Great Lakes ‘seasons’ may reflect a warming trend

Scrutinizing a 139-year record of Great Lakes water levels, a UW–Madison scientist has discovered a dramatic shift in the seasonal changes in water levels on the Great Lakes.

The finding, reported here today, May 24, at a meeting of the International Association of Great Lakes Research by UW–Madison climatologist John D. Lenters, is further evidence that the effects of global warming on natural systems could be far reaching and significant.

“The bottom line is that over this 139-year period, the annual rising and falling of Lakes Ontario and Erie has gotten earlier” by about a month, Lenters says describing results of an analysis of long-term trends in Great Lakes water levels.

The findings, Lenters says, also show that the range of Lake Ontario’s “annual cycle” increased from 17 to 22 inches, a change in volume equivalent to 90 billion cubic feet of water. While Lake Erie does not show the same increase, the one month early arrival of seasonal high and low water levels mirrors that of Lake Ontario.

In the Great Lakes, explained Lenters, there is an annual ebb and flow of lake levels influenced by such things as precipitation, snowmelt and evaporation over the Great Lakes basin. In the spring and summer, lake levels rise reflecting such things as precipitation and spring snowmelt. In the fall and winter, lake levels recede as a result of evaporation of the relatively warm lake water.

These shifts, says Lenters, are essentially hydrological representations of the seasons, and “what I am finding is a shifting of the seasons.”

These shifts are independent, Lenters says, of annual variability in lake levels that may reflect, for example, a drought year, or a year when rainfall exceeds normal precipitation averages.

“At this time, the most likely explanation for the observed trends appears to be earlier spring snowmelt in association with higher springtime temperatures in the Great Lakes region,” Lenters says. “Climate is almost definitely responsible, but exactly how it is responsible is unknown.”

Lenters’ analysis was made using records of monthly mean lake levels from 1860 to 1998 from four stations around the Great Lakes, including stations along Lakes Superior, Huron, Ontario and Erie. Lake Michigan is included in the study as part of Lake Huron since the two lakes are hydraulically connected.

Large shifts in the water cycles of Lakes Superior and Michigan-Huron were also found, but for fewer months of the year. The result is a different and less dramatic seasonal shift for those lakes, says Lenters.

“It is not clear why Lakes Superior and Michigan-Huron are behaving differently, but it may be related to differences in regional climate, or the fact that Erie and Ontario are the furthest downstream lakes.

“If warming continues, we may begin to see the same consequences in Lakes Superior, Michigan and Huron,” he says. “For example, following the warm El Niño winter of 1997-1998, all five Great Lakes reached their annual maximum nearly two months earlier than normal.”

It is likely that the changes observed in the lakes are part of a larger systemic change spurred by increased levels of carbon dioxide in the atmosphere and resulting warming trends, according to the Wisconsin climatologist. Similar long-term shifts in lake ice and river flow in the Great Lakes and Upper Mississippi basins have already been observed by scientists.

Lenters is a staff scientist in the Climate, People and Environment Program of the UW–Madison Institute for Environmental Studies. His work was supported by a grant from NASA’s Upper Midwest Regional Earth Science Application Center.