By Dennis O'Brien
The Baltimore Sun
Global warming is widely feared for its long-term effects on the planet. But concerns that it's affecting our weather today remain controversial.
Some scientists believe warming was a factor in this year's hurricane season, which saw four major storms hit Florida and cause a record $25 billion in damage. The four hurricanes were the most to strike a single state in the same year since 1886.
"Four hurricanes in a five-week period could be a harbinger of things to come," said Dr. Paul Epstein, associate director of the Center for Health and the Global Environment at Harvard Medical School.
Epstein is among a group of scientists from Harvard and the National Center for Atmospheric Research - a consortium of 68 universities funded by government and private grants - who argue that a rise in global sea temperatures is putting more moisture into the air, increasing the chances not only for more intense hurricanes, but also for more rainfall and severe storms throughout the year.
And they say this is just the beginning. Warming, they contend, is an underlying factor contributing to droughts in the Midwest, heat waves in Europe and typhoons in Japan.
"There's a very consistent story here," said James J. McCarthy, a professor of biological oceanography at Harvard and a climate change expert. "On every continent, it is evident there are impacts."
Other scientists dispute that gloomy appraisal.
"You can never attribute a single season, or even several seasons, to something like global warming," said Gerry Bell, a meteorologist with the National Oceanic and Atmospheric Administration's Climate Prediction Center.
Bell and other NOAA scientists say it's wrong to blame warming for an increase in the number or severity of recent storms.
"There's no long-term trend in hurricanes that can be traced to warming," said Christopher Landsea, a meteorologist with NOAA's hurricane research division in Miami. "This is a natural cycle, driven by the ocean and the atmosphere both."
Bell said that blaming global warming for recent storms is overlooking the obvious.
"It's like if you see someone take a blowtorch to a house and the house burns down, then arguing that it was the summer heat that caused the fire and overlooking the blowtorch," Bell said. "We don't need to be looking for some amorphous signal that may or may not be there."
The storms that swept through Florida this year are part of a natural weather cycle - one that alternates between calm and stormy periods every few decades, according to Landsea and Bell. Such natural climate variability has been occurring since the last Ice Age.
Known as the Atlantic multi-decadal signal, it causes an increase in storm activity and can be linked to a variety of forces, including ocean temperatures and the amount of rainfall in Africa and the Amazon basin.
"We don't understand what the causes are for this multi-decadal signal, but they're what we expect," Bell said.
One of the most active hurricane periods, they note, occurred in the 1880s and 1890s, before the industrial revolution gave rise to exhaust and smokestack emissions widely blamed for global warming.
"In terms of hurricanes, warming wasn't a factor back then, and yet it was probably the worst period we know of," Landsea said.
The current hurricane activity began in 1995 when ocean temperatures in the tropical Atlantic climbed at least 1 degree Fahrenheit above normal. Monsoons also dumped heavier than average rains in West Africa, and the Amazon basin became drier. Since then, the number of major storms out of the tropics has doubled.
From 1970 to 1995, there was a stretch of relative calm, with few hurricanes, Bell and Landsea said.
The current cycle of increased storm activity each fall could last another 30 years, Bell said, but another extended period of relative calm will follow, he said.
Even so, both sides agree that the planet is warming up and that the long-term consequences could be serious.
Since scientists began recording temperature ranges in the 1800s, the 10 warmest years on record have all occurred since 1990, according to federal records. The International Panel on Climate Change, a group representing scientists from 120 countries, concluded three years ago that the 20th century was probably the warmest in the past 1,000 years.
Greenhouse gases -- mainly carbon dioxide -- trap the Earth's heat and are expected to increase sea surface temperatures by about 2 degrees Fahrenheit over the next century. Air temperatures near the Earth's surface are expected to rise by 6 to 8 degrees, according to the panel.
"We don't want to be causing global climate change, period," Bell said. "No one would say we shouldn't address these issues."
But the Harvard group claims that the warmer ocean temperatures allow more moisture to evaporate into the air, increasing the chances for thunderstorms each summer and hurricanes each fall. At a news briefing last month, they called for government efforts to reduce greenhouse gases, for stricter building codes in coastal areas and for land-use policies that minimize flooding by retaining forests and open space.
They point to a study last month in the Journal of Climate that linked the strength of future hurricanes to global warming. While the study stopped short of predicting more hurricanes in the years ahead, it concluded that when hurricanes do hit, they are likely to be more severe.
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The study analyzed the likelihood and strength of hurricanes under different warming scenarios. "In almost any situation, we found an increase in intensity," said Thomas Knutson, the lead author and a researcher at the NOAA.
So the arguments will continue. But both sides agree on one thing: the need for more study.
"There's just a lot about this we don't know," Landsea said.
Distributed by the Los Angeles Times-Washington Post News Service