Link Between Tropical Warming And Greenhouse Gases Stronger Than Ever, Say Scientists

The new evidence linking past tropical ocean temperatures to levelsof atmospheric greenhouse gases is published in this week's ScienceExpress, the on-line publication of the journal Science. The authorsare Martin Medina-Elizalde, graduate student in the Department of EarthScience and the Interdepartmental Program in Marine Science at UC SantaBarbara, and David Lea, professor in UCSB's Department of Earth Scienceand the Marine Science Institute.

The link between increased atmospheric greenhouse gas and globaltemperatures underlies the theory of global warming, explained theauthors. This link can be established by computer climate models ormodern observations. Another way to study the link is throughpaleoclimate observations where past climate is reconstructed throughnatural archives. This latest study is based on such paleoclimateobservations; the scientists analyzed the chemical composition offossil plankton shells from a deep sea core in the equatorial Pacific.

"The relationship between tropical climate and greenhouse gasesis particularly critical because tropical regions receive the highestproportion of solar output and act as a heat engine for the rest of theearth," said Lea.

Modern observations of tropical sea surface temperature indicate a riseof one to two degrees Fahrenheit over the last 50 years, a trendconsistent with rising carbon dioxide in the atmosphere due to fossilfuel combustion, according to the authors. The paleoclimate evidencefrom this new study supports the attribution of the tropicaltemperature trend to the ever-increasing greenhouse gas burden in theatmosphere.

The research described in this week's article demonstrates that overthe last 1.3 million years, sea surface temperatures in the heart ofthe western tropical Pacific were controlled by the waxing and waningof the atmospheric greenhouse effect. The largest climate mode shiftover this time interval, occurring ~950,000 years before the present(the mid-Pleistocene transition), has previously been attributed tochanges in the pattern and frequency of ice sheets.

The new research suggests instead that this shift is due to a change inthe oscillation frequency of atmospheric carbon dioxide abundances, ahypothesis that can be directly tested by deep drilling on theAntarctic Ice Cap. If proved correct, this theory would suggest thatrelatively small, naturally occurring fluctuations in greenhouse gasesare the master variable that has driven global climate change on timescales of ten thousand to one million years.