How likely is a huge Arctic methane pulse? We find disagreement among scientists
- 24 Jul 2013, 17:45
- Freya Roberts
Rising temperatures in the Arctic Ocean could see 50
billion tonnes of methane that's currently frozen in the seabed
released into the atmosphere,
a comment piece published in the journal Nature argues.
According to the modelling, the resulting climate
change impacts could cost the globe an extra $60 trillion. But the
scenario is unlikely, other scientists say.
A warming Arctic
The shallow seabeds off the coast of Eastern
Siberia contain what scientists believe is a huge store of frozen
methane, which is a powerful greenhouse gas.
The Arctic is warming about
twice as fast as the global average, in part
because the sea ice is in
long term decline, meaning the Arctic ocean
absorbs more heat.
If ocean waters warm too much, deposits of
frozen methane in the seabed could start to thaw and escape into
the atmosphere, accelerating climate change, the authors of the
Their paper uses an economic model and a
scenario proposed in one
scientific study to calculate that if 50
billion tonnes of methane currently trapped in the East Siberian
sea bed was to thaw and be released to the atmosphere over a period
of ten years, the resulting climate change impacts from this pulse
of methane into the atmosphere could cost 60 trillion dollars.
Expressing the consequences of huge and sudden
methane release as an economic cost is new, and that makes this
first-of-a-kind study an interesting thought experiment. But the
idea that such a sudden methane burst could occur as a result of
melting permafrost (frozen ground) has been around for a
The important question is how likely such a
scenario is. In particular, there is scientific disagreement about
how quickly methane could be released. The scientists we spoke to
suggested the authors have chosen a scenario that's either
implausible, or very much at the upper limit of what we can
reasonably expect. Dr Vincent Gauci, a researcher at the Open
University and director of the MethaneNet research
network explained to Carbon Brief:
"It's not a given all
the methane will end up in the atmosphere. Some could be oxidised
[broken down] in the water by bacteria, and some could remain in
the sediments on the seafloor."
Dr Gauci told us that the authors had made
an "enormous leap" assuming that the entire 50 billion
tonnes of frozen methane trapped in ocean sediments would end up in
the atmosphere over a ten-year period.
Those sentiments were mirrored by
Archer from the University of Chicago, who
researches ocean sediments and methane. He told us even if the
ocean warms, most of the methane released by thawing permafrost
could stay in the seabed or dissolve in seawater. Professor Archer,
who blogs at Realclimate
, described the scenario as "totally unjustified",
"No one has proposed any
mechanism for releasing methane that wouldn't take centuries, not
just a few years."
Speaking on twitter
, NASA Scientist Dr Gavin Schmidt, who runs Realclimate, said
the study assumed a "highly
unlikely" scenario of "50 Gt increase in
emissions in short time" and that there was no evidence in ice
cores of these fast intense blips in past warm periods.
The location of the East Siberian Arctic Shelf containing
frozen methane (Adapted from
Shakhova et al., 2010)
The thickness of the permafrost layer in question is one
reason scientists might disagree about the length of time needed to
release 50 billion tonnes of methane. Dr Julian Merton from the
University of Sussex explained to us that permafrost doesn't
respond quickly to rising temperatures:
"Permafrost hundreds of
metres thick simply doesn't warm or thaw much in ten years on
account of its thermal inertia."
But Professor Wadhams told us the layer of
undersea frozen methane off the coast of East Siberia may only be
20 metres thick, and so more susceptible to Arctic warming. He
agreed there is a question about how much methane will be released
and how quickly, but he told us the process could happen sooner
rather than later:
"Summer [Arctic sea] ice
retreat now is enough to expose the shallow sea beds containing
permafrost, so we are already in danger."
He has also expanded on his views
on the Guardian website.
While scientists are often keen to warn about
'unknown unknowns' and tipping points in the climate system, the
likelihood of such methane pulses is hotly contested among experts
who work in this area. While there is research suggesting the
possibility, other scientists think undersea frozen methane could
be relatively stable for the time being.
Although the new paper uses 50 billion tonnes of
methane over ten years as the basis of its calculations, the
authors also looked at the effects of a smaller or slower
The modelling showed that economic costs and
methane emissions increased at a similar rate. So if only half the
methane emissions materialise, the economic impact would also be
halved. As Wadhams explains:
"If a better estimate of
likely [methane] release comes up, you can adjust the figures down
(or up) accordingly."
The economics modelling also showed that if
methane was released more slowly, over 30 years rather than ten,
the costs would stay largely the same.
But the modelling doesn't examine whether much
slower methane release, say over centuries, would lead to the same
sorts of costs.
Methane is a powerful greenhouse gas, causing
lots of warming while it's present in the atmosphere. But it breaks
down much faster than other greenhouse gases, so very slow steady
release could be "massively less costly", according to Prof
The paper has a core message that climate
impacts will lead to economic costs. It considers a particularly
scary scenario so it's perhaps not surprising that the costs are
However there isn't agreement on the likelihood
of such extreme scenarios among the scientific community. The $60
trillion topline gives no sense of how likely the release of that
much methane is over such a short time period.
But if this scenario is at the extreme end of
what's possible, that isn't a reason to dismiss the risks of
methane release altogether. Any extra methane release from the
region as it warms reduces the remaining carbon budget to stay
within a certain level of global warming - making the task of
Even if less methane is released into the
atmosphere than this scenario suggests, and even if that process
occurs more slowly than one best estimate suggests, the effects of
thawing methane can't be ignored. But while carbon
dioxide emissions continue to rise as they are, the possibility of
methane burps like this are probably not the thing to be worrying
Whiteman et al., (2013)
Vast costs of Arctic change. Nature 499; p401