Methane emissions associated with the conversion of marshland to cropland and climate change on the Sanjiang Plain of northeast China from 1950 to 2100

Abstract. Wetland loss and climate change are known to alter regional and global methane (CH₄) budgets. Over the last six decades, an extensive area of marshland has been converted to cropland on the Sanjiang Plain in northeast China, and a significant increase in air temperature has also been observed there, while the impacts on regional CH₄ budgets remain uncertain. Through model simulation, we estimated the changes in CH₄ emissions associated with the conversion of marshland to cropland and climate change in this area. Model simulations indicated a significant reduction of 1.1 Tg yr⁻¹ (0.7–1.8 Tg yr⁻¹) from the 1950s to the 2000s in regional CH₄ emissions. The cumulative reduction of CH₄ from 1960 to 2009 was estimated to be ~36 Tg (24–57 Tg) relative to the 1950s, and marshland conversion and the climate contributed 86% and 14% of this change, respectively. Interannual variation in precipitation (linear trend with P > 0.2) contributed to yearly fluctuations in CH₄ emissions, but the relatively lower amount of precipitation over the period 1960–2009 (47 mm yr⁻¹ lower on average than in the 1950s) contributed ~91% of the reduction in the area-weighted CH₄ flux. Global warming at a rate of 0.3 ° per decade (P < 0.001) has increased CH₄ emissions significantly since the 1990s. Relative to the mean of the 1950s, the warming-induced increase in the CH₄ flux has averaged 19 kg ha⁻¹ yr⁻¹ over the last two decades. In the RCP (Representative Concentration Pathway) 2.6, RCP 4.5, RCP 6.0 and RCP 8.5 scenarios of the fifth IPCC assessment report (AR5), the CH₄ fluxes are predicted to increase by 36%, 52%, 78% and 95%, respectively, by the 2080s compared to 1961–1990 in response to climate warming and wetting.