Carbon dioxide role in past climate revealed

Researchers at the British Antarctic Survey (BAS) and the University of California, Santa Cruz have discovered that Earth’s last great global warming period, 3 million years ago, may have been caused by levels of CO2 in the atmosphere similar to today’s.

Reporting this week in a leading Earth Science journal, Geochemistry Geophysics Geosystems, the scientists describe how they tested two widely held ideas that attempted to explain the balmy conditions on Earth at that time. Their findings clearly demonstrate that studying past climates can help us to understand the likely impact of greenhouse gas emissions and global warming.

BAS Principal Investigator Dr Alan Haywood said, ‘There are two schools of thought about past warm intervals. Many scientists suggest that they were caused by ocean currents (like the Gulf Stream) moving greater amounts of warm water from the tropics to the polar regions. Others speculate that increased levels of CO2 in the atmosphere initiated warming all over the planet. We used the latest supercomputing technology combined with chemical analysis of seabed sediments to make a sophisticated reconstruction of past sea temperatures. If the warming was caused by ocean currents, we would expect to see cooling at the tropics and warming at the poles. Conversely, if CO2 was the cause then we would expect both the tropics and the poles to warm. The sea temperature pattern we found points the finger squarely at CO2 rather than the ocean currents. This is a real breakthrough for those of us investigating past climate – we’ve made a major contribution to a long standing argument and our findings are critical to understanding how climate may respond to emissions of greenhouse gases in the future’.

Clues to past sea-surface temperature come from tiny marine algae that live near the surface. They produce chemicals called alkenones that record the sea temperature. When the algae die they sink and become part of the seabed. Therefore, a record of past sea temperatures is stored within the sediments. Sea-surface temperatures were also predicted using a climate model running on a sophisticated supercomputer based at Manchester. This is capable of billions of calculations per second.