AUDIE CORNISH, HOST:
During the late summer and early fall, the water level on the Great Lakes usually drops several inches. This year, three of those lakes, Superior, Michigan and Huron, have seen the opposite happen - rising water levels. Joining us to talk about why that is is Drew Gronewold. He's a hydrologist at the National Oceanic and Atmospheric Administration. He works in the Great Lakes Environmental Research Laboratory. Welcome to the show.
DREW GRONEWOLD: Thanks, Audie.
CORNISH: Help us understand how much water we're talking about and, specifically, where you think the water's come from.
GRONEWOLD: Sure. So in addition to the lakes - Lake Superior and Lake Michigan-Huron - being so large, when we look at the water level rise over the past year and a half or so, we're talking about Lake Superior between - over the past 18 months or so, has gone up almost three quarters of a meter, which is quite a lot of rise, you know, on the order of a foot and a half to two feet. And the same is true for Michigan-Huron. And that is really an awfully lot of water when we're considering that that's a foot and half of water over the earth's largest freshwater surfaces. And the water came primarily from precipitation. But what's interesting is that it stayed in the system longer than we expected it to, in part because it's been cooler than average, and there's been less evapotranspiration - that is, transfer of water out of the soil by processes that draw it out of the soil through plants and out of the soil - but also less evaporation off the lakes themselves. And as a result, more water has stayed in the system.
CORNISH: So is there any sense that this might have happened anyway, that it's part of the natural lifecycle of the lakes?
GRONEWOLD: A lot of the ups and downs of water levels in the past are very closely connected with changes in precipitation. Like any freshwater system, you get more rain, water levels go up. You get less rain; they tend to go down. Over the past 20 years, there have been two changes in water levels that are more closely associated with changes in temperature and evaporation than necessarily precipitation. One of those was in the late 1990s, when water levels actually went down by about three feet over the course of just a couple of years. And they stayed low for 15 years because water temperatures were high, and evaporation rates stayed very high. And then, just a couple of years ago, a relatively unpredicted phenomenon occurred, which was the deformation of the Arctic polar vortex - or the polar vortex phenomenon, as a lot of people refer to it as - that caused a tremendous amount of cold air to locate right over the Great Lakes. And all of the sudden, we have cold water temperatures. We have unprecedented ice cover, and water levels start rising again. And what's important, Audie, is that these phenomenon are very hard to predict, yet they lead to a lot of water level variability.
CORNISH: In the end, what are the lessons here? Is there anything you're going to be looking for in terms of an effect beyond the Great Lakes region?
GRONEWOLD: Sure. Well, one of the things that we're learning about here, Audie, has to do with the types of changes that we're experiencing, whether it's regarding a coastline or whether it's regarding a large freshwater system, that the changes we're experiencing are dramatic. And they may be tied to larger climate change phenomena that are driving the system. You know, one of the things we often do is compare this to the phenomenon of sea level rise on the marine coasts and some of the water levels around New York City and other cities on the East Coast. The rate of change there is on the order of 10 inches to a foot per century. So we're talking here about 10,000 miles of coastline across the Great Lakes that are fluctuating on the scale, sometimes, of one to two feet every year or so. It's really a tremendous rate of change that we experience here.
CORNISH: Drew Gronewold, thanks so much for talking with us.
GRONEWOLD: No problem. Thank you, Audie.
CORNISH: NOAA's Drew Gronewald. He's also a professor of civil and environmental engineering at the University of Michigan. Transcript provided by NPR, Copyright NPR.