Australia: The Land Where Time Began

A biography of the Australian continent 

Climate Change - Very Rapid Changes

The authors1 point out that the climate of the Earth can change very rapidly, in either direction, either warming fiercely or sharply cooling (1). In the past there have been massive climate shifts, some being complete in as little as a few years (2). At the close of the Younger Dryas cold period about 11,600 years ago, the global climate very suddenly warmed, associated with strongly increased levels of atmospheric greenhouse gases, especially methane. Radio carbon (14C) data was used by Petrenko et al. to identify the sources of the additional methane (3), in (Science 324, no. 5926 (April 24, 2009 2009): 506-08.).

Radio Carbon is present in biological methane of the present, but is virtually absent from old geological sources of methane, which allows any input from fossil sources such as methane hydrates to be detectable in methane that has been extracted from gas bubbles in ice. As to extract a few hundred thousand 14C atoms per time sample studied, it is necessary to extract the methane from 1,000 kg of ice. At Pakitsoq, on the margin of the ice, West Greenland, where there is an outcrop of the time horizon, Petrenko mined the ice as he would a geological seam. Very large corrections were needed, especially to allow for the effect of cosmic rays, which made it difficult to make sense of the measurements. The absolute concentrations 14CH4 are meaningless because of these corrections, as well as others, though there is meaning in the relative changes in a time sequence of measurements, which vary as sources change during a warming event.

The new 14C results indicate that much of the new methane that sustained the warming came from wetlands, at least in the early stages of the warming event, though they do not indicate the trigger that initiated the end of the Younger Dryas. Changes in the emissions are also indicated by less direct methods, including measuring the inter[polar gradient (4), the hydrogen isotope ratio, that is expressed as δD (5), and the carbon isotope ratio (δ13C) (6) in the ice core record of methane. During episodes of glaciation boreal wetlands would have been essentially shut down, though as soon as the melted they could be rapidly reactivated. There appears to have been little change between cool and warm periods with regards to the sources of burning biomass (7). The decomposition of northern clathrates (northern methane) may have been injected into the atmosphere as warming continued, with wetlands, permafrost thawing and the clathrate emissions all reinforcing each other as part of a feedback loop (8). It is implied by the data obtained by Petrenko et al. that wetlands were the main driver, though such a scenario is unrestrained.

The authors1 suggest a possible explanation for the Younger Dryas ending so suddenly may have been an initial outburst of methane, possibly from a fossil source such as methane clathrate triggered by an episode of increased insolation in the Arctic. The resulting climatic warming then initiating strong emissions from wetlands in the tropics as well as in the Arctic regions (8), leading to a further response of decomposing clathrates, with the release of any gas pools that had been trapped beneath them. According to the clathrate gun hypothesis (11) hydrate decomposition plays a dominant role, rather than emissions from wetlands, in being the main driver of climate change at this time. Large methane emissions are also generated by organic matter in thawing permafrost (12) In ice cores from Greenland detailed comparisons between the records of methane and warming, as seen in the isotope signals of nitrogen and argon (13,14), suggest that prior to the main rise of methane, warmer air reached Greenland, which therefore challenges the clathrate gun scenario (11).

The hypothesis that a methane outburst from clathrate hydrates drove the change have been disputed by use of the δD results (5). Shallow hydrate and decaying sources in permafrost can be depleted, though marine hydrates which are supplied by gas from deep geological sources are deuterium-rich relative to terrestrial sources of methane. As the global methane budget is not in balance, the interpretation of the D/H signature over the decades when the atmospheric methane concentration was rising most sharply, is complex. The initial trigger remains uncertain, though what is clear is that a major role in driving the main increase in methane is played by a very rapid wetland response. The closing of the Younger Dryas was also accompanied by a large increase in the atmospheric concentration of N2O (15), also a greenhouse gas, from sources such as soil and areas of marine upwelling.

The authors1 suggest analogies with the modern Arctic are clear, especially as the effects of global warming are expected to be felt most strongly in the Arctic. A decreased area of summer ice cover, earlier springs and later ice formation may cause Arctic wetlands and permafrost regions may result in radical changes in next few decades. In the summer of 2007 (16) there was a hidden decrease of ice cover. In recent times very warm summer weather has occurred in the Arctic that the authors1 suggest may possibly be driven by global warming. New wetland sources (17,18) may be triggered by this as well as emissions of fossil and thermokarst methane (12,19).

The authors1 ask if the Arctic could be preparing shift gear again. If such a shift were to happen tomorrow on the scale and with the speed of changes in the past, that includes intensified emissions from wetlands, decaying permafrost, and the breakdown of hydrates on the Arctic continental margins and slopes, they suggest the consequences could be very severe. With crops, and possibly even nations, far from the Arctic failing, adding more urgency to the task of deciphering how climates of the past have changed and at what speed these changes occurred. The authors1 suggest there is a strong case for horizontal ice mining at the correct depth in the polar ice caps, far from surface cosmic ray in situ production. There is also an urgent need for the diminishing tropical ice caps  to assess the past history of other influences on methane, such as the burning of biomass. According to the authors1 it is wise to understand past changes if we are to prepare for possible future changes.

Sources & Further reading

  1. Nisbet, E. G., and J. Chappellaz. "Shifting Gear, Quickly." Science 324, no. 5926 (April 24, 2009 2009): 477-78.


Author: M. H. Monroe
Last updated 25/05/2013
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