In hot water. Oceanic plant life thrived during a greenhouse period at the end of the Paleocene--and perhaps helped cool the planet again.

Did Marine Plants Cool Ancient Greenhouse?

About 55 million years ago, Earth endured a dramatic bout of global warming 150,000 years long. A new study shows that those toasty times spawned a bounty of oceanic plant life, which perhaps in turn may have helped cool the fevered planet.

Many scientists are increasingly worried about the current rise of global temperatures, but things were far worse during the late Paleocene and early Eocene periods. In a geological instant, the Earth warmed by 5° to 7°C after some event--perhaps an earthquake or the eruption of oceanic volcanoes--sparked the release of methane from the sea floor. When this gas reached the atmosphere, it was converted to carbon dioxide. In the resulting greenhouse conditions, high latitudes warmed more than low latitudes. This probably impacted ocean life. Computer models suggest that the winds would slack off, leaving the oceans sluggish, says marine geologist Birger Schmitz of the University of Göteborg, Sweden. That in turn would mean fewer nutrients would be whipped up from the ocean floor, stunting plant life--mostly plankton--at the surface.

Now, a team of researchers led by paleoceanographer Santo Bains of the University of Oxford in the United Kingdom says that's not what happened. Examining sediment from drilling cores taken off Florida and Antarctica, the team looked specifically for barite, a well-preserved mineral formed by oceanic plants. Barite deposits increased about four-fold at the beginning of the hot spell 55 million years ago and dropped to pre-warming levels when temperatures cooled, the team reports in the 14 September issue of Nature. That tight correlation suggests that oceanic plant life formed part of a "biological pump" by drawing carbon dioxide out of the atmosphere, thus lowering the temperature, the team says.

The finding challenges the existing idea of what happens during a greenhouse period, Schmitz says, and it should fuel more research; for instance, he says, it's not quite clear where the plants' nutrients came from if they weren't stirred up from the bottom. While an ocean plant explosion may well have helped Earth cool off by sequestering carbon dioxide, humanity shouldn't count on the same effect to avert disaster from modern-day global warming, Schmitz adds. Not only did it take tens of thousands of years for the plants to kick in, but carbon dioxide is also pumped into the atmosphere at a much higher rate today.