Thanks, but why didn't you post the full article here so it would archived for further use? :)
Bubbles prompt climate-change rethink
Argon traces hint that carbon dioxide did not lead life out of the freezer, but followed.
14 March 2003
Carbon dioxide certainly warms our planet, but it might not turn on the heat, reveals a new analysis of ancient Antarctic ice.
"Our data suggest that the warming came first, then carbon dioxide increased," says Jean Jouzel of the Pierre-Simon Laplace Institute in Gif-sur-Yvette, France1. Something else probably extraterrestrial got the warming going, his team concludes.
Aside from water vapour, carbon dioxide is the major warming influence on our planet. But it's hard to work out which comes first: a rise in carbon dioxide levels or a slight warming. Why? Because even a slight temperature hike increases atmospheric carbon dioxide, through its effects on forests and oceans.
Pioneering a new technique, Jouzel's team has probed air bubbles trapped in 240,000- year-old ice laid down as snow when the Earth was warming up at the end of a massive ice age.
They compared the ratio of two forms of the atmospheric gas argon in the bubbles, and looked at their carbon dioxide content. The argon ratio changes relative to the temperature of the air at the time it was trapped, the team argues.
They saw a temperature rise, followed by greater warming caused by rising carbon dioxide levels, that tallied well with evidence from the surrounding ice and other climate records. "We were surprised to find that these indicators agreed," says Jouzel.
Other researchers are also surprised. Other ice records had already pointed to warming as a trigger for further warming. However, vagaries in the rate at which ice is deposited in different parts of the Antarctic makes firm conclusions about the actual age of bubbles difficult to draw, says glaciologist Martin Siegert of the University of Bristol, UK.
"Making sense of individual ice records is hard enough, let alone getting them to agree with others," he says. If they are right, however, Jouzel's team has found good evidence for heat, not gas, beginning the end of an ice age.
It doesn't change our understanding of today's global warming, Siegert says carbon dioxide levels are already increasing, so what got it started is somewhat irrelevant.
Nor does it mean that carbon dioxide is any less important as a greenhouse gas. Like many researchers before, Jouzel's team argues that a subtle shift in the Earth's orbit around the Sun triggered a minute amount of warming. "But you need carbon dioxide to amplify the effect," Jouzel says.
It could, however, be important for the future. Climate models, such as those used to forecast change, are based on past events, so pinning down what went on improves their predictive power. Jouzel's team is now checking more recent records to see what preceded other ice ages.
Caillon, N. et al. Timing of atmospheric CO2 and Antarctic temperature changes across termination III. Science, 299, 1728 - 1731, (2002). |
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