Impact of the biosphere on atmospheric aerosol and climate

理解气候变化是当今科学面临的最重要的挑战之一。大气微粒（气溶胶）对地球有重要的降温作用，部分抵消了工业时期的温室气体变暖。量化这种气溶胶冷却是对气候变化做出更准确预测的关键一步。在评估人为（人造）颗粒排放对气候的影响方面已经付出了相当大的努力。这一建议的重点是天然生物气溶胶，这方面的研究要少得多，而且了解也很少。然而，在我们完全了解人类是如何改变大气和气候之前，我们需要了解自然状况。植被向大气释放有机物质（这就是众所周知的森林松树气味的原因），在那里它们可以反应形成颗粒。植被也会直接释放花粉和真菌孢子等颗粒。观测表明，这些天然的生物源性气溶胶排放可以在大部分大气中主导总颗粒负荷。这些粒子通过反射阳光和使云层更厚、反射性更强来冷却气候。我们还知道，雨滴是在这些微小的生物颗粒上形成的，因此它们可以在控制全球降雨模式方面发挥作用。尽管具有如此明显的重要性，但目前的气候模型对生物颗粒源的代表非常有限，因此它们对调节气候的重要性尚不为人所知。随着气候的变化，生态系统将做出反应：植物的光合作用等活动被改变，植被分布也发生了变化。这些变化改变了生物源性气溶胶排放，改变了大气中颗粒的数量，从而对气溶胶冷却产生影响。这种气候反馈可能抑制或放大人为的全球变暖。也就是说，随着全球变暖，来自森林的颗粒排放可能会增加，有助于冷却气候。这样，森林就可以充当全球的“空调”。但我们不知道这种影响有多重要，因为这些相互作用还没有被气候模型处理过。热带森林砍伐正在迅速改变土地覆盖，但对气候的影响尚不清楚，因为气候模型没有处理植被和气候之间所有必要的相互作用。人们还对有意修改土地利用以减缓气候变化感兴趣。例如，造林可以将二氧化碳吸收到植被中，并可以作为碳汇来抵消化石燃料使用产生的温室气体排放。但人们对土地利用变化对气候的影响知之甚少。最近的研究表明，高纬度地区（例如欧洲）的植树造林实际上可能会使气候变暖，因为深色的森林植被比白雪覆盖的农田吸收更多的阳光。但是这些先前的研究忽略了森林的颗粒排放及其对气候的影响。我自己的研究表明，这些来自森林的颗粒导致了重要的降温，可以在土地利用变化的净气候影响中发挥重要作用。在这项研究中，我将大大提高我们对生物圈中自然粒子的理解。我将首次在气候模型中纳入植被颗粒排放的综合处理。我将回答陆地生物圈排放如何调节全球气溶胶和气候的基本问题。最后，通过对不同生态系统-气候相互作用的综合评估，我将提高我们对土地利用变化如何改变气候的理解。

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.