Increased greenhouse gas emissions could lead to permanently hotter summers in coming decades, according to Stanford scientists. Image: Dori, Wikimedia Commons
and much of the Northern Hemisphere are likely to experience an irreversible
rise in summer temperatures within the next 20 to 60 years if atmospheric
greenhouse gas concentrations continue to increase, according to a new climate
study by Stanford
Univ. scientists. The
results will be published in Climatic Change Letters.
In the study,
the Stanford team concluded that many tropical regions in Africa, Asia, and South America could see “the permanent emergence of
unprecedented summer heat” in the next two decades. 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 the study’s lead author, Noah Diffenbaugh, an assistant
professor of environmental Earth system science and fellow at the Woods
Institute for the Environment at Stanford. The study is co-authored by Stanford
research assistant Martin Scherer.
“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,'” Diffenbaugh said. “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 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 said. “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% of seasons in the early 21st century (2010-2039)
exceeding the late-20th century maximum,” the authors wrote.
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.
This dramatic shift in seasonal temperatures could have severe
consequences for human health, agricultural production and ecosystem
productivity, Diffenbaugh said. As an example, he pointed to record heat waves
in Europe in 2003 that killed 40,000 people.
He also cited studies showing that projected increases in summer temperatures
in the Midwestern United States could reduce the harvest of staples, such as
corn and soybeans, by more than 30%.
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
“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.