Climate Change Indicators in the United States: Great Lakes Water Levels and
Temperatures - www.epa.gov/climate-indicators - Updated August 2016
Great Lakes Water Levels and Temperatures
This indicator measures water levels and surface water temperatures in the Great Lakes.
Background
The Great Lakes, which are Lake Superior, Lake
Michigan, Lake Huron, Lake Erie, and Lake
Ontario, form the largest group of freshwater
lakes on Earth. These lakes support a variety of
ecosystems and play a vital role in the economy
of the eight neighboring states and the Canadian
province of Ontario, providing drinking water,
shipping lanes, fisheries, recreational
opportunities, and more.
Water level and water temperature are two
important and interrelated indicators of weather and climate change in the Great Lakes. Water level
(the height of the lake surface above sea level) is influenced by many factors, including precipitation,
snowmelt runoff, drought, evaporation rates, and people withdrawing water for multiple uses. Water
temperature is influenced by many factors, too, but most directly by air temperature.
In recent years, warmer surface water temperatures in the Great Lakes have contributed to lower water
levels by increasing rates of evaporation and causing lake ice to form later than usual (see the Lake Ice
indicator), which extends the season for evaporation.1 Lower water levels in the Great Lakes forced
ships to reduce their cargo tonnage by 5 to 8 percent between 1997 and 2000, which increased shipping
costs. Lower water levels can also affect water supplies, the usability of infrastructure such as docks and
piers, and shoreline ecosystems. These types of disruptions from low water levels are expected to
continue as the climate changes.2
Another possible effect of warmer water, reduced ice cover, and increased evaporation is a
corresponding increase in precipitation over nearby land, especially "lake effect" snow (see the Snowfall
indicator).3 Rising water temperatures are also expected to expand the ranges of and give new
advantages to some invasive species such as the zebra mussel, and to encourage the growth of certain
waterborne bacteria that can make people ill.4,5
About the Indicator
This indicator analyzes water levels and surface water temperatures in the Great Lakes. Water levels are
recorded by gauges along the shore of each lake, some of which have been operated since the 1800s.
Pre-1918 data came from one water level gauge per lake. Data since 1918 have come from a designated
set of gauges in each lake. Figure 1 shows annual water level anomalies, or differences, in feet
oEPA
Direction of flow
Lake
Superior
\
Lake
Michigan
/ Lake /
Huron /
C
*
Lake
Ontario
/
Lake
Erie
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Climate Change Indicators in the United States: Great Lakes Water Levels and
Temperatures - www.epa.gov/climate-indicators - Updated August 2016
compared with the average water levels in each lake from 1860 to 2015. Lakes Michigan and Huron are
combined because they are connected at the same water level.
Surface water temperatures are measured by satellites. Figure 2 shows annual average temperatures
over the entire surface of each lake, along with the pattern of daily temperatures over the course of the
year. This figure's data begin in 1995, which was the first year with complete satellite data for all five
lakes.
• Water levels in the Great Lakes have fluctuated since 1860. Over the last few decades, they
appear to have declined for most of the Great Lakes (see Figure 1). The most recent levels,
however, are all within the range of historical variation.
• Since 1995, average surface water temperatures have increased slightly for each of the Great
Lakes (see Figure 2).
• Recent increases in water temperature have mostly been driven by warming during the spring
and summer months (see Figure 2). These trends could relate in part to an earlier thawing of
winter ice (see the Lake Ice indicator).
Key Points
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Climate Change Indicators in the United States: Great Lakes Water Levels and
ItiF C«rjrm. Temperatures - www.epa.gov/climate-indicators - Updated August 2016
Figure 1. Water Levels of the Great Lakes, 1860-2015
Lake Superior
T
v TO
I860 1880 1900 1920 1940 1960 1980 2000 2020
4
3
2
1
0
-1
-2
-3
-4
1860
4
3
2
1
0
-1
-2
-3
-4
1860
1880 1900 1920 1940 1960 1980 2000 2020
1880 1900 1920 1940 1960 1980 2000 2020
1860 1880 1900 1920 1940 1960 1980 2000 2020
Year
This figure displays how water levels in each of the Great Lakes have changed since 1860. For each year,
the shaded band shows the range of monthly average water levels, and the line in the middle shows the
annual average. The graph uses the 1981 to 2010 average as a baseline for depicting change. Choosing a
different baseline period would not change the shape of the data over time. Lakes Michigan and Huron
are shown together because they are connected at the same water level.
Data source: NOAA, 2016s
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Climate Change Indicators in the United States: Great Lakes Water Levels and
Temperatures - www.epa.gov/climate-indicators - Updated August 2016
Figure 2. Surface Water Temperatures of the Great Lakes, 1995-2015
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Lake Superior
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Climate Change Indicators in the United States: Great Lakes Water Levels and
Temperatures - www.epa.gov/climate-indicators - Updated August 2016
Indicator Notes
While climate change influences water levels, human activities such as dredging can also play a role. For
example, the St. Clair river opening was enlarged in the 1910s, 1930s, and 1960s, contributing to greater
outflows from Lakes Michigan and Huron.8 Similarly, natural year-to-year variability and other factors
such as human use and wastewater discharges can influence water temperatures.
Data Sources
Water level data were provided by the Canadian Hydrographic Service and the National Oceanic and
Atmospheric Administration's Center for Operational Oceanographic Products and Services, and can be
downloaded from: www.glerl.noaa.gov/data/dashboard/data. Surface water temperature data were
provided by the National Oceanic and Atmospheric Administration's Great Lakes Environmental
Research Laboratory (satellite data at: https://coastwatch.glerl.noaa.gov).
1 Gronewold, A.D., V. Fortin, B. Lofgren, A. elites, C.A. Stow, and F. Quinn. 2013. Coasts, water levels, and climate
change: A Great Lakes perspective. Climatic Change 120:697-711.
2 Posey, J. 2012. Climate change impacts on transportation in the Midwest. U.S. National Climate Assessment,
Midwest Technical Input Report.
3 Burnett, A.W., M.E. Kirby, H.T. Mullins, and W.P. Patterson. 2003. Increasing Great Lake-effect snowfall during
the twentieth century: A regional response to global warming? J. Climate 16:3535-3542.
4 Rahel, F.J., and J.D. Olden. 2008. Assessing the effects of climate change on aquatic invasive species. Conserv.
Biol. 22(3):521—533.
5 Kanoshima, I., L. Urmas, and J.-M. Leppanen. 2003. The influence of weather conditions (temperature and wind)
on cyanobacterial bloom development in the Gulf of Finland (Baltic Sea). Harmful Algae 2:29-41.
6 NOAA (National Oceanic and Atmospheric Administration). 2016. Great Lakes water level observations.
Accessed April 2016. www.glerl.noaa.gov/data/dashboard/data.
7 NOAA (National Oceanic and Atmospheric Administration). 2016. NOAA CoastWatch, Great Lakes node.
Accessed February 2016. https://coastwatch.glerl.noaa.gov/.
8 Quinn, F.H. 1985. Temporal effects of St. Clair River dredging on Lakes St. Clair and Erie water levels and
connecting channel flow. J. Great Lakes Res. ll(3):400-403.
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