An unexpected acceleration in methane growth is threatening to negate or reverse efforts to stave off climate change by reducing carbon dioxide emissions. Although scientists don’t know where all the extra methane is coming from, it’s clear that drastically reducing emissions from man-made sources will be necessary to meet the goals of the 2015 Paris climate agreement, researchers say.
Methane is a powerful greenhouse gas with more than 25 times carbon dioxide’s ability to trap heat in the atmosphere over a 100-year period. It issues from many natural sources, such as microbes in the digestive tracts of cows and soggy wetlands, as well as man-made sources, such as natural gas wells, coal mines, and human-lit fires.
For thousands of years, levels of methane hovered below 1,000 parts per billion (ppb) in Earth’s atmosphere. When the Industrial Revolution began around 1750, however, atmospheric methane levels started to rise. Although the overall trend has been consistently upward, the rate of increase for methane emissions has accelerated and slowed fitfully since detailed measurement began in the 1980s. By the end of the 20th century methane growth had slowed, and it looked as if the amount in the air had stabilized. Then in 2007, growth began again.
In a new study, Nisbet et al. looked at air samples from around the globe and found that the growth rate of methane sped up yet again in 2014, leading to a total of 1,850 ppb in the air by 2018 and increasing quickly. That’s more than double preindustrial levels—and a much faster increase than scientists anticipated when world leaders came together in 2015 to sign the Paris Agreement, a commitment to limit global warming to well below 2°C.
It’s not clear whether the sudden increase in methane is from natural or man-made sources or whether the growth has been because of a reduction in the destruction of methane, but there is some evidence that natural sources have increased in response to climate warming. Because methane produced by microbes is richer in the carbon-12 isotope and thus lighter than methane emitted by the fossil fuel industry, which has more carbon-13, the team was able to use the gases’ distinct isotopic signatures to estimate the abundance of methane from both sources. The team found that the proportion of the lighter isotope has grown, reversing the trend of the past two centuries.
There are several potential explanations for the increase in lighter methane. One is an increase in emissions from tropical regions, where cattle are abundant and where warmer temperatures and increased flooding could be increasing methane emissions from wetlands. Another explanation is that the atmosphere is losing its ability to cleanse itself of methane with hydroxyl, which breaks methane down and is better at destroying the lighter gas. A third explanation is that a decrease in methane biomass burning, which is relatively rich in the heavier isotope, has camouflaged a rise in fossil fuel emissions.
Regardless of what’s causing the increase, the growth in methane makes it much more difficult to reach the goals of the United Nations Paris Agreement. Anthropogenic emissions have to be reduced sharply if we are to meet the Paris goal, the authors write. There are many possible ways to do this, including curbing leaks from natural gas wells, cutting coal use, and reducing the burning of tropical grassland and forests. (Global Biogeochemical Cycles, https://doi.org/10.1029/2018GB006009, 2019)
—Emily Underwood, Freelance Writer