In the ancient past temperatures on Earth appeared to have been much warmer than today. It is possible that temperatures may rise as high as then based on current climate change projections. The new study, by National Center for Atmospheric Research (NCAR) scientist Jeffrey Kiehl, will appear as a Perspectives piece in this week’s issue of the journal Science. Building on recent research, the study examines the relationship between global temperatures and high levels of carbon dioxide in the atmosphere tens of millions of years ago. It warns that, if carbon dioxide emissions continue at their current rate through the end of this century, atmospheric concentrations of the greenhouse gas will reach levels that last existed about 30 million to 100 million years ago, when global temperatures averaged about 29 degrees F higher than now (in the high eighties F).