Abrupt seasonal fluxes of methane from northern lakes and ponds

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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.