Permafrost catchments in transition: hydrological controls on carbon cycling and greenhouse gas budgets

北极正在经历快速的气候变化，对“冰冻世界”（“冰冻圈”）产生了巨大的影响，包括海冰的深度，范围和持续时间的减少，陆地上的季节性积雪，冰盖/冰川的退缩，以及永久冻土（“至少连续两年保持在0摄氏度或以下的地面”）的融化。这不仅对地方和区域生态系统和人类社区很重要，而且对整个地球系统的运作也很重要。越来越多的证据表明，永久冻土中冻结的有机物（通常长达数千年）现在正在解冻，使其可供土壤生物分解，并释放出二氧化碳（CO2）和甲烷（CH 4），两者都是温室气体（GHG）作为副产品。现在的一个主要问题是，由于永久冻土含有1672千万亿立方米，（1 Pg = 10亿吨）的有机碳（C），约占全球地下有机碳总储量的50%，永久冻土层约占25%（2300万平方公里）的北半球陆地表面，融化引起的温室气体从永冻土释放到大气中可能导致全球变暖的加速。这被称为对全球变化的“积极的地球化学反馈”;换句话说，这是全球碳循环和气候系统中无意的副作用。不幸的是，相互作用的生物，对冻土CO2和CH 4排放的化学和物理控制（和融化的永久冻土）环境对大气的影响是许多猜测的主题，因为科学界对C和C之间的相互作用了解不够。永久冻土系统中的水循环较温暖和干燥的土壤可能释放更多的CO2，而较温暖/潮湿的土壤可能释放更多的CH 4。永冻层融化也会导致水在景观中流动的方式发生变化（因为冻土通常不透水），一些地区可能会变得更干燥，而另一些地区则会变得更潮湿。CO2和CH 4排放的相对比例如何变化，以及它们的绝对数量，对于整体“全球变暖潜能值”（GWP）至关重要，因为这两种气体作为温室气体具有不同的潜力。还需要考虑从土壤向淡水中释放的C，因为下游的“脱气”和有机物质的分解也会影响从淡水中释放的CO2和CH 4，或C向湖泊/海洋的输送。总而言之，预测永冻土地区的全球升温潜能值在科学上具有挑战性，水（水文）和碳循环之间的相互作用知之甚少。在这个项目中，我们认识到水文过程在北极和寒带地区的碳通量中发挥的关键作用。在加拿大西北部的永久冻土集水区（包括已知永久冻土正在融化的地区），我们将测量从大气中捕获的C（通过光合作用），其在植物和土壤中的分布，以及从土壤到淡水的C运输和交付的生物，物理和化学控制，并最终以CO2和CH 4的形式进入大气。从本质上讲，我们希望“关闭C循环”。对水循环和碳循环关键过程的实地测量，包括地球化学示踪剂和最先进的碳、氢和氧同位素方法，将通过计算机建模联系起来。该项目团队与加拿大、美国和英国的合作伙伴一起，在将永久冻土环境中的水和碳循环联系起来方面处于独特的地位，我们将提供有关气候变暖和永久冻土融化对北方高纬度地区温室气体排放的潜在后果的基本科学知识。无论是对直接依赖北极苔原/北方森林生态系统维持生计和文化特性的当地人民，还是对必须应对和预测气候和环境变化的潜在后果的全球社会，该项目都将是在理解/预测能力方面向前迈出的重要一步。

The Arctic is undergoing rapid climatic change, with dramatic consequences for the 'Frozen World' (the 'cryosphere'), including reductions in the depth, extent and duration of sea ice, and seasonal snow cover on land, retreat of ice sheets/glaciers, and melting of permafrost ("ground that remains at or below 0 degrees C for at least two consecutive years"). This is important not only for local and regional ecosystems and human communities, but also for the functioning of the entire earth system. Evidence is growing that organic matter frozen in permafrost soils (often for many millennia) is now thawing, making it available for decomposition by soil organisms, with the release of carbon dioxide (CO2) and methane (CH4), both greenhouse gases (GHGs), as by-products. A major concern now is that, because permafrost soils contain 1672 petagrams (1 Pg = 1 billion tonnes) of organic carbon (C), which is about 50% of the total global below-ground pool of organic C, and permafrost underlies ~ 25% (23 million km2) of the N hemisphere land surface, a melting-induced release of GHGs to the atmosphere from permafrost soils could result in a major acceleration of global warming. This is called a 'positive biogeochemical feedback' on global change; in other words, an unintentional side-effect in the global C cycle and climate system.Unfortunately, the interacting biological, chemical and physical controls on CO2 and CH4 emissions from permafrost (and melting permafrost) environments to the atmosphere are the subject of much speculation because the scientific community does not know enough about the interactions between C and water cycling in permafrost systems. Warmer and drier soils may release more CO2, while warmer/wetter soils might release more CH4. Permafrost thawing also causes changes in the way water flows though the landscape (because frozen ground if often impermeable to water), and some areas may become drier, while others wetter. How the relative proportions of CO2 and CH4 emissions change, and their absolute amount, is critical for the overall 'global warming potential' (GWP) because these two gases have different potency as GHGs. Release of C from soils into freshwaters also needs to be taken into account because down-stream 'de-gassing' and decomposition of organic materials also influences releases of CO2 and CH4 from freshwater, or delivery of C to lakes/oceans. All-in-all, predicting the GWP of permafrost regions is scientifically challenging, and the interactions between the water (hydrological) and C cycles are poorly known.In this project we recognise the key role that hydrological processes play in landscape-scale C fluxes in arctic and boreal regions. In permafrost catchments in NW Canada (including areas where permafrost is known to be thawing) we will measure the capture of C from the atmosphere (through photosynthesis), its distribution in plants and soils, and the biological, physical and chemical controls of C transport and delivery from soils to freshwaters, and ultimately to the atmosphere as CO2 and CH4. In essence we wish to 'close the C cycle'. Field-based measurements of key processes in the water and C cycles, including geochemical tracer and state-of-the-art C, hydrogen and oxygen isotope approaches, will be linked by computer modelling. The project team, together with partners in Canada, the US and UK, is in a unique position to link the water and C cycles in permafrost environments, and we will deliver essential scientific knowledge on the potential consequences of climate warming, and permafrost thawing, for GHG emissions from northern high latitudes. Both for local peoples directly dependent on arctic tundra/boreal forest ecosystems for their livelihoods and cultural identity, and for the global community who must respond to, and anticipate, potential consequences of climate and environmental change, this project will represent a significant step forward in understanding/predictive capacity.