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Poring over the Past

CLIMATE SCIENCE
The concentration of CO2 in the atmosphere is understood to be a primary controller of climate, and high values are thought to be the main cause of a number of extended warm periods over the geological history of Earth. Additionally, atmospheric CO2 exerts a fundamental influence on the carbonate chemistry of the ocean, due to the solubility of CO2 in seawater, and therefore on calcifying marine organisms. A direct record of atmospheric CO2 concentration is available for less than the past 1 million years, so earlier periods must be studied by modeling or via proxies.

Haworth et al. use measurements of the density of stomata, the pores through which gas exchange occurs in plant leaves, in an extinct conifer to estimate the partial pressure of atmospheric CO2 during the mid-Cretaceous. This technique takes advantage of the observation that stomatal density is negatively correlated to the concentration of CO2 in the air. They find that CO2 was between 560 and 1200 parts per million over that period, values that fall on the low side of the range of previous estimates based on other techniques and are compatible with other evidence for relatively cool mid-Cretaceous climates. — HJS 

Geology 33 , 749 (2005).