Impact of the biosphere on atmospheric aerosol and climate

Understanding climate change is one of the most important challenges facing science today. Atmospheric particles (aerosols) have an important cooling impact on the Earth, partly counteracting greenhouse gas warming over the industrial period. Quantifying this aerosol cooling is a critical step to making more accurate predictions of climate change. Considerable effort has been put into assessing the climate impact of anthropogenic (man-made) particle emissions. This proposal focuses on natural biogenic aerosol, which has been much less studied and is poorly understood. However, we need to understand the natural situation before we can fully understand how mankind is changing the atmosphere and the climate. Vegetation emits organic species to the atmosphere (the cause of the well known forest pine smell) where they can react to form particles. Vegetation also directly emits particles such as pollen and fungal spores. Observations have shown that these natural biogenic aerosol emissions can dominate the total particle load in much of the atmosphere. The particles cool the climate by reflecting sunlight and by making clouds thicker and more reflective. We also know that rain droplets form on these tiny biological particles and so they can play a role in controlling worldwide rainfall patterns. Despite such obvious importance, current climate models have a very limited representation of biological particle sources and so their importance for regulating climate is not well known. As the climate changes ecosystems will respond: plant activity such as photosynthesis is modified and vegetation distributions are changed. These changes modify biogenic aerosol emissions changing the amount of particles in the atmosphere with consequent impacts on aerosol cooling. Such climate feedbacks may dampen or amplify anthropogenic global warming. That is, as the world warms particle emissions from the forests may increase helping to cool the climate. In this way forests may act as a global 'air conditioner'. But we don't know how important this effect is because these interactions have not yet been treated by climate models. Tropical deforestation is rapidly changing land-cover but the impacts on climate are not well known because climate models do not treat all the necessary interactions between vegetation and climate. There is also interest in intentionally modifying land-use to mitigate climate change. For example, afforestation absorbs carbon dioxide into vegetation and could be used as a carbon-sink to offset greenhouse gas emissions from fossil fuel use. But the impact to climate of such changes to land-use is poorly known. Recent studies suggest that afforestation at high-latitudes (for example, in Europe) may actually warm the climate because dark forest vegetation absorbs more sunlight than snow-covered farmland. But these previous studies ignored particle emissions from the forest and their impact on climate. My own research has shown that these forest-derived particles result in an important cooling that can play a significant role in the net climate impact of land-use change. In this fellowship I will substantially improve our understanding of natural particles from the biosphere. I will include a comprehensive treatment of vegetation particle emissions in climate models for the first time. I will answer the fundamental question of how terrestrial biosphere emissions regulate global aerosol and climate. Finally, through an integrated assessment of different ecosystem-climate interactions I will improve our understanding of how changes to land-use alter climate.