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VOLCANOES were more destructive in ancient history. Not because they were
bigger, but because the carbon dioxide they released wiped out life with greater
ease.

Paul Wignall from the University of Leeds was investigating the link between
volcanic eruptions and mass extinctions. Not all volcanic eruptions killed off
large numbers of animals, but all the mass extinctions over the past 300 million
years coincided with huge formations of volcanic rock. To his surprise, the
older the massive volcanic eruptions were, the more damage they seemed to
do.

Wignall calculated the “killing efficiency” for these volcanoes by comparing
the proportion of life they killed off with the volume of lava that they
produced. He found that size for size, older eruptions were at least 10 times as
effective at wiping out life as their more recent rivals.

The Permian extinction, for example, which happened 250 million years ago, is
marked by floods of volcanic rock in Siberia that cover an area roughly the size
of western Europe. Those volcanoes are thought to have pumped out about 10
gigatonnes of carbon as carbon dioxide. The global warming that followed wiped
out 80 per cent of all marine genera at the time, and it took 5 million years
for the planet to recover.

Yet 60 million years ago in the late Palaeocene there was another huge amount
of volcanic activity and global warming but no mass extinction. Some animals did
disappear but things returned to normal within tens of thousands of years. “The
most recent ones hardly have an effect at all,” Wignall says. He ignored the
extinction which wiped out the dinosaurs at the end of the Cretaceous, 65
million years ago, because many scientists believe it was primarily caused by
the impact of an asteroid.

Wignall thinks that older volcanoes had more killing power because more
recent life forms were better adapted to dealing with increased levels of
CO₂. Ocean chemistry may also have played a role. As the supercontinents
broke up and exposed more coastline there may have been more weathering of
silica rocks. This would have encouraged the growth of phytoplankton in the
oceans, increasing the amount of CO₂ absorbed from the atmosphere.

Vincent Courtillot, director of the Paris Geophysical Institute in France,
says that Wignall’s idea is provocative. But he says it is incredibly hard to do
these sorts of calculations. He points out that the killing power of volcanic
eruptions depends on how long they lasted. And it is impossible to tell whether
the huge blasts lasted for thousands or millions of years.

Courtillot also adds that it is difficult to estimate how much lava
prehistoric volcanoes produced, and that lava volume may not necessarily
correspond to carbon dioxide or sulphur dioxide emissions.