Annual winter maximum ice cover trend on the Great Lakes since the 1970s

A new <a href="https://www.climate.gov/news-features/featured-images/great-lakes-ice-cover-decreasing-over-last-40-years" target=n>report</a> from NOAA's climate website indicates that there has been a slight downward trend in the annual maximum extent of Great Lakes ice cover since the 1970s.

Despite the trend, the annual maximum extent remains highly variable and can still be dramatically different from one year to the next.

<img src="https://vortex.accuweather.com/adc2004/pub/includes/columns/climatewx/2018/590x236_07111900_icecover_avg_1973_2018.png"/>

Lake Erie typically has the highest percentage of ice cover during a given season due to the shallow nature of the lake.

The dashed line on the graph below shows the longer-term trend, which is just slightly downward.

<img src="https://vortex.accuweather.com/adc2004/pub/includes/columns/climatewx/2018/590x259_07111905_screen-shot-2018-07-11-at-3.04.06-pm.png"/>

Breaking it down by individual lakes, the researchers found that lakes Superior, Huron, Erie and St. Clair are losing ice cover more quickly than others over the period.

Influences from man-made climate change and natural climate influences such as the ENSO, AMO, NAO and the PDO are likely having an impact on the long-term trend and the large year-to-year swings of extent that we see in the chart above.

The report also notes that less ice on the Great Lakes during the winter means more open water and a higher risk for heavy lake-effect snow, as cold air crossing the open water is able to pick up moisture and deposit it in narrow bands of heavy snow downwind of the lakes. Less ice can also lead to greater evaporation from the lake surface, leading to lower lakes levels.

The highest annual ice extent maximum for the Great Lakes as a whole was 94.7 percent back in 1973. The lowest was 11.9 percent in 2002.