Rock Doc

From time to time I give public talks on climate change – those large-scale changes geologists have been studying since the 1830s. At those talks I’m often asked a basic question about climate that, until now, has stumped scientists. Here’s the background.

In the 1830s, a Swiss naturalist named Louis Agassiz started promoting the idea that Europe had once been enveloped in a cold time in which large areas had been covered in glacial ice. He called that interval “the Ice Age.”

Working in this country in later decades, geologists studying glacial debris and soil layers came up with the idea that there had really been multiple episodes of extreme glacial advances. By 1900 most geologists agreed there had been at least four bitter intervals during which massive glaciers had covered Canada, with a sheet of ice extending down into the upper Midwest and New England.

Today, geologists believe there have been numerous cold times during the past 2.5 million years. Those long, bitter intervals have been separated by milder times like the present. The current warm interval has now lasted about 10,000 years. It’s really no different from the previous warm times except that human civilization has grown up within it.

But what triggered the start of the Ice Age? That’s the question I’m often asked by members of the public. After all, most of Earth’s history has been much warmer than the present and not marked by periodic advances of giant glaciers.

A team of researchers recently put forward a hypothesis that addresses the question of what may have started the Ice Age. They studied wind-blown dust in north central China, near the Tibetan plateau. That dust reflects changes in temperature and monsoons.

The idea coming out of the research is that the salinity of the Pacific Ocean was changed when North and South America were joined by the creation of the land bridge that now links them. The salinity change created more sea ice, which, in turn, led to changes in wind patterns, with intensified monsoons. Finally, the new wind and rain regime led to increased snowfall at high latitudes – and thus were born the massive glaciers geologists have long believed in.

Thomas Stevens of the University of London was one of the researchers who recently put forth the new work.

“Until now, the cause of [the Ice Age] had been a hotly debated topic,” Stevens told ScienceDaily. “Our findings suggest a significant link between ice sheet growth, the monsoon, and the closing of the Panama Seaway, as North and South America drifted closer together.”

Once the Panama region took its present shape, a feedback cycle in climate was established. More sea ice promoted more precipitation of snow, creating the conditions for the growth of massive glaciers in the northern parts of our hemisphere.

If the new hypothesis holds up, it will address one question about geologically recent climate change on Earth. And it’s another example of how numerous factors influence climate. In this case, a dash of plate tectonics moving land masses closer together led to climate changes half a world away. Or so some now think.

Dr. E. Kirsten Peters, a native of the rural Northwest, was trained as a geologist at Princeton and Harvard. This column is a service of the College of Agricultural, Human, and Natural Resource Sciences at Washington State University.