According to the United Nations’ 2011 Revision of World Urbanization
Prospects, global urban population is expected to gain more than 2.5 billion
new inhabitants through 2050. Such sharp increases in the number of urban
dwellers will require considerable conversion of natural to urban landscapes,
resulting in newly developing and expanding megapolitan areas.
Could climate impacts arising from built environment growth pose additional
concerns for urban residents also expected to deal with impacts resulting from
global climate change?
In the first study of its kind, attempting to quantify the impact of rapidly
expanding megapolitan areas on regional climate, a team of researchers from
Arizona State University and the National Center for Atmospheric Research has
showed that local maximum summertime warming resulting from projected expansion
of the urban Sun Corridor could approach 4 C.
This finding, reported in Nature
Climate Change, establishes that this factor can be as important as warming
that results from increased levels of greenhouse gases.
Arizona’s Sun Corridor is the most rapidly growing megapolitan area in the
U.S. Nestled in a semi-arid environment, it is composed of four metropolitan
areas: Phoenix, Tucson, Prescott, and Nogales. With a population
projection expected to exceed 9 million people by 2040, the developing Sun
Corridor megapolitan provides a unique opportunity to diagnose the influence of
large-scale urbanization on climate, and its relation to global climate change.
“We posed a fundamental set of questions in our study, examining the
different scenarios of Sun Corridor expansion through mid-century,” says
Matei Georgescu, lead author and assistant professor in the School of
Geographical Sciences and Urban Planning in ASU’s College of Liberal Arts and
Sciences. “We asked, ‘what are the summertime regional climate
implications, and how do these impacts compare to climate change resulting from
increased emissions of greenhouse gases?'”
The study’s authors used projections of Sun Corridor growth by 2050
developed by the Maricopa Association of Governments, the regional agency for
metropolitan Phoenix tasked with providing long-range and sustainably oriented
planning. Incorporating maximum- and minimum-growth scenarios into a regional
climate model, the researchers compared these impacts with experiments using an
urban representation of modern-day central Arizona. Their conclusions indicate
substantial summertime warming.
“The worst-case expansion scenario we utilized led to local maximum summer
warming of nearly 4 C,” said Georgescu. “In the best-case scenario,
where Sun Corridor expansion is both more constrained and urban land use
density is lower, our results still indicate considerable local warming, up to
about 2 C.”
An additional experiment was conducted to examine an adaptation where all of
the buildings were topped by highly reflective white or “cool” roofs.
“Incorporating cool roofs alleviated summertime warming substantially,”
says Georgescu, “reducing the maximum local warming by about half. But
another consequence of such large-scale urbanization and this adaptation
approach also include effects on the region’s hydroclimate.”
The cool roofs, like the maximum-growth scenario without this adaptation
approach, further reduce evapotranspiration—water that evaporates from the soil
and transpires from plants.
Ultimately, comparison of summertime warming resulting from Sun Corridor
expansion to greenhouse gas-induced summertime climate change shows that
through mid-century the maximum urbanization scenario leads to greater warming
than climate change.
However, the authors say that pinning precise figures on the relative
contribution of each effector is difficult.
“The actual contribution of urban warming relative to summertime climate
change warming depends critically on the path of urbanization, the conversion
of natural to urban landscapes, and the degree to which we continue to emit
greenhouse gases,” says Alex Mahalov, a coauthor of the Nature Climate Change article and principal investigator of the
National Science Foundation grant, “Multiscale Modeling of Urban Atmospheres in
a Changing Climate,” which supported the research.
“As well as providing insights for sustainable growth of the Sun Corridor
and other rapidly expanding megapolitan areas, this research offers one way to
quantify and understand the relative impacts of urbanization and global
warming,” says Mahalov, who is the Wilhoit Foundation Dean’s Distinguished
Professor in the School of Mathematical and Statistical Sciences at ASU.
The group conducted their numerical simulations using an ensemble-based
approach. By modifying their model’s initial conditions and repeating their
simulations a number of times, they were able to test the robustness of their
results. In all, nearly half of a century of simulations were conducted.
“By incorporating differing Sun Corridor growth scenarios into a
high-performance computing modeling framework with projections obtained from
Maricopa Association of Governments, we quantified direct hydroclimatic impacts
due to anticipated expansion of the built environment,” Mahalov added.
Simulations were conducted at ASU’s Advanced Computing Center (A2C2).
Georgescu says that one take-home message from this study is that the
incorporation of sustainable policies need to extend beyond just greenhouse gas
emissions. He also stresses the importance of extending adaptation strategies
beyond the focus on mere average temperature.
“Truly sustainable adaptation, from an environmental standpoint, must extend
to the entire climate system, including impacts on temperature and hydrology,”
Source: Arizona State University