1. Methane’s Global Warming Potential (GWP)
The image featured in a video and poster produced by Sam Carana1 (2012a).
IPCC2 figures were used to create the blue line. The red line is based on figures in a study by Shindell et al.3, which are higher as they include more effects. This study concludes that methane's GWP would likely be further increased when including ecosystem responses.
Ecosystem responses can be particularly strong in the Arctic. As mentioned on the poster, further warming in the Arctic can cause accelerated ice loss and trigger further releases of methane from sediments under the sea.
Release of methane from sediments is particularly worrying in areas such as the East Siberian Arctic Shelf (ESAS), where the sea is rather shallow (image below), causing much of the methane to enter the atmosphere without being oxidized in the water.
Furthermore, low water temperatures and long sea currents in the Arctic Ocean are not very friendly toward bacteria that might otherwise break down methane in the water.
As said, release of methane from sediments is particularly worrying in the Arctic, where much of the water is rather shallow, as illustrated on the image below.
As said, Shindell's GWP figures do not include all indirect effects. Accelerated loss of sea ice and weakening of methane stores, due to the additional local warming of methane locally, can have a dramatic impact in case of large abrupt methane release in the Arctic.
Such local warming can cause accelerated loss of the amount of snow and ice and of its capacity to reflect sunlight back into space, causing further warming, in a vicious circle of feedbacks.
The images below are from Flanner4 (2011) and shows that ice in the Arctic can cool areas by more than 30 Watts per square meter, and in summer by up to 70 Watts per square meter.
Apart from decline of snow and ice, additional methane releases could also dramatically increase accelerate local warming.
Some 50 Gt6 is ready for abrupt release at any time in the East Siberian Arctic Shelf area (ESAS) alone (image left, from Semiletov7, 2012).
The image below, from Sam Carana8 (2011), illustrates the danger of the situation in the Arctic, where high levels of greenhouse gases, combined with the impact of aerosols such as soot, can cause high summer temperature peaks.
High temperatures in the Arctic will speed up loss of sea ice, resulting in even further warming that weakens stores of methane in the form of hydrates and free gas in sediments under the water, in a vicious cycle that threatens to lead to runaway global warming.
|For more details on feedbacks, see extended version of this image and discussion at
The IPCC2 rates methane's Radiative Efficiency (in W m–2 ppb–1) at 3.7 x 10–4 and gives methane a perturbation lifetime of 12 years9. The IPCC9 defines perturbation time as the time it takes for a perturbation to be reduced to 37% of its initial amount. At the same time, the IPCC10 gives methane a global mean atmospheric lifetime of 8.4 years, which is the time it takes for half a perturbation to be broken down.
Methane's lifetime will be extended as the burden rises, due to hydroxyl depletion. The IPCC11 estimates that this methane feedback effect amplifies the climate forcing of an addition of methane to the current atmosphere by lengthening the perturbation lifetime relative to the global atmospheric lifetime of methane by a factor of 1.4.
A NASA12 (2009) article discussing Shindell's work mentions that increases in global methane emissions have caused a 26% decrease in hydroxyl.
Prather et al.13 (2012) derive a present-day atmospheric lifetime for methane (CH4) of 9.1 years.
In case of large abrupt releases of methane in the Arctic, much of the methane may persist there for decades and thus amplify local warming dramatically.
This is the case because the methane originated in one location, unlike other types of methane releases that occur gradually around the world.
4. Abrupt release of 1Gt of methane in the Arctic
What would the impact be of abrupt release of 1Gt of methane in the Arctic, compared to the total global carbon dioxide emissions from fossil-fuel burning, cement manufacture, and gas flaring? The image below, from Sam Carana16 (2012b), gives a rather conservative impact, showing a rapid decline toward a small residual impact as carbon dioxide.
The additional warming that this will cause in the Arctic will make the sea ice decline even more dramatically than is already the case now. The combined impact of sea ice loss and methane is huge, and threatens to trigger further releases of methane in the Arctic, with their joint impact accumulating as illustrated in the image below, also from Sam Carana16 (2012b).
1. Sam Carana (2012a), Video and poster - methane in the Arctic
2. IPCC, Climate Change 2007: Working Group I: The Physical Science Basis, Table 2.14
3. Drew Shindell et al. (2009), Improved Attribution of Climate Forcing to Emissions.
Presentation at Symposium, November 30, 2010
6. Natalia Shakhova et al. (2008)
EGU General Assembly 2008
7. Semiletov et al. (2012)
On carbon transport and fate in the East Siberian Arctic land–shelf–atmosphere system
10. IPCC, TAR (2001) Working Group I: The Scientific Basis, 4.1.1 Sources of Greenhouse Gases
11. IPCC, TAR, 04 (2001), Atmospheric Chemistry and Greenhouse Gases, Executive Summary
12. NASA (2009), Interactions with Aerosols Boost Warming Potential of Some Gases
13. Prather et al. (2012), Reactive greenhouse gas scenarios: Systematic exploration of uncertainties and the role of atmospheric chemistry
14. Taraborelli et al (2012), Hydroxyl radical buffered by isoprene oxidation over tropical forests
15. Euan G. Nisbet (2002), Have sudden large releases of methane from geological reservoirs occurred since the Last Glacial Maximum, and could such releases occur again?
16. Sam Carana (2012b), How much time is there left to act?