A long-term decrease in carbon dioxide levels will render the world as inhospitable as Venus, where temperatures today are about 400C.
Although rising carbon dioxide levels due to the past 100 years of industrialisation threaten to cause global warming, this is nothing compared with the threat posed by a much longer term decrease in carbon dioxide.
Ken Caldeira and James Kasting, from Pennsylvania State University, have taken a closer look at the geological events that are likely to end life on Earth. They conclude in today's Nature that carbon dioxide depletion is the key.
Once carbon dioxide levels fall below a certain threshold, plant photosynthesis, on which all other life depends, will become increasingly difficult. The planet will become rapidly hotter and water will evaporate into space. Once the water is gone, all life would become impossible, even the production and consumption of situation comedies and seasonal news-substitutes.
Tyler Volk, professor of applied science at New York University, said the trend since first life evolved billions of years ago is for carbon dioxide to decrease. 'Current rising levels of CO2 from industrial and agricultural sources are, by comparison, just a blip on the geological landscape.' Even though the predictions mean we can 'breathe easy for about a billion more years', Professor Volk said, the work raises issues that are 'hauntingly familar'.
While the human race is now concerned with rising carbon dioxide levels, in millennia to come it may have to think of ways of putting vast amounts of the gas into the atmosphere. 'We can envisage incinerating the white cliffs of Dover in giant solar furnaces and burning off the carbon dioxide in the rocks,' he said.