ICAAP: Increasing Carbon Accumulation in Arctic Peatlands

预测未来的气候变化是人类面临的最大的科学和社会挑战之一。虽然人类活动的碳排放是未来气候变化的主要决定因素，但地球系统的反应也极其重要。地球系统过程为气候变化提供“反馈”，要么加强温室气体浓度和温度的上升趋势(正反馈)，要么有时抑制它们(负反馈)。陆地全球碳循环和气候形成了一个重要的反馈回路。随着大气中二氧化碳浓度和温度的上升，由于二氧化碳的施肥效应和高纬度地区生长季的延长(这是一个负反馈)，植物的碳固定增加。但与此同时，温度升高会导致土壤中储存的碳分解增加，从而导致更多的二氧化碳被释放回大气(这是一个积极的反馈)。这些相互竞争的过程的平衡对泥炭地尤为重要，因为它们是非常大的碳库。北半球泥炭地拥有的碳量与包括森林在内的世界上所有活着的植被中储存的碳量大致相同，因此确定这个庞大的碳库对未来气候变化的反应尤为关键。一种假设是，气候变暖将增加泥炭地土壤的分解速度，以至于未来将释放大量碳。然而，绝大多数泥炭地位于相对寒冷和潮湿的地区，过去积累速度变化的证据表明，对于这些地区来说，气候变暖可能会导致生产力的提高，这超过了对腐烂速度的任何增加的补偿，从而导致整体碳封存的增加。此外，在北极最北部地区，泥炭地有可能进一步横向扩张，增加泥炭堆积的总面积。我们打算回答的问题是，北极泥炭地积累的变化加上寒冷地区泥炭地分布的增加是否会导致其碳储存能力的全面增加。我们的方法将是使用泥炭地中存储的化石记录数据和卫星数据的新组合来测试一个全球模型，该模型模拟了北极地区泥炭地植被范围和碳积累速率的变化。如果我们能够证明该模型在模拟过去的变化时表现良好，那么我们就可以自信地使用它来预测未来数百年内因应气候变暖而发生的变化。我们知道，过去1000年来北极气候的波动本应足以推动泥炭积累速率和横向扩散的变化，因此我们将重点放在这一时期的分析上。特别是，我们知道，在过去的150-200年里，特别是在过去的30-40年里，气温有所上升。如果我们的假设是正确的，即气温升高会导致北极泥炭地的积累和扩散增加，我们预计会在泥炭积累和扩散的化石记录中以及植被变化的卫星数据中看到这一点的证据。我们以前的工作和我们新的试点研究表明，我们可以重建累积速率变化，并且我们提出的遥感技术可以探测到自20世纪80年代中期以来泥炭地植被的增加，因此我们对我们的方法充满信心。该模型将为下个世纪和2300年之前的不同气候变化情景提供北部泥炭地碳储量变化的估计。如果我们能够证明北极泥炭地的碳储量存在潜在的增加甚至没有变化，这将从根本上改变我们对北极陆地碳库在调节气候变化中所起作用的看法。

Predicting future climate change is one of the biggest scientific and societal challenges facing humankind. Whist carbon emissions from human activities are the main determinant of future climate change, the response of the earth system is also extremely important. Earth system processes provide 'feedbacks' to climate change, either reinforcing upward trends in greenhouse gas concentrations and temperature (positive feedbacks) or sometimes dampening them (negative feedbacks). A crucial feedback loop is formed by the terrestrial global carbon cycle and the climate. As carbon dioxide concentrations in the atmosphere and temperature rise, carbon fixation by plants increases due to the CO2 fertilisation effect and the lengthening of the growing season at high latitudes (this is a negative feedback). But at the same time, increasing temperatures lead to increased decomposition of the carbon stored in soils and this results in more carbon dioxide being released back to the atmosphere (this is a positive feedback). The balance of these competing processes is especially important for peatlands because they are very large carbon stores. Northern Hemisphere peatlands hold about the same amount of carbon that is stored in all the world's living vegetation including forests, so determining the response of this large carbon store to future climate change is especially critical. One hypothesis is that warming will increase decomposition rates in peatland soils to such an extent that large amounts of carbon will be released in the future. However, the vast majority of peatlands are in relatively cold and wet areas and evidence from past changes in accumulation rates suggest that for these regions, warming may lead to increased productivity that more than compensates for any increase in decay rates, leading to increased carbon sequestration overall. Furthermore, in the northernmost areas of the Arctic, there is potential for further lateral expansion of peatlands, increasing the total area over which peat accumulates. We intend to answer the question of whether changes in accumulation in Arctic peatlands plus the increased spread of peatlands in cold regions will lead to an overall increase in their carbon storage capacity.Our approach will be to use a novel combination of data from the fossil record stored in peatlands together with satellite data to test a global model that simulates changes in both carbon accumulation rates and the extent of peatland vegetation over Arctic regions. If we can demonstrate that the model performs well in simulations of past changes, we can then confidently use it to make projections of future changes in response to warming for several hundred years into the future. We know that fluctuations in Arctic climate over the past 1000 years should have been sufficient to drive changes in peat accumulation rates and lateral spread, so we are focusing our analyses on this period. In particular, we know there were increases in temperature over the last 150-200 years and especially over the last 30-40 years. If our hypothesis that increased temperature leads to increasing accumulation and spread of Arctic peatlands is correct, we expect to see the evidence for this in the fossil record of peat accumulation and spread, and also in satellite data of vegetation change. Our previous work and our new pilot studies show that we can reconstruct accumulation rate changes and also that our proposed remote sensing techniques can detect peatland vegetation increases since the mid-1980s, so we are confident in our methodology. The model will provide estimates of northern peatland carbon storage change for different climate change scenarios over the next century and longer term to the year 2300. If we can show that there is a potential increase or even no change in carbon storage in Arctic peatlands, it will radically change our perception of the role of the Arctic terrestrial carbon store in mediating climate change.