The relatively warm winters of the past several years set the stage for the high Lake Michigan water surface temperatures recorded in July, lake experts at the National Oceanic and Atmospheric Administration said today.

"Based on the last six years, both 1998 and 1999 are warmer than average," said Michael McCormick, an oceanographer at NOAA's Great Lakes Environmental Laboratory.

As a result of the warmer temperatures the Great Lakes are at a 30-year low, three to nine inches below their long-term averages, because of lack of rain and mild winters. While providing wider beaches for swimmers and those living along the shore, lower water levels are causing problems for some boat owners who need to seek deeper water for docking and recreational boating.

Long-term temperature records in the Great Lakes exist at various municipal water intake points. However, only recently has it been possible to obtain lakewide measurements of water temperatures. NOAA's Great Lakes Environmental Laboratory (GLERL) in Ann Arbor, MI, has developed a Great Lakes Surface Environmental Analysis (GLSEA) that uses satellite-derived water temperature data.

Based on 50 years of modeled data which reconstructs temperature data based on meteorological records, July 1999 is the third warmest year of the last 50 years for Lake Michigan, following 1998 and 1995. The preceding winter temperatures of those two years were also warmer than normal. Inferences on climate changes in the Great Lakes water temperature require much longer data records, the scientists say.

Prolonged heat waves and calm conditions can heat up the very near surfaces and near shore waters. For example, conditions during the past week have caused the average near surface temperatures in Lake Michigan to exceed those measured at any time during the last six years. Further modeling analyses suggest that all the Great Lakes have shown higher-than-normal monthly water surface temperatures for the past 18 months.

Because of the large volumes of water and extreme depths, the Great Lakes' lake-wide temperatures respond slowly to day-to-day shifts in air temperature and are more representative of seasonal or longer changes in weather.

The higher than average water temperatures are likely to increase the late summer and fall evaporation, which will further reduce seasonal water levels, said Frank Quinn, senior hydrologist at GLERL.

Lower levels also mean that the lake freighters that carry iron ore, coal, and limestone between Great Lakes ports such as Duluth, South Chicago, and Toledo cannot travel fully loaded because of the low water levels in the harbors and connecting channels.

According to the Lake Carriers Association, an organization that represents U.S. flag vessel operators on the Great Lakes, "Vessels working the Great Lakes forfeit anywhere from 70 to 270 tons of cargo for each one inch reduction in loaded draft." The association reports an 8.6 percent decrease in shipments of iron ore, coal, and stone in April - the most recent figure available - compared with the same period last year.

The outlook is for lower levels during the coming fall and winter because of the normal seasonal decline in water levels, Quinn said.

NOAA's mission is to describe and predict changes in the earth's environment and to conserve and manage wisely the nation's coastal and marine resources.