Climate Change



The marine environment of Casco Bay is changing due to a combination of forces, including a changing regional climate, ocean acidification, invasive species and coastal development. Resiliency is the ability of the natural and human environment to absorb and recover from the changing climate, including increased severe storms, sea level rise, and resulting changes such as ocean acidification and increased invasive species. Social and economic resilience is closely tied to environmental resilience.

Long-term data on conditions over the past century, largely from a weather monitoring station at what is now the Portland Jetport,  reveal clear trends in Casco Bay’s climate. While the record shows significant year-to-year variability, long-term trends are clear. Overall, conditions are warmer, especially in the winter, and wetter than in the past. Ocean data collected over the past century at Portland Harbor show a rising sea level.

Climate models for the watershed show further summer temperature increases of 2 to 6 degrees F by mid-century and by the end of the century, climate scientists expect that Portland will have 15-30 fewer winter days with temperatures below 32 degrees F. Sebago Lake’s ice-out in the spring occurs 23 days earlier than it did in 1807. Between 2004 and 2013, the Gulf of Maine warmed at a rate of .41 degrees F per year – faster than 99 percent of the world’s ocean.

Maine is experiencing increases in both total annual precipitation and extreme precipitation events. Intense rain events typically occurred about once a year in the 1940s but are now occurring in Portland about three time a year. And the number of extreme precipitation events – categorized as coastal floods, flash floods, floods, heavy rain, and tropical storms – has increased dramatically.

In the past two decades, sea level rise increased 130 percent faster than the average rate in the last 100 years. Studies indicate that sea level could rise another 2 to 10 feet by 2100.

And the ocean is acidifying at a rate at least 100 times faster than at any other time in the past 200,000 years. Approximately one quarter of human emissions of C02 is being absorbed by the ocean, causing seawater to become more acidic.

Resilience is the ability of an ecosystem or a coastal community to remain healthy and vibrant in the face of external pressures like climate change.  Resilient coastal ecosystems are less likely to collapse in the face of change and are more likely to continue to provide environmental goods and services despite external pressures.  Protecting and enhancing resilience of coastal ecosystems and coastal economies are important policy goals in an era of coastal change.


Oyster shell recycling project.  One of the potential negative effects of acidification is to slow the growth and development of juvenile oysters and clams. With support from the U.S. Environmental Protection Agency’s Climate Ready Estuaries Program, CBEP is engaged in a multi-year project to test the use of recycled shell as natural materials for coastal restoration, shoreline erosion protection, and ameliorating the impact of coastal acidification.  In 2019 Maine Coastal Program collected oyster shell from nine restaurants in Portland and cured it to reduce the risk of spreading invasive species or pathogens. In 2019-2020, Bigelow Laboratory for Ocean Sciences conducted an experiment to test use of shell in “upwellers” to remediate impacts of low pH waters in oyster aquaculture. In 2021, Downeast Institute is studying the use of oyster shell hash (crushed shell) to reduce the acidity of tidal flats and to affect the abundance of shellfish, including softshell clams and quahogs.

Salt Marsh Tidal restoration.  In recent years, CBEP staff have led or partnered on several tidal restoration projects in Casco Bay’s salt marshes.  Projects such as the Thomas Bay Marsh restoration project at Adams Road in Brunswick, in which an undersized and corroded culvert was replaced with a larger aluminum pipe arch, have enhanced the Bay’s salt marshes by providing more frequent and extensive tidal inundation into upstream wetlands, delivering salt and sediments to areas and in the process, enhancing system resilience to stressors such as sea level rise.

Living shorelines pilot project. Through a collaborative project with the Maine Coastal Program at DMR, the Maine Geological Survey, The Nature Conservancy, Maine Coast Heritage Trust, and local municipalities, three ‘living shoreline’ demonstration sites were constructed at sites around the Bay in the summer and fall of 2020.  Where suitable, living shorelines, which are typically constructed of natural materials such as wood, rocks, shell, and plants, can be utilized to stabilize eroding coastal banks while providing ecosystem benefits. Unlike hard shorelines like seawalls or rip-rap, which impede the growth of plants and animals, living shorelines are intended to grow over time.  CBEP staff are monitoring these demonstration sites to evaluate their effectiveness and provide data to state and federal regulators in the hope that their use will expand in Maine. Read more here.

Volunteer-Based Marine Invasive Species Monitoring (MIMIC). We support our partner Wells Estuarine Research Reserve in coordinating volunteer-based monitoring for presence of invasive species at several Casco Bay locations. Read more here.

For More Information

Read our Climate Change Summary Report (2017) here, Climate Change Fact Sheet here, and Climate Change in the Casco Bay Watershed: Past, Present, and Future (2009). Check out our Resources page for many other documents and reports.

Contact CBEP at (207) 780-4820 or