The annual timing of the formation and disappearance of ice in aquatic systems represents a valuable proxy for climate change and provides an early warning for potential responses that may occur within these ecosystems. The strength as a climate proxy includes the broad spatial distribution and regional, continental, and global abundance of sites, the annual resolution of the data, data records that are often longer than other direct measures of climate such as air temperature, and the relative ease and precision of measuring freeze and breakup dates. Taking advantage of the opportunities provided by ice data, a group of scientist led by NTL researchers assembled and analyzed 39 records of ice-on and ice-off dates across the Northern Hemisphere that were at least 100 years long to determine if any long-term trends were apparent. The analysis revealed later ice on dates and earlier ice off dates for 38 of the 39 records. The date of freezing decreased in a linear fashion at a rate of 5.8 days/100 years later, while the breakup date occurred 6.5 days sooner per 100 years earlier. Also apparent was an increase in inter-annual variability in ice phenology dates in the last half of the 20th century. The strength of these trends vary across the Northern Hemisphere and over the 100-150 years of record. In the North American Great Lakes region, the rate of ice change has been particularly rapid over the past 25 years, and many lakes are unexpectedly warming at a rate that is faster than atmospheric warming temperature increases.
These long-term trends in lake and river ice phenologies provide evidence that freshwater ecosystems are responding to climate shifts. The increases are generally consistent with scenarios for greenhouse gas -- forced climate warming, but recent evidence suggests that other global-scale atmospheric changes, including the recent trend of "global brightening" (increased radiation of the earth's surface) may also influence regional and continental patterns of ice formation and break-up.
Regardless of the specific mechanism, the cross-site synthesis of long-term ice records drew sharp attention to global-scale changes in lakes and the role of lakes as sentinels of climate change. Once attention had been drawn to the ongoing responses of lakes to climate forcings, subsequent investigations have identified a range of ecological consequences of lake warming and modification of the timing and duration of ice cover, including increases in spring algal blooms and alteration of lake chemistry. Ultimately, changes in ice phenology will influence provisioning and regulating ecosystem services. Since ice phenology is one of the few freshwater variables that is directly driven by climate and that is broadly available and easily measured, the timing of ice-on and ice-off serves as a valuable indicator of the consequences of climate change on freshwater ecosystems on local, regional and continental scales.