Climate Feedback

A never-before-seen megadrought made an appearance this morning at the last day of the AGU Chapman Conference. Paul Aharon of the University of Alabama says his latest observations are the first to suggest that drought affected the southeastern United States from about 13,000 to 11,800 years ago - during the so-called Younger Dryas cool period.

The evidence comes from the De Soto Caverns in Alabama. This cave has already offered up rich history of a non-palaeoclimatological kind: it holds a Native American burial ground and an abandoned moonshine distillery from the 1930s, when good-timing Alabamans used to shoot down the stalactites.

Continue reading "AGU Chapman Conference: Megadrought in Dixieland" »

Posted by Anna Barnett on June 19, 2009
Categories: AGU Chapman conference on Abrupt Climate Change, Anna Barnett, Palaeoclimate | Permalink | Comments (0) | TrackBacks (0)

At the AGU Chapman conference today, Yale archaeologist Harvey Weiss took the prize for an abrupt climate change picture worth a thousand words. Excavating an Akkadian palace in Tell Leilan, Syria, in 2006 and 2008, Weiss's team found one room with a grain storage vessel smashed on the floor. Lying next to it were a standard litre measure used for rationing grain, and the tablet on which a bureaucrat had been recording the rationing. The artifacts date from about 2190 B.C., when cities and towns of the Akkadian empire in Mesapotamia were being abandoned en masse as the region suffered crushing drought.

"This site is the Pompeii of ancient Mesapotamia," says Weiss. "They walked away."

Weiss reviewed evidence that a rapid change in storm tracks in the North Atlantic - yet to be satisfactorily explained - dried out the Tigris and Euphrates valley 4,200 years ago. And that valley wasn't alone. Around the same time, deflection of the Indian Monsoon hit the Nile with a drought, and Egypt's Old Kingdom went down. The extreme events are also mirrored in North America from New Jersey to the Yukon. In a separate talk today, glaciologist Lonnie Thompson showed a new ice core data* from Huascarán in Peru, the highest tropical mountain, with a huge spike in dust deposition around this time. The dust probably blew off an aridifying West Africa, Thompson says.

Continue reading "AGU Chapman Conference: "They walked away"" »

Posted by Anna Barnett on June 18, 2009
Categories: AGU Chapman conference on Abrupt Climate Change, Anna Barnett, Extreme Events, Palaeoclimate, Society | Permalink | Comments (0) | TrackBacks (0)

As the Earth has alternated between glacial and inter-glacial periods, the steep climatic ups and downs have gone hand in hand with changes in atmospheric carbon dioxide levels. But where was the CO2 going to and coming from? Scientists have pointed to the ocean – currently a vast sponge for the greenhouse gas.

But a talk at the AGU Chapman Conference today by palaeoclimatologist Lowell Stott of the University of Southern California suggests a radically different reservoir: pools of liquid carbon dioxide trapped in seafloor hydrothermal vents.

These pools were spotted in the mid 2000s unleashing bubbles of liquid CO2 from the Okinawa trough in the Pacific Ocean (see the video here).

The CO2 pools form when one oceanic plate buckles under another and carbonates in the sediment break down under the intense heat. Perforations around underwater volcanic vents can allow CO2 droplets to escape and bubble up to the surface, but where the seawater is cold enough it effectively freezes the CO2 into a solid, or hydrate, form that acts as a lid. NOAA has a further explanation and diagram.

Stott points out that the carbon isotope signatures in some mid-latitude ocean sediment don’t tally with the conventional view of carbon entering the ocean system via photosynthesizing algae. The chemistry of the sea-vent carbon is a much better match, he argues. What's more, unpublished work by Stott and colleagues shows that past changes in deep sea temperatures around the vents would have been sufficient to destabilize hydrate caps and thus modulate the vents' release of CO2 in time with the rising and falling atmospheric CO2 concentrations.

Continue reading "AGU Chapman: Could seafloor vents control atmospheric CO2?" »

Posted by Anna Barnett on June 17, 2009
Categories: AGU Chapman conference on Abrupt Climate Change, Anna Barnett, Ocean science, Palaeoclimate | Permalink | Comments (0) | TrackBacks (0)

Today's theme at the AGU Chapman Conference on Abrupt Climate Change is that big baddie of climatic tipping points, the shutdown (and rebooting) of the Atlantic Meridional Overturning Circulation, or AMOC. Could this massive system go down again? Tom Delworth of NOAA took on that question and offered up some interesting new modelling evidence.

When running strong, AMOC carries heat from the Southern Hemisphere northward. It's thought that some of the past coolings under scrutiny here stem from slowing or stopping of this conveyor belt. AMOC's future change in response to greenhouse gas increases was recently considered in an assessment report on abrupt change by the US Climate Change Science Program, which Delworth helped author.

While some state-of-the-art models suggest the circulation could slow this century - a 25-30% decrease is the report's best estimate - not a single one forecasts another shutdown in that time. That led the US panel to evaluate AMOC shutdown as "very unlikely", in the parlance of the IPCC - meaning a less than 10% probability. The lack of support from models meant they couldn't set the likelihood any higher, says Delworth - but on the other hand, the possibility of flawed simulations kept them from setting it lower, at "extremely unlikely". But Delworth's new work validates the model results.

Continue reading "AGU Chapman: Meridional madness" »

Posted by Anna Barnett on June 16, 2009
Categories: AGU Chapman conference on Abrupt Climate Change, Anna Barnett, Climate prediction, Extreme Events, Palaeoclimate | Permalink | Comments (0) | TrackBacks (0)

A scant 21,000 years ago, Columbus, Ohio, was blanketed by the Laurentide ice sheet. Today it is home to the Byrd Polar Research Centre at Ohio State University, where this morning I sat in a glacially air-conditioned lecture hall watching an animation of that sheet flickering rapidly back and forth across Columbus and the rest of the northern parts of the continent. Such strobe-light climate change from the Earth's past is the focus of the AGU Chapman Conference on Abrupt Climate Change, being held here this week.

Though it's a fairly small gathering of 150 experts, it doesn't have the annual reunion feeling of some meetings; many of the people here seem never to have met before. We've got palaeoscientists of various persuasions: they reconstruct climatic history via models, ice, sediment, or - as geochemist Henry Pollack described his work on borehole temperature records to me over hors d'oerves - by "taking the Earth's temperature through its rectum". (Jokes about giant thermometers ensued.)

The common thread is figuring out what caused abrupt changes in the past - and what that implies about the prospects for their return. Says Richard Alley, a Penn State glaciologist and IPCC author, "The IPCC reports are the most optimistic thing we can put forward, because the projections are smooth. If you look at any palaeo record, there are bumps. This meeting is all about the bumps."

One innovative study on show today applied the models behind those smooth future projections to the bumpy past record. Bette Otto-Bliesner of NCAR says her group's research is the first to feed palaeo data into an IPCC-style coupled global climate model and run it continuously for several thousand years during the last deglaciation - rather than just taking snapshots in time, as was done previously. Having given the model instructions about what the greenhouse gas levels, sea ice extent, and meltwater flows should be, they found that it beautifully reproduced the bumpy North Atlantic temperature record.

Continue reading "AGU Chapman: It's all about the bumps" »

Posted by Anna Barnett on June 15, 2009
Categories: AGU Chapman conference on Abrupt Climate Change, Anna Barnett, Climate prediction, Palaeoclimate | Permalink | Comments (0) | TrackBacks (0)