The Fall and Rise of the Global Climate Model

Global models are an essential tool for climate projections, but conventional coarse-resolution atmospheric general circulation models suffer from errors both in their parameterized cloud physics and in their representation of climatically important circulation features. A notable recent study by Terai et al. (2020, https://doi.org/10.1029/2020ms002274) documents a global model capable of reproducing the regime-based effect of aerosols on cloud liquid water path expected from observational evidence. This may represent a significant advance in cloud process fidelity in global models. Such models can be expected to give a better estimate of the effective radiative forcing of the climate. If this advance in cloud process representation can be matched by advances in the representation of circulation features such as monsoons, then such models may also be able to navigate the complex tangle between spatially heterogeneous aerosol-cloud interactions and regional circulation patterns. This tight link between aerosol and circulation results in anthropogenic perturbations of climate variables of societal importance, such as regional rainfall distributions. Upcoming global models with km-scale resolution may improve the regional circulation and be able to take advantage of the Terai et al. (2020, https://doi. org/10.1029/2020ms002274) improvement in cloud physics. If so, an era of significantly improved regional climate projection capabilities may soon dawn. If not, then the improvement in cloud physics might spur intensified efforts on problems in model dynamics. Either way, based on the rapid changes in aerosol emissions in the near future, learning to make reliable projections based on biased models is a skill that will not go out of style. Plain Language Summary Human activity releases particles into the atmosphere. Some of these particles are small enough to remain suspended in air, forming an aerosol. Aerosols interact with clouds to make them brighter or dimmer. Clouds are very good sunlight reflectors, so changing their properties even slightly results in large changes to how much sunlight is absorbed by the Earth. Over time, this changes the rate at which the climate warms. It also affects the regional circulation, that is, recurring weather patterns at the regional scale. This is especially important to society when it affects regions that experience intense rainfall, such as the summer monsoon. If circulation patterns change, locations that used to experience intense rain may receive far less rain than they are used to, and vice versa. To predict how human activity will change regional rainfall in the future requires using models. Unfortunately, the models that are currently available struggle with both the interactions between aerosols and clouds and with some regional circulation features, so their regional predictions are not as reliable as society needs them to be. A recent study describes a model that appears to be much better at aerosol-cloud interactions than previous global models, potentially solving half the problem. Many groups are currently at work on a new generation of models that may be better at regional circulation, which would solve the other half, although it may be good to temper any optimism until this has actually been demonstrated. If these two advances can be combined within the same model, then that model will be able to provide a much more reliable estimate of future regional climate. But model development is hard and slow work, while aerosol emissions are changing rapidly, so it is also important to find ways to extract useful information from imperfect models.

Automated summary

Global models are essential for climate projections, but conventional models have errors in both cloud physics and circulation features. Recent research shows a global model capable of accurately representing the impact of aerosols on clouds, potentially improving cloud process fidelity. Future models with km-scale resolution may enhance regional circulation and improve regional climate projection capabilities.