Washington: Scientists claim that a drop in carbon dioxide could be the driving force that led to the Antarctic ice sheet's formation.
   
A team at Yale and Purdue universities analysed molecules from ancient algae found in deep-sea core samples to come to the conclusion, a journal reported.
   
The key role of the greenhouse gas in one of the biggest climate events in Earth's history supports carbon dioxide's importance in past climate changes and implicates it as a significant force in present and future climate.
   
The scientists pinpointed a threshold for low levels of carbon dioxide below which an ice sheet forms in the South Pole, but how much the greenhouse gas must increase before the ice sheet melts -- which is the relevant question for the future -- remains a mystery.
   
Team leader Prof Matthew Huber said roughly a 40 per cent decrease in carbon dioxide occurred prior to and during the rapid formation of a mile-thick ice sheet over the Antarctic approximately 34 million years ago.
   
"The evidence falls in line with what we would expect if carbon dioxide is the main dial that governs global climate; if we crank it up or down there are dramatic changes.”
   
"We went from a warm world without ice to a cooler world with an ice sheet overnight, in geologic terms, because of fluctuations in carbon dioxide levels," he said.

For 100 million years prior to the cooling, which occurred at the end of the Eocene epoch, Earth was warm and wet. Mammals and even reptiles and amphibians inhabited the North and South poles, which then had subtropical climates.
   
Then, over a span of about 100,000 years, temperatures fell dramatically, many species of animals became extinct, ice covered Antarctica and sea levels fell as the Oligocene epoch began.
   
Mark Pagani, the Yale geochemist and a team member, said polar ice sheets and sea ice exert a strong control on modern climate, influencing the global circulation of warm and cold air masses, precipitation patterns and wind strengths, and regulating global and regional temperature variability.
   
"The onset of Antarctic ice is the mother of all climate 'tipping points'. Recognising the primary role carbon dioxide change played in altering global climate is a fundamentally important observation," he said.
   
There has been much scientific discussion about this sudden cooling, but until now there has not been much evidence and solid data to tell what happened, Huber said.
   
The team found the tipping point in atmospheric carbon dioxide levels for cooling that initiates ice sheet formation is about 600 parts per million. Prior to the levels dropping this low, it was too warm for the ice sheet to form.
   
At Earth's current level of around 390 parts per million, the environment is such that an ice sheet remains, but carbon dioxide levels and temperatures are increasing.

(Agencies)