Connecticut State Council on Environmental Quality

2025 CEQ Annual Report


Water


Rivers, Lakes, and Estuaries                 Water of Long Island Sound               Swimming               Clamming and Oystering                 Drinking Water

The Warming and Rising Waters of Long Island Sound


   the sun, earth and a thermometer that identifies indicators that are affected by a warmer climate or those that affect the climate.

quick summary box indicating that the most recent year's data improved from the previous year, improved from the previous ten-year average and not applicable to meet goal.  

Water Temperatures

In 2025, the average annual surface and bottom water temperatures for Long Island Sound were 58.4 degrees Fahrenheit (°F) and 55°F, respectively.39   Annual variations in water temperatures and water levels in the Sound are less important than longer term trends. The longer-term trends for average annual surface and bottom temperatures of the Sound from 1995 through 2025 have been rising. While the long-term impact of warmer water in the Sound is unknown, species diversity and biomass remain high, although there has been a shift to more warm water tolerant species. 


 

Water Levels

The relative sea level trend (Station 8467150 Bridgeport) has increased by 3.43 millimeters/year based on monthly mean sea level data from 1964 to 2025, which is equivalent to a change of one foot, two (2) inches in 100 years.40  The Connecticut Institute for Resilience and Climate Adaptation (CIRCA) anticipates that sea level could be one (1) foot eight (8) inches higher than the national tidal datum in Long Island Sound by 2050.41  Sea level rise has multiple impacts on the Connecticut shoreline, including increased erosion rates, increased frequency and severity of flooding where property and infrastructure can be damaged, and loss of coastal wetlands and important habitat

Goal: While there are no goals for water temperature or sea level rise in Long Island Sound, it is assumed that a significant increase in temperature and water level is not a desired outcome.


Technical Note: *The vertical axis in the yearly average temperatures of Long Island Sound chart above has been shortened, beginning at 40.0°F rather than the customary zero. Surface and bottom water temperature data were incomplete for 2024 and 2025.

 

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39 DEEP; Long Island Sound Water Quality and Hypoxia Monitoring Program, personal communication from K. O’Brien-Clayton, January 27, 2026.
40 National Oceanic and Atmospheric Administration (NOAA), Tides and Currents, Relative Sea Level Trend 8467150 Bridgeport; tidesandcurrents.noaa.gov/sltrends/sltrends_station.shtml?id=8467150.
41Connecticut Institute for Resilience & Climate Adaptation (CIRCA), James O’Donnell, Sea Level Rise in Connecticut Final Report February 2019; circa.media.uconn.edu/wp-content/uploads/sites/1618/2019/10/Sea-Level-Rise-Connecticut-Final-Report-Feb-2019.pdf.