by Verity Payne, via the Carbon Brief
Two new research papers published today improve our understanding of the planet’s methane emissions, and might raise worries about the role of the gas in warming the planet. The first suggests that there may be extensive methane deposits under the Antarctic ice sheets. Meanwhile, the second concludes that emissions of the gas from Arctic permafrost have been underestimated.
Methane is a potent greenhouse gas — accounting for around 14 per cent of the warming effect of current man made greenhouse gas emissions. Recent research has focused on measuring emissions from methane sources, both natural and manmade.
Antarctic methane reservoirs
Scientists have been particularly interested in methane emitted from the Arctic. This is because the region is warming particularly rapidly. In addition, methane released from melting permafrost and escaping methane hydrate deposits could exacerbate climate change. But research published today in the journal Nature suggests for the first time that there might also be large stores of methane at the other end of the planet, under the Antarctic ice sheet.
Plants thrived on Antarctica before the continent was covered by ice some 35 million years ago. Lab experiments show that microbes living beneath the ice are able to convert plant remains into methane, and scientists calculate that half of the West Antarctic Ice Sheet (1 million square kilometers) and a quarter of the East Antarctic Ice Sheet (2.5 million square kilometers) could cover carbon-rich sediments containing up to 4 billion metric tons of methane in the form of methane hydrates. These are an ice-like substance formed when methane and water combine.
The researchers suggest methane could be released if ice sheets retreat as global warming continues. According to study co-author Slawek Tulaczyk, professor of Earth and planetary sciences at UC Santa Cruz, the findings underline “the need for continued scientific exploration of remote sub-ice environments in Antarctica, because they may have far greater impact on Earth’s climate system than we have appreciated in the past.”
Media outlets have reported the findings widely, with most headlines focusing on potential impacts of escaping methane. For example, Reuters headlines the story ‘ Antarctic methane could worsen global warming — scientists’, while the UK Press Agency goes for ‘ Methane fear beneath Antarctic ice’.
So how realistic is the prospect of enough of the Antarctic melting to release methane that might be beneath the ice?
The study’s authors highlight that “significant uncertainty exists” in their estimates of methane reservoirs beneath Antarctic ice. But they calculate that Antarctic ice sheet retreat at the rate of 1,000 square kilometres per year — comparable to previous episodes of ice sheet collapse — could result in enough methane release to affect atmospheric methane levels.
At the moment, this isn’t happening. The West Antarctic ice sheet is losing ice mass, but slower than an ice sheet which is collapsing. And with most of the potential methane under the East Antarctic ice sheet, it looks unlikely that there will be significant methane release from Antarctica soon.
Arctic methane underestimated
What about Arctic methane sources? Another new Nature paper finds ten times more carbon than previously thought is escaping from coastal permafrost in the East Siberian Arctic Shelf — an amount that dwarfs emissions from land and submarine permafrost in the region.
The escaping carbon has been trapped for tens of thousands of years, but with summer sea ice declining, Arctic coastlines are becoming more vulnerable to erosion from waves and storms. With roughly three-quarters of the Arctic coastline made of permafrost, it is perhaps not surprising that the crumbling coastline is releasing more methane and carbon dioxide than previously estimated.
The scientists warn that erosion of permafrost coasts might worsen as the Arctic warms, and that this will have “consequences for the temperatures all over the world.”
Other recent research also suggests that methane emissions from the Arctic have been previously underestimated. Such stories have tended in the past to lead to claims of a “ methane time bomb” or “Arctic apocalypse”, since enough methane escape could lead to further warming, which would lead to more methane escape, and so-on, in a self-reinforcing warming cycle. This time, though, the coverage has been more restrained, with less dramatic headlines and an informative segment about the research on the BBC Radio 4’s Today programme.
Wetlands dominate in the near future?
Despite recent research suggesting that we’ve underestimated Arctic methane sources, there isn’t yet evidence to suggest that these sources are having a significant effect on atmospheric methane. For example, weekly air samples from Arctic sites show that there is as yet no direct atmospheric evidence that emissions of methane or carbon dioxide from the Arctic are changing.
Scientists studying the composition of methane in the air over Siberia find that during the summer most methane emissions come from wetlands, and in winter most methane comes from leaking gasfields. Even though there are plumes of methane in the seawater nearby, suggesting methane escaping from seabed methane hydrate deposits, only a negligible amount appears to be making it into the atmosphere.
In fact the majority of methane emissions into the atmosphere are currently from wetlands — such as marshes, bogs, peatland and paddy fields. And climate model projections of increasing methane emissions over the coming century are largely controlled by wetland methane emissions.
“When we’ve seen big methane emissions over the past 30–40 thousand years … the isotopes just don’t support big hydrate emissions. They do support changes in wetland extent and productivity having a bigger control over the amount of methane in the atmosphere.”
Important warming feedbacks from methane emissions in the polar regions — particularly to the Arctic — have the potential to occur. But different feedbacks are likely to act over different timescales, and the possibility of sudden and catastrophic methane release may be overstated. In a recent review of the subject, methane experts write:
“We speculate that, in the short term (few decades), melting permafrost will result in larger wetland areas; when combined with warmer temperatures, this will result in increased [methane] emissions. The impact of a warming Arctic will probably only slowly increase emissions from hydrates”.
So although polar methane hydrate sources will probably become important over the long term, particularly if Arctic warming continues, in the nearer future it looks like wetlands will remain the major determining factor in global methane emissions.
This piece was originally published at the Carbon Brief and was reprinted with permission.