Study foresees sultry, snow-starved N.E.

Summers in Boston could feel like July in South Carolina by the end of the century if global warming is allowed to continue unchecked, according to a scientific report that gives the most detailed projection yet of the effects of climate change on the Northeast.

Temperatures in the city could exceed 90 degrees Fahrenheit on as many as 64 days a year, compared with an average of about 10 days now, says the study by a team of 14 scientists and the Union of Concerned Scientists, an environmental advocacy group based in Cambridge. For the nine states from Pennsylvania to Maine, the report predicts more severe rainstorms and a decline of as much as 50 percent by the year 2100 in the number of days with snow on the ground, changes that could harm tourism, agriculture, and the economy.

``The very notion of the Northeast as we know it is at stake," said Cameron Wake, one of the lead authors of the report and a research associate professor at the University of New Hampshire's Climate Change Research Center.

The regional and local impacts of global warming have been notoriously difficult to predict, because climate models have not been able to analyze small geographic areas reliably.

Climate scientists not involved in the research said the authors' methods and conclusions appeared reasonable, although they said that it was more important to focus on the overall warming trend than on specific predictions and that precipitation was particularly difficult to predict. A paper on many of the study's findings is undergoing a second round of peer review for the Bulletin of the American Meteorological Society.

Annual average temperatures in New England and the Northeast have warmed about 2 degrees Fahrenheit since 1970, a period when carbon dioxide emissions from power plants and cars began increasing significantly. Many scientists believe the temperature increase is occurring at least in part because of a human-induced greenhouse effect, the result of carbon dioxide building up in the atmosphere and holding in heat. But local fluctuations in climate also could be responsible.

The study, which took two years to complete, made projections in 30-year periods through the end of this century under two scenarios: If the world curbs its current carbon dioxide emissions by about 75 percent by the year 2050 and if it does not. Under the first scenario, average annual temperatures could rise 3.5 to 6.5 degrees Fahrenheit by the end of the century. Under the second scenario, temperatures could rise between 6.5-12.5 degrees.

The report paints a bleak picture for those who love winter sports. Days with snow cover across the Northeast will decline by at least 25 percent and as much as 50 percent under the two scenarios. There will be more winter precipitation, the scientists said, but much of it will be rain. The average winter could become like last year, when dozens of dog sled races, ice-fishing derbies, and winter carnivals were canceled across New England because of snowless conditions and patchy lake ice . The study projects that winter temperatures could increase by more than 9 degrees Fahrenheit by the end of the century.

Summers will be hotter, too, with 100-degree days becoming common. Today, the thermometer hits that mark in Boston about once a year. Under the higher emissions scenario, that number could increase to as many as 24 days in Boston by 2100; in Buffalo, it could increase to 14 days. The growing season will become longer, and rainy days will have more rain.

The scientists used three types of global climate models to come up with the Northeast predictions. Then they used each model to run three separate computer simulations of the weather during the last 100 years and compared the results to historical weather data collected over the last century. The three models' results closely matched the actual data, giving the scientists confidence in the models to make predictions.

``The bottom line is the models do quite well," said Katharine Hayhoe, a lead author of the report and a climate scientist at Texas Tech University. If anything, Hayhoe said, the models tended to underestimate the observed changes in the Northeast.

The scientists called for the Northeast states, which together are the seventh-largest emitter of carbon dioxide when ranked among the nations of the world, to make more use of renewable energy sources such as wind, drive fewer gas-guzzling vehicles, and make buildings more energy-efficient.

``The near-term emissions choices we make in the Northeast and throughout the world will help determine the climate and quality of life our . . . grandchildren experience," Wake said.

A coalition of seven Northeast states is working on a plan that would freeze carbon dioxide emissions from power plants and then reduce them by 2020. Massachusetts was part of the Regional Greenhouse Gas Initiative, but Governor Mitt Romney withdrew in 2005.

On Tuesday, Senators John F. Kerry of Massachusetts and Olympia J. Snowe of Maine introduced bipartisan legislation for the United States to freeze carbon dioxide emissions in 2010 and eventually reduce emissions to 65 percent below the 2000 levels.

Beth Daley can be reached at bdaley@globe.com.