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THE controversy over whether a huge asteroid really wiped out three-quarters of life on Earth, including the dinosaurs, has taken another twist. New data gleaned from fossil plants shows that the impact 65 million years ago sparked a dramatic change in climate.

The asteroid that slammed into Mexico at the end of the Cretaceous period gouged out a crater the size of Maryland. The smash converted huge quantities of carbonate rock into carbon dioxide, and earthquakes and fires ravaged the planet in its aftermath.

This might have created a sudden greenhouse effect that caused the mass extinction, but some scientists doubt that the asteroid was responsible (New Scientist, 4 May 2002, p 28). They say that dinosaurs were already in decline, and a series of volcanic eruptions at the Deccan Traps in India could have prompted the critical climate swing.

Now David Beerling at the University of Sheffield and his team think they have evidence for a climate switch so rapid that it has to be linked to the impact. The researchers found that fossilised plants after the extinction had fewer pores on their leaves, as the more CO₂ there is, the easier it is for plants to extract carbon for photosynthesis.

From these measurements, the team estimates that CO₂ levels rose by a factor of four or five within 10,000 years of the impact. This would have led to an average global warming of 7.5 °C. For the Deccan eruptions to produce this effect, they would have had to occur within these 10,000 years, not the 2 million years thought.