Did you know that freshwater lakes and rivers are unevenly distributed across the Earth? Most freshwater lakes and rivers are in the Northern Hemisphere in regions where air temperatures drop below the freezing point of 32 degrees Fahrenheit, forming ice. Observations of lake freezing and thawing patterns began before scientists started recording and studying them. Early observations were conducted for religious and cultural reasons, for curiosity, and for practical reasons-such as transportation over ice or open water. Ice plays an important role for lakes because it determines how much heat is absorbed by water and how much water evaporates into the atmosphere. Ice increases a lake's ability to reflect solar radiation (albedo), controlling water temperature. Ice is also an important component of aquatic ecosystems where animal and plant species have adapted their life cycles to include freezing conditions.
Changes in ice phenology-the cyclical timing of events from freezing to thawing-can be used as indicators of climate change impacts on freshwater ecosystems. Ice phenology is influenced to a large extent by variations in air temperatures. Warmer air temperatures and warmer winters reduce the amount and duration of lake ice cover.
Scientific studies show that changes in freeze-up and break-up dates correspond to an increase in air temperature over the past 150 years. In the Northern Hemisphere, there is evidence that lake freeze-up is occurring an average of 5.8 days later per 100 years, and ice break-up is occurring 6.5 days earlier per 100 years. In the United States, the number of days with temperatures below freezing decreased by four days per year over the period from 1948-1995. A study conducted in small inland lakes in the Great Lakes Region shows a warming trend during the cold season from 1916-2007; not only is ice duration decreasing, but ice thinning is occurring at a rate of 0.003 inches (0.01 cm) per decade since the 1970s.
Changes in ice phenology can lead to warmer lake temperatures, increases in lake heat storage and delayed ice formation. The nutrient and chemical composition of lake water also changes with warmer winters, including changes in water color and concentrations of calcium and magnesium. Ice cover reductions affect lake ecosystems. Ice cover usually reduces the amount of oxygen that reaches the bottom of lakes, killing off some species of large bodied fish. This helps maintain the balance between large fish and their prey species. Reduced ice cover results in more oxygen reaching the bottom of the lake - this means that fewer large bodied fish die over the winter and their demand for food will be greater, disrupting the balance in lake ecosystems. Reduced ice cover can also have positive effects, for example, by extending the open water shipping season. But, vessels have to reduce their cargo load because water levels tend to drop due to increased evaporation.
You can see the changes in the duration of ice cover, date of first freeze and date of first thaw for eight U.S. lakes from 1850-2010 below. Visit EPA's Climate Change Indicators in the United States report for more information.
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