This enabled them to establish past seawater acidity, which in turn was dictated by the amount of atmospheric CO2. The researchers estimate that between about 60 and 52 million years ago, CO2 concentrations reached more than 2,000 ppm. Since about 24 million years ago, concentrations appear to have remained below 500 ppm and were more stable than before, although transient intervals of CO2 reduction may have occurred during periods of rapid cooling approximately 15 and 3 million years ago. There are still too many unknowns involved in climate prediction. But the sweltering ice-free world of the Eocene does warn us of what might happen if a runaway greenhouse effects sets in.
Some researchers still doubt that human activities are inducing rapid climate change.
They highlight the inconsistencies between the temperature records taken at the Earth's surface, which show rapid warming over the last century, and the data produced by satellite and balloon studies. Climate models generally predict that temperatures should increase in the upper air as well as at the surface if increased concentrations of greenhouse gases are causing the warming recorded at ground level.