Solving an ice age mystery

People who study the behavior of Earth's climate have been pondering a mystery of the last ice age. Why did the size of the Northern ice sheets fluctuate so dramatically, pushing global sea level up and down? 

Many suspected a great cause for such great effects, such as changes in Earth's orbit, but the timing didn't seem right.  Now researchers report that they have found the surprising answer in minor changes in one small place -- the narrow ocean passage between Alaska and Russia known as the Bering Strait.

These ice age events may sound like ancient history, but this is a hot topic for researchers trying to understand the climate system's natural ability to make big changes abruptly.  Climate is on the move, and researchers look to changes in the past for clues to how it might react to higher concentrations of greenhouse gases caused by modern industrial activity.

"Even small processes, if they are in the right location, can amplify changes in climate around the world," noted Aixue Hu, a scientist at the National Center for Atmospheric Research and leader of the international study team. 

The Bering Strait, just 50 miles wide, is the Northern Hemisphere's main gateway -- by way of the Arctic Ocean -- between the Pacific Ocean and its relatively fresh water and the saltier North Atlantic.

Using new computing power and a model developed at NCAR, the team ran simulations that revealed a distinct pattern of events that lasted 70,000 years.  Reporting in this week's edition of the journal Nature Geoscience, the researchers describe a circular process that goes like this:

• Changes in Earth's orbit cooled the climate, causing ice sheets to expand over time. Trapping more water in ice caused sea levels to fall as much as 100 feet, and eventually the flow through the Bering Strait was choked off by a land bridge between Asia and North America.
• This changed the speed of ocean currents which transport heat into the North Atlantic from the Tropics, like a great conveyor belt.  When the Pacific flow shut off, the water of the Atlantic  became saltier, and heavier, so it dropped more quickly to the ocean's depths, speeding up the conveyor. 
• The Northern Hemisphere warmed up, as much as 3 degrees Fahrenheit, enough to cause the ice sheets of North America and Greenland to melt and shrink over thousands of years -- raising sea levels again, reopening the Bering Straits, pushing Pacific water back into the North Atlantic, slowing the conveyor, cooling the Atlantic, and so on…

The map, courtesy of Nature, modified by NCAR, shows the effect on ocean temperatures and ice sheets when the Pacific's flow through the Bering Strait was closed by fallen sea level.

The pattern fell apart about 34,000 years ago, scientists say. At that point in its 95,000-year orbit, Earth was so far from the Sun at certain times that ice sheets continued to grow even when the Bering Strait was closed.  By 10,000 years ago, Earth had moved closer to the Sun during the northern winter, causing the ice sheets to shrink back, reopening the strait and leading to the stable climate that cradled the rise of civilization.