Expect the Unexpected: Cold Snaps, Heat Waves, and Extreme Weather

Frigid winter. Cold spring. Wet and cool August. Warming climate? When the conditions you experience extravagantly defy the AVERAGE expectations described by climate experts, remember that climate VARIABILITY might also be changing.

Averages make communication convenient, but variability (the range of possible outcomes) may be more important to talk about when it comes to climate. As an example, although the annual average (mean) air temperature on the western shores of Lake Superior is a cool 38 °F (3.3 °C), Duluthians need to be prepared to dress for a tropical 95 °F (35 °C) and a bone-chilling -25 °F (-32 °C).

Precipitation, streamflow, groundwater levels, temperature, and humidity (a.k.a. hydrometeorological variables) are often expressed as annual or seasonal means. As the graph shows, a mean can stay constant as variability changes and that variability can remain consistent even if the mean changes. Climate change can take the form of a trend in mean value (i.e. longer summer growing season), an alteration of variability (i.e. extreme precipitation and droughts), or both at the same time. Some climatologists suggest that wildly swinging highs and lows, and extreme precipitation and droughts will be the new norm around Lake Superior.

Schematic showing the effect on extreme temperatures when (a) the mean temperature increases, (b) the variance increases, and (c) when both the mean and variance increase for a normal distribution of temperature.3

Linking specific events, like a particularly cold month or Lake Superior's near-record low water level in 2007, to the larger climate change picture is difficult without the perspective of time and an understanding of confounding factors. For example, experts suggest that in its geologically brief history Superior has exhibited two lake level cycles, one lasting about 160 years and the other about 30.1 The cycles don't operate with clockwork precision and could have synchronously hit their low points in 2007. Interestingly, torrential rains in September and October followed the summer drought of 2007; about 10 inches of rain fell (25 cm), shocking hydrologists and the public alike by abruptly raising Superior to a near normal level.2

A day, a season, and even a year are little more than dots in a very long line of climate data. The scattered dots produce the variance that informs climate modeling. Record water levels, torrential rain, and extreme temperatures might be anomalies. But, perhaps it's wise to start expecting the unexpected.

To see Minnesotaís on-the-record weather extremes check out: Wikipedia: List of Minnesota weather records.


1 Thompson, T. 2007. Reconstructing past lake levels in the Lake Superior Basin. Presentation at the Making a Great Lake Superior Conference. www.seagrant.umn.edu/superior2007/presentations/pdf/Thompson-Reconstructing-Past-Lake%20Levels.pdf

2 U.S. Army Corps of Engineers, Detroit District. 2008. Great Lakes Update. Frequently Asked Questions on Current Water Levels. Vol. No. 173. http://www.lre.usace.army.mil/_kd/go.cfm?destination=ShowItem&Item_ID=6291

3 Intergovernmental Panel on Climate Change. 2001. Climate Change 2001: The Scientific Basis. IPCC Third Assessment Report. Cambridge Univ. Press, 881 pp. www.grida.no/publications/other/ipcc_tar/

For additional information, read: If itís that Warm, How Come itís so Darned Cold? An Essay on Regional Cold Anomalies within Near-Record Global Temperature by climate scientists James Hansen, Reto Ruedy, Makiko Sato, and Ken Lo, Jan 27, 2010. www.columbia.edu/~jeh1/mailings/2010/20100127_TemperatureFinal.pdf

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