Warming ‘seesaw’ turns extra sunlight into global greenhouse

Earth's most recent shift to a warm climate began with intense summer sun in the Northern Hemisphere, the first pressure on a seesaw that tossed powerful forces between the planet's poles until greenhouse gases accelerated temperature change on a global scale. Climate scientists, led by a group from the University of Wisconsin-Madison, used computer models to provide the strongest support yet to a case made nearly 90 years ago by mathematician Milutin Milankovitch. "Milankovitch actually calculated the changes in insolation — the amount of heat coming into the Earth system from sunlight — in the Northern Hemisphere by astronomy," says Feng He , a scientist at UW-Madison's Center for Climatic Research. "He did that before World War I, and that equation is still fairly accurate, even though he didn't have anything like the computing resources we do." Milankovitch's work showed glacial rise and decline tied to cycles of summer sun — as opposed to winter conditions, which were intuitively expected to hold sway. A glacier that could survive summer without substantial ice loss was well set to grow even during sunny winters. The importance of summer sun has been confirmed repeatedly by studies of physical climate records, largely ice and sediment cores taken from glaciers and the bottom of oceans around the world. But the progression of warming — from north to south, or south to north — during the last great glacial melt about 20,000 years ago is hard to follow, and has proven a bone of contention among those studying ancient climate.