Global warming could stifle cleansing summer winds across parts of the northern United States over the next 50 years and worsen air pollution, U.S. researchers said on Saturday.

Further warming of the atmosphere, as is happening now, would block cold fronts bringing cooler, cleaner air from Canada and allow stagnant air and ozone pollution to build up over cities in the Northeast and Midwest, they predicted.

“The air just cooks,” said Loretta Mickley of Harvard University’s Division of Engineering and Applied Sciences. “The pollution accumulates, accumulates, accumulates, until a cold front comes in and the winds sweep it away.”

Mickley and colleagues used a computer model, an approach commonly used by climate scientists to predict weather and climate changes.

She told a meeting of the American Association for the Advancement of Science that the model predicted a 20-percent decline in summer cold fronts out of Canada.

“If this model is correct, global warming would cause an increase in difficult days for those affected by ozone pollution, such as people suffering with respiratory illnesses like asthma and those doing physical labor or exercising outdoors,” she said.

World temperatures have risen by an average of 1 degree Fahrenheit (0.6 degree Celsius) over the past century.

Earlier this week 141 nations signed the U.N. Kyoto Protocol aimed at cutting the so-called greenhouse gas emissions that fuel global warming.

It imposes caps on carbon dioxide emissions, mainly from burning fossil fuels in power plants, factories and cars, in 35 developed nations.

The United States, which produces the most pollution of any country, has refused to sign it.

The model used by Mickley and her colleagues incorporates things such as the sun’s luminosity, topography of the planet, the distribution of the oceans, the pull of gravity and the tilt of the Earth’s axis, as well as predicted warming.

They fed in gradually increased levels of greenhouse gases at rates projected by the Intergovernmental Panel on Climate Change. What they found surprised them.

“The answer lies in one of the basic forces that drive the Earth’s weather - the temperature difference between the hot equator and the cold poles,” Mickley said.

In the middle latitudes, low-pressure systems and accompanying cold fronts help redistribute heat by carrying warm air to the poles and replacing it with cool air. Warming slows that process down, Mickley’s team found.