A 3.19-kilometer column of ice drilled from Antarctica by European scientists during 2004 provided researchers with the oldest and most detailed record of climate change obtained to date, stretching back more than 740,000 years. The ice core indicated that today’s greenhousegas concentrations in the atmosphere are by far the highest for at least that long, by at least 30 percent. Ice cores are valuable records of the Earth’s past climate because they record variations in temperatures as well as concentrations of gases such as carbon dioxide and methane that contribute to the greenhouse effect. The new core, drilled in Eastern Antarctica, doubled the available paleoclimatic record when it was analyzed during 2005.
Furthermore, the rate at which humankind’s burning of fossil fuels is changing the composition of the atmosphere is extraordinary by natural standards. ‘‘The rate of increase [in greenhouse gases] is more than 100 times faster than any rate we can detect from the ice cores we have seen so far," said Thomas Stocker, a member of the European Project for Ice Coring in Antarctica (EPICA), a consortium of laboratories and Antarctic logistics operators from 10 nations.
The new core confirms evidence from ocean sediments that the Earth has endured several ice ages during the last 740,000 years, each separated by a warmer interglacial. While ice ages typically last about 100,000 years, the interglacial periods usually are much shorter, averaging (very roughly) 10,000 years each. EPICA's findings indicate an "extremely strong" 100,000-year cycle for ice ages during at least the last 500,000 years, with the present interglacial most closely resembling another about 430,000 years ago. The proportion of carbon dioxide in the atmosphere follows a well-defined cycle, rising to about 280 parts per million (ppm) during interglacial periods, declining to between 180 and 200 ppm during ice ages. In 2008, the carbon-dioxide level was 385 ppm.
The interglacial 430,000 years ago lasted about 28,000 years, longer than most. Why has the present interglacial been so long? It partially may be a matter of "how much, where, and during what season the Sun's energy reaches the planet,"according to one measurement used by the EPICA scientists. The shape of the Earth's orbit around the sun (which determines Earth's distance from the sun during different seasons) and its interaction with the
seasonal cycle seem to fit the waxing and waning of ice ages noted in ice cores.
Because the present interglacial is about 12,000 years old, the EPICA scientists put little credibility in global warming contrarians' assertions that burning fossil fuels may forestall a new ice age any time soon. "Given the similarities between this earlier warm period and today, our results may imply that without human intervention, a climate similar to the present one would extend well into the future". Given rising levels of greenhouse gases, however, the team finds any assumption that climate will remain stable "highly unlikely". The fact that human activity has raised the atmosphere's carbon-dioxide level to about 30 percent above the natural range for the last 740,000 years or more provokes scientists to wonder just how much the Earth's temperature will rise in coming years, especially if carbondioxide levels continue to rise. "In everything we have got up to now, temperature and greenhouse gases are absolutely in step with each other," Wolff said. "I don't see any particular reason this shouldn't continue into the future. It is worrying".