Scientific American: Just How Sensitive Is Earth's Climate to Atmospheric Carbon Dioxide?

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http://www.scientificamerican.com/article.cfm?id=how-sensitive-is-climate-to-carbon-dioxide

News - October 8, 2009

Just How Sensitive Is Earth's Climate to Atmospheric Carbon Dioxide?

Two new studies look far back in geologic time to determine how sensitive the global climate is to atmospheric CO2 levels

By David Biello

http://www.scientificamerican.com/blog/post.cfm?id=is-350-the-right-number-to-stop-cli-2009-05-04">Carbon dioxide levels climbing toward a doubling of the 280 parts per million (ppm) concentration found in the preindustrial atmosphere pose the question: What impact will this increased greenhouse gas load have on the climate? If relatively small changes in CO2 levels have big effects—meaning that we live in a more sensitive climate system—the planet could warm by as much as 6 degrees Celsius on average with attendant results such as changed weather patterns and sea-level rise. A less sensitive climate system would mean average warming of less than 2 degrees C and, therefore, fewer ramifications from global warming.

Human civilization is now running an experiment (and without a control) that will definitively determine the answer. Scientists, however, have also realized that history can be a guide: Two new papers published in Science this week examine the http://www.sciencemag.org/cgi/content/abstract/sci;326/5950/248?maxtoshow=&HITS=10&hits=10&RESULTFORMAT=&fulltext=hai+cheng&searchid=1&FIRSTINDEX=0&resourcetype=HWCIT">historical record preserved in a stalagmite and http://www.sciencemag.org/cgi/content/abstract/sci;1178296v1?maxtoshow=&HITS=10&hits=10&RESULTFORMAT=&fulltext=tripati&searchid=1&FIRSTINDEX=0&resourcetype=HWCIT">microscopic seashells, respectively, to offer some clues].

Earth scientist Aradhna Tripati of the University of California, Los Angeles's Department of Earth and Space Sciences and her colleagues extracted a record of past atmospheric concentrations of CO2 stretching back 20 million years from the shells of tiny creatures known as foraminifera buried in a column of ocean mud and rock. The microscopic animals build shells of calcium carbonate out of minerals in seawater—a process that is affected by the water's relative pH (acidity), which is, in turn controlled by the level of CO2 in the atmosphere. More CO2 in the atmosphere means a http://www.scientificamerican.com/article.cfm?id=more-acid-ocean-corrodes-sea-life">more acidic ocean.

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"Climate systems are well linked worldwide, such as sea-level, CO2, ice sheet(s), the Asian monsoon, regional temperature and precipitation," Cheng says. "So a change in one of them could trigger changes in others." And that might mean the climate is too sensitive to tolerate current levels of CO2 without changing the conditions that have allowed human civilization to flourish in the past 10,000 years.