Methane and monsoons

摘要

Methane is a potent greenhouse gas, and influences the levels of other atmospheric constituents. The huge increase, of about 150%, to nearly 1,800 parts per billion by volume (p.p.b.v.) in atmospheric concentration over the past two centuries is clearly caused by human activities. However, methane concentration also increased significantly, from about 550 to 700 p.p.b.v., over the previous 5,000 years - the later part of the (interglacial) Holocene epoch that began some 10,000 years ago. There has been intense debate about whether this rise was also anthropo-genic or was due to changes in natural sources and sinks. Using models of climate, vegetation and emissions, Singarayer et al. show how the increase could have arisen from natural causes (page 82 of this issue).