Is it man or nature that has caused the recent increase in methane emissions?
Klassekampen, 6 Jan 2017
When climate change hits the news, it's often about records being broken. Globally, 2016 was the hottest year ever measured. Never before was there so little ice in the Arctic as this winter. And the concentration of CO2 in the atmosphere hasn't been this high in 800.000 years.
But there's another greenhouse gas that has also risen to a record high: methane. A gas with 28 times the global warming potential of CO2, even though its rise in the atmosphere has gotten less attention. That's undeserved, because it remains unclear how this increase came to be.
That is, the rise of methane in the atmosphere was slowing down in the 1980's, even though CO2 was still rising strongly. Actually, after the fall of the Soviet Union, there have been years where the concentration of methane was diminishing! That was undeniably good climate news. But unfortunately, it didn't last. In the past decade, the amount of methane in the atmosphere has been rising again, and rather quickly in the past few years. Where does all this methane come from? Is it man or is it nature?
The Arctic may still come with a few surprises
To understand the recent turn in methane concentrations, and to solve this problem, we must first know how much and where this greenhouse gas is emitted. "The Global Carbon Project", a group of scientists that I'm part of, have elaborated on this in a publication last December.
For example, we do not only emit methane through the use of fossil fuels, but agriculture and livestock contribute a lot as well. Cows have a relatively high amount of methane in their breath (a bigger problem than what comes out at the other end!), and permanently flooded rice paddies are also a large source. Forest fires and municipal waste contribute further to human emissions of methane.
We think that we understand our own contribution relatively well. A lot of uncertainty on the recent increase in methane concentrations comes from a lack of knowledge of natural sources. These comprise about 40% of the whole, and the largest natural emissions are from swamps, bogs and other wetlands.
These ecosystems are a large source of methane, precisely because they are so wet. In a dry soil, old plant material is quickly broken down and converted back to CO2, but the responsible microbes need oxygen to be able to do so. Oxygen is absent in a flooded soil, the quick breakdown of plant remains stops, and plant material accumulates. A small fraction of that plant material can still be broken down by other microbes, and methane is released in the process.
These methane producing microbes work harder at higher temperatures, and this may be an explanation for the recent increase of methane in the atmosphere. The world is becoming warmer because of our doing, so more methane should be produced as a consequence, right? But it isn't that simple.
No other area in the world is warming as fast as the Arctic. Combine this with the thawing of old plant material in ancient permafrost, and it's obvious that ideal conditions may have formed for an increase of methane. Nonetheless, we still haven't been able to ascertain a large increase in methane emissions from the high North. It is possible that the Arctic has become drier in many places due to permafrost thaw. But we don't know this for certain, since the loss of sea ice has a large influence on the temperature and precipitation in the Arctic. Moreover, there are large uncertainties on how this will develop in the future. Because of these uncertainties, the Arctic may still come with a few surprises.
Another, often cited, surprise are the so-called gas hydrates – large stores of methane captured in an ice structure – which are found under high pressures and low temperatures below the ocean floor. These could destabilise due to the warming of the North Pole region. But for now they appear to remain quiet. Research from NILU has shown that methane that is escaping from the sea floor off the coast of Svalbard doesn't reach the atmosphere, it dissolves in the ocean water. In addition, earlier estimates of large amounts of methane emitted from the East-Siberian Sea have been strongly corrected downwards by several recent studies.
No, in a study from last September, a group of scientists was pointing to very different causes. Wetter conditions and higher temperatures have facilitated an increase in emissions from the tropics, not just from swamps, but also rice paddies. And although the fossil fuel industry may not be the main culprit this time, through our rice consumption we may still be responsible for it ourselves.
This text first appeared in Klassekampen on 6 Jan 2017