Time's Secrets

Contents Updated: Thursday, August 05, 1999

The End of Earth's Summer.

Dr Maureen Raymo says that the turn from a benign warm climate to the present cold regime which started about 40 million years ago was caused by the upthrust of the Tibetan plateau and to a lesser extent the Rocky mountains. Before 40 million years ago for 250 million years there were no icecaps, seasons varied little and the climate was warm. Alaska, Greenland, Antarctica and Tibet were all green and lush and there seemed to have been few deserts. The southern Pacific ocean was 15 degrees warmer.

In 1986 Dr Raymo realised that the formation of the Tibetan plateau had a profound effect on several factors in the climate equation. The fragment of a continent called India that broke loose and sped across the Indian ocean to impact into southern Asia pushed up the Tibetan plateau and the Himalayas. The average height of these peaks is now 15,000 feet and the weather of the region has somehow to push past such huge blocks of land. The prevailing winds were carrying tropical moisture into the Gobi area and to Siberia, but as the land rose the winds were pushed up and cooled. Rain fell and as the land pushed higher, more rain fell until now all the rain falls in the monsoon season on the southern slopes of the Tibetan mass and the Gobi and Siberia are in a rain shadow.

The large amounts of rain that fell were effectively dilute carbonic acid and began to dissolve the rocks of the plateau. Calcium silicate was attacked by the dissolved CO2 and formed silica and calcium carbonate which washed down the slopes. The Tibetan plateau, 4 percent of the planet's land mass produces 25 percent of its mud which is washed down into the ocean by eight large rivers. The Ganges and Brahmaputra carry 323 million tons of solids a year, the Yangtze, 166 million tons and the Indus and Mekong, 60 million tons each a year. Over 40 million years, that is a lot of mud but the plateau continued to rise up pushed by the momentum of the sub-continent moving north. The net effect was that vast amounts of CO2 were taken out of the air and the Greenhouse Effect reduced, cooling the planet. It is this cooling that eventually led to icecaps. The fall in temperature led to less overall rainfall and arid zones appeared and the differences between the seasons became sharper. Jungles retreated and 15 million years ago grass appeared and grasslands started to form.

Of course there are many effects on global climate but the presence of high plateaus, just where they are, is a major influence. The closing of the Panama isthmus and the eastern end of the Mediterranean might have been important too because they cut out ocean currents that spread the earth's heat about. The closure of the gap between the oceans in central America diverted the hot currents northwards where it stimulated snowfall over the Arctic but the overall effect of having the gap open is actually worse for the north—the heat doesn't get there at all and it would have been colder. The air currents blocked off by the Tibetan plateau compensated.