Just How Sensitive Is Earth's Climate to Atmospheric Carbon Dioxide?Two new studies look far back in geologic time
Scientific American: 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 chells 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 more acidic ocean...
'Modern-day levels of carbon dioxide were last reached about 15 million years ago,' Tripati says, when sea levels were at least 25 meters higher and temperatures were at least 3 degrees C warmer on average. 'During the middle Miocene, an [epoch] in Earth's history when carbon dioxide levels were sustained at values similar to what they are today [300 to 500 ppm], the planet was much warmer, sea level was higher, there was substantially less ice at the poles, and the distribution of rainfall was very different.'
Further, 'at no time in the last 20 million years have levels of carbon dioxide increased as rapidly as at present,' Tripati adds; CO2 concentrations have climbed from 280 ppm to 387 ppm in the past 200 years. And 'our work indicates that moderate changes in carbon dioxide levels of 100 to 200 parts per million were associated with major climate transitions and large changes in temperature' -- indicative of a very sensitive climate.
