Monday, June 28, 2010

What event caused the end of the last ice age?

Scientists are still anything but certain in what event led to the end of the last ice age that resulted in warmer climate and the birth of human civilization. From the geological point of view we know that ice sheets in the northern hemisphere began to collapse and warming spread quickly to the south. Most scientists support the theory that the event that led to warming was an orbital shift that caused more sunlight to fall across Earth's northern half.

The latest interesting scientific report that tries to put more light into the event(s) that ended the last ice age comes from the team of scientists from the Columbia University. They believe that not one event, but actually entire chain of events caused the end of the last ice age.

According to their theory the events began with the melting of the large northern hemisphere ice sheets that happened 20,000 years ago. The melting ice sheets reconfigured the planet's wind belts, pushing warm air and seawater south, and pulling carbon dioxide from the deep ocean into the atmosphere, allowing the planet to heat even further.

Our planet regularly goes into an ice age every 100,000 years or so, and this is caused by the shifts in Earth's orientation toward the sun. At the peak of the last ice age that happened around 20,000 years ago, Earth's orbit shifted. This event lead to more summer sunlight began falling on the northern hemisphere, melting those massive ice sheets and sending icebergs and fresh water into the North Atlantic Ocean.

The report also says that freshening of the North Atlantic triggered a series of cold spells in Greenland and northern Europe by shutting down the Gulf Stream current, which usually carries warm water north from the equator. Sea ice spread across the North Atlantic, bringing bitter cold winters to Europe and profoundly reshaping the planet's wind belts. And with the North Atlantic covered in ice, the tropical trade winds shifted south, bringing dry spells across much of Asia and rain to normally arid regions of Brazil. The displaced winds moved not only rain further south, but hot air and warm seawater, heating up the southern hemisphere.

The report also states that ice core records show that between 18,000 and 11,000 years ago atmospheric carbon dioxide levels rose from 185 parts per million to 265 parts per million. (Current levels are 393 parts per million, after the heavy industrial expansion across the globe). The infusion of carbon dioxide came just as the planet's orientation was shifting, and summer sunlight to the northern hemisphere was declining, at about 11,000 years ago. The scientists have also concluded that the boost in carbon dioxide may have even prevented Earth from falling into another ice age.

This is one way to answer the question why does the Earth, when it appears so firmly in the claws of an ice age, start to warm? Most scientists agree that carbon dioxide played the crucial role in warming of the planet but up to this point scientists didn't have sound explanations in explaining the early warming in the southern hemisphere, where glaciers in Patagonia and New Zealand were melting before carbon dioxide levels rose significantly.

There were some scientists that supported a theory that a change in ocean currents, triggered by the freshening of the North Atlantic, caused this early warming. But computer models using ocean circulation to explain the rapid warming in the south have been unable to recreate the large temperature jumps seen in the paleoclimate record. Now, with the evidence for shifting southern hemisphere westerlies, the rapid warming looks to be finally explained.