In This Issue

During the Miocene period, from 25 to 12 million years ago, climate change drove plants and animals to evolve toward their modern forms and distribution. The fossil and geological records show considerable fluctuation in global temperature during this time, but not all proxies for atmospheric CO2 data show a link between CO2 and Miocene climate. Wolfram Kürschner et al. tap a new source of CO2 data: the stomatal density of leaves, which supports a close link between atmospheric CO2 and temperature. Plants, which take up CO2 for growth, regulate the number of leaf stomata, meaning that more CO2 means fewer stomata. The authors obtained the ‘‘stomatal index’’ as a function of atmospheric CO2 for Ginkgo biloba as well as several species of laurel, two living and one extinct. With this information, the authors could estimate the atmospheric CO2 for the Miocene by examining fossil leaves of the four species. They found that the graph of atmospheric CO2, based on this proxy, closely paralleled the fossil temperature data. The causal links among CO2, temperature, and evolution are extremely complex, the authors report. — K.M.