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Atmospheric carbon dioxide levels are higher than any point in the last 2.1 million years, report researchers writing in the journal Science.
Analyzing the shells of single-celled plankton buried under the Atlantic Ocean off the coast of Africa, Bärbel Hönisch, a geochemist at Columbia University’s Lamont-Doherty Earth Observatory, and her colleagues dtermined that peak CO2 levels over the last 2.1 million years averaged only 280 parts per million. By comparison current CO2 levels stand at 385 parts per million, or 38% higher than the long-term peak.
  Annual mean atmospheric CO2 concentrations (ppm), 1850-2008 [top]; Monthly atmospheric CO2 records from sites in the SIO air sampling network 2005-June 2009 [bottom]. |
The findings provide new insight on the role of CO2 in global cycles of cooling and warming, suggesting that a shift towards longer glacial cycles — producing a major ice age about every 100,000 years instead of every 40,000 — some 1.2 million to 500,000 years ago was the result not of declining CO2 levels, but another factor, perhaps Earth’s orbit and tilt, which determines how much sunlight falls on each hemisphere, or changes in how previous ice ages affected subsequent ice ages.
“Previous studies indicated that CO2 did not change much over the past 20 million years, but the resolution wasn’t high enough to be definitive,” said Hönisch. “This study tells us that CO2 was not the main trigger, though our data continues to suggest that greenhouse gases and global climate are intimately linked.”
B. Hönisch et al. Atmospheric Carbon Dioxide Concentration Across the Mid-Pleistocene Transition. SCIENCE VOL 324 19 JUNE 2009