Michael Mann, a climatologist at Pennsylvania State University in University Park, also uses the analogy of loaded dice. If a die loaded in favor of sixes is rolled once and a six comes up, nobody can say whether the six showed because of the switch.
But when the die is rolled many times and six shows on a third of the rolls, the loading effect is clear.
"It's the same thing with heat waves," he said.
"It may be we are getting to see the effect of loading the die."
The link between the current heat waves and global warming is "a little complex," said Kevin Trenberth, head of the climate analysis section at the National Center for Atmospheric Research in Boulder, Colorado.
According to Trenberth, the immediate cause of heat waves is a weather pattern known as an anticyclone, or a high-pressure ridge. Anticyclones lead to dry conditions.
"That means all the heat is going into raising temperature rather than evaporating moisture," he said. "If you have wet conditionsif the ground is wetthat tends to act somewhat as an air conditioner."
Trenberth said only 7 of the last 25 years have had above normal precipitation over land. And a recent statistical analysis of global temperature records indicates that the number of really hot days and really hot nights is increasing almost everywhere around the world.
"The extremes of temperature are generally increasing in line with what we expect with global warming," he said. "These heat waves are a particular manifestation of that."
Derrick Ryall, head of government meteorological research at England's Hadley Centre, says climate scientists expect extreme weather like heat waves and droughts to increase as Earth warms.
But the climate also has "lots of natural variability," he added.
The trick, he says, is to tease the climate change signals from the natural variability.
"Seeing hot conditions is perfectly consistent with our predictions [of global warming]," he said. "But we cannot blame any one event on climate change."
"It's a probability game."
Signs of Warming
According to climate scientists, other signs of global warming abound:
Spring is arriving by February in New England, threatening the job of Pennsylvania's Punxsutawney Phil, the groundhog of Groundhog Day fame.
Glacier ice is retreating all over the world, from the European Alps to Kenya's Mount Kilimanjaro to Glacier National Park in Montana.
Warm climate zones have shifted higher in the mountains of Central America. Several species of frog have nowhere left to move and are rapidly going extinct.
The National Oceanic and Atmospheric Administration recently said the first half of 2006 was the warmest ever for the U.S.
Mann, the Pennsylvania State climatologist, says proving an effect of global warming on day-to-day weather is difficult.
But amid all these other signs of global warming, he said, "to the extent it's ever possible to say individual events are due to climate change, this [summer's rash of heat waves] comes pretty close.
"It certainly looks like we are seeing those loaded dice."
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