Climate Variability and Change in the Great Lakes Watershed

2.0 CLIMATE VARIABILITY AND CHANGE  

IN THE GREAT LAKES WATERSHED

Has climate and hydrology in the Great Lakes watershed changed significantly during the 20th

century?  In what ways has it changed and by how much?  This chapter explores recent trends in

selected climatological and hydrological parameters.  Primarily, it draws from published

literature at the global and national scale and relates results to the Great Lakes region.  

Key trends include:

•

Mean annual air temperature increases; most warming occurs in winter and spring  

and the least in fall;

•

Minimum air temperature increases more than maximum air temperature;

•

Frost-free period lengthens and other temperature indices change;

•

Annual precipitation increases;

•

Ratio of snow to total precipitation decreases;  

•

Extreme precipitation increases in the United States and shows no trend in Canada;

•

Snow cover (depth, areal coverage, and duration) is reduced;

•

Both wet and dry periods increase;  

•

The onset of the spring melt (freshet) is earlier;  

•

Great Lakes water levels respond to climate variability but show no long-term

changes;

•

Dates for freezing of lakes are later and dates for ice off are earlier; and

•

Timing of phenological events is changing in North America and Europe.

2.1 DETECTING CHANGES

Large-scale processes such as population growth, land use change (agricultural expansion and

urbanization), and intensification of land uses confound the detection of changing climate and

hydrology.  Data quality is also an issue.  Changes in station location, instrumentation,

observing practices, urbanization, and exposure introduce inhomogeneities to the observations

that need to be corrected.  Canadian and American observing practices and methods of

Ňcorrecting” data can be dissimilar and differences are noted at the border.  Here we have

reported results from trend analysis using the longest period of record available with the most

homogeneous data.  

2.1.1  Air Temperature

Mean annual air temperature increases.  

The global mean temperature has increased 0.6 ±  0.2  Celcius over the 20th century.  Analysis of the

instrumental record since 1861 suggests that the warmest decade globally may have been the

1990s and 1998 may have been the warmest year (IPCC, 2001: 26).  Paleoclimate data for the

Northern Hemisphere corroborates this; it also indicates that the temperature increase of the 20th

century was the largest during the past 1000 years.

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