-atmosphere energy balance (reflected solar radiation, outgoing infrared radiation, and net energy balance). As Patrick’s post mentions, these are amongst the most fundamental aspects of global warming.
The key result is shown in the figure below. When you constrain the models on the basis of how well the match these observational constraints (pink band, red line), they project more warming than the unconstrained model results suggest (grey band, dashed line), and the range is slightly reduced. For example, if we follow and RCP4.5 concentration pathway, this would suggest we would warm about as much as suggested by the original RCP6 results. Also, the mean ECS value from the constrained models is 3.7oC, with a likely range from 3oC to 4.2oC (right-hand panel below).
Of course, this is simply one study, so one should be careful of simply accepting it. However, it is suggestive and is – I think – not the first time a study has suggested that models that better represent recent observations have higher climate sensitivities than the model mean. This would seem to indicate, as the paper says, that
achieving any given global temperature stabilization target will require steeper greenhouse gas emissions reductions than previously calculated.
Steve Forden made a good point on Twitter. If you consider the left-hand panel of the figure, the difference between the constrained model results and the original results is not particularly significant for the lower emission pathway. Therefore what I quote at the end of the post is probably not really true for a temperature target of around 2oC (i.e., the result in this paper doesn’t really change what emission pathway we would need to follow to achieve such a target).