Northeast US one of the most warming regions: study

The coastal Northeast — from Maine to Delaware — is warming faster than most of the United States due to dramatic changes in ocean and atmospheric conditions over the North Atlantic, according to a new study.

The study, published in Nature Climate Change on Thursday, showed that not only are Northeast winters getting warmer, as the authors expected, but there is also rapid summer warming along the Atlantic coast. In many parts of this region, which includes the densely populated cities of New York and Boston, the researchers found that warming over the past century exceeded 2 degrees Celsius – the total global rate countries have pledged to avoid in the Paris Agreement for the climate of 2015.

This warming of the coasts is nothing short of “excellent”, lead author Ambarish Karmalkar, a professor at the University of Massachusetts-Amherst said in a press statement accompanying the research.

“Some of the biggest ones [population] the centers in the US are experiencing the greatest degree of warming,” said Karmalkar, who conducted the study with Columbia University climate scientist Radley Horton.

The authors investigated several previous studies and re-analyzed existing datasets to explore the link between rising sea surface temperatures and a slowing of the Atlantic Ocean's current system – called the Atlantic Meridional Overturning Circulation (AMOC), according to the study.

Describing the AMOC “as a conveyor belt that carries warm, salty water from the tropics north to Greenland, where it cools and sinks,” the authors explained that this cold water then flows south in deep-water currents. However, as the climate warms and Greenland's glaciers melt, this conveyor belt is slowing down.

“One consequence of this slowing of the conveyor belt is greater ocean warming off the Northeast coast, which explains the sharp increase in ocean temperatures in the Mid-Atlantic Bight and Gulf of Maine regions,” Karmalkar said.

The AMOC is also linked to rising temperatures in northeastern European coastal cities due to a weather phenomenon called the North Atlantic Oscillation (NAO), which steers winds blowing over the Atlantic Ocean from the US to Europe, the researchers said .

The NAO has settled into a pattern in recent decades that has strengthened the influence of ocean air on the East Coast, the authors explained. Combined with the slowing of the AMOC, this results in warmer ocean air blowing to the northeast and accelerating warming trends over land.

Although northeastern coastal cities may already have experienced 2 degrees Celsius of warming, Karmalkar said he does not see a direct link between this region-specific warming and overall global warming, which stands at about 1.2 degrees Celsius so far.

“To stay below 2 degrees Celsius of global warming, we will need a global effort to reduce greenhouse gas emissions,” he told The Hill. “This certainly implies strong mitigation in states along the northeast coast, but this alone will not help meet the 2 degrees Celsius global warming target set in Paris.”

For the northeast coast, however, the authors said a fundamental dilemma is whether the AMOC current — which is at its slowest in the past millennium — has occurred because of human influences or internal variability.

The researchers, acknowledging the limitation of their study, said the next step should involve more sophisticated models that delve into daily weather conditions, as the nonlinear effects of temperature effects can be masked by seasonal averages.

But what is clear from the study, they concluded, is that the “social and environmental vulnerability” of the heavily populated northeast coast warrants “more nuanced climate assessments” than previously available.

And without the incorporation of improved high-resolution data into regional climate estimates, scientists and policymakers “may understate warming in this densely populated region,” Karmalkar said in the press statement.

Fortunately, he told The Hill, these capabilities are readily available to scientists, who now have access to high-resolution observational data sets that show temperature and humidity changes at local scales.

“But in terms of understanding future projections, we need to better understand the ability of our current generation of climate models to capture these highly localized trends,” Karmalkar said.

Horton, the study's co-author, stressed that recent extreme weather events, such as the Pacific Northwest heat wave and flooding in the Northeast, “have exceeded climate model predictions and led to large impacts on vulnerable urban populations.”

“When we look at how to reduce vulnerability, we can't limit our vision of the possible to the relatively narrow band of outcomes suggested by climate models,” Horton told The Hill. “And we need to realize that the impacts can escalate in complex ways that our sectoral models simply cannot envision.”