Lake and river ice cover

Ice break-up dates from selected European lakes and rivers (1835-2006) and the North Atlantic Oscillation (NAO) index for winter 1864-2006

Note: Time series of ice break-up dates from selected European lakes and rivers. Data smoothed with a 7-year moving average

Data source:

Benson, B. and Magnuson, J., 2000 (updated 2006). Global lake and river ice phenology database. In: Boulder, C.O., National Snow and Ice Data Center/World Data Center for Glaciology.Bauernfeind, E. and U.H. Humpesch 2001. Die Eintagsfliegen Zentraleuropas (Insecta: Ephemeroptera) Bestimmung und Ökologie. Verlag des Naturhistorischen Museums, Wien Austria.

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Past trends

An analysis of long (more than 150 year) ice records from lakes and rivers throughout the northern hemisphere by Magnuson et al. (2000) indicated that for a 100 year period, ice cover has been occurring on average 5.7 +/- 2.4 days later (+/- 95 % confidence interval), while ice break-up has been occurring on average 6.3 +/- 1.6 days earlier, implying an overall decrease in the duration of ice cover at a mean rate of 12 days per 100 years. These results do not appear to change with latitude, or between North America and Eurasia, or between rivers and lakes.
Changes in ice parameters mostly show trends that are in agreement with observed local temperature increases. Air temperature is the key variable determining the timing of ice break-up (Palecki and Barry, 1986; Livingstone, 1997).
A few longer time-series reveal reduced ice cover (a warming trend) beginning as early as the 16th century, with increasing rates of change after about 1850 (see Figure 1). The early and long-term decreasing trend in the ice break-up dates is the result of the end of the Little Ice Age, which lasted from about 1400 to 1900 (Kerr, 1999). In the 20th century, the effects of the North Atlantic Oscillation on the ice regime of European inland waters appear to be stronger than the effects of increasing temperatures.
Studying ice cover information from 11 Swiss lakes over the last century, Franssen and Scherrer (2008) found that ice cover was significantly reduced in the past 40 years, and especially during the past two decades.
Ice cover of lakes in southern Sweden is more sensitive to climate change than those in the north, where mean winter temperatures are below zero most of the winter. A study of 196 Swedish lakes along a latitudinal temperature gradient revealed that a 1 ^oC air temperature increase caused an up to 35 days earlier ice break-up in Sweden's warmest southern regions with annual mean air temperatures around 7 ^oC. It caused only about 5 days earlier break-up in Sweden's coldest northern regions where annual mean air temperatures are around - 2 ^oC (Weyhenmeyer et al., 2004; Weyhenmeyer, 2007). Ice break-up in Finland has also become significantly earlier from the late 19th century to the present time, except in the very north (Korhonen, 2006).

Projections

Future increases in air temperature associated with climate change are likely to result in generally shorter periods of ice cover on lakes and rivers. The most rapid decrease in the duration of ice cover will occur in the temperate region where the ice season is already short or only occurs in cold winters (Weyhenmeyer et al., 2004). As a result, some of the lakes that now freeze in winter and that mix from top to bottom during two mixing periods each year (dimictic lakes) will  potentially change into monomictic (mixing only once) open-water lakes with consequences for vertical mixing, deep-water oxygenation, nutrient recycling and algal productivity. This may lead to an alteration in the ecological status of normally ice-covered lakes in temperate regions.
Regional climate model projections for northern Germany, based on the IPCC high emissions SRES A2 and intermediate emissions B2 climate scenarios, imply that for the Muggelsee, the percentage of ice-free winters will increase from about 2 % now to more than 60 % by the end of the century (Livingstone and Adrian, submitted). In contrast, increases in mean annual air temperature are likely to have a much smaller effect on lakes in very cold regions (e.g. northern Scandinavia) until these also reach the threshold of having winter temperature close to zero.