Accelerated methane emission from permafrost regions since the 20th century

Carbon dioxide has been increasing in the atmosphere since the industrialization, resulting in global warming, which in turn enhanced methane emissions from permafrost regions. However, methane emitted into the atmosphere is quickly oxidized, such that the intensity and threat of methane emission in the past are severely underestimated. Here we show that the ratio between annual changes in atmospheric oxygen and carbon dioxide concentrations (ΔO₂/ΔCO₂ index) dropped dramatically between 1945 and 1958. There are no such abrupt changes in the fossil fuel combustion curve. Considering that atmospheric methane has the residence time less than ∼9 years through biological and chemical processes, this crisis is attributed to the World War II, which had disturbed gas hydrates underneath ocean floors and permafrost regions. Consistently, the land-ocean temperature index fluctuated more severely in permafrost regions than the global average during the WWII, indicating stronger greenhouse effects induced by methane emission. This is supported by the changes in the ΔO₂/ΔCO₂ index, coupled with higher atmospheric methane concentrations and lower δ¹³CO₂ in permafrost regions in the last three decades, suggesting accelerated methane emission. The anthropogenic carbon dioxide emission has been pushing the accelerated thawing of permafrost, which promotes methane emission that further intensifies permafrost thawing and global warming. Such a positive feedback at an inflection point of Earth climate may break the glacial-interglacial cycle and initiate a long-lasting greenhouse period.