Scientists are starting to unravel the secrets of how the last ice age ended. Researchers from Harvard University, the University of Wisconsin and Oregon State University say that global rises in carbon dioxide in the atmosphere were to blame.
Alterations in the Earth's orbit could have played a part in global warming around 20,000-10,000 years ago, but was not as significant as the rise in CO2.
Jeremy Shakun, the study's main author and a Harvard visiting postdoctoral fellow, says, "Orbital changes are the pacemaker. They're the trigger, but they don't get you too far. Our study shows that CO2 was a much more important factor, and was really driving worldwide warming during the last deglaciation."
The findings from the American researchers are published in the latest issue of the journal Nature.
The team collected 80 samples of ice and sediment from the Earth's core in various locations around the globe.
"We have ice cores from Greenland, people have cored the sea floor all around the world, they've cored lakes on the continents, and they have worked out temperature histories for all these sites."
Following research on ice cores in the Antarctic, it has long been realised that during the last million years, mass melting of glaciers and peaks in carbon dioxide have been linked. However, it has been difficult to pin down the causes and effects of carbon dioxide and global warming from the geological evidence.
The evidence from the ice core suggested the amount of carbon dioxide rose following temperature rises, which has been seized upon by those who are sceptical of global warming.
Some climate experts hit back, demonstrating that the time delay between the rises in temperature and carbon dioxide suggests that greenhouse gases did not spark the climate change, but amplified it.
But the American team found a bigger issue that although carbon dioxide levels in ice core air bubbles mirror measurements across the atmosphere, temperatures taken just in the ice mirror conditions confined to the Antarctic.
Most experts in the field consider that the initial cause was a slight alteration in the orbit of the earth that led to additional sun in the north hemisphere. This caused layers of melting ice in North America and Europe and produced vast amounts of fresh water that altered the pattern of sea currents.
The cycle of the currents is similar to a worldwide conveyor belt that moves heat. Nowadays, the oceans increase temperatures in the north by a few degrees as they cross the equatorial Atlantic, says Jeremy Shakun.
However, if the conveyor stops, temperatures rise in the south, as the heat is no longer being removed, and the wind patterns change, ice caps melt and carbon is trapped in deep oceans.
As more carbon dioxide goes into the atmosphere, temperatures rise and the ice age begins to end, he explains. The study once again shows that nowadays it is vital that humans cut their carbon footprint and help reduce the effect of greenhouse gases on climate change.
The study was backed by the he National Science Foundation and National Oceanic and Atmospheric Administration Climate and Global Change Fellowship. Research was carried out at Oak Ridge National Laboratory, at Tennessee, USA.