What caused global warming 40 million years ago?

Whatever you think about today's prognosis for global warming, there's no denying that the planet has been a lot hotter in the past. Now we know some of that heat was generated by carbon dioxide emissions 40 million years ago.

At that time, called the Middle Eocene, the Earth was going through a long phase of cooling that would eventually create the ice sheets at the poles. But suddenly, the planet grew a lot warmer. Until now, nobody was quite sure how or why that happened. But a group of researchers reported yesterday in Science that it was most likely caused by a huge increase in carbon dioxide in the atmosphere.

A summary of the study from Science explains:

Using a technique called organic molecular paleothermometry, Peter Bijl and colleagues patched together records of sea surface temperature and atmospheric carbon dioxide concentrations across the Middle Eocene period. They found that during this interval of short-term warming, vast amounts of carbon dioxide were injected into the atmosphere, and that a sea surface temperature increase of as much as 6 degrees Celsius accompanied the atmospheric carbon dioxide rise. Such a causal link between rising carbon dioxide levels and sea temperatures is consistent with recent models that predict sea surface temperature increases of 2 to 5 degrees Celsius for a doubling of carbon dioxide.

So we have a good historical precedent for the idea that temperatures on the planet will rise if we keep pumping carbon dioxide into the atmosphere. What we don't know is how this previous global warming event happened.

Lots of carbon had to have been released into the atmosphere pretty quickly. One possible culprit is volcanoes, which were fairly common while the continents were drifting northward. But the researchers say the amount of carbon that would need to be pumped into the atmosphere to cause this kind of climate change can't be traced back to volcanoes alone. That leaves us with a big question mark.

Brief invasion by pollution-loving aliens?

Read the full scientific paper here [PDF], or a less-technical summary of it via Science