Thursday, October 25, 2012

Gulf Stream destabilizing methyl hydrates

Thus Nature:

The Gulf Stream is an ocean current that modulates climate in the Northern Hemisphere by transporting warm waters from the Gulf of Mexico into the North Atlantic and Arctic oceans1, 2. A changing Gulf Stream has the potential to thaw and convert hundreds of gigatonnes of frozen methane hydrate trapped below the sea floor into methane gas, increasing the risk of slope failure and methane release3, 4, 5, 6, 7, 8, 9. How the Gulf Stream changes with time and what effect these changes have on methane hydrate stability is unclear. Here, using seismic data combined with thermal models, we show that recent changes in intermediate-depth ocean temperature associated with the Gulf Stream are rapidly destabilizing methane hydrate along a broad swathe of the North American margin. The area of active hydrate destabilization covers at least 10,000 square kilometres of the United States eastern margin, and occurs in a region prone to kilometre-scale slope failures. Previous hypothetical studies3, 5 postulated that an increase of five degrees Celsius in intermediate-depth ocean temperatures could release enough methane to explain extreme global warming events like the Palaeocene–Eocene thermal maximum (PETM) and trigger widespread ocean acidification7. Our analysis suggests that changes in Gulf Stream flow or temperature within the past 5,000 years or so are warming the western North Atlantic margin by up to eight degrees Celsius and are now triggering the destabilization of 2.5 gigatonnes of methane hydrate (about 0.2 per cent of that required to cause the PETM). This destabilization extends along hundreds of kilometres of the margin and may continue for centuries. It is unlikely that the western North Atlantic margin is the only area experiencing changing ocean currents10, 11, 12; our estimate of 2.5 gigatonnes of destabilizing methane hydrate may therefore represent only a fraction of the methane hydrate currently destabilizing globally. The transport from ocean to atmosphere of any methane released—and thus its impact on climate—remains uncertain.
A number of outlets have picked up on this story, and it's easy to see why. This is another classic we-thought-it-would-take-thousands-of-years moment. In recent years methyl hydrate deposits in the Arctic, and especially the shallow deposits in the East Siberian Arctic Shelf, have grabbed the spotlight. Another recent study made headline when it warned of large methane deposits under Antarctica. It seemed that methane was on the move North and South, and the poles grabbed most of the popular attention. But:

Methane hydrates are over over the place. Including places in the ocean dramatically warmed by shifting ocean currents. So there's that.

Key points from the study include:

1. Methyl hydrate deposits are being destabilized by warming oceans right now.
2. We don't know how much of this carbon will make it into the atmosphere, vs contributing to the acidification of the oceans.
3. The study looked at part of the North American coastline, but this process is likely unfolding in other parts of the world as well.
4. Reports of the death of the clathrate gun hypothesis have been greatly exaggerated. This is only one of many recent studies to illustrate that carbon-cycle feedbacks have the potential to add large amounts of greenhouse gases to the atmosphere. How fast? Not overnight, but not necessarily over thousands of years, either.
5. With vulnerable carbon stores in the Arctic, the Antarctic, and on the continental shelves in between, it is becoming painfully clear that anthropogenic global warming is a game of Russian roulette played with a semiautomatic.

Friday, October 19, 2012

Climate deniers losing the argument

  • Americans’ belief in the reality of global warming has increased by 13 percentage points over the past two and a half years, from 57 percent in January 2010 to 70 percent in September 2012. At the same time, the number of Americans who say global warming is not happening has declined nearly by half, from 20 percent in January 2010 to only 12 percent today.
  • For the first time since 2008, more than half of Americans (54%) believe global warming is caused mostly by human activities, an increase of 8 points since March 2012. Americans who say it is caused mostly by natural changes in the environment have declined to 30 percent (from 37% in March).
  • A growing number of Americans believe global warming is already harming people both at home and abroad. Four in ten say people around the world are being harmed right now by climate change (40%, up 8 percentage points since March 2012), while 36 percent say global warming is currently harming people in the United States (up six points since March).
  • In addition, they increasingly perceive global warming as a threat to themselves (42%, up 13 points since March 2012), their families (46%, up 13 points), and/or people in their communities (48%, up 14 points). Americans also perceive global warming as a growing threat to people in the United States (57%, up 11 points since March 2012), in other modern industrialized countries (57%, up 8 points since March), and in developing countries (64%, up 12 points since March).
  • Today over half of Americans (58%) say they are “somewhat” or “very worried” - now at its highest level since November 2008.
  • For the first time since 2008, Americans are more likely to believe most scientists agree that global warming is happening than believe there is widespread disagreement on the subject (44% versus 36%, respectively). This is an increase of 9 percentage points since March 2012.
This is a welcome reminder that the climate blogosphere is a tiny, tiny community, and the irrational, unpersuadable right-wing ideologues who relentlessly seek domination over it do not reflect the broader, disengaged public. Nor is the public responding to pro-science perspectives. If I had to guess, I'd say they are simply responding to the evidence that they are seeing with their own eyes.

Saturday, October 6, 2012

Three papers on the Antarctic ice

There is a lot of carbon under that ice.
Potential methane reservoirs beneath Antarctica Wadham et al (2012)
Abstract: Once thought to be devoid of life, the ice-covered parts of Antarctica are now known to be a reservoir of metabolically active microbial cells and organic carbon1. The potential for methanogenic archaea to support the degradation of organic carbon to methane beneath the ice, however, has not yet been evaluated. Large sedimentary basins containing marine sequences up to 14kilometres thick2 and an estimated 21,000 petagrams (1Pg equals 1015g) of organic carbon are buried beneath the Antarctic Ice Sheet. No data exist for rates of methanogenesis in sub-Antarctic marine sediments. Here we present experimental data from other subglacial environments that demonstrate the potential for overridden organic matter beneath glacial systems to produce methane. We also numerically simulate the accumulation of methane in Antarctic sedimentary basins using an established one-dimensional hydrate model3 and show that pressure/temperature conditions favour methane hydrate formation down to sediment depths of about 300metres in West Antarctica and 700metres in East Antarctica. Our results demonstrate the potential for methane hydrate accumulation in Antarctic sedimentary basins, where the total inventory depends on rates of organic carbon degradation and conditions at the ice-sheet bed. We calculate that the sub-Antarctic hydrate inventory could be of the same order of magnitude as that of recent estimates made for Arctic permafrost. Our findings suggest that the Antarctic Ice Sheet may be a neglected but important component of the global methane budget, with the potential to act as a positive feedback on climate warming during ice-sheet wastage.

That ice is melting faster than we thought it would.

Dynamics of the last glacial maximum Antarctic ice-sheet and its response to ocean forcing -- Fogwill et al (2012)
Abstract: Retreat of the Last Glacial Maximum (LGM) Antarctic ice sheet is thought to have been initiated by changes in ocean heat and eustatic sea level propagated from the Northern Hemisphere (NH) as northern ice sheets melted under rising atmospheric temperatures. The extent to which spatial variability in ice dynamics may have modulated the resultant pattern and timing of decay of the Antarctic ice sheet has so far received little attention, however, despite the growing recognition that dynamic effects account for a sizeable proportion of mass-balance changes observed in modern ice sheets. Here we use a 5-km resolution whole-continent numerical ice-sheet model to assess whether differences in the mechanisms governing ice sheet flow could account for discrepancies between geochronological studies in different parts of the continent. We first simulate the geometry and flow characteristics of an equilibrium LGM ice sheet, using pan-Antarctic terrestrial and marine geological data for constraint, then perturb the system with sea level and ocean heat flux increases to investigate ice-sheet vulnerability. Our results identify that fast-flowing glaciers in the eastern Weddell Sea, the Amundsen Sea, central Ross Sea, and in the Amery Trough respond most rapidly to ocean forcings, in agreement with empirical data. Most significantly, we find that although ocean warming and sea-level rise bring about mainly localized glacier acceleration, concomitant drawdown of ice from neighboring areas leads to widespread thinning of entire glacier catchments—a discovery that has important ramifications for the dynamic changes presently being observed in modern ice sheets.

When that ice melted previously, global carbon dioxide levels rose dramatically over only two hundred years.

Abrupt change in atmospheric CO2 during the last ice age – Ahn et al. (2012)
Abstract: “During the last glacial period atmospheric carbon dioxide and temperature in Antarctica varied in a similar fashion on millennial time scales, but previous work indicates that these changes were gradual. In a detailed analysis of one event we now find that approximately half of the CO2 increase that occurred during the 1500-year cold period between Dansgaard-Oeschger (DO) events 8 and 9 happened rapidly, over less than two centuries. This rise in CO2 was synchronous with, or slightly later than, a rapid increase of Antarctic temperature inferred from stable isotopes.”
Citation: Ahn, J., E. Brook, A. Schmittner, and K. J. Kreutz (2012), Abrupt change in atmospheric CO2 during the last ice age, Geophys. Res. Lett., doi:10.1029/2012GL053018.
Comment: This is all very new science, but these three very different papers with different subjects and different methods seem together to suggest a coherent narrative; ocean warming rapidly triggers widespread decay of the Antarctic ice sheets, which uncovers significant amount of carbon. That carbon makes its way into the atmosphere, in amounts significant enough to warm the climate further.

The Arctic permafrost feedback appears (to an outsider, like me) to be gaining widespread acceptance as a significant contributor to global warming both in the near term (the next century) and in the longer term (a few centuries.) Now we are trying to nail down the scale of the feedback. Meanwhile, we are starting to get some science that suggests a similar carbon-cycle feedback could unfold in the South, scale and speed unknown.