By the year 2100, New Yorkers could face 25 days when the mercury tops 100 degrees Fahrenheit and no longer be able to ski on natural snow in winter if the current reliance on fossil fuels does not end, scientists said in a report released yesterday.

The Northeast Climate Impacts Assessment report predicted that climate changes would drastically affect the economy, health, and quality of life in the Northeast region of the country if there are no changes to current emissions. A switch to cleaner energy resources and new mitigation protocols could make the effects less severe, the chairman of the report team, Peter Frumhoff, said.

“The whole character of New York is poised to change dramatically,” Mr. Frumhoff said from the New York Botanical Gardens. “We can’t stop climate change altogether; it’s already with us because of the emissions we’ve already set in motion, but our choices today have a huge impact.”

The target to avert the worst environmental impact is for an 80% emissions reduction by mid-century, which Mr. Frumhoff said is achievable if renewable energy resources such as solar, geothermal, and wind energy are used more frequently.

Other consequences of the climate shift could include elevated sea levels, which can cause more frequent coastal flooding and affect real estate values. Plants sensitive to climate change could produce more pollen, affecting asthma and allergy sufferers. Hotter and drier summers punctuated by heavy rainstorms could create favorable conditions for dangerous insect-borne illnesses.

A critic of the report, Myron Ebell, said it was “clearly designed” to scare people, pointing to historical data that suggests less-alarming statistics.

“If you rely on historical trends, we’re in the midst of a very modest warming trend,” Mr. Ebell, the director of energy and global warming policy at the Competitive Enterprise Institute, said. “If they base their predictions on that, then it would be a much smaller change in temperature with much smaller impacts than they are predicting. What they’re relying on is highly dubious computer models.”

The city’s director of long term planning and sustainability, Rohit Aggarwala, said he has already seen the effects and is ready to make a difference.

“The time for action is now,” Mr. Aggarwala said, pointing to Mayor Bloomberg’s PlaNYC, which outlines 127 environmentally sustainable initiatives for the city that includes both long- and short-term projects.

“The choice is between doing something now and allowing our children to pay for our unwillingness to face long-term consequences,” he said.