WASHINGTON – People in Philadelphia would swelter through as many as 30 days over 100 degrees each summer. The entire Northeast ski industry except western Maine would likely go out of business. And spruce and hemlock forests – as well as song birds such as the Baltimore oriole – would all but disappear from New Jersey to the Canada border.

These are some of the conclusions of a two-year study by the public interest group Union of Concerned Scientists of the effects of global warming in the Northeast if current greenhouse gas emission patterns around the world continue unabated. Winters will be on average 8 to 12 degrees warmer by the end of the century and summers 6 to 14 degrees hotter.

Given those conditions, they concluded in a report released Wednesday, the environment of the Northeast would be transformed, and Boston, Atlantic City, New York and other cities would all be subject to disastrous flooding on a regular basis.

“The bad news is that the character of the Northeast will change dramatically under the business-as-usual scenario,” said Peter Frumhoff of the Union of Concerned Scientists and one of the lead authors of the report. “But on the other side, we say that the worst of the damage can be mitigated if we act soon.”

The world’s leading climate scientists concluded in February that it is “unequivocal” that the planet’s climate is warming and that it is “very likely” that heat-trapping emissions from the burning of fossil fuels and other human activities have caused most of the change.

The UCS report – put together with the help of more than 50 top university and government-based researchers – is one of numerous efforts to build on and localize those global conclusions, which have been released in recent months and years by the Intergovernmental Panel on Climate Change. Frumhoff was a lead author as well on the panel’s just-published impacts and mitigation report.

Some have argued that the effects of global warming will be positive as well as negative, and Frumhoff acknowledged that there would be some winners in a warmer Northeast. Farmers, for instance, would have a longer growing season, and residents in more northern areas might use less fuel to keep warm in the winter.

But overall, he said, the effect will be overwhelmingly disruptive and costly. Even under the best-case emission-lowering scenario, global warming will bring unprecedented damage to the coastline and will require enormous expenditures to maintain and replace roads, bridges and other infrastructure. Apple orchards in the Northeast would likely wither, dairy farmers would have great difficulty protecting against the rising heat, and lobstermen south of Maine and cod fishermen plying the Georges Bank would lose their livelihoods.

Because of the rising temperatures, the Northeast would be susceptible to longer and more severe droughts because of the increased evaporation of rainfall. In a similar finding, scientists wrote in the journal Science in April that long-term droughts would likely become common in the American Southwest as the planet warms.

Some of the changes are already inevitable, the report says, because greenhouse gases emitted today stay in the atmosphere for decades. Nonetheless, the report says that a low-emission alternative – featuring much-improved fuel efficiencies and a transition to alternative and renewable energy resources – is achievable and would leave the Northeast with a climate similar to today’s.

But it will not be easy, since it would require industrialized nations to reduce their carbon dioxide emissions to 80 percent below levels in 2000 and for developing nations to make substantial cuts, too.

The new report, which was peer-reviewed and will be published in the journal Mitigation and Adaptation Strategies for Global Change, points to efforts by many Northeast states to reduce greenhouse gas emissions as a sign that the worst-case scenarios can be avoided. Most have joined the Regional Greenhouse Gas Initiative, the first multistate, market-based plan to reduce harmful emissions from power plants, and have mandated increases in the use of alternative energy sources and reductions in auto tailpipe emissions.