Emissions from agriculture, including cattle and rice cultivation, are likely driving a steep rise in global methane emissions, says a new report.
The Global Methane Budget, which is regularly updated by an international group of scientists including researchers form Imperial, charts the sources and sinks of global methane from our atmosphere.
Its 2016 update, released this week, shows that methane emissions are rising in line with one of the worst-case scenario greenhouse gas predictions.
Global methane emissions are rising at a particularly alarming rate, and our analysis shows that increased human activities are the most likely cause.
– Dr Apostolos Voulgarakis
Methane is a greenhouse gas much like carbon dioxide, which traps heat inside our atmosphere. Methane is a much more powerful greenhouse gas, but is more quickly removed from the atmosphere by natural processes, so its effects do not last as long as those of carbon dioxide.
However, the report shows that methane in the atmosphere has increased by 150 percent since the year 1750, and is responsible for 20 percent of the global warming produced by all greenhouse gases so far.
Methane is produced from a range of sources often associated with biological breakdown or decay. Anthropogenic emissions (those related to human activities) account for 60 percent of all global methane emissions, and include emissions from landfills and livestock farming. Methane also makes up a portion of fossil fuels, particularly natural gas.
Natural emissions are produced largely in wetlands.
The 2016 Global Methane Budget update suggests the most likely explanation for the rapid rise in atmospheric methane concentrations in recent decades is increased emissions from the agricultural sector, rather than from fossil fuels or natural wetlands.
This includes increased livestock farming, which accounts for 60 percent of anthropogenic emissions, and rice cultivation, which creates a habitat for methane-producing microbes and accounts for 10 percent of anthropogenic emissions.
Dr Apostolos Voulgarakis from the Department of Physics at Imperial contributed to the report as an expert on how methane is naturally destroyed in the atmosphere, in particular where methane is removed as a result of interactions with a molecule called a hydroxyl radical (OH).
Chemical reaction with hydroxyl radicals is the most important loss process for methane in the Earth system, and therefore understanding the levels, distribution and changes in hydroxyl is key for understanding methane abundances and evolution.
He said: “Global methane emissions are rising at a particularly alarming rate, and our analysis shows that increased human activities are the most likely cause.
“There is also increasing evidence that changes in the hydroxyl radical have played a significant role. However, it remains a major challenge for scientists to explain why those changes in hydroxyl occurred, as this chemical is extremely short-lived and thus very hard to measure and model.”
Large and immediate benefits
Dr Voulgarakis said: “Reducing methane will have large and immediate benefits. Methane mitigation offers rapid climate benefits and economic, health and agricultural side benefits that are highly complementary to CO2 mitigation.
"However, only if deep and decisive measures to reduce CO2 emissions take place simultaneously, will methane mitigation efforts be able to matter for the long-term future."
Methane also directly impacts human health by contributing to the formation of low-level ozone. Ozone in the upper atmosphere protects the surface of the Earth from harmful ultraviolet rays, but at lower levels is an irritant to the respiratory system.
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