Tropical wetlands drove methane changes since the last glacial

Methane (CH₄) is a greenhouse gas that has a global warming potential per emitted molecule about 28 times greater than carbon dioxide. For the last 800000 years its atmospheric concentration can be reconstructed from ice cores, and the concentration was always higher for the warm interglacial periods than for the cold glacial periods. Between the last glacial maximum (LGM, ca. 20000 years ago) and the preindustrial state (PI, about 200 years ago), the concentration doubled from ca. 350 parts per billion (ppb) to about 700 ppb, and it has more than doubled again from the industrial revolution to the present level of 1900 ppb. The latter increase is due to human activities – mainly the use of fossil fuels, but also agriculture and waste facilities. The former increase, however, occurred before the Anthropocene. What drove this natural increase in the atmospheric CH₄ concentration? And what drove the massive swings in atmospheric methane during this period, with the methane concentration going down and up by more than 20% over the course of a few years?

The authors found that the long-term increase in atmospheric methane originates mainly in tropical wetlands, with the Amazon and Indonesia playing very prominent roles. Furthermore, they could show that the large swings in atmospheric methane during the Bølling-Allerød (14.7 – 12.9 ka BP) and Younger Dryas (12.9 – 11.7 ka BP) periods were likely caused by circulation changes in the Atlantic Ocean, as an induced change in the Atlantic Meridional Overturning Circulation displays a signature extremely similar to the methane changes recorded in ice cores.