Quantifying the impact of atmospheric aerosol on diffuse radiation and the Amazon biosphere

Trees in the Amazon are getting bigger absorbing large amounts of carbon dioxide from the atmosphere; equivalent to almost 10% of anthropogenic greenhouse gas emissions. This "carbon sink" slows the buildup of atmospheric carbon dioxide and therefore plays an important role in regulating climate change. At the moment we do not understand why the Amazon forest is behaving in this way. One potential reason is changes to the amount of tiny particles in the atmosphere called aerosols. We know that human activity has significantly changed the amount of aerosol over the Amazon: each dry season, humans start forest fires across southern parts of the Amazon basin (the so called "Arc of deforestation"). These fires emit large quantities of smoke aerosol into the atmosphere, scattering and absorbing radiation from the sun. This scattering of the sun's rays decreases the amount of direct radiation reaching the surface but increases the amount of diffuse radiation. It has been known for a long time that increases in diffuse radiation can increase plant productivity. For example, gardeners place "diffuser sheets" inside greenhouses that increase diffuse radiation in the similar way to atmospheric particles. The reason why diffuse radiation increases plant productivity is that it penetrates deeper into the canopy and illuminates leaves that would otherwise be in the shade. These previously shaded leaves can do more photosynthesis. A recent study showed that this effect can be very important, increasing the amount of carbon taken up by forests globally by 25%. It is therefore likely that deforestation fires (a major source of carbon dioxide emissions) are at least partly contributing to the sink of carbon dioxide in undisturbed forests in the Amazon basin.

So far this effect has not been carefully studied for forests in the Amazon. Firstly, it is not well known exactly how much the amount of aerosol particles over the Amazon has changed. Second, no one has quantified how this change in aerosol particles has altered the amount of diffuse radiation. And finally we do not know whether these changes in diffuse radiation are sufficient to explain any of the observed changes in forest biomass across the Amazon. In this project we will use a suite of atmospheric aerosol, radiation and land-surface models to explore the problem. We will use a global model of atmospheric aerosol to simulate how aerosol distributions have changed over the Amazon over the last few decades. We will input these changes in aerosol into an atmospheric radiation model to quantify how amounts of diffuse radiation have changed. Finally, we will use these changes in diffuse radiation as input to a land-surface model to quantify how it impacts Amazon forests. At each step in the process we will use observations from ground-sites and satellite sensors to test how well the models explain observed changes. The project will lead to a much better understanding of how Amazon aerosol has changed and how this impacts diffuse radiation and the biosphere.

Our proposal will have a direct impact on national and international climate prediction centres, climate policy and the Intergovernmental Policy on Climate Change (IPCC). The proposal will develop and improve a global aerosol model (GLOMAP) that is embedded in the UK Chemistry and Aerosol Model and as part of the Met Office Unified Model and the the European Centre for Medium-Range Forecasting (ECMWF) Integrated Forecasting System. Developments to the GLOMAP model feed directly into these other models resulting in improved representation of aerosol processes and climate impacts. The Institute for Climate and Atmospheric Science (ICAS) has established collaborations with the Met Office. ICAS has been working with the Met Office for several years on a new chemistry-aerosol-climate which will be used for climate prediction including future IPCC assessments. The new Met Office academic partnership is a cluster of research excellence that includes the University of Leeds. Improving our understanding of aerosol processes is one of 3 key areas identified for the Leeds - Met Office collaboration. This proposal will contribute directly to this partnership. ICAS also collaborates with the ECMWF through the EU Global Monitoring for Environment and Security MACC project.

Specifically in this project, our user engagement will include:

1. CEH: We will engage with CEH through our project partner Lina Mercado. She will be involved closely with the work we propose using the Met Office/CEH JULES model. We will aim to write a joint publication based on the assessment of JULES output against observations across the Amazon.

2. Met Office: Engagement with the Met Office will continue through ongoing collaborations including the Met Office - Leeds academic partnership and the NCAS-funded collaboration (UKCA). We propose a comprehensive evaluation of the model over the Amazon which will be more detailed than any carried out previously in this regions. Through this process we will improve treatment of aerosol over this important region ultimately resulting in improvement to the UK's climate modelling capability.

3. RAINFOR: RAINFOR is a network of more than 20 institutional partners in South America, Europe and the United States. The main focus of RAINFOR is to understand the behaviour of tropical forests in the face of atmospheric change, involving both fieldwork and analysis to describe spatial patterns, detect change, and to attribute it to various potential drivers. We will engage with RAINFOR through Oliver Phillips who leads the UK coordination of RAINFOR and is a project partner on the proposal. We aim to write a joint publication.

4. ECMWF: The project will improve the understanding and model treatment of biomass burning aerosol which can be a substantial air quality problem in many regions of the world. Through the MACC project improved knowledge from this project will feed into the ECMWF model. Therefore our project will ultimately help ECMWF provide publically available air quality forecasts.