Abrupt seasonal fluxes of methane from northern lakes and ponds

甲烷是一种强大的、寿命长的温室气体，其辐射强迫潜力仅次于二氧化碳。了解区域尺度上的地球甲烷循环对于评估甲烷减排计划的有效性、检测受气候影响的甲烷生物源和甲烷汇的变化以及预测并可能减少未来的甲烷排放是必要的一步。自1990年代以来，大气甲烷的增长速度有所放缓，但仍显示出相当大的年际变化，这无法得到充分解释。一些变化是由于天气对以生物方式产生甲烷的系统的影响造成的。当在北半球检测到与温暖天气条件有关的大气甲烷异常增加时，通常认为是湿地和泥炭地造成的。然而，小型湖泊和池塘通常被忽视，因为它们是甲烷排放的潜在主要来源。湖泊历来被认为是甲烷的次要排放者，因为夏季月份的扩散通量可以忽略不计。尽管从20世纪90年代开始的测量一直表明，北方湖泊在春季和秋季排放了大量甲烷，但这种观点一直持续到最近。在冬季，冰盖将湖水与大气隔离，水柱变得贫氧和分层。甲烷在底部沉积物中的产生量增加，气体通过水柱传播，一些富含甲烷的气泡向上上升，并在冬季末期向下增厚时被困在冰盖中。在春天，当冰融化时，气体就会释放出来。通过温度的变化和风的影响，湖泊水柱混合，更深层次的甲烷积聚损失到大气中。在夏季，水柱再次分层，甲烷再次聚集在底部沉积物中。当水柱在秋天变得热不稳定并最终倾覆时，深层的甲烷再次被释放，尽管在秋天似乎有更大比例的甲烷被细菌消耗。湖泊在水的化学成分和盆地的几何形状上有所不同。因此，很难确定所有的湖泊都会有这样的行为，但对许多人来说，这似乎是可能的。这项拟议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聚情况。重点将放在春季的积聚和排放上，因为这些气体最不可能受到消耗甲烷的细菌的影响。然而，秋季还将对部分湖泊的甲烷排放量进行详细测量。然后，将使用遥感数据将测量结果扩展到区域级别，提供对春季和秋季甲烷通量的自下而上的估计。这些结果将与使用英国气象局弥散模型确定的自上而下的估计进行比较，该模型反向计算全球监测站测量的大气甲烷浓度的气团路径，以确定在通过一个区域时必须向空气中添加多少甲烷。通过比较这两种方法的估计，将提供对北方湖泊季节性甲烷通量的潜在影响的独立评估。此外，还将测量甲烷(C，H)和水(H)中碳和氢原子的轻和重版本，以评估它们作为示踪剂的潜在用途，以唯一地识别不同纬度的湖泊释放的甲烷。如果成功，这项拟议的研究有可能改变我们对甲烷循环的看法，因为它最终证明，对天气敏感的第二个主要生物甲烷来源有助于大气甲烷增长率的年复一年的变化。

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.