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Greenhouse-gas levels highest for 650,000 years

Greenhouse-gas levels highest for 650,000 years
Published online: 24 November 2005
Greenhouse-gas levels highest for 650,000 years
Climate record highlights extent of man-made change.
Michael Hopkin

Current levels of greenhouse gases in the atmosphere are higher than at any 
time in the past 650,000 years, say researchers who have finished 
cataloguing air bubbles trapped for millennia inside Antarctic ice. The 
record, which extends back over the past eight ice ages, shows that today's 
concentrations of carbon dioxide and methane far outstrip those in the past.

Atmospheric carbon dioxide levels have risen 200 times faster over the past 
50 years than at any other time during this period, says Thomas Stocker of 
the University of Bern, Switzerland, who led the analysis.

The researchers studied air bubbles preserved in ice drilled from the 
Antarctic ice sheet as part of the European Project for Ice Coring in 
Antarctica (EPICA). The ice core represents a logbook of the state of the 
world's climate (see 'Frozen time') and goes back 210,000 years further than 
previous records.

After searching ice spanning the period of 390,000-650,000 years before 
present, Stocker's team has discovered that carbon dioxide levels in the 
atmosphere did not exceed 290 parts per million during that time. Today, 
that figure is around 375 parts per million.

The situation is similar for methane: during this period, levels hovered 
around 600 parts per billion. Today's atmospheric methane concentration is 
well over 1,700. Stocker and his colleagues report the results in 

Unprecedented push

The burning of fossil fuels in the industrial era has pushed greenhouse-gas 
levels far beyond their natural fluctuations, says Stocker. "This is really 
something unprecedented," he says. Humans, by releasing fossil fuels from 
their imprisonment underground, are now adding greenhouse gases to the 
atmosphere on top of those released as part of natural climate cycles.

The news comes as world leaders plan to attend a United Nations climate 
change conference in Montreal, Canada, which begins on 28 November. 
Delegates will discuss current efforts to reduce greenhouse emissions, and 
what plans should follow on from the initial phase of the Kyoto Protocol, 
which ends in 2012.

The past four ice ages and their intervening warm periods are thought not to 
have been typical. Glacial cycles before this had longer, cooler intervening 
periods than more recent ones. Researchers are unsure why this is, although 
they hope the ice cores may hold some clues.

Unnatural changes

The newly analysed ice does show that although the climate is in constant 
flux, it is capable of producing extended warm phases even when carbon 
dioxide levels are stable, says Stocker. Two places in the record, for 
example, are marked by periods of almost 30,000 years when temperature 
hardly changed at all. And the beginning of these 'interglacial' phases was 
not linked to rises in carbon dioxide.

That's not to say that current rises in temperature are due to natural 
shifts, as some climate-change sceptics have claimed. "The CO2 emitted now 
is not part of the natural cycle," Stocker points out.

"In the palaeorecord there's no human activity driving the change," says 
Chris Jones, of the Hadley Centre for Climate Prediction and Research in 
Exeter, UK. The current challenge facing climate modellers is to work out 
the one-way effect of the huge spike in greenhouse gases now being pumped 
into our skies by human activities.

SiegenthalerU., et al. Science, 310. 1313 - 1317 (2005).
SpahniR., et al. Science, 310. 1317 - 1321 (2005).
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