Abrupt seasonal fluxes of methane from northern lakes and ponds

甲烷是一种强大的长寿命温室气体，其辐射强迫潜力仅次于二氧化碳。在区域尺度上了解地球的甲烷循环是评估甲烷减排计划的有效性、探测受气候影响的甲烷生物源和汇的变化、预测并可能减轻未来甲烷排放的必要步骤。自20世纪90年代以来，大气甲烷的增长速度已经放缓，但它继续显示出相当大的年度变化，这无法充分解释。有些变化是由于天气对甲烷生物生成系统的影响造成的。当在北半球检测到大气中甲烷的异常增加与温暖的天气条件有关时，湿地和泥炭地通常被认为是原因。然而，小湖泊和池塘通常被忽视为潜在的主要甲烷排放源。湖泊历来被认为是甲烷的次要排放者，因为夏季的扩散通量可以忽略不计。这种观点一直持续到最近，尽管从20世纪90年代开始的测量一直表明，在春季和秋季，北部湖泊排放了大量的甲烷。在冬季，冰盖将湖水与大气隔绝，水柱变得缺氧并分层。底部沉积物中的甲烷产量增加，气体通过水柱扩散，一些富含甲烷的气泡向上上升，并在冬末冰层变厚时被困在冰层中。在春天，当冰融化时，气体被释放出来。通过温度的变化和风的影响，湖泊水柱的混合和更深层次的甲烷积累被损失到大气中。在夏季，水柱再次分层，甲烷再次积聚在底部沉积物中。当水柱在秋天变得热不稳定并最终倾覆深层甲烷时，甲烷再次被释放出来，尽管大部分甲烷似乎在秋天被细菌消耗了。湖泊在水的化学成分和盆地的几何形状上各不相同。因此，很难确定所有的湖泊都会以这种方式行事，但对许多湖泊来说，这似乎是可能的。这项拟议中的研究将测量北美一系列生态区春季和秋季湖泊中甲烷的积累情况。重点将放在春季的积累和排放上，因为这种气体最不可能受到消耗甲烷的细菌的影响。然而，对甲烷排放的详细测量也将在秋季对一部分湖泊进行。然后，这些测量将利用遥感数据扩大到区域水平，提供春季和秋季甲烷通量的“自下而上”估计。这些结果将与一个“自上而下”的估计进行比较，该估计是由英国气象局的弥散模型确定的，该模型通过全球监测站测量大气甲烷浓度来反向计算气团的路径，以确定在通过一个地区的过程中必须向空气中添加多少甲烷。比较这两种方法的估计值将提供对北部湖泊季节性甲烷通量潜在影响的独立评估。此外，还将测量甲烷（C， H）和水(H)中碳和氢原子的轻、重形态，以评估它们作为独特识别不同纬度湖泊释放的甲烷的示踪剂的潜在用途。如果成功，这项研究有可能通过最终证明第二个主要的对天气敏感的生物甲烷来源对大气甲烷增长率的逐年变化有贡献，从而使我们对甲烷循环的认识发生阶梯式变化。

Methane is a powerful long-lived greenhouse gas that is second only to carbon dioxide in its radiative forcing potential. Understanding the Earth's methane cycle at regional scales is a necessary step for evaluating the effectiveness of methane emission reduction schemes, detecting changes in biological sources and sinks of methane that are influenced by climate, and predicting and perhaps mitigating future methane emissions. The growth rate of atmospheric methane has slowed since the 1990s but it continues to show considerable year-to-year variability that cannot be adequately explained. Some of the variability is caused by the influence of weather on systems in which methane is produced biologically. When an anomalous increase in atmospheric methane is detected in the northern hemisphere that links to warm weather conditions, typically wetlands and peatlands are thought to be the cause. However, small lakes and ponds commonly are overlooked as potential major sources of methane emissions. Lakes historically have been regarded as minor emitters of methane because diffusive fluxes during summer months are negligible. This notion has persisted until recently even though measurements beginning in the 1990s have consistently shown that significant amounts of methane are emitted from northern lakes during spring and autumn. In the winter time the ice cover isolates lake water from the atmosphere and the water column become poor in oxygen and stratified. Methane production increases in bottom sediment and the gas spreads through the water column with some methane-rich bubbles rising upwards and becoming trapped in the ice cover as it thickens downward in late winter. In spring when the ice melts the gas is released. Through changes in temperature and the influence of wind the lake water column mixes and deeper accumulations of methane are lost to the atmosphere. In summer the water column stratifies again and methane accumulates once more in the bottom sediments. When the water column become thermally unstable in the autumn and eventually overturns the deep methane is once again released although a greater proportion of it appears to be consumed by bacteria in the autumn. Lakes differ in the chemistry of their water as well as the geometry of their basins. Thus it is difficult to be certain that all lakes will behave in this way but for many it seems likely. The proposed study will measure the build-up of methane in lakes during spring and autumn across a range of ecological zones in North America. The focus will be on spring build-up and emissions because that gas is the least likely to be influenced by methane-consuming bacteria. However, detailed measurements of methane emissions will also be made in the autumn at a subset of lakes. The measurements will then be scaled to a regional level using remote sensing data providing a 'bottom-up' estimate of spring and autumn methane fluxes. Those results will be compared to a 'top-down' estimate determined using a Met Office dispersion model that back-calculates the path of air masses for which the concentration of atmospheric methane has been measured at global monitoring stations in order to determine how much methane had to be added to the air during its passage through a region. Comparing estimates by these two approaches will provide independent assessments of the potential impact of seasonal methane fluxes from northern lakes. In addition measurements of the light and heavy versions of carbon and hydrogen atoms in methane (C, H) and water (H) will be measured to evaluate their potential use as tracer for uniquely identifying methane released by lakes at different latitudes. If successful the proposed study has the potential to yield a step-change in our perception of the methane cycle by demonstrating conclusively that a second major weather-sensitive source of biological methane contributes to year-to-year shifts in the growth rate of atmospheric methane.