Dramatic rises in atmospheric methane are threatening to derail plans to hold global temperature rises to 2C, scientists have warned.
In a paper published this month by the American Geophysical Union, researchers say sharp rises in levels of methane – which is a powerful greenhouse gas – have strengthened over the past four years. Urgent action is now required to halt further increases in methane in the atmosphere, to avoid triggering enhanced global warming and temperature rises well beyond 2C.
“What we are now witnessing is extremely worrying,” said one of the paper’s lead authors, Professor Euan Nisbet of Royal Holloway, University of London. “It is particularly alarming because we are still not sure why atmospheric methane levels are rising across the planet.”
Methane is produced by cattle, and also comes from decaying vegetation, fires, coal mines and natural gas plants. It is many times more potent as a cause of atmospheric warming than carbon dioxide (CO2). However, it breaks down much more quickly than CO2 and is found at much lower levels in the atmosphere.
During much of the 20th century, levels of methane, mostly from fossil fuel sources, increased in the atmosphere but, by the beginning of the 21st century, it had stabilised, said Nisbet. “Then, to our surprise, levels starting rising in 2007. That increase began to accelerate after 2014 and fast growth has continued.”
Studies suggest these increases are more likely to be mainly biological in origin. However, the exact cause remains unclear. Some researchers believe the spread of intense farming in Africa may be involved, in particular in tropical regions where conditions are becoming warmer and wetter because of climate change. Rising numbers of cattle – as well as wetter and warmer swamps – are producing more and more methane, it is argued.
This idea is now being studied in detail by a consortium led by Nisbet, whose work is funded by the Natural Environment Research Council. This month the consortium completed a series of flights over Uganda and Zambia to collect samples of the air above these countries.
“We have only just started analysing our data but have already found evidence that a great plume of methane now rises above the wetland swamps of Lake Bangweul in Zambia,” added Nisbet.
However, other scientists warn that there could be a more sinister factor at work. Natural chemicals in the atmosphere – which help to break down methane – may be changing because of temperature rises, causing it to lose its ability to deal with the gas.
Our world could therefore be losing its power to cleanse pollutants because it is heating up, a climate feedback in which warming allows more greenhouse gases to linger in the atmosphere and so trigger even more warming.
In 2016, in Paris, nations agreed to cooperate to hold global temperature rises to 2C above preindustrial levels and, if possible, to keep that rise to under 1.5C. It was recognised that achieving this goal – mainly by curbing emissions of CO2 from fossil fuels – would always be difficult to achieve. Accelerating increases in a different greenhouse gas, methane, means that this task is going to be much, much harder.
This point was backed by Martin Manning of Victoria University of Wellington in New Zealand. “Methane is the gas … that keeps us to a 2C rise in global temperatures. And even more significantly, we do not really know why.”
If nothing can be done about this, he added, then even more cuts will have to be made in CO2 emissions. Continued increases in methane levels will only make this situation worse, he said.
This point was backed by Nisbet. “It was assumed, at the time of the Paris agreement, that reducing the amount of methane in the atmosphere would be relatively easy and that the hard work would involve cutting CO2 emissions.
“However, that does not look so simple any more. We don’t know exactly what is happening.
“Perhaps emissions are growing or perhaps the problem is due to the fact that our atmosphere is losing its ability to break down methane.
“Either way we are facing a very worrying problem. That is why it is so important that we unravel what is going on – as soon as possible.”