Exploring new climate feedbacks in the Arctic

北极变暖的速度是地球速度的两倍以上，这是因为特定的反馈，如雪的反照率反馈。发现和量化其他反馈对于可靠预测北极和全球气候至关重要。政府间气候变化专门委员会最新的气候预测没有包括多年冻土的融化和由此产生的以二氧化碳和CH4形式的大气中的永久冻土层碳的释放，这表明我们对北极气候反馈的理解不足。我将研究与多年冻土热状况演变及其碳收支相关的反馈。变暖有利于苔原上灌木的生长，由于灌木中积雪较深提供了额外的隔热，导致了永久冻土变暖的加剧。这种影响必须量化。我们首先观察到灌木的雪热传导性较低，从而增强了它们的多年冻土变暖潜力。或者，灌木通过在秋天吸收太阳辐射，可能导致积雪融化，形成高导热系数的融冻结壳，可能逆转灌木的变暖效应。这些复杂的雪-灌木-冻土相互作用将通过：(1)观察气象、灌木特性与雪物理特性之间的相关性；(2)测量灌木对冬季地表能量收支的影响。这需要测量太阳辐射穿透&gt；10厘米深时的反照率和垂直辐照度分布；(3)利用DART模型进行三维辐射转移模拟，以了解和预测灌木对三维辐射场、积雪中的能量沉积和雪的变质作用的影响；(4)通过测量辐射转移来量化夏季土壤阴影，从而全年了解灌木对地表辐射平衡和对永冻土热力制度的影响；(5)实施陆面和雪物理模型的研究结果。 灌木的生长还通过增加生物量和凋落物的积累来导致碳的固存，可能会减缓融化的旧碳的微生物呼吸。我将研究努纳维克州乌米尤贾克附近土壤碳储量的演变，那里的苔原地衣正受到矮小的桦树灌木的入侵。那里的土壤几乎不含旧碳，这有利于新碳的量化。生物量和凋落物将被量化，灌木年龄将通过树种年代学获得。将分析凋落物中有机物的结构，以了解其降解动力学，并量化中长期储存潜力。 最后，生物学家和我将合作研究雪、气候和旅鼠种群动态之间的相互作用。我们将检验这样一种假设，即秋季的降雨通过导致坚硬的基础雪层，阻碍旅鼠在冬季取食和繁殖的能力，从而防止种群破坏。

The Arctic is warming at over twice the planetary rate because of specific feedbacks such as the snow-albedo feedback. Discovering and quantifying other feedbacks is essential for reliable projections of Arctic and planetary climate. The thawing of permafrost and the resulting atmospheric release of permafrost carbon as CO2 and CH4 has not been included in the latest IPCC climate projections, illustrating our insufficient understanding of Arctic climate feedbacks. I will study feedbacks relevant to the evolution of the permafrost thermal regime and to its carbon budget. Warming favors shrubs growth on tundra, leading to enhanced permafrost warming because of the extra thermal insulation provided by deeper snow in shrubs. This effect must be quantified. We first observed that the snow thermal conductivity was lower in shrubs, enhancing their permafrost warming potential. Alternatively, shrubs, by absorbing solar radiation in the fall, can lead to snow melt and the formation of melt-freeze crusts of high thermal conductivity, possibly reversing the shrub warming effect. These complex snow-shrubs-permafrost interactions will be studied by: (1) observing the correlation between meteorology, shrub properties and snow physical properties; (2) measuring the impact of shrubs on the winter surface energy budget. This requires measuring albedo but also vertical irradiance profiles as solar radiation penetrates >10 cm depth in the snow; (3) performing 3-D radiation transfer simulations using the DART model to understand and predict shrub impact on the 3-D radiation field, energy deposition in the snowpack and on snow metamorphism; (4) quantify soil shading in summer by measuring radiation transfer, therefore gaining a year-round understanding of shrub impact on the radiation budget of the surface and on the permafrost thermal regime; (5) implementing findings in a land surface and snow physics model. Shrub growth also leads to carbon sequestration by enhancing biomass and litter buildup, possibly mitigating the microbial respiration of thawed old carbon. I will study the evolution of carbon stocks of soils near Umiujaq, Nunavik, where lichen tundra is being invaded by dwarf birch shrubs. Soils there contain almost no old carbon, facilitating the quantification of new carbon. Biomass and litter will be quantified, and shrub age obtained by dendrochronology. The structure of the organic matter in litter will be analyzed to understand its degradation kinetics and quantify mid- to long-term storage potential. Lastly, biologists and I will collaborate on the interactions between snow, climate and the population dynamics of lemmings. We will test the hypothesis that rain on snow events in the fall, by leading to a hard basal snow layer, impede the ability of lemmings to feed and reproduce in winter, thus preventing a population irruption.