Lethargic Response to Aerosol Emissions in Current Climate Models

The global temperature trend observed over the last century is largely the result of two opposing effects—cooling from aerosol particles and greenhouse gas warming. While the effect of increasing greenhouse gas concentrations on Earthu0027s radiation budget is well constrained, that due to anthropogenic aerosols is not, partly due to a lack of observations. However, long-term surface measurements of changes in downward solar radiation (SDSR), an often used proxy for aerosol radiative impact, are available worldwide over the last half century. We compare SDSR changes from ∼1,400 stations to those from the Coupled Model Intercomparison Project Version 5 global climate simulations over the period 1961–2005. The observed SDSR shows a strong early downward trend followed by a weaker trend reversal, broadly consistent with historical aerosol emissions. However, despite considerable changes to known aerosol emissions over time, the models show negligible SDSR trends, revealing a lethargic response to aerosol emissions and casting doubt on the accuracy of their future climate projections.