WASHINGTON - Scientists from across the country testified at a congressional hearing Wednesday that global warming was a real threat that could cause heat waves and alter eco-systems by the second half of the century.

But scientists in Maryland say global warming is more than a threat. It's here.

Already, the Chesapeake Bay is rising and temperatures are creeping upward. If greenhouse gas emissions are not controlled, scientists and environmentalists say global warming could have serious consequences.

And Maryland, with 3,100 miles of tidal shoreline, is among the three states at greatest risk from global warming, said Mike Tidwell, director of the Chesapeake Climate Action Network. He said the other states, based on total shoreline miles, are Florida and Louisiana, where global warming could flood coastal areas, bring tropical diseases and alter growing seasons.

"There are a lot of reasons to be concerned," said Tidwell.

The Environmental Protection Agency said temperatures rose 2.4 degrees in College Park over the past century. While some warming is natural, it is also caused by human emissions of carbon dioxide that trap heat in the atmosphere.

Scientists do not all agree how much of warming is caused by human activity, but most agree that the increased heat melts ice at the poles, raising ocean levels and altering climates.

The Chesapeake Bay's relative sea level has risen about a foot in the past 100 years and is expected to rise at least 2 more feet by 2100, said Donald F. Boesch, president of the University of Maryland Center for Environmental Science. A change of about 4 inches every 100 years is part of natural effect that causes the land to sink, but the rest is attributed to global warming, he said.

"Sea level is going to rise. There's not any serious question," Boesch said.

He said higher sea levels can lead to more storm surges, like those that flooded parts of Washington and Baltimore when Hurricane Isabel hit last year.

While the change in actual temperature would be gradual, the Chesapeake Futures report that Boesch edited said that winters in Washington by 2090 could be as mild as those of 20th-century Atlanta.

Wednesday's hearing was held by Sen. John McCain, R-Ariz., who charged the Bush administration is not taking the problem of global warming seriously. McCain is pushing his own proposal, the Climate Stewardship Act, which would try to limit greenhouse gases by giving industries market incentives to limit emissions.

Frank Maisano, a self-described energy lobbyist at Wednesday's hearing, said he did not dispute the science behind global warming, but called McCain's bill the wrong approach. Maisano said a reduction in greenhouse-gas emissions is more likely to take place if it is market-driven, not forced as it would be under the Climate Stewardship Act.

William O'Keefe, president of the George C. Marshall Institute, a science policy think tank, cautioned that it is difficult for scientists to predict so far in the future. He said it is also difficult for scientists to discern how much warming stems from emissions and how much is natural.

Peter Frumhoff, director of the global environment program at the Union of Concerned Scientists in Massachusetts, acknowledged that modeling systems were imperfect.

"Our crystal balls are clouded," he said.

But Frumhoff and Daniel Cayan said that does not mean the threat of global warming is not real.

"Something is happening. Something will happen," said Cayan, a research meteorologist at the Scripps Institution of Oceanography in California. "It's a very serious phenomenon that we're dealing with."

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Copyright © 2004 University of Maryland Philip Merrill College of Journalism