PerspectiveAtmospheric Science

Shifting Gear, Quickly

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Science  24 Apr 2009:
Vol. 324, Issue 5926, pp. 477-478
DOI: 10.1126/science.1172001

Earth's climate can change gear very quickly, either sharply warming or fiercely cooling (1). Past shifts of this kind were massive, and some took place within a few years (2). About 11,600 years ago, at the end of the Younger Dryas cold period, the planet warmed very suddenly, with strong increases in atmospheric greenhouse gases, especially methane. On page 506 of this issue, Petrenko et al. use radiocarbon (14C) data to identify the sources of the additional methane (3).

14C is essentially absent in old geological methane sources but is present in biological methane. Thus, any fossil input—for example, from methane hydrates—should be detectable in methane extracted from bubbles in ice. However, the measurement effort required is heroic: 1000 kg of ice are needed to obtain a few hundred thousand 14C atoms per sample of time studied. Petrenko et al. mined the ice like a geological seam at Pakitsoq on the West Greenland ice margin (see the figure), where the time horizon outcrops. Making sense of the measurements is difficult, however, as very large corrections must be applied, especially for the effect of cosmic rays. Because of these and other corrections, the absolute 14CH4 concentrations are meaningless. However, the relative changes in a time sequence of measurements do carry meaning, varying as sources shifted during the warming event.

Although the new 14C results do not elucidate the nature of the initial trigger that ended the Younger Dryas, they suggest that much of the new methane sustaining the warming came from wetlands, at least in the earlier part of the warming event. Less direct approaches to this problem—such as measuring the interpolar gradient (4), the hydrogen isotope ratio (expressed as δD) (5), and the carbon isotope ratio (expressed as δ13C) (6) in the ice core methane record—also point to changes in wetland emissions. Boreal wetlands would have been essentially shut down during glacial episodes but could have switched on quickly in ice-free areas. In contrast, biomass burning sources were stable, showing little change between cool and warm times (7). As a warming event is sustained, decomposing northern methane hydrates (clathrates) may inject fossil methane into the air, with wetland, thawing permafrost, and clathrate emissions reinforcing each other in a feedback loop (8). Petrenko et al.'s data leave such a scenario unconstrained but imply that wetlands were the main driver.

Pakitsoq from the south.

By mining large amounts of ice from Pakitsoq on the West Greenland ice margin for radiocarbon, Petrenko et al. elucidate some of the processes behind a rapid climate transition that began about 11,600 years ago.

CREDIT: V. V. PETRENKO/UNIVERSITY OF COLORADO

A possible explanation for the sudden end of the Younger Dryas is that, at a time of high Arctic insolation, an initial outburst of methane—perhaps from a geological source such as methane clathrates—triggered global warming, initiating both strong wetland emission in the tropics and north (8), and further hydrate responses as the thermal shock penetrated the permafrost (9, 10), freeing methane from decomposing clathrate hydrates and releasing gas pools trapped beneath them. The clathrate gun hypothesis (11) postulates the dominant role of hydrate decomposition, as opposed to wetland emission, as the principal driver of change. Organic matter in thawing permafrost can also generate large emissions of methane (12). Detailed comparison in Greenland ice cores between the methane record and warming, as shown in nitrogen and argon isotopic signals (13, 14), suggests that warmer air reached Greenland before the main rise in methane, thus challenging the clathrate gun scenario (11).

δD results (5) have been used to argue against the hypothesis that an outburst of methane from clathrate hydrates drove the change. However, although marine hydrates supplied by gas from deep geological sources are enriched in deuterium relative to terrestrial methane sources, shallow hydrate and decaying permafrost sources can be depleted. Moreover, the interpretation of the D/H signature during the decades of most sharply rising atmospheric methane is complex, because the global methane budget is not in equilibrium.

The jury thus remains out on the initial trigger, but it is clear that a very rapid wetland response plays a major role in driving the main methane increase. In addition, the end of the Younger Dryas is accompanied by a large increase of another greenhouse gas: N2O (15), produced, for example, in soils and marine upwelling areas.

There are clear analogies with the modern Arctic, especially because global warming is expected to be strongest in the Arctic. Within the next few decades, reduced summer ice cover, earlier springs, and later freeze-up may cause radical change in Arctic wetland and permafrost regions. There was a sudden decrease in ice cover in summer 2007 (16). Very warm summer weather, possibly driven by global warming, has occurred recently in the Arctic. This may trigger new wetland sources (17, 18) as well as fossil and thermokarst methane emissions (12, 19).

Could the Arctic be preparing to shift gear again? If a shift on the scale and rapidity of past changes were to happen tomorrow— including intensified methane emissions from wetlands, decaying permafrost, and hydrate breakdown on Arctic continental margins and slopes—then the consequences for humanity could be very severe. Far from the Arctic, crops could fail and nations crumble. It is thus essential to decipher what took place in the past.

The hard labors of Petrenko et al. in mining tons of ice show the way, as there is plenty more ice to extract on outcropping blue ice fields at both poles. There is now a strong case for horizontal ice mining at the right depths in polar ice caps, far from surface cosmic ray in situ production. Large-sample ice collection from vanishing tropical ice caps is also urgently needed to assess the past history of other factors influencing methane, such as biomass burning. Humanity is triggering great changes; it would be wise to document and understand the past before we face the future.

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