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updated 6/6/2011 5:20:37 PM ET 2011-06-06T21:20:37

Expect extra-toasty summers to be a mainstay if greenhouse gas levels continue to rise, according to a new report suggesting the tropics and the Northern Hemisphere may see an irreversible bump in summer temperatures within the next 20 to 60 years.

In the study to be published later this month in the journal Climate Change, Stanford University researchers conclude that many tropical regions in Africa, Asia and South America could see "the permanent emergence of unprecedented summer heat " in the next two decades.

The middle latitudes of Europe, China and North America, including the United States, are likely to undergo extreme summer temperature shifts within 60 years, the researchers found.

"According to our projections, large areas of the globe are likely to warm up so quickly that, by the middle of this century, even the coolest summers will be hotter than the hottest summers of the past 50 years," said lead study researcher Noah Diffenbaugh, an assistant professor of environmental Earth system science and fellow at the Woods Institute for the Environment at Stanford University.

Though no single extreme weather event can be linked to global warming, scientists say that as the planet warms, we should expect more extremes, such as heat waves. Diffenbaugh and Stanford research assistant Martin Scherer wondered whether one extreme pattern — heat waves — would become more normal. "At what point can we expect the coolest seasonal temperatures to always be hotter than the historically highest temperatures for that season?" Diffenbaugh said in a statement.

The researchers analyzed more than 50 climate model experiments, which included computer simulations of the 21st century (when greenhouse gas emissions are expected to increase) and those of the 20th century that have accurately predicted climate over the last 50 years. They also looked at historical data from weather stations around the world.

Results showed the tropics are heating up the fastest. "We find that the most immediate increase in extreme seasonal heat occurs in the tropics, with up to 70 percent of seasons in the early 21st century (2010 to 2039) exceeding the late-20th century maximum," the authors wrote.

Even so, wide swaths of North America, China and Mediterranean Europe are also likely to enter into a new heat regime by 2070, they found.

Diffenbaugh was surprised at how quickly the new, potentially destructive heat regimes are likely to emerge, given that the study was based on a relatively moderate forecast of greenhouse gas emissions in the 21st century.

"The fact that we're already seeing these changes in historical weather observations, and that they match climate model simulations so closely, increases our confidence that our projections of permanent escalations in seasonal temperatures within the next few decades are well-founded," Diffenbaugh said.

This dramatic shift in seasonal temperatures could have severe consequences for human health and agricultural production, Diffenbaugh said, citing as an example the 2003 heat waves in Europe that killed 40,000 people. Studies have also shown that projected increases in summer temperatures in the Midwestern United States could reduce the harvest of corn and soybeans by more than 30 percent, he added.

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