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A study scheduled to publish in the journal Climatic Change Letters finds many tropical regions in Africa, Asia, and South America could see “the permanent emergence of unprecedented summer heat” in the next two decades while middle latitudes of Europe, China and North America—including the United States—are likely to undergo extreme summer temperature shifts within 60 years.

“When scientists talk about global warming causing more heat waves, people often ask if that means that the hottest temperatures will become the new normal,” says Noah Diffenbaugh, assistant professor of environmental earth system science at Stanford University and the study’s lead author.

“That got us thinking—at what point can we expect the coolest seasonal temperatures to always be hotter than the historically highest temperatures for that season?”

Climate models, past and future
To determine the seasonal impact of global warming in coming decades, Diffenbaugh and research assistant Martin Scherer analyzed more than 50 climate model experiments, including computer simulations of the 21st century when global greenhouse gas concentrations are expected to increase, and simulations of the 20th century that accurately “predicted” the Earth’s climate during the last 50 years.

The analysis revealed that many parts of the planet could experience a permanent spike in seasonal temperatures within 60 years.

“We also analyzed historical data from weather stations around the world to see if the projected emergence of unprecedented heat had already begun,” Diffenbaugh says. “It turns out that when we look back in time using temperature records, we find that this extreme heat emergence is occurring now, and that climate models represent the historical patterns remarkably well.”

According to both the climate model analysis and the historical weather data, 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 (2010-2039) exceeding the late-20th century maximum,” the authors write.

Tropical regions may see the most dramatic changes first, but wide swaths of North America, China, and Mediterranean Europe are also likely to enter into a new heat regime by 2070, according to the study.

Environmental impact
The shift in seasonal temperatures could have severe consequences for human health, agricultural production, and ecosystem productivity, Diffenbaugh says, citing record heat waves in Europe in 2003 that killed 40,000 people and previous studies that project increases in summer temperatures in the Midwestern United States could reduce the harvest of staples, such as corn and soybeans, by more than 30 percent.

Diffenbaugh was surprised to see 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.”

The research was supported by the National Science Foundation, the Department of Energy, the National Institutes of Health and the World Bank.

More news from Stanford University: http://news.stanford.edu/