The magazine of the european research area European Commission Copenhagen, a missed chance?

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The good 

greenhouse effect

Four billion years ago, the sun 

was still a young star, 30 % cooler 

than today, and the earth, without 

the greenhouse effect to heat it, 

would probably never have known 

life. This life it owes to carbon 

oxysulphide (COS) derived 

from the volcanic activity that 

it experienced during several 

millennia, according to a study 

published in August 2009 in 

Proceedings of the National Acade-

my of Sciences (PNAS). 

The authors, chemist Matthew 

Johnson and his colleagues 

at Copenhagen University and 

a team from the Tokyo Institute 

of Technology, found in ancient 

rocks a strange distribution of 

sulphur isotopes that could not 

have resulted from geological 

processes. Convinced that an atmos-

pheric factor had come into play, 

they irradiated sulphur dioxide (SO

2

) 

with sunlight at different wave-

lengths, and actually managed to 

reproduce the distribution of the 

sulphur isotopes in the rocks. 

For researchers, these results clearly 

identify carbon oxysulphide, which 

was formed from sulphur ejected 

from volcanoes, as the best candi-

date for explaining the greenhouse 

effect experienced by the earth 

in its infancy. They believe that 

a cover of atmospheric COS, which 

is more efficient than CO

2

, would 

have been enough to make good 

the 30 % less energy received from 

the sun. And it would have facili-

tated the emergence of life all 

the more as, like ozone (O

3

), it halts 

ultraviolet radiation. 

But how then, with such a cover, 

do we explain the ice age that 

the earth apparently experienced 

2.5 billion years ago? Precisely 

by the appearance of life, which 

resulted in the release of large 

amounts of oxygen. In an oxidized 

atmosphere, the volcanic sulphur 

would no longer have produced 

COS, but sulphated aerosols 

having the opposite effect. In 

other words, we need to reckon 

with the gases that were present 

in the atmosphere.