Representation of aerosol effects in models

In many climate models, the complex interaction between aerosol and clouds is represented by a single equation. Each model has a different equation (and some do more complicated things and have many many equations...). The inherent sensitivity of the model to aerosol is determined by this equation, but the overall response to a change in aerosol is influenced by lots of other things too: how long aerosol stays in the atmosphere after it's emitted; where the clouds are, and how many of them there are; and how much aerosol there was in the model atmosphere 200 years ago.

We present a simple framework for quantifying the contributions to differences in the strength of model responses to aerosol changes in our article in Geophysical Research Letters.

Aerosol effects on the hydrological cycle

Aerosol affects the hydrological cycle through a range of mechanisms: perturbing the net radiation input at the surface and the top of the atmosphere, altering the microphysics of clouds, and changing the temperature structure of the atmosphere. The response of the climate system is complex. We intend to identify metrics of the hydrological cycle likely to be influenced by aerosol, and whether there are any discernable influences on these metrics in CMIP5 and observations.

As part of this work, we have shown that the global temperature hiatus in the mid-twentieth century, and the coincident decrease in precipitation, is likely to have been strongly influenced by anthropogenic aerosol forcing. You can read more about this work in Ellie's blog, our science highlight, or in the article in Environmental Research Letters.