CYCLOPS: Carbon Cycling Linkages of Permafrost Systems

陆地生态系统目前吸收了化石燃料燃烧释放到大气中的四分之一的二氧化碳，从而减缓了气候变化的速度。随着条件变得更有利于植物生长，大多数模型预测，在21世纪，高纬度地区将吸收更多碳。然而，大量的碳储存在北方和北极的永久冻土中，气候变暖可能会导致这些碳中的一部分释放到大气中。最近将多年冻土融化纳入大规模模型模拟表明，多年冻土反馈可能会显著降低21世纪陆地生态系统预测的全球碳净吸收，这将对气候变化的速度产生重大影响。在预测永久冻土融化速度和确定对比生态系统融化的影响方面仍然存在巨大的不确定性，许多关键过程从碳-气候模型中缺失。首先，不同植物群落在隔离土壤和保护冻土方面的作用没有得到很好的量化，大多数模型中没有苔藓等关键类群。此外，火灾干扰可以大大加速永久冻土的融化，因此，保护永久冻土的植物群落从火灾中恢复的能力可能在确定永久冻土的弹性方面发挥关键作用。其次，不同的生态系统对融化的反应可能不同，对温室气体排放的影响也不同。在自由排水的生态系统中，融化可能会导致碳的净释放，因为之前冻结的有机物质的分解增加了。另一方面，当泥炭地发生融化时，土壤下沉可以有效地提高地下水位，从而导致碳的积累。然而，这种潜在的负面反馈可能会被更强大的温室气体甲烷的增强释放所抵消。重要的是，在这些不同的生态系统中，对永久冻土融化的全方位反馈目前并没有反映在基于过程的模型中。为了解决这些问题，我们将开展有针对性的实地考察活动，以确定(1)不同植物群落在保护不同土壤类型以及未燃烧和火灾干扰的生态系统中的永久冻土方面所起的作用，以及(2)永久冻土融化对自由排水系统和泥炭地系统中二氧化碳和甲烷通量的影响。通过与加拿大合作伙伴的链接，将从正在进行多年冻土监测的一系列实地地点收集数据，包括：(1)两个不同的北方泥炭地，它们的永久冻土范围不同，并且存在永久冻土退化；(2)加拿大北部三种重要森林类型内的烧毁和未烧毁地点。我们的美国合作伙伴将提供阿拉斯加连续永冻带内已燃烧和未燃烧的潮湿酸性冻土带的数据。火灾现场的空间可变植被恢复使植被和永久冻土之间的关系得到了详细的测试，而冻土带、森林和泥炭地之间的比较提供了对不同生态系统中永久冻土融化影响的洞察。关键是，这些数据将用于开发、参数化和评估基于过程的植被-土壤-永久冻土相互作用的详细模型。深入展现植被与永久冻土之间的联系将改善对永久冻土融化速度的预测。该模型将首次模拟自由排水与湿地生态系统中的生物地球化学反馈(甲烷和二氧化碳)的全范围。此外，通过与英国气象局科学家的联系，我们的模型将与英国联合陆地环境模拟器(Jules)相耦合，允许运行区域模拟，与气候模型相耦合。最终，我们的项目将改善对永久冻土融化速度和后果的预测，并有助于确定对21世纪气候变化的潜在影响。

Terrestrial ecosystems currently absorb one quarter of the carbon dioxide released by fossil fuel burning into the atmosphere, and thus reduce the rate of climate change. As conditions become more favourable for plant growth, most models predict that high latitudes will take up more carbon during the 21st century. However, vast stores of carbon are frozen in boreal and arctic permafrost, and warming may result in some of this carbon being released to the atmosphere. The recent inclusion of permafrost thaw in large-scale model simulations has suggested that the permafrost feedback is potentially so significant that it could reduce substantially the predicted global net uptake of carbon by terrestrial ecosystems during the 21st century, with major implications for the rate of climate change.Large uncertainties remain in predicting rates of permafrost thaw and in determining the impacts of thaw in contrasting ecosystems, with many of the key processes missing from carbon-climate models. Firstly, the role that different plant communities play in insulating soils and protecting permafrost is poorly quantified, with key groups such as mosses absent in most models. In addition, fire disturbance can substantially accelerate permafrost thaw, and hence the ability of permafrost-protecting plant communities to recover from fire may play a key role in determining permafrost resilience. Secondly, different ecosystems may respond differently to thaw with contrasting effects on release of greenhouse gasses. In free-draining ecosystems, thaw may result in the net release of carbon due to increased decomposition of previously frozen organic matter. On the other hand, when thawing takes place in peatlands, soil subsidence can effectively raise the water table, which could result in carbon accumulation. However, this potential negative feedback may be offset by enhanced release of the more powerful greenhouse gas, methane. Importantly, the full range of feedbacks to permafrost thaw in these contrasting ecosystems is not currently reflected in process-based models. To address these issues, we will undertake directed fieldwork campaigns to determine (1) the role that different plant communities play in protecting permafrost within different soil types, and in unburned and fire-disturbed ecosystems, and (2) the impacts of permafrost thaw on fluxes of carbon dioxide and methane in free-draining versus peatland systems. Through links to Canadian partners, data will be collected from a range of field sites where permafrost monitoring is ongoing, including: (i) two contrasting boreal peatlands differing in permafrost extent, and where there is permafrost degradation; (ii) burnt and unburned sites within three important forest types in boreal Canada. Data will be provided from burnt and unburned moist acidic tundra within the continuous permafrost zone in Alaska by our US partners. The spatially variable vegetation recovery at the fire sites allows relationships between vegetation and permafrost to be tested in detail, while comparisons between the tundra, forest and peatland sites provide insights into the impacts of permafrost thaw in contrasting ecosystems.Critically, these data will be used to develop, parameterise and evaluate a detailed process-based model of vegetation-soil-permafrost interactions. The in-depth representation of vegetation-permafrost linkages will improve predictions of rates of permafrost thaw. The model will be the first to simulate the full range of biogeochemical feedbacks (methane and carbon dioxide) in free-draining versus wetland ecosystems. Furthermore, through links with Met Office scientists, our model will be coupled to the Joint UK Land Environment Simulator (JULES), allowing regional simulations to be run, coupled to a climate model. Ultimately, our project will improve predictions of both the rates and consequences of permafrost thaw, and help determine the potential impacts on 21st century climate change.