Ice Age Ice Sheets & Western US Climate

During the last ice age some 20,000 years ago, the climate of the western US would have been unrecognizable to us. An ice sheet covered most of Canada, extending into the northern United States. The Southwest was wet and lush, and much of western Utah was covered by a massive lake. The climate was surprisingly different then, but what is perhaps truly surprising is how quickly that foreign world changed into something like the one we know today.

A recent study published in Geophysical Research Letters clocks that past rate of climatic change by examining the mechanisms driving it. According to the study’s authors, Juan Lora and colleagues, the shift in North America’s ancient climate occurred within less than 1,000 years. Geologically speaking, that’s fast.

To arrive at their answer, the researchers looked at how those massive glaciers in the north might have shifted the storm track. It’s believed that the glacier’s role worked something like this:

During the last ice age, thick ice sheets in the North forced the jet stream, or storm track, southward. The storm track’s southward pull carried precipitation from the Pacific Ocean toward what are now the Southwest states, special-delivering the wet winter weather we grudgingly tolerate here in the Pacific Northwest to the states of California, Nevada, and Arizona.

Since the last ice age, North America’s massive ice sheets have, of course, melted. As a consequence, the storm track has shifted north, creating the dry Southwest and wet Pacific Northwest we know today.

Paleoclimatologists—researchers of the Earth’s paleoclimate, or ancient climate—have known this for a while. What isn’t as well known is how fast these sorts of shifts occurred. But that’s changing. A growing body of literature is now pointing to the conclusion that these paleoclimate shifts occurred in relatively short order, geologically speaking that is. Reenter Lora and colleagues’ recent study.

To track the relationship of those northern glaciers to the southerly storm track, the researchers used a coupled climate model to simulate the glacial retreat. This allowed them to examine the retreat’s effect on atmospheric circulation and estimate when the climate of the western US started to look more like it does today.

The researchers estimate an abrupt reorganization of the climate occurred around 14,000 years ago, when their simulations started to show ice sheet volumes dropping dramatically. The ice sheet’s effect on the atmosphere happened somewhat circuitously.

All that cold, melted freshwater from these ice sheets flowed into the Pacific, lowering the temperature of the ocean’s surface water. That freshwater was not only colder than the salty seawater, it was lighter too. As a result, the freshwater floated on top of the seawater, creating a surface layer of cold water. Because sea surface temperatures are a strong driver of wind patterns, this influx of cold freshwater caused a reorganization of North Pacific atmospheric circulation that altered wind patterns, weakened the low-pressure system off the Aleutian Islands, and set up a high-pressure system over the Great Basin.

These changes in circulation shifted the storm track nearly seven degrees (~800 km; 500 miles) to the North. And we all know the outcome: the Pacific Northwest became lush and the Southwest started drying out, setting up the climate we have today.

STUDY: Geophysical Research Letters

Citation: Lora, Juan M., Jonathan L. Mitchell, and Aradhna E. Tripati. “Abrupt reorganization of North Pacific and western North American climate during the last deglaciation,” Geophysical Research Letters, (2016). doi: 10.1002/2016GL071244.

Photo Caption: Mount Rainier and the Nisqually Glacier as seen from Nisqually Vista in the winter. (Photo Credit: Mount Rainier National Park, some rights reserved.)

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