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，以及陆地上季节性积雪的减少，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，冻结在永久冻土中的有机物质（通常已冻结数千年）正在解冻，使其可被土壤生物分解，同时释放出二氧化碳（CO2）和甲烷（CH4）这两种温室气体（ghg）作为副产品。现在的一个主要问题是，由于永久冻土土壤含有1672吨（1 Pg = 10亿吨）有机碳(C)，约占全球地下有机C总量的50%，而永久冻土占北半球陆地表面的25%（2300万平方公里），融化引起的温室气体从永久冻土土壤释放到大气中可能导致全球变暖的严重加速。这被称为全球变化的“积极生物地球化学反馈”；换句话说，这是全球C循环和气候系统中一个意想不到的副作用。不幸的是，由于科学界对永久冻土系统中C和水循环之间的相互作用了解不够，因此对永久冻土（和融化的永久冻土）环境向大气排放的CO2和CH4的相互作用的生物、化学和物理控制是许多猜测的主题。温暖和干燥的土壤可能释放更多的CO2，而温暖/潮湿的土壤可能释放更多的CH4。永久冻土层的融化也会改变水流的方式（因为冻土通常不透水），一些地区可能会变得更干燥，而另一些地区可能会变得更湿润。二氧化碳和甲烷排放量的相对比例如何变化，以及它们的绝对数量，对于总体的“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效力。从土壤中释放到淡水中的碳也需要考虑在内，因为下游的“脱气”和有机物质的分解也会影响淡水中二氧化碳和甲烷的释放，或碳向湖泊/海洋的输送。总而言之，预测永久冻土区的全球升温潜能值在科学上具有挑战性，而且人们对水（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北方地区景观尺度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碳（通过光合作用），其在植物和土壤中的分布，以及碳从土壤到淡水的运输和输送的生物、物理和化学控制，最终以CO2和CH4的形式进入大气。从本质上讲，我们希望“关闭C循环”。对水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，将通过计算机模型联系起来。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影响的基本科学知识。对于直接依赖北极苔原/北方森林生态系统维持生计和文化认同的当地人民，以及必须应对和预测气候和环境变化潜在后果的国际社会来说，该项目将在理解/预测能力方面向前迈出重要一步。

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.