Greenhouse Gas Levels
Highest In 650,000 Years
Climate record highlights extent of man-made change.
By Michael Hopkin
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.
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.
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.
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.
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 Science1,2.
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.
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.
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.
analyzed 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.
to say that current rises in temperature are due to natural shifts,
as some climate-change skeptics have claimed. "The CO2 emitted now
is not part of the natural cycle," Stocker points out.
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
SiegenthalerU., et al. Science, 310. 1313 - 1317 (2005). SpahniR.,
et al. Science, 310. 1317 - 1321 (2005).