The response of the Arctic regions to changing climate

人为燃烧化石燃料通过提高大气中的二氧化碳浓度影响气候系统。但是，当受到气候变化的影响时，自然地球系统的某些方面是否会对碳循环产生重大影响？这种气候-碳（C）循环反馈可能是积极的--也就是说，它们可能导致自然系统目前减缓人为CO2排放的能力下降，甚至可能迫使自然系统成为CO2的直接来源。这种积极的反馈是令人担忧的一个主要原因，它们的启动可以被视为气候“临界点”。据推测，北极陆地表面可能是一个这样的“引爆点”，全球变暖足以诱导土壤碳损失大于任何额外的下拉C通过增强苔原（灌木）和北方森林生长在一个温暖的，CO2富集的环境。此外，气候变暖将影响能源和水循环，北方地区积雪减少，降低了地表反射率，从而导致进一步变暖。在现实中，能源、水和碳循环都是密切相关的，需要进行交互式建模，以提供对未来的可靠估计。全球环流模型（GCM）旨在模拟气候系统和全球碳循环，并在此过程中提供气候系统潜在“临界点”的指针。但他们对北极地区的预测只能与该地区地表模型中描绘的水文、生态和能量相互作用一样好。因此，这个项目有两个目标-提供更多的物理现实的陆面模型，然后看看这种增强模拟未来气候的影响。北极地区是否最终会通过增加未来大气二氧化碳水平来加剧人类引起的气候变化？北极地区的陆地表面建模是复杂的。要做到这一点，我们需要捕捉植被如何在温暖的环境中生长和扩张，以及这如何改变土壤碳储量。我们还需要模拟雪与植被的相互作用-积雪将随着气候变化而变化，并将影响能量输入，水循环，冻土和植被分布。例如，较深的雪将出现在高大植被的地区，因此植被结构不仅影响雪融化的时间，而且影响热状况，因为较深的雪实际上隔离了土壤。因此，对土壤呼吸和植被生长产生了连锁效应。该项目将对所有这些特征进行动态建模，以便评估未来温度和降雪模式对北极生态系统的影响。将广泛利用专业人员和其他方面在过去十年中，特别是在国际极地年开发的现有观测数据集。北极陆地表面的这一新知识将被引入泛北极网格建模系统。将综合当地和区域的行为，以确定陆地-大气CO2净通量。然而，未来陆地表面行为的重大变化可能会对气候系统的其他方面产生强烈的反馈，例如表面温度和土壤湿度。因此，该项目的最后一个组成部分是进行耦合陆地-大气模拟，从而捕获所有反馈。我们将通过与哈德利中心（一个世界领先的气候系统建模中心，他们用他们的GCM家族进行预测）现有和正在进行的合作来实现这一目标。这种联系将允许对北极陆地表面是否会通过一个不受欢迎的气候“临界点”进行最终评估，从而对现有的变暖进行反馈，无论是通过进一步加剧局部变暖还是通过全球碳循环。

The anthropogenic burning of fossil fuels is affecting the climate system via raised atmospheric CO2 concentrations. But are there aspects of the natural Earth system that, when forced by imposed climate change, have themselves a major impact on the carbon cycle? Such climate-carbon (C) cycle feedbacks might be positive - that is, they may cause a reduction in the current capability of natural systems to mitigate anthropogenic CO2 emissions, or may even force natural systems to become direct sources of CO2. Such positive feedbacks are a major cause for concern, and their initiation could be regarded as a climate 'tipping point'. It has been hypothesised that the Arctic land surface could be one such 'tipping point', whereby global warming is sufficient to induce soil C losses greater than any extra draw-down of C through enhanced tundra (shrub) and boreal forest growth in a warmer, CO2-enriched environment. In addition, a warmer climate will impact on the on the energy and water cycles, with less snow cover in Northern Latitudes reducing surface reflectivity and so inducing additional warming. In reality the energy, water and C cycles are all strongly linked and need to be modelled interactively to provide robust estimates of the future. Global Circulation Models (GCMs) are designed to emulate the climate system and the global carbon cycle, and in the process, provide pointers to potential 'tipping points' in the climate system. But their predictions for the Arctic region will only be as good as the hydrology, ecology and energy interactions depicted in the land surface model for that region. This project therefore has two aims - to provide much more physical realism in land surface models, and then to see how this enhancement impacts on modelled future climate. Does the Arctic region eventually enhance human-induced climate change by increasing future levels of atmospheric carbon dioxide? Modelling the land surface for the Arctic region is complicated. To get this correct, we will need to capture how the vegetation may grow and expand in a warmer environment, and how this might change soil C stocks. We also need to model how the snow interacts with vegetation - snow cover will change with climate, and will influence the energy inputs, the water cycle, the frozen ground and the vegetation distribution. For example, deeper snow will occur in areas of tall vegetation and thus vegetation structure influences not only the timing of snow melt but also the thermal regime as deeper snow actually insulates the soils. There is thus a knock-on effect on soil respiration and vegetation growth. This project will model all of these features, dynamically, such that the impact of future temperature and snowfall patterns on the Arctic ecosystems can be assessed. Extensive use will be made of existing observational datasets developed by the PIs and others over the last decade and, in particular the International Polar Year. This new knowledge of the Arctic land surface will be introduced within a pan-Arctic gridded modelling system. The local and regional behaviours will be integrated to determine net land-atmosphere CO2 fluxes. However, major future changes in land surface behaviour might have strong feedbacks on other aspects of the climate system e.g. surface temperatures and soil moisture. Hence, the last component of this project is to make coupled land-atmosphere simulations, thereby capturing all feedbacks. We will achieve this through our existing and on-going collaboration with the Hadley Centre (a world leading centre for modelling the climate system, who make predictions with their family of GCMs). This link will allow a final assessment to be made of whether the Arctic land-surface could pass an unwelcome climate 'tipping point', and thus feedback on existing warming, either locally through enhancing warming further or through the global C cycle.