Water shortages as a result of rising temperatures will not do as much damage as feared, evidence from ancient trees suggests

* Alok Jha, science correspondent

It is generally acknowledged that a warming world will harm the world’s forests. Higher temperatures mean water becomes more scarce, spelling death for plants – or perhaps not always.

According to a study of ancient rainforests, trees may be hardier than previously thought. Carlos Jaramillo, a scientist at the Smithsonian Tropical Research Institute (STRI), examined pollen from ancient plants trapped in rocks in Colombia and Venezuela. “There are many climactic models today suggesting that … if the temperature increases in the tropics by a couple of degrees, most of the forest is going to be extinct,” he said. “What we found was the opposite to what we were expecting: we didn’t find any extinction event [in plants] associated with the increase in temperature, we didn’t find that the precipitation decreased.”

In a study published todayin Science, Jaramillo and his team studied pollen grains and other biological indicators of plant life embedded in rocks formed around 56m years ago, during an abrupt period of warming called the Paleocene-Eocene Thermal Maximum. CO2 levels had doubled in 10,000 years and the world was warmer by 3C-5C for 200,000 years.

Contrary to expectations, he found that forests bloomed with diversity. New species of plants, including those from the passionflower and chocolate families, evolved quicker as others became extinct. The study also shows moisture levels did not decrease significantly during the warm period. “It was totally unexpected,” Jaramillo said of the findings.

Klaus Winter of the STRI added: “It is remarkable that there is so much concern about the effects of greenhouse conditions on tropical forests. However, these horror scenarios probably have some validity if increased temperatures lead to more frequent or severe drought as some of the current predictions suggest.”

Last year, researchers at the Met Office Hadley Centre reported that a 2C rise above pre-industrial levels, widely considered the best-case scenario, would still see 20-40% of the Amazon die off within 100 years. A 3C rise would see 75% of the forest destroyed by drought in the next century, while a 4C rise would kill 85%.

Jaramillo found that the plants he studied seemed to become more efficient with their water use when it became more scarce. But he also cautioned that future risks for the world’s plant species did not end with climate change. Human action would continue to determine the fate of the world’s forests, he said.

“What the fossil record is showing is that plants have already the genetic variability to cope with high temperature and high levels of CO2.

“Rather than global warming, the [trouble] for tropical plants is deforestation. The fossil record shows that, when you don’t have humans around, the plants can deal with high temperatures and CO2.”