New test results are providing further evidence that the concentration of atmospheric carbon dioxide and Earth’s surface temperature are inextricably linked. Nearly thirty-four million years ago, the Earth underwent a transformation from a warm and high-carbon dioxide greenhouse state to a lower-CO2, variable climate of the modern icehouse world. Massive ice sheets grew across the Antarctic continent, major animal groups shifted, and ocean temperatures decreased by up to 5 degrees. Various studies of how this drastic change affected temperatures on land have had mixed results. Some show no appreciable terrestrial climate change; others find cooling of up to 8 degrees and large changes in seasonality. Now, a group of American and British scientists have used a new chemical technique to measure the change in terrestrial temperature associated with this shift in global atmospheric CO2 concentrations.