By Gail Gallessich

New evidence from past climate records provides some of the strongest indications yet of a direct link between tropical warmth and higher greenhouse gas levels, say scientists at UC Santa Barbara.

The new evidence was published last month in Science Express, the online publication of the journal Science. The authors are Martin Medina-Elizalde, a graduate student in the Earth Science Department (formerly geological sciences) and the Interdepartmental Program in Marine Science, and David Lea, professor of earth science.

The link between increased atmospheric greenhouse gases and global temperatures underlies the theory of global warming, explained the authors. They added that the present, steady rise in tropical temperatures due to global warming will have a major impact on global climate, and could intensify destructive hurricanes, such as Katrina and Rita.

This link can be established by computer climate models or modern observations. Another way is through paleoclimate evidence where past climate is reconstructed by analyzing calcium carbonate shells of ancient microscopic sea life. This latest study is based on scientists’ analysis of the chemical composition of fossil plankton shells (foraminifera) from deepsea cores.

“The relationship between tropical climate and greenhouse gases is critical, because tropical regions receive the highest proportion of sunshine and act as a heat engine for the rest of the Earth,” said Lea.

Modern observations of tropical sea surface temperature indicate a rise of one to two degrees Fahrenheit over the last 50 years, a trend consistent with rising carbon dioxide in the atmosphere due to fossil fuel combustion, according to the authors.

The research described in the article demonstrates that over the last 1.3 million years, sea surface temperatures in the heart of the western tropical Pacific were controlled by the waxing and waning of the atmospheric greenhouse effect. The largest climate mode shift over this time interval, which occurred ~950,000 years before the present (the mid-Pleistocene transition), has previously been attributed to changes in the pattern and frequency of ice sheets.