Quantifying Sources Of Uncertainty In Climate Cooling By Aerosol Particles

GLOMAP Showcase

Human emissions of tiny particles of dust or soot in the air (aerosol particles) can cool the climate by increasing the number of water droplets that are created when clouds form. The magnitude of this cooling, however, is extremely uncertain. Reducing this uncertainty is an important step in improving climate forecasts.

Researchers in the aerosol group at the School of Earth and Environment, with the support of the CEMAC team, have developed a new way of quantifying the different sources of uncertainty that contribute to this cooling, with surprising results. They found that although it is increases in number man-made aerosol particles that causes the aerosol cooling effect, is natural aerosol particles (emitted by volcanoes and sea spray) that contribute most to the uncertainty in the cooling. This means that to improve confidence in our calculations this aerosol effect, we first have to understand what aerosol existed in the pre-industrial atmosphere, before human activities created a lot of additional aerosol particles.

This analysis required the researchers to run what is called a perturbed physics ensemble. This is a set of simulations where the computer model is run a large number of times, with similar but different settings of key uncertain parameters. A statistical emulator was then used to calculate which of the uncertain parameters contributed most to the uncertainty in the results. The CEMAC team helped setup the simulations, automate the submission and monitoring of large numbers of simulations and helped develop the software to analyse the dataset produced. The figure on the left shows model-calculated aerosol forcing (top) and the uncertainty (bottom).

During this work we contributed to an article on the technical challenges in, but importance of, examining uncertainty in climate models in Scientific Computing World.

Reference:
Carslaw KS; Lee LA; Reddington CL; Pringle KJ; Rap A; Forster PM; Mann GW; Spracklen DV; Woodhouse MT; Regayre LA; Pierce JR (2013) Large contribution of natural aerosols to uncertainty in indirect forcing, Nature, 503, pp.67-71.