By Joseph Milton

Most scientists have assumed that, as carbon dioxide levels increase and the Earth warms, plant species diversity in the rainforests will start to dwindle, with plants unable to adapt to the heat. But a new study suggests that the opposite may be true. In the past, rising atmospheric carbon dioxide and higher temperatures actually drove the evolution of far greater numbers of new rainforest plant species than were wiped out.

But don't trade in your electric car for a gas-guzzler just yet--if rainfall drops as temperatures rise, or if they rise too rapidly, the outcome for rainforest diversity could be much less positive.

For clues to how rainforest diversity will be affected by increasing atmospheric carbon dioxide and the corresponding rise in temperatures, Carlos Jaramillo, a palaeobiologist at the Smithsonian Tropical Research Institute in Panama, and his colleagues decided to look at what happened during similar conditions in the past.

One such episode in Earth's history occurred 56.3 million years ago and is called the Palaeocene-Eocene Thermal Maximum (PETM). Within 10,000-20,000 years, the world warmed by 3-5 degrees Celsius and atmospheric carbon dioxide doubled to around two and a half times the levels we see today. These unusually warm conditions lasted for around 200,000 years.

Pollen clues

To find out how this ancient climate change affected rainforest plants, Jaramillo and his team analyzed fossilized pollen trapped in rock cores from rainforests in Colombia and Venezuela. They spent seven years locating appropriate sites and taking samples, then used a battery of dating techniques to ensure that they were examining cores formed before, during and after the thermal maximum--a relatively narrow time window in geological terms. The results were published November 12 in Science.

Although some plant species disappeared, many more new species arose. That included entire families, suggesting that the increased temperatures and carbon dioxide levels actually boosted biodiversity. "What we found was exactly the opposite of what we were expecting," says Jaramillo. "The diversity of the tropical forest increased really fast over a very short amount of time."

The pollen fossil record shows that some important plant families, such as Myrtaceae, which includes eucalyptus, and Passifloraceae--the passion flowers--made their first appearance during the thermal maximum. The tropics have remained the most species-diverse area of the world ever since.

This might sound like good news for the rainforest in the face of contemporary climate change. However, Guy Harrington, a palaeobiologist at the University of Birmingham, UK, warns that any positive effects on plant diversity could be canceled out if temperatures rise too quickly for plants to adapt. "It's the rate--how fast you're turning up the heater--that's the most important thing," he says.

The availability of water could be critical too, he adds. There was no shortage of water during the PETM, but the effects of future climate change on rainfall in the tropics are uncertain.

The beneficial effects of a hotter and more carbon dioxide-rich world may also be limited to the tropics. Harrington has studied fossil deposits from the period in North America, and says that many native species died out there as temperatures rose. He adds that higher latitudes are more likely to suffer from extreme conditions or lack of water.

Despite the caveats, Harrington fears that "uninformed climate skeptics will probably use this as evidence" to say that action on global warming isn't necessary. Jaramillo agrees: "Of course I'm worried some people will look at this and say 'we shouldn't care about global warming'," he says, "but this is what the fossil record is telling us."

Jaramillo believes that there is a more pressing threat to the diversity of tropical rainforests. "Deforestation is the real enemy," he says, "not the increase in temperature and carbon dioxide."