35th Anniversary logo for Friends of Casco Bay

Casco Bay Heats Up

30 years of temperature data show waters in Casco Bay are warming rapidly.

Casco Bay is warming rapidly, according to Friends of Casco Bay’s 30-year-and-growing data set. Temperatures in the Bay have warmed 3° Fahrenheit on average since 1993. At a rate of 1°F per decade, the warming trend suggests the nearshore environment in Maine’s most populated region will continue to see dramatic changes in the coming years.

“This rise in water temperature marks an enormous shift,” says Staff Scientist Mike Doan. “It’s a stark reminder that climate change is altering the Bay in a fundamental way. And not only is the temperature increasing, but the rate of increase has continued to rise, too.” 

30 years of seasonal temperature data show Casco Bay has warmed by approximately 3°F since 1993. These data are from Friends of Casco Bay’s three Sentinel Sites in the Bay, which are located offshore in Broad Sound, nearshore by Clapboard Island, and at the mouth of Portland Harbor. These are water column data, which are collected at regular intervals from the water’s surface to the bottom of the Bay.

We completed our 30th year of collecting seasonal water temperature data in October. Our full 30-year data set shows Casco Bay’s three warmest years on record have all occurred in the past five years, between 2018 and 2022. These data confirm that warming conditions in the Bay align with those observed in the Gulf of Maine and that the region’s waters are warming faster than global averages. 

Scientists have linked rising marine temperatures to shifts in species distribution. Valuable cold-water fisheries like lobster are migrating north. Green crabs, well-known for decimating softshell clam populations and ecologically critical eelgrass meadows, have grown in number in Casco Bay as waters have warmed. 

Staff Scientist Mike Doan collects water quality data from Friends’ research vessel, R/V Joseph E. Payne. Mike uses a data sonde – a scientific device that measures water quality characteristics – to measure temperature, salinity, dissolved oxygen, and other parameters.

“Rising water temperatures cause so many impacts,” says Casco Baykeeper Ivy Frignoca. “A significant rise in temperature can lower the amount of oxygen in the water, cause ill health for cold water plants and animals, and signal an end to a species’ ability to live here. How do we help the Bay adapt to these changes?” 

Slowing the rate that Casco Bay is warming will require accelerated regional and national efforts to reduce reliance on fossil fuels and turn to renewable energy. For many people living in the watershed however, changing energy policy to reduce carbon emissions can feel beyond their influence, says Executive Director Will Everitt. 

“Locally, we have a limited ability to control carbon emissions across the nation or beyond our borders. But we can control the pollution we put into the Bay,” says Will. “Everything we do now to improve the health of marine ecosystems can help buy us time in the face of the long-term impacts of climate change. Actions like limiting the use of pesticides and fertilizers, reducing stormwater pollution, and developing our towns in ways that do not harm water quality matter. Especially in highly populated areas like the shores of Casco Bay. ”