Overexplaining or underexplaining methane’s role in climate change

Methane lies at the nexus of climate and air quality, being both a major anthropogenic greenhouse gas—causing about one-half of the warming of carbon dioxide—and a precursor of tropospheric ozone pollution. Over the industrial era, atmospheric methane abundances rose from about 720 parts per billion (ppb) (10−9 mole fraction) to over 1,850 ppb today. Humans have driven this change largely through agriculture, waste, and fossil fuel emissions. The community’s regular review of the science of atmospheric methane via the Intergovernmental Panel on Climate Change (IPCC) Assessment Reports [First Assessment Report (FAR), 1990 (1); Second Assessment Report (SAR), 1995 (2); Third Assessment Report (TAR), 2001 (3); Fourth Assessment Report (AR4), 2007 (4); and Fifth Assessment Report (AR5), 2013 (5)] has maintained both a scientific interest and political urgency as nations seek to mitigate near-term climate change and keep the overall warming to less than 2 °C (6⇓⇓–9). The recent history (Fig. 1), based on ref. 10, shows a complex overall growth with different rates and even a pause from 2000 to 2006. The many conflicting reports of this recent variability (11–23) suggest that it remains unexplained, or perhaps overexplained. Past work has separately used measurements of methane, its isotopes, and related gases to interpret the methane history. Two new publications (24, 25) combine these complementary data into a consistent Bayesian modeling framework and use advanced statistical methods to match all observations simultaneously subject to the prior constraints. Notably, they advance our understanding of what could have caused the variability. The similarities and differences of optimal solutions that emerge from both studies teach us about the information contained in present observations, as well as their limits. (A) Timeline of the international assessments …