TRENTON, N.J. – The climate of the nine Northeastern states could become like that of the South by the end of the century unless greenhouse gas emissions are steadily lowered, the Union of Concerned Scientists warned in the most detailed regional report yet on the issue.

The report, released Wednesday, states longer, much hotter summers, warmer winters with less snow and other changes fueled by global warming could put significant strain on the power grid and health care system, along with farms, forests, marine fisheries, recreation and tourism. The impact could be much lower, though, if carbon dioxide emissions are reduced by 3 percent each year, according to the advocacy group and university scientists who worked on the report.

Their collaboration, called the Northeast Climate Impacts Assessment, spent two years using multiple state-of-the-art climate models and weather records to project likely climate changes from New Jersey and Pennsylvania up to Maine through 2099.

“This has enormous implications for human health. It puts a lot of stress on the energy system. It could lead to blackouts,” Katherine Hayhoe, an associate professor of geosciences at Texas Tech University and a lead author of the report, told reporters during a teleconference.

If power plant and auto emissions of carbon dioxide – considered the main culprit of global warming – continue unabated, average temperatures in the Northeast could rise between 6.5 degrees and 12.5 degrees by the end of the century, she said. If society shifts to cleaner, renewable energy sources, the temperature increase would be halved, she said.

The study said, without any change, New Hampshire’s climate will be more like that of South Carolina by the end of the century; with lower greenhouse emissions, it would be like North Carolina. In the New York-New Jersey-Connecticut region, without changes the climate would be like eastern Virginia’s by century’s end; with lower emissions, it would be more like Georgia’s.

With no changes, Boston could see its number of 90-degree-or-higher summer days jump from one to 40 and New York City could have 70.

“We’re beginning to see the climate in the Northeast changing” already, with average temperatures rising 1.5 degrees in the summer and 4 degrees in the winter between 1970 and 2000, said the other lead author, Cameron Wake, an associate professor at the University of New Hampshire’s Climate Change Research Center.

Hayhoe said the report shows winter precipitation is likely to increase, but mostly as rain, and snow would be on the ground for much shorter periods. Meanwhile, summers could become up to six weeks longer and spring could start earlier, affecting plants, wildlife and recreation tied to the cycle of seasons.

The report targets the Northeast because it is the world’s seventh-largest source of emissions, ranked behind the U.S. as a whole and five other nations, and because the region’s leaders have already taken steps to reduce emissions and could serve as a national model with innovations in policy and technology.

Mike MacCracken of The Climate Institute, a former head of the interagency group that did climate assessments under a Clinton-era research program, called the report “a high-quality job” with plausible results. He said its findings are similar to those of the federal program’s last Northeast regional assessment several years ago.

“These give pretty reliable indications of the amount of change” without being overly precise, MacCracken said.

Doug Inkley, senior science adviser at the National Wildlife Federation, said the report was done by top-tier scientists and backs up his group’s research showing a warmer climate in the Northeast will push out temperature-sensitive species from sugar maple and northern pine trees to songbirds and trout.

“I certainly hope people would act on it,” Inkley said. “This report is yet another wake-up call we cannot ignore.”

John R. Christy, director of the Earth System Science Center at University of Alabama-Huntsville, said regional analyses his center has done indicate the latest climate models “do not have the predictive skill needed, especially with regard to rain and snow.”

He said the report’s recommendations – mostly centered on replacing or upgrading buildings, cars and appliances with more energy-efficient ones – won’t have much effect on the total amount of carbon dioxide going into the atmosphere, partly because energy demand will continue to grow.