Maybe you missed the headlines July 24, when Nature ran a commentary warning that a 50-billion-ton burst of methane from the East Siberian Sea would boost global warming enough to cost world economies $60 trillion — about the world’s one-year gross economic output.
Methane packs a hothouse punch: gram for gram, it produces about 25 times as much warming as carbon dioxide. And so that methane “burp” — from an Arctic Ocean backwater — could spike planetary temperatures by 0.6°C.
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The commentary raised a roiling row among scientists — we promise to return to it shortly — and it got us thinking about carbon releases and global warming in the North, scene of the fastest warming on the globe.
By itself, permafrost, land that never completely thaws, covers 24 percent of the Northern hemisphere land surface, and recent measurements show rapid thawing in some areas, together with bubbling streams of methane and releases of carbon dioxide.
This huge area stores vast amounts of carbon, which can be converted into carbon dioxide or methane. Offshore, colossal stores of methane hydrate, a frozen cage holding natural gas molecules, can break down in a warming sea. Submarine hydrates have been found at 40 sites globally.
In March, Japan was the first to extract methane hydrate from an offshore well.
Greenhouse gases from the Arctic are likely to cause much less warming than burning fossil fuels and obliterating temperate and tropical forests. But if the Arctic dump of greenhouse gas is big enough, it could stimulate more warming that releases more gas that causes more warming.
Such a runaway warming is a nightmare scenario that could lead to uncontrollable warming. But with so many moving parts, it is extremely difficult to study. What do we know about the release of carbon dioxide and especially methane from the Arctic? Are the rates going up?
Let’s return to our “news hook” — the July Nature commentary by Gail Whiteman of Erasmus University in the Netherlands. The study figured the economic cost of warming that would result if 50 billion tons of methane entered the atmosphere from the East Siberian Sea over 10 years and raised the average global temperature 0.6°C.
Due mainly to sea level rise, the hefty price tag — $60 trillion (with a “t”) — would be paid by mostly by poor countries in Africa, Asia and South America.
How likely is an unaffordable and catastrophic “methane bomb”?
When measured against the 600 million tons of methane entering the atmosphere each year, 5 billion tons a year is huge. And although methane oxidizes within seven years in the atmosphere, atmospheric concentrations have been rising for decades.
The Whiteman study built on research by Natalia Shakhova, of the University of Alaska, who reported in 2010 that the East Siberian Sea was annually releasing about 8 million tons of methane to the air, “on par with previous estimates of methane venting from the entire World Ocean.”
Shakhova and colleagues wrote that the subsea methane is escaping because “the permafrost ‘lid’ is clearly perforated… .”
Fifty billion tons of methane is a big tank of gas, and according to Peter Wadhams, a professor of ocean physics at Cambridge University and a co-author of the “methane bomb” analysis, “It is not important how fast you release the methane, you get the same total cost in our model.”
$60 trillion = big bucks!
That price tag attracted a wave of skepticism. Many of Whiteman’s critics pointed to a 2011 study by Carolyn Ruppel of the U.S. Geological Survey, which concluded that “Catastrophic, widespread dissociation of methane gas hydrates will not be triggered by continued climate warming at contemporary rates (0.2ºC per decade; IPCC 2007) over timescales of a few hundred years.”
But Wadhams, who studies ocean physics, says the sun now warms the sea in summer because the Arctic ice cap is melting. The East Siberian Sea is only 45 meters deep on average, so warmer water is reaching the seabed to thaw the hydrate. Ruppel’s study, Wadhams concluded, “does not reflect awareness of this new mechanism.”
Critics also pointed to ice-core records showing rises in methane that followed past warming spells. How, they asked, could cause follow effect? “This is a dubious argument,” Wadhams says. Methane is an unstable molecule, and if it is destroyed in ice-core samples, past levels of atmospheric methane are uncertain. “There is a whole load of research that needs to be done.”
Want more on the methane bomb?
Hunting for volume
Methane hydrates, a frozen cage holding methane molecules, require a specific range of temperatures and pressures that exist in the “hydrate stability zone.” In a recent study1, Stephen Hunter, of the School of Earth and Environment at the University of Leeds (United Kingdom) looked at changes in the stability zone after expected rises in sea level and ocean temperature.
Hunter’s calculations balanced warming of the bottom water, which can thaw hydrate, against higher pressure due to rising sea level, which enhances hydrate stability. He found about a 0.04 percent shrinkage in volume by 2100. That sounds trivial, but since these stability zones hold an estimated 500 billion to 3 trillion tons of methane hydrate, that means hydrates are adding to about 1 million tons of methane to the atmosphere each year.
But it’s not really that simple, since methane can oxidize into carbon dioxide while rising through soil or water. Hunter adds that the certainty of his estimate is unknown, and that it ignores methane trapped in subsea permafrost, which is exactly the source at issue in the East Siberian Sea.
We’re not going to solve this, but have to agree with a contributor to the Dot Earth blog at the New York Times: “Even if it turns out that rapid methane degassing isn’t in the cards, you still do have to worry about those several trillion metric tons of near-surface carbon and how secure they are.”
Landward, ho! A sink, then a source?
You might think the situation on land, where permafrost also stores methane and releases methane and carbon dioxide, would be easier to study, but the permafrost region is vast, distances are long, mosquitoes are ferocious, and doing anything there is incredibly expensive. The permafrost region encircling the North Pole stores a staggering 1.7 trillion tons of carbon — more than twice what’s in the atmosphere, and its fate matters.
Just anticipating how much methane will come from a particular ecosystem is difficult, explains David McGuire, of the University of Alaska:
Methane generating microbes are anaerobic, meaning they thrive in flooded conditions. When permafrost thaws, there’s plenty of water, but if it drains away, methane production can fall
Methane is not the only product that can be produced when microbes eat carbon-rich organic matter
Organic matter that was microbially digested before it froze may not feed microbes after thawing
On land, soil and vegetation can play opposite roles in the carbon budget. Rising atmospheric carbon dioxide accelerates plant growth, removing carbon from the air, but warming increases microbial action, which releases methane and carbon dioxide.
And so the warming-induced loss of carbon from the surface soil “is almost balanced out by vegetative carbon increase from better growing conditions,” says Charles Koven of Lawrence Berkeley Laboratory2.
Although near-term plant growth will outweigh the loss of permafrost carbon, soil contains vastly more carbon than plants, and so in the long term, Koven says, “with more warming, we expect permafrost carbon to play a stronger role, and northern lands will shift from absorbing carbon dioxide to releasing greenhouse gases later this century.
Feedback due to methane and carbon dioxide looks like a real threat, Koven says. “We see warming that leads to more warming.”
Edward Schuur, professor of biology at the University of Florida in Gainesville, surveyed a group of experts on Arctic warming issues to predict carbon releases in terms of “CO2 equivalents,” which translates the warming impact of methane in terms of carbon dioxide. The experts said that, in the highest warming scenario, permafrost would release a total of 19 to 45 billion tons of carbon dioxide by 2040 and 162 to 288 billion tons by 21003.
We wondered why Schuur surveyed experts rather than plow through models and publications. “The scientific community relies on model projections to understand future carbon releases,” he said, “but you have to understand that the models are just beginning to represent the processes that are described from field studies.” Thus the alternative is to “either run a lot of models that we know have flaws,” or to talk to experts “who have an understanding of the system and knowledge of measurements.”
The carbon-release numbers are roughly two to five times larger than some previous estimates because carbon resides deep in the permafrost, Schuur says. Scientists who looked below the top three feet “found organic carbon buried over thousands of years, and once you have found this large quantity frozen, there is the potential for some of it to be unfrozen.”
The larger estimates also reflect a better picture of how permafrost thaws. “It’s not only about the temperature, but there are all sorts of changes to the ecosystem,” says Schuur. “When the permafrost thaws, the ecosystem can collapse, so releases can happen much faster.” Warming also speeds the growth of microbes that consume carbon and produce methane or carbon dioxide.
The stakes in this climate feedback are considerable, Schuur says. “When we consider climate change, we think about the drivers, burning fossil fuel and changing land from forest to agriculture,” but ecosystem responses must be considered. “Humans are influencing the global carbon cycle, and there are responses that are outside our control that could amplify or mitigate the changes. It’s profound. This is a fundamental thing that sometimes gets lost in the discussion of what to do about climate change.”
Somebody is going to pay
Even if the East Siberian Sea scenario does not end up costing a full $60 trillion, Schuur says releases of greenhouse gases in the Arctic will have planetary impact and global costs. But the positive feedback scenarios we have explored are not inevitable, says McGuire. “A lot of our research is focused on improving our ability to make better predictions of what might happen. Currently, the opinion among scientists and the range from current models varies from very little to a lot.”
Even if there is only a slender chance of catastrophic feedback, McGuire says, a “primary reason” to study the issue is ensuring that feedback will not undercut restrictions on greenhouse gases. “If policy decisions are made to mitigate the rise in atmospheric CO2 , a positive feedback from release of methane, or CO2 from thawing permafrost could compromise those efforts.”
Decades after climate scientists began considering feedbacks that can slow or hasten warming, the picture grows more tangled. “The tricky thing with the feedback problem is that you end up caring about the interactions between all the feedbacks,” says Koven, whether they are related to clouds, radiation, the carbon cycle or changes in forests or permafrost. “What ends up really mattering is the total interaction between those feedbacks. Methane is one feedback among many that we are trying to get a handle on.”
Some warming feedbacks are quick, as when a tropical forest dries and burns. But the feedbacks due to permafrost thawing, Koven says, “will happen slowly, on the 100-year scale, and it will take a long time to be able to detect the effects. But once it gets started, we expect those to last a very long time.”
–David J. Tenenbaum
Terry Devitt, editor; S.V. Medaris, designer/illustrator; Yilang Peng, project assistant; David J. Tenenbaum, feature writer; Amy Toburen, content development executive
- Sensitivity of the global submarine hydrate inventory to scenarios of future climate change, S.J. Hunter et al, Earth and Planetary Science Letters 367 (2013)105–115 ↩
- Permafrost carbon-climate feedbacks accelerate global warming, Charles D. Koven et al, PNAS, September 6, 2011, vol. 108, no. 36, 14769–14774 ↩
- Expert assessment of vulnerability of permafrost carbon to climate change, E. A. G. Schuur et al, Climatic Change (2013) 119:359–374 ↩
- Gas hydrate primer ↩
- Energy in methane hydrate. ↩
- Methane bubbles ↩
- Rapid Arctic thawing could be economic timebomb ↩
- Arctic methane release scenario may be very misleading ↩
- What are gas hydrates? ↩
- A victim of climate change ↩