The moderate global warming that has already occurred as a result of human emissions is responsible for about 75 percent of daily heat extremes, and about 18 percent of precipitation extremes, scientists reported Monday.
Especially hot days of a sort that occurred only once every 30 years or so before the Industrial Revolution are now occurring every six or seven years, the scientists found. Such scorching days could become near-annual events by late this century even if emissions are brought under control, the researchers said, and could occur several times a year if they are not.
The new research builds on those predictions of frequency and attempts to answer the question: To what degree are the changes attributable to human-caused climate change?
“People can argue that we had these kinds of extremes well before human influence on the climate — we had them centuries ago,” said Erich M. Fischer, lead author of a study published Monday by the journal Nature Climate Change. “And that’s correct. But the odds have changed, and we get more of them.”
The study by Dr. Fischer and his colleague Reto Knutti, of the Swiss Federal Institute of Technology in Zurich, is not the first to attribute large-scale changes in extreme weather to human influence on the climate. But it is among the first to forecast, at a global scale, how those extremes might change with continued global warming.
The question is important because while a gradual increase in average temperatures can have profound ecological consequences, it is weather extremes that have the greatest effect on human society. A 1995 heat wave in Chicago killed hundreds of people, and a 2003 heat wave in Europe killed an estimated 70,000.
Scientists believe both were made more likely by the human emissions that are warming the planet, and heat on that scale will become commonplace if emissions are allowed to continue unabated. For now, though, such heat extremes — Chicago temperatures were near or above 100 degrees for four days running that July — are still rare, which makes them difficult to study in a statistical sense.
For their paper, Dr. Fischer and Dr. Knutti focused on more common heat and precipitation extremes, the sort occurring about every three years. To perform a global study, they defined extremes not on an absolute scale, but in terms of what local weather is like — after all, a hot day in North Dakota might be pretty routine in Texas.
These weather extremes still have important consequences, even if they are not as severe as those of great heat waves. Such hot days can, for example, cut farm yields sharply and drive up food prices.
The researchers found that 75 percent of the daily heat extremes in the present-day climate were a consequence of the planetary warming that has occurred since the 19th century, a global average rise of about 1.5 degrees Fahrenheit. They found a smaller effect on precipitation, but it was still significant, with about 18 percent of the extremes in today’s climate attributable to global warming. That is a planetary average, Dr. Fischer said, with the effect being small in some regions and much larger in others.
If efforts to control emissions are unsuccessful, and temperatures by the end of the century rise as much as some experts fear, both heat and precipitation extremes will skyrocket, with human-caused global warming likely to be responsible for half the precipitation extremes and perhaps 90 percent of the heat extremes in that climate, the researchers found.
The change being seen today in precipitation patterns was predicted decades ago, based on the elementary fact that warmer air can hold more moisture. Other researchers have found that the increase is leading to heavier rainstorms across large parts of the United States, with the biggest effect occurring in the Northeast. At the same time, warmer temperatures are worsening the effects of droughts when they do occur, as in California over the last few years.
“The bottom line is that things are not that complicated,” Dr. Knutti said. “You make the world a degree or two warmer, and there will be more hot days. There will be more moisture in the atmosphere, so that must come down somewhere.”
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