The rapid warming of the 20th century has reversed 900 years of cooling in the Gulf of Maine, according to a new study led by the Woods Hole Oceanographic Institution, co-authored by the University of Maine and funded by the National Science Foundation. 

The Gulf of Maine has undergone recent, rapid ocean warming, but the lack of long-term instrument records has made it difficult for scientists to put this warming into historical context. The longest continuous instrumental record available for the Gulf of Maine is a sea surface temperature record from Boothbay Harbor that extends back to 1905, and little is known about the Gulf’s water properties before that station was installed.

To gain greater insight about what warming and cooling patterns were like for the Gulf of Maine in the past, scientists developed a 300-year-long geochemical record from shells of a clam known as the ocean quahog, Arctica islandica, in the western Gulf of Maine. The shells have been proven in previous studies to be valuable proxies because they are long-lived and faithfully record environmental conditions as they precipitate their shells in annual increments, gathering isotopes with valuable data along the way.

Each of the isotopes collected from the shells served as a proxy for a property of the water in the region at a given time. For example, oxygen isotopes can be used as a proxy for seawater temperature and salinity, while nitrogen isotopes can be used as a proxy for water mass source. The scientists compared the records from the shells with 1,000-year-long climate model simulations known as the Community Earth System Model-Last Millennium Ensemble, a global climate model developed by the National Center for Atmospheric Research, which considers orbital, solar, volcanic, greenhouse gas, aerosol and land use changes over the last millennium.

“Combining precisely dated geochemical data from the clam shells with state-of-the-art climate models provides a powerful method for understanding climate change in the Gulf of Maine. We can see how local conditions are influenced by large-scale patterns through time,” says Karl Kreutz, co-author of the study, director of the School of Earth and Climate Sciences and professor in the Climate Change Institute.

The results suggest that the Gulf of Maine underwent a long-term cooling over the last 1,000 years driven mainly by volcanic forcing. However, this trend was significantly reversed by warming that began in the late 1800s, around the time of the Industrial Revolution began adding greenhouse gasses to the atmosphere while the behavior and position of the Gulf Stream shifted. 

The simulations suggest that the warming in the most recent century has been more rapid than any other 100-year period in the region’s last 1,000 years. 

“The climate changes that ecosystems and coastal communities are now being forced to adapt to are different from what has occurred in the recent past. That’s important to know when developing policies and decision support tools,” Kreutz says.