Aerosols and Climate

Aerosols – small particles in the atmosphere – have a profound effect on many aspects of our environment.

The particles derive from natural processes like sea spray and wildfires, but in our polluted world the main source is from human activities like fossil fuel burning. Aerosols affect Earth’s climate as strongly as greenhouse gases, acting to cool the planet by reflecting the sun’s rays back to space. The Intergovernmental Panel on Climate Change has concluded that industrial aerosols have acted as a significant brake on the increase in global temperatures over the last 30 years or so.

Aerosols are also air pollutants that cause the premature death of tens of thousands of people every year in the UK alone. Air pollution is estimated to cost the UK more than obesity, but our understanding of how aerosol pollution will respond to expensive mitigation strategies is still quite limited.

Aerosols have even more interesting and important effects on weather. For example, clouds and rainfall can be quite different in polluted air compared to clean air, and storm systems can intensify more rapidly. We need to understand these effects because they may lead to changes in damaging weather over many years. Ultimately these effects may turn out to be more important to society than a small change in temperature.

Society needs accurate predictions of aerosols to understand the climate and to predict air pollution episodes. At present, the uncertainty in the magnitude of their effect on climate dominates the overall uncertainty, which is hampering progress with climate prediction. The open questions concerning impacts on precipitation and other weather phenomena are even more challenging to answer.

Aerosol is an exciting research topic, not only because of its obvious importance to society, but also because aerosol research draws on many branches of science – the chemistry of air pollutants, the physics of the weather, the biology of emissions from trees, and the computer science needed to make predictions. It’s a highly challenging field of research that puts students in a stimulating environment working with a wide range of different scientists.

What are the big future challenges and what is Leeds doing?

We think there are four big challenges:

1. How will global aerosol will evolve in a future climate? The main question is how the natural sources and sinks of aerosol will change in a warmer climate with different patterns of precipitation. All natural sources of aerosol are likely to change in some way in future (wildfires, sea spray, dust, biogenic emissions from plankton and vegetation. We reviewed these effects in a recent paper. This is a highly challenging modern-day Earth system problem that requires collaboration across the sciences of atmospheric science, chemistry, biology, meteorology, etc. All aerosol (whether from human activities or nature) will also respond to anticipated changes in atmospheric chemistry. In Leeds we are developing the necessary Earth System models and studying the multiple interactions between aerosol processes and other parts of the Earth system.
    
2. How do changes in aerosol affect global weather patterns? Research shows that long-term trends in drought (such as in the Sahel) may be induced by the spatially patchy cooling of the Earth's surface caused by aerosols. The enormous aerosol concentrations over parts of Asia probably also affect the Monsoon circulation, impacting the rainfall that sustains over a billion people. As weimplement policies to reduce aerosol pollutants, how will this affect global weather? In Leeds we have co-developed the HadGEM-UKCA model which is capable of simulating the global and regional responses to changes in aerosol.
    
3. How is local extreme weather affected by aerosol? Extensive research, including at Leeds, shows that extreme precipitation events (e.g., thunderstorms) can be dramatically affected by changes in aerosol. Weather models don't properly take account of these processes. Other studies have shown that extreme precipitation events are changing over time, which is likely to be caused by a complex interplay of changes in atmospheric thermodynamics and aerosols. Although we probably understand how average rainfall will change, we have very little understanding of how extreme events will change in future. In Leeds we are working in partnership with the Met Office to study the interactions between aerosols and meteorology on local scales.
    
4. What are the key processes we need to put in models to predict all of the above? This is not a small question. Most aerosol processes are already too complex to include in high resolution weather and climate models. Choosing where to put our research effort for maximum gain in model fidelity (realism) will become a major area of research in its own right. Otherwise we will waste time stuffing our models full of superfluous complexity. In Leeds we are pioneering the use of model emulators that will enable us to identify the most important uncertainties in models as part of the AEROS project.