Recent increases in annual, seasonal, and extreme methane fluxes driven by changes in climate and vegetation in boreal and temperate wetland ecosystems

Climate warming is expected to increase global methane (CH₄) emissions from wetland ecosystems. Although in situ eddy covariance (EC) measurements at ecosystem scales can potentially detect CH₄ flux changes, most EC systems have only a few years of data collected, so temporal trends in CH₄ remain uncertain. Here, we use established drivers to hindcast changes in CH₄ fluxes (FCH₄) since the early 1980s. We trained a machine learning (ML) model on CH₄ flux measurements from 22 [methane-producing sites] in wetland, upland, and lake sites of the FLUXNET-CH₄ database with at least two full years of measurements across temperate and boreal biomes. The gradient boosting decision tree ML model then hindcasted daily FCH₄ over 1981–2018 using meteorological reanalysis data. We found that, mainly driven by rising temperature, half of the sites (n = 11) showed significant increases in annual, seasonal, and extreme FCH₄, with increases in FCH₄ of ca. 10% or higher found in the fall from 1981–1989 to 2010–2018. The annual trends were driven by increases during summer and fall, particularly at high-CH₄-emitting fen sites dominated by aerenchymatous plants. We also found that the distribution of days of extremely high FCH₄ (defined according to the 95th percentile of the daily FCH₄ values over a reference period) have become more frequent during the last four decades and currently account for 10–40% of the total seasonal fluxes. The share of extreme FCH₄ days in the total seasonal fluxes was greatest in winter for boreal/taiga sites and in spring for temperate sites, which highlights the increasing importance of the non-growing seasons in annual budgets. Our results shed light on the effects of climate warming on wetlands, which appears to be extending the CH₄ emission seasons and boosting extreme emissions.