Climate Science

Leafing Through Time

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Science  11 Sep 2009:
Vol. 325, Issue 5946, pp. 1320
DOI: 10.1126/science.325_1320c

Climate changes in Earth's history can inform our understanding of anthropogenic greenhouse gas forcing, but the resolution in the rock record for most events spans much longer periods (thousands to millions of years) than the rapid changes occurring today. For example, much of the Mesozoic (from about 250 to 65 million years ago) was warmer than today, and atmospheric CO2 levels then were generally higher, but resolving individual events during this time requires developing detailed proxies for both CO2 levels and paleoclimate and then correlating these different records in time. Retallack has attempted such an accounting for the past 300 million years, using the stomata of fossil Ginkgo leaves to infer past atmospheric CO2 levels (plants have fewer stomata when CO2 levels are high) and also measuring the occurrence and depth of carbonate nodules, which reflect precipitation and temperature, in more than 3700 paleosols in the western United States. In all, 40 noticeably wet excursions are indicated in the paleosol record, and many of these can be correlated, albeit with some uncertainties, with the inferred CO2 record (though the records don't fully overlap). With the caveat that there are assumptions and extrapolations in many steps of such an analysis, the results suggest that Mesozoic warmth included numerous extreme warm-wet excursions associated with extremely high CO2 levels.

Geol. Soc. Am. Bull. 121, 1441 (2009)

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