Great Lakes Environmental Assessment and Mapping Project

Warming water temperatures

Warming air temperatures in the Great Lakes region will bring about warmer water temperatures. Summer surface water temperatures appear to be increasing across the Great Lakes basin.

  • Larger temperature increases have been observed in the upper Great Lakes relative to the lower Great Lakes.
  • Summer water temperatures appear to be warming at a faster rate than mean annual air temperatures in the upper Great Lakes. This is likely due to an earlier onset of summer stratification, which may last longer and become established at shallower depths, and to the decreasing duration and extent of winter ice cover.1,2


Model projections predict that water surface temperatures between 2000 and 2100 will warm at rates ranging from 0.37-0.93°C per decade in Lake Superior to between 0.20 and 0.60°C per decade in Lake Erie.3 These temperature changes are expected to:

  • Affect the metabolism and possibly the growth rates of ectothermic organisms, especially fish.
  • Alter the ranges and abundances of many species due to changes in thermal habitat.
  • Affect the timing of seasonal events such as spring blooms of algae and the onset of fish spawning.
  • Promote the spread of nuisance algae and invasive species.


Note that some environmental alterations caused by climate change have the potential to be beneficial for some species. For example warmer water temperatures likely will increase ecosystem productivity unless nutrients or other factors become limiting, and may expand the thermal habitat available to some native fishes.


Mapping warming water temperatures as a Great Lakes stressor

  • Daytime water surface temperature data from AVHRR satellite imagery were obtained from the NOAA CoastWatch program for the warmest three summer months (July 1-September 30) in 1994-2010.
  • We computed annual mean temperatures for each pixel during this three month window.
  • To calculate a water warming index, we used least squares linear regression (relating mean surface water temperature to calendar year); the slope of this regression line represents the average summer temperature change per year during the warmest period of the year.

Spatial distribution of warming water temperatures in the Laurentian Great Lakes (Inset: Western Lake Superior).


Data Sources: 

1. Austin, J.A. and S.M. Colman. 2007. Lake Superior summer water temperatures are increasing more rapidly than regional air temperatures: A positive ice-albedo feedback. Geophys Res Lett 34(6).
2. Dobiesz, N.E. and N.P. Lester. 2009. Changes in mid-summer water temperature and clarity across the Great Lakes between 1968 and 2002. Journal of Great Lakes Research 35(3):371-384.
3. Trumpickas, J., B.J. Shuter, and C.K. Minns. 2009. Forecasting impacts of climate change on Great Lakes surface water temperatures. Journal of Great Lakes Research 35(3):454-463.