Lots of vegetation bad for animals

Plants use CO2 during photosynthesis, and with CO2 at 10 times the present level, growth conditions were exceptional in the Devonian. Yet the world was struck by mass extinctions in which 70% or more of all species disappeared. Volcanic eruptions and meteor strikes have been blamed, but an alternative theory says plants are responsible.

clouds, less of the sun’s energy is reflected back into the atmosphere. Instead, it will heat Earth’s oceans and surface, contributing further to global warming. And once activated, such a negative spiral takes a long time to reverse. After the warming during the PETM 56 million years ago, 170,000 years passed before the climate stabilised.

Rapid change kills life

This marked change of temperature during the PETM may be extremely fast in geological terms, but it’s still very slow compared with today’s temperature rise. To find a period that is closer to the present climate scenario, scientists must take a look at a disaster that was triggered by an external event. At the end of the Cretaceous 66 million years ago, a mass destruction event wiped out not only the dinosaurs that had ruled Earth for 160 million years, but 75% of all animal species. The mass destruction was most probably due to a violent and sudden temperature drop caused by a meteor the size of Mount Everest striking Earth near the town of Chicxulub on the Yucatan Peninsula in Mexico. The energy release corresponds to one billion Hiroshima nuclear bombs, and the collision sent huge quantities of dust and soot into the atmosphere, blocking out the sunlight. Temperatures fell suddenly and drastically. According to research, the event affected the climate for up to 5000 years and limited biodiversity for 20,000 years after the strike.

Things went very differently with the slower temperature rise during the PETM, because life had time to adapt. Animal species reacted by shrinking in order to

become better at regulating their heat, while biodiversity actually ended up increasing. Whether temperatures rise or fall, it is the speed of the climate change which is the key to how life is affected.

CO2 explosion since 1800

When it comes to the CO content of the atmosphere, we are again now experiencing an unusually rapid rise. For almost a million years up until the year 1800, the CO level was fairly constant: 200-300 ppm (parts per million), corresponding to 0.02-0.03%. But within the past 200 years, it has risen to 400 ppm. Depending on what we do to prevent further increases, the CO level by 2300 is expected to be 400-700 ppm or, worst case, 2000 ppm. And again the levels themselves aren’t unprecedented – it has reached 7,000 ppm before. It’s the rate of increase which makes this such a dramatic climate change.

Indeed if charted over hundreds of millions of years, the CO content of the atmosphere has been on the decrease. Over the past 542 million years, the CO content of the atmosphere has been reduced to

WALTER MYERS/SCIENCE PHOTO LIBRARY Arthropods originated during this period, such as the ichthyostega which lived in swamps. The first trees ruptured the rock The air’s high content of CO2 helped plants establish a presence on dry land some 370 million years ago. Low bushes developed into the ancestor of trees, Archaeopteris. The trees grew deep into cracks and cavities in the ground, their hefty roots rupturing the rock.

Nutrients in the water CO2 from the air was dissolved in water to produce carbon dioxide, which entered the cracks and weathered the rock, liberating nutrients that washed into oceans, lakes and streams. Algae caused depletion The nutrients made algae thrive. After their death, they sank to the bottom where they were broken down by oxygenconsuming bacteria, whose oxygen consumption caused oxygen depletion, making animals suffocate.