June 14, 2007
Reduced greenhouse gas emissions required to avoid dangerous
increases in heat stress, researchers say
A study led by a Purdue University researcher projects a 200 percent
to 500 percent increase in the number of dangerously hot days in the
Mediterranean by the end of the 21st century if the current rate of
greenhouse gas emissions continues. The study found France would be
subjected to the largest projected increase of high-temperature
The study also showed a reduction in greenhouse gas emissions could
reduce the intensification of dangerously hot days projected in the
scenario by up to 50 percent.
“Rare events today, like the 2003 heat wave in Europe, will become
much more common as greenhouse gas concentrations increase,” said
Noah Diffenbaugh, the Purdue assistant professor of earth and
atmospheric sciences who led the study. “The frequency at which that
scale of event occurs at high greenhouse gas concentrations is
staggering. Rare events become the norm, and the extreme events of
the future are unprecedented in their severity.”
A 2003 heat wave led to 15,000 deaths in France and almost 3,000 in
Italy. The researchers found that global warming causes summer
temperatures to dramatically exceed the range that was correlated
with the increased number of deaths.
“The thresholds of the 2003 event are substantially exceeded in the
future in both of our research scenarios,” said Diffenbaugh, who is a
member of Purdue’s Climate Change Research Center. “This research is
about understanding the response to different emissions levels. We
find that decreases in greenhouse gas emissions greatly reduce the
impact, but we see negative effects even with reduced emissions.
Technological and behavioral changes that are made now will have a
big influence on what actually happens in the future.”
In addition to the human health risks, extremely high temperatures
could impact the economy of this region, which includes metropolitan
areas such as Rome, Paris and Barcelona, said Jeremy Pal,
co-researcher and associate professor of civil and environmental
engineering at Loyola Marymount University.
The study covered the entire Mediterranean area, which includes 21
countries in Europe, Africa and Asia. Major cities covered in the
study include: Prague, Zurich, Bucharest, Athens, Istanbul, Tel Aviv,
Cairo, Algiers and Casablanca.
“When high temperature extremes increase, it could have significant
negative impacts on human health, water resources, agriculture and
energy demand,” Pal said.
The results of the study, which originated at the International
Centre for Theoretical Physics in Trieste, Italy, will be published
in the Friday (June 15) issue of Geophysical Research Letters.
In addition to Diffenbaugh and Pal, Filippo Giorgi of the
International Centre for Theoretical Physics and Xuejie Gao of the
National Climate Center in Beijing are co-authors of the paper. The
researchers used a supercomputer in the National Climate Center in
Beijing to run the climate model.
The model offers a resolution of 20 kilometers, about 12.5 miles, and
is believed to have the highest spatial resolution available for the
Mediterranean region. Much like increased resolution in a photo makes
a clearer picture and allows one to zoom in without blurring the
image, the powerful resolution of the climate model allows
researchers to gather detailed information about particular areas.
Giorgi, who is head of the Earth System Physics Section of the
International Centre for Theoretical Physics, said the Mediterranean
is of interest because it is one of the most susceptible areas to
climate changes – both climatically and socially.
“In the global warming scenario, there is more warming and drying in
the Mediterranean than in other regions of the world, which makes the
Mediterranean a climate change ‘hotspot,'” Giorgi said. “The
Mediterranean also is a very vulnerable region to climate change in
terms of the impacts to the way of life of those who live there.”
The researchers found that this warming and reduced precipitation
contribute to a preferential warming of the hottest days of the year.
“We found that the hottest days of the year, or the ‘hot tail,’ warm
more than the typical summer days warm,” Diffenbaugh said. “One might
expect that an average warming of four degrees would equate to each
day warming by four degrees, but in fact the hottest days warm quite
a bit more.”
This is due, in large part, to a surface moisture feedback. The
surface gets dryer as it gets hotter and the dry soil leads to less
moisture in the area and less evaporative cooling. The locations of
intensified warming on hottest days of the year matched the locations
where surface drying occurred, Diffenbaugh said.
With the projected shift to more severe temperatures, the daily
temperatures currently found in the hottest two weeks of the summer
instead are found in the coldest two weeks of the summer in the
future climate scenario, Diffenbaugh said.
“The hottest temperatures we are used to experiencing will become the
normal temperatures of the summer, and the hot periods will be
magnified,” Diffenbaugh said. “Take Paris: If we look at the
temperatures that occurred there during the heat wave in 2003, when
15,000 people died, those temperatures are exceeded a couple dozen
times every year in the future projection. That means that severe
heat waves, such as those rare events that have occurred in the past
couple of years, are likely to become far more common.”
The study used the National Weather Service Heat Index in the
analysis of the heat stress response to increasing greenhouse gas
concentrations. The researchers found that areas most likely to face
substantial increases in the dangerous heat index were concentrated
largely in coastal areas.
“Coastal regions were more affected than inland regions, which is of
particular importance because many large cities in the Mediterranean
are on the coast,” Giorgi said. “This is the first time this
amplification signal over coastal areas could be seen and quantified.
Coastal regions are particularly vulnerable because they will likely
be affected by other important climate change related stresses, such
as a rising sea level.”
Diffenbaugh said without the high resolution of the climate model,
the researchers would not have been able to identify the coastal
“This underscores the importance of advancing our technology and
examining a number of scenarios in great detail,” he said. “If we
want to quantify the risks associated with climate change, it is
critical to understand the local and regional impacts as well as the
For the study’s standard future scenario, the research group used a
commonly accepted emissions scenario from the Intergovernmental Panel
on Climate Change that assumes greenhouse gas emissions continue to
increase exponentially. The reduced emissions scenario incorporated a
reduction in population growth and greater environmental concern,
The researchers are currently using the high-resolution model to
further evaluate the effects that increased temperatures and surface
drying could have on agriculture and energy and water resources.
This research was funded by the Italy-USA collaborative agreement on
climate change research and in part by the National Science
The Purdue Climate Change Research Center is affiliated with Purdue’s
Discovery Park. The center promotes and organizes research and
education on global climate change and studies its impact on
agriculture, natural ecosystems and society. It was established in
2004 to support Purdue in research and education on regional scale
climate change, its impacts and mitigation, and adaptation
strategies. The center serves as a hub for a range of activities
beyond scientific research, including teaching, public education and
the development of public policy recommendations.
Heat Stress Intensification in the Mediterranean Climate Change Hotspot
Noah S. Diffenbaugh, Jeremy S. Pal,
Filippo Giorgi and Xuejie Gao
We find that elevated greenhouse gas concentrations dramatically
increase heat stress risk in the Mediterranean region, with the
occurrence of hot extremes increasing by 200 to 500% throughout the
region. This heat stress intensification is due to preferential
warming of the hot tail of the daily temperature distribution, with
95th percentile maximum and minimum temperature magnitude increasing
more than 75th percentile magnitude. This preferential warming of the
hot tail is dictated in large part by a surface moisture feedback,
with areas of greatest warm-season drying showing the greatest
increases in hot temperature extremes. Fine-scale topographic and
humidity effects help to further dictate the spatial variability of
the heat stress response, with increases in dangerous Heat Index
magnified in coastal areas. Further, emissions deceleration
substantially mitigates heat stress intensification throughout the
Mediterranean region, implying that emissions reductions could reduce
the risk of increased heat stress in the coming decades.