So it may well be, no, it is likely that once the underlying physics is properly understood, climate models will emerge that produce an ECS value considerably smaller than 1.8C. What could such a model look like? To find out we look at the main source of the variation between the CMIP6 models: the positive feedback on water vapour (AR4, refs 6,7). The idea goes back to Manabe & Wetherald (8) who reasoned as follows: a warming due to CO2 increase leads to an increase in the water vapour content. Water vapour is also a ‘greenhouse gas’, so there is extra warming. This mechanism is assumed to ‘amplify’ the primary effect of CO2 increase. Vary the strength of the coupling and add the influence of clouds and you have a whole series of models that all predict a different ECS.
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