Across the northern United States, a popular tradition for many decades has been guessing the date of ice breakup ("ice-out") on local lakes. Not only is it a local news story and harbinger of spring, but ice-out has numerous practical ramifications as well. It marks the end of winter activities on the lake, such as ice fishing and travel across the ice, and the beginning of summer activities, such as open-water fishing.

In earlier days, ice-out was important for commercial activities such as log drives and steamship navigation. As a result, for many lakes ice-out data are available since the middle of the nineteenth century. Moosehead Lake, the largest lake in Maine, has continuous data since 1848; Sebago Lake in southern Maine has sporadic data since 1807. Areas outside New England also maintain iceout statistics. Minnesota has at least four lakes with more than 100 years of data, and the longest record is 139 years. Lakes Mendota and Monona, which flank the city of Madison, Wisconsin, have records extending back more than 150 years (Hodgkins, 2000).

Ice-out dates are quite variable from year to year and depend on air temperature, the flux of meltwater into the lake, and the discharge of the outlet stream. For example, the earliest and latest ice-out dates for Moosehead Lake differ by forty-five days. Nevertheless, scientists from the U.S. Geological Survey analyzed ice-out dates for New England lakes and found that the average date of ice-out was nine days earlier in 2000 than it was in 1850. Generally, ice-out data are tabulated in terms of day of the year (the Julian date) rather than the calendar date. For example, February 20 is Julian day 51. The data indicate generally steady ice-out dates until 1900, decreasing (earlier) dates until roughly 1950, a slight increase in ice-out dates until about 1970, and decreasing dates thereafter. Despite the wide variability from year to year, the overall pattern is consistent across numerous lakes.

There are many complexities in the use of ice-out data, largely due to variations in what defined "iceout." Sometimes a marker placed on the ice has been used to signal ice breakup (Hodgkins, 2002). In some places the marker was even a junked car (an environmentally dubious practice). In other places the criterion has been the ability to cross the lake by boat without being blocked by ice. Still other places have used visual estimates of ice cover. Such estimates can be quite subjective. It may take several days for ice to clear out, and ice may persist in restricted coves long after most of a lake is ice-free.

Unfortunately, it is not always clear what criterion was used for many early records. Also, ice-out data are extremely "noisy"--that is, the range in the data is much larger than the long-term change in average date. For example, for Maine's Moosehead Lake, the best-fit trend line for the ice-out data shows a nine-day decrease in ice-out day between 1849 and 2005, but the variation between earliest and latest ice-out is forty-five days. Even from year to year, the variations can be quite large: In 2001 ice-out occurred on day 124, dropping to day 110 in 2002 and increasing to day 127 in 2003.

Ice-out data for any individual lake are usually so variable that there is a significant possibility that the data, just by chance, happen to show a decrease over time. For example, if one flips a coin and keeps score of the difference between heads and tails, even though the flips are completely random there may be long runs where the difference increases or decreases steadily. However, when data from many lakes, especially those separated by large distances, show the same pattern, the statistical significance of the data becomes far greater. Taking all these considerations into account, ice-out data for lakes across the northern United States show a broad trend of earlier ice breakup. Climate change skeptics have not paid much attention to ice-out data. Their most common approach has been to use anecdotal data--that is, point to unusually late ice-out on some particular lake, rather than examine long-term trends.

References

1. Hodgkins, Glenn A., and Ivan C. James. "Historical Ice-Out Dates for Twenty-nine Lakes in New England." U.S. Geological Survey Open-File Report 02-34. Denver, Colo.: U.S. Geological Survey, 2002.

2. Hodgkins, Glenn A., Ivan C. James, and T. G. Huntington. "Historical Changes in Lake Ice-Out Dates as Indicators of Climate Change in New England, 1850-2000."

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