What is the impact of increasing boreal forest fires on Arctic climate and sea ice?

北方森林覆盖了北亚和北美的大片地区，主要覆盖西伯利亚、加拿大和阿拉斯加。近年来，这些森林受到火灾增加的影响，部分原因是夏季较温暖，春季降雪较少。这些大火释放出大量的烟雾，可以被风向北吹到北极。一旦这些烟雾到达北极，它既可以通过反射阳光来冷却气候，也可以使气候变暖(如果烟雾更接近地表，或者如果小烟雾颗粒到达表面的冰雪，因为它们可以使其变暗并吸收更多阳光)。由于我们预计地球各地的气温将继续变暖，北部地区的大火很可能会继续下去，甚至变得更加常见。我们计划利用气候模型研究烟雾可能对北极气候和海冰产生的影响。过去，这些气候模型没有考虑到这些北方大火的增加，我们的工作将帮助我们确定这是否是正确理解北极气候和海冰未来变化的重要过程。随着我们为帮助联合国气候变化专门委员会提供信息的下一代气候模型做准备，我们的结果也将有助于突出北方森林火灾对北极气候变化的重要性。北方森林火灾产生的生物质燃烧可以通过气溶胶，特别是黑碳从源头地区向北极的大气输送，影响北极气候和海冰。在大气中，气溶胶可以有正(变暖)或负(冷)辐射强迫，这取决于它们的海拔高度。黑碳也可以沉积在海冰或雪的表面，在那里它具有正的辐射强迫，因为它使它变暗，并吸收更多的阳光。虽然完全耦合的气候模型模拟了这些过程，但生物质燃烧的量被规定为集合强迫。在帮助为联合国气候变化专门委员会提供信息的最新一套气候模型运行中，从2015年到2100年的模拟使用了对北方生物质燃烧排放的固定预测，这些预测没有预测到森林火灾的增加。然而，在现实世界中，我们观察到过去10年北方地区生物质燃烧的急剧增加，部分原因是夏季变暖和春季雪盖减少(这本身是由温暖的泉水推动的)，黑碳的排放量已经是2015-2100年气候模型使用的两倍多。我们计划通过运行气候模型模拟来研究北方生物质燃烧增加对北极气候和海冰的影响，根据最近观察到的这些火灾的增长来规定气溶胶排放的增加。我们还计划研究风模式对北方森林火灾烟雾传输的影响，因为众所周知，某些天气模式可能会增加森林火灾的可能性。天气和火灾之间的这种耦合作用目前在气候模型中是缺失的，我们的工作将帮助我们确定风，除了火灾的数量、严重性和季节性之外，在确定有多少黑碳被输送到北极方面有多重要。我们的成果将有助于让利益相关者了解北方森林在未来几十年对北极气候的重要性。随着社区为下一轮气候模型模拟做准备，我们的结果将帮助模拟社区了解北方森林火灾增加以及火灾和风对北极气候和海冰的相互作用的重要性。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Boreal forests cover large areas of northern Asia and North America, mainly over Siberia, Canada, and Alaska. In recent years, these forests have been affected by an increase in fires, caused in part by warmer summers and less snow in the spring. These fires emit large amounts of smoke which can be blown by winds northward to the Arctic. Once this smoke gets to the Arctic, it can both cool the climate by reflecting sunlight, but also warm it (if the smoke is nearer the surface, or if the small smoke particles reach the ice and snow on the surface, as they can darken it and absorb more sunlight). As we expect temperatures to continue to warm everywhere on the planet, it is likely that large boreal fires will continue and even become more common. We plan to study the impact that the smoke may have on Arctic climate and sea ice using climate models. In the past, these climate models have not taken into consideration an increase in these boreal fires, and our work will help us determine if this is an important process to get right for understanding future changes in Arctic climate and sea ice. As we prepare for the next generation of climate models that help inform the United Nations’ panel on climate change, our results will also help highlight the importance boreal forest fires may have for climate change in the Arctic.Biomass burning from boreal forest fires can impact Arctic climate and sea ice via the atmospheric transport of aerosols, particularly black carbon, from source regions into the Arctic. While in the atmosphere, aerosols can have either a positive (warming) or negative (cooling) radiative forcing, depending on their elevation. Black carbon can also be deposited onto the sea ice or snow at the surface, where it has a positive radiative forcing because it darkens it and absorbs more sunlight. While fully coupled climate models simulate these processes, the amount of biomass burning is prescribed as a set forcing. In the most recent set of climate model runs that help inform the United Nations’ panel on climate change, simulations from 2015 to 2100 used fixed projections of boreal biomass burning emissions that did not anticipate an increase in forest fires. However, in the real world we have observed a dramatic increase in boreal biomass burning over the last 10 years, in part due to warmer summers and reduced spring snow cover (which itself is driven by warmer springs), and the emissions of black carbon are already more than double what the climate models used over 2015-2100. We plan to study the impact that an increase in boreal biomass burning has on Arctic climate and sea ice by running climate model simulations that prescribe an increase in aerosol emissions based on the recent observed growth in these fires. We also plan to study the impact that wind patterns have on the transport of smoke from boreal forest fires, as it is known that certain weather patterns can promote the likelihood of forest fires. This coupled interaction between weather and fires is currently missing in climate models, and our work will help us determine how important winds, in addition to the amount, severity and seasonality of fires are in determining how much black carbon is transported into the Arctic. Our results will serve to inform stakeholders of the importance of boreal forests for Arctic climate in the coming decades. As the community prepares for the next round of climate model simulations, our results will help inform the modeling community on the importance of both increasing boreal forest fires and the interaction of fires and winds on Arctic climate and sea ice.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.