Substantial methane emissions from abandoned coal mines in China.

China has shut down and abandoned a substantial number of coal mines since 1999, which have been releasing methane (CH4) for many years. However, the characteristics of China's abandoned mine methane (AMM) emissions are still unclear; this is a concerning knowledge gap because coal mines are the largest contributor to China's anthropogenic CH4 emissions. This study used two methods to estimate China's historical AMM emissions over the past 40 years (1980-2020) and to project its AMM emissions to 2060 which is the target year for China's carbon neutrality goal. The results show that China's AMM emissions increased substantially from 0.11 ± 0.03 million tons per year (Mt/yr) (3.1 ± 0.84 Mt/yr CO2-eq) in 1980 to 4.7 ± 0.94 Mt/yr (131 ± 26 Mt/yr CO2-eq) in 2020. An accelerated growth rate was found during 1998-2005, with AMM emissions rapidly increasing by approximately three times, which was consistent with the high number of mine shutdowns. In 2019, we found that AMM emissions had become the fourth largest anthropogenic source in China, higher than the national anthropogenic CH4 emissions of individual United Nations Framework Convention on Climate Change Annex I countries excluding the United States of America and the Russian Federation. If unabated, China's AMM emissions are projected to peak at 8.7 ± 2.6 Mt/yr in 2040 and reach approximately one-third of China's anthropogenic carbon dioxide emissions in 2060. This study reveals that understanding AMM emissions can help more accurately quantify China's total CH4 emissions and guide their future mitigation.