by Judith Curry
It will be difficult — perhaps impossible — to avoid large methane releases in the East Siberian Sea without major reductions in global emissions of CO2.- Gail Whiteman, Chris Hope, Peter Wadhams
Nature has published a Commentary by Whiteman, Hope and Wadhams entitled Vast Costs of Arctic Change. Excerpts:
As the amount of Arctic sea ice declines at an unprecedented rate. the thawing of offshore permafrost releases methane. A 50-gigatonne (Gt) reservoir of methane, stored in the form of hydrates, exists on the East Siberian Arctic Shelf. It is likely to be emitted as the seabed warms, either steadily over 50 years or suddenly. Higher methane concentrations in the atmosphere will accelerate global warming and hasten local changes in the Arctic, speeding up sea-ice retreat, reducing the reflection of solar energy and accelerating the melting of the Greenland ice sheet. The ramifications will be felt far from the poles.
To quantify the effects of Arctic methane release on the global economy, we used PAGE09. This integrated assessment model calculates the impacts of climate change and the costs of mitigation and adaptation measures. An earlier version of the PAGE model was used in the UK government’s 2006 Stern Review on the Economics of Climate Change to evaluate the effect of extra greenhouse-gas emissions on sea level, temperature, flood risks, health and extreme weather while taking account of uncertainty7. The model assesses how the net present value of climate effects varies with each tonne of carbon dioxide emitted or saved.
We ran the PAGE09 model 10,000 times to calculate confidence intervals and to assess the range of risks arising from climate change until the year 2200, taking into account sea-level changes, economic and non-economic sectors and discontinuities such as the melting of the Greenland and West Antarctic ice sheets (see Supplementary Information; go.nature.com/rueid5). We superposed a decade-long pulse of 50 Gt of methane, released into the atmosphere between 2015 and 2025, on two standard emissions scenarios. First was ‘business as usual’: increasing emissions of CO2 and other greenhouse gases with no mitigation action (the scenario used by the Intergovernmental Panel on Climate Change Special Report on Emissions Scenarios A1B). Second was a ‘low-emissions’ case, in which there is a 50% chance of keeping the rise in global mean temperatures below 2°C (the 2016r slow scenario from the UK Met Office). We also explored the impacts of later, longer-lasting or smaller pulses of methane.
In all of these cases there is a steep global price tag attached to physical changes in the Arctic, notwithstanding the short-term economic gains for Arctic nations and some industries.
The methane pulse will bring forward by 15–35 years the average date at which the global mean temperature rise exceeds 2°C above pre-industrial levels — to 2035 for the business-as-usual scenario and to 2040 for the low-emissions case (see ‘Arctic methane’). This will lead to an extra $60 trillion (net present value) of mean climate-change impacts for the scenario with no mitigation, or 15% of the mean total predicted cost of climate-change impacts (about $400 trillion). In the low-emissions case, the mean net present value of global climate-change impacts is $82 trillion without the methane release; with the pulse, an extra $37 trillion, or 45% is added . These costs remain the same irrespective of whether the methane emission is delayed by up to 20 years, kicking in at 2035 rather than 2015, or stretched out over two or three decades, rather than one. A pulse of 25 Gt of methane has half the impact of a 50 Gt pulse.
The economic consequences will be distributed around the globe, but the modelling shows that about 80% of them will occur in the poorer economies of Africa, Asia and South America. The extra methane magnifies flooding of low-lying areas, extreme heat stress, droughts and storms.
An article on this paper in the Guardian includes the following reactions from climate scientists:
Not everyone agrees that imminent methane release is plausible. Nasa’s Gavin Schmidt has previously argued that the danger of such a methane release is low, whereas scientists like Prof Tim Lenton from Exeter University who specialises in climate tipping points, says the process would takethousands if not tens of thousands of years, let alone a decade.
If Prof Wadhams is correct in his forecast that the summer sea ice could be gone by 2015, then we might be closer to the tipping point than we realise.
Selected statements from Wadhams:
Given present trends in extent and thickness, the ice in September will be gone in a very short while, perhaps by 2015. In subsequent years, the ice-free window will widen, to 2-3 months, then 4-5 months etc, and the trends suggest that within 20 years time we may have six ice-free months per year.
I think that most Arctic specialists would agree that this scenario is plausible.
More comments from climate scientists in this post at the Carbon Brief:
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 Professor David 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”, saying:
“No one has proposed any mechanism for releasing methane that wouldn’t take centuries, not just a few years.”
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.”
Melting of the Arctic Sea Ice
Climate Dialogue has an excellent post on Melting of the Arctic Sea Ice, which includes essays by myself, Walt Meiers and Ron Lindsay, as well as an extended summary by the moderators. If you haven’t visited this discussion, I encourage you to do so. I was the nominal skeptic in the bunch, but the disagreement among us wasn’t all that large. None of the three subscribed to the ‘spiral of death’ scenerio whereby an ice free arctic is plausibly ice free within a few years.
With regards to climate models, there is a new paper by Jiping Liu in PNAS that infers from CMIP5 climate model simulations that the Arctic will be ice free in September by around 2054-58. Liu et al. selected the climate model runs that agreed most closely with the observed sea ice decline. So even the climate models with a CO2 sensitivity that is arguably too high don’t predict an imminent ice free Arctic.
Methane hydrates and contemporary climate change
Given the dire consequences of a major methane gas release triggered by ongoing warming the latest science now says that such a release of methane will not happen for several hundred years. Nature has a paper by Carolyn Ruppel entitled Methane hydrates and contemporary climate change. From the conclusion:
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. Most of Earth’s gas hydrates occur at low saturations and in sediments at such great depths below the seafloor or onshore permafrost that they will barely be affected by warming over even 1000 yr. Even when CH4 is liberated from gas hydrates, oxidative and physical processes may greatly reduce the amount that reaches the atmosphere as CH4.
JC comment: The plausibility of Wadhams’ scenario rests on two assumptions:
- the ‘spiral of death’ loss of arctic sea ice
- connection of the sea ice loss to a massive release of methane hydrates into the atmosphere on the time scale of a decade
Each of these assumptions is highly implausible, based upon my understanding; the combination of these two assumptions into a single scenario seems impossible to me.
So, if you are not a fan of climate models, I suspect that you really will not like impact assessment models used by Wadhams et al..