Science News - Aimee Cunningham
When days and nights get too hot, city dwellers are the first to run into trouble
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HEAT ISLANDS
Heat claims more lives than floods, hurricanes and other weather-related disasters. How will cities cope as temperatures rise? Ultraforma/iStockphoto |
Some victims were found at home. An
84-year-old woman who’d spent over half her life in the same Sacramento,
Calif., apartment died near her front door, gripping her keys. A World
War II veteran succumbed in his bedroom. Many died outside, including a
hiker who perished on the Pacific Crest Trail, his water bottles empty.
The
killer? Heat. Hundreds of others lost their lives when a stifling air
mass settled on California in July 2006. And this repeat offender’s rap
sheet stretches on. In Chicago, a multiday scorcher in July 1995 killed
nearly 700. Elderly, black residents and people in homes without air
conditioning were hardest hit. Europe’s 2003 heat wave left more than
70,000 dead, almost 20,000 of them in France. Many elderly Parisians
baked to death in upper-floor apartments while younger residents who
might have checked in on their neighbors were on August vacation. In
2010, Russia lost at least 10,000 residents to heat. India, in 2015,
reported more than 2,500 heat-related deaths.
Year in and year
out, heat claims lives. Since 1986, the first year the National Weather
Service reported data on heat-related deaths, more people in the United
States have died from heat (3,979) than from any other weather-related
disaster — more than floods (2,599), tornadoes (2,116) or hurricanes
(1,391). Heat’s victim counts would be even higher, but unless the
deceased are found with a fatal body temperature or in a hot room, the
fact that heat might have been the cause is often left off of the death
certificate, says Jonathan Patz, director of the Global Health Institute
at the University of Wisconsin–Madison.
As greenhouse gases
accumulate in the atmosphere, heat’s toll is expected to rise.
Temperatures will probably keep smashing records as carbon dioxide,
methane and other gases continue warming the planet. Heat waves
(unusually hot weather lasting two or more days) will probably be
longer, hotter and more frequent in the future.
Beyond deaths,
researchers are beginning to document other losses: Heat appears to rob
us of sleep, of smarts and of healthy births. “Heat has the ability to
affect so many people,” says Rupa Basu, an epidemiologist with the
California Environmental Protection Agency’s Office of Environmental
Health Hazard Assessment in Oakland. “Everybody’s vulnerable.”
Many
people see heat as more of an annoyance than a threat, but climate
change, extreme heat and human health are entwined. “There might not be a
huge burden of disease from heat-related illness right now in your
community,” says Jeremy Hess, an emergency medicine physician and public
health researcher at the University of Washington in Seattle. “But give
it another 20 years, and it might be a more significant issue.”
Nowhere but up
The
number of days each year above 95° Fahrenheit (35° Celsius) is expected
to rise across the United States, and average summer temperatures will
reach new heights if greenhouse gas emissions remain high. The maps
below compare late 20th century temperatures to projections for the
mid–21st century.
Adaptation has limits
The
human body can’t tolerate excessive heat. The biological and chemical
processes that keep us alive are best carried out at a core temperature
of 36° to 37° Celsius (96.8° to 98.6° Fahrenheit), with slight variation
from person to person. Beyond that, “the body’s primary response to
heat is to try and get rid of it,” says Jonathan Samet, dean of the
Colorado School of Public Health in Aurora. Blood vessels in the skin
dilate and heart rate goes up to push blood flow to the skin, where the
blood can release heat to cool down. Meanwhile, sweating kicks in to
cool the skin.
With repeated exposure to high temperatures, the
body can become more efficient at shedding excess heat. That’s why a
person can move from cold Minneapolis to steamy Miami and get used to
the higher heat and humidity. But there is a limit to how much a person
can adjust, which depends on the person’s underlying health and the
ambient temperature and humidity. If the outside is hotter than the
body, blood at the skin surface won’t release heat. If humidity is high,
sweating won’t cool the skin. Two scientists proposed in 2008 that
humans cannot effectively dissipate heat with extended exposure to a
wet-bulb temperature, which combines heat and humidity, that is greater
than 35° C.
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| In New Delhi during a May 2015 heat wave, a man wipes his brow. That year, heat claimed more than 2,500 lives in the South Asian country. Tsering Topgyal/AP Photo |
Forced to regulate heat without a break,
the body gets worn out. Heat exhaustion leads to weakness, dizziness
and nausea. If a person doesn’t cool off, heat stroke is likely — and
likely fatal. The ability to regulate heat breaks down and core body
temperature reaches or exceeds 40° C. A person suffering heat stroke may
have seizures, convulsions or go into a coma.
No one is immune to
heat, but it hits some groups harder than others. The elderly,
considered the most vulnerable, have fewer sweat glands and their bodies
respond more slowly to rising temperatures. Children haven’t fully
developed the ability to regulate heat, and pregnant women can struggle
due to the demands of the fetus. People with chronic diseases like
diabetes, cardiovascular disease and obesity can have trouble
dissipating heat. And, of course, people living in poverty often lack
air conditioning and other resources to withstand sweltering conditions.
Collateral damage
Researchers are discovering more ways
that heat can hurt. Take sleep: The onset and duration of sleep is
sensitive to temperature. The body cools down as it prepares to sleep;
this decrease in core temperature is a signal to bring on the z’s. Body
temperature stays low throughout the night, then rises just before
awakening. A good night’s rest is a cornerstone of health.
Hot
nights make for bad sleep, according to a study combining responses to a
U.S. Centers for Disease Control and Prevention sleep survey of 765,000
U.S. residents from 2002 to 2011 with data on nighttime temperatures
during that period. The higher the nighttime temperatures, the more
nights respondents reported getting too little shut-eye.
The effect hit low-income respondents and the elderly hardest, the researchers reported in May 2017 in
Science Advances.
The ability to think and calculate may take a beating in the heat,
according to a small study presented in January in Austin, Texas, at
the American Meteorological Society’s annual meeting. Researchers from
Harvard University tested undergraduate students for 12 days — the time
before, during and after a heat wave. Twenty-four lived in buildings
with air conditioning and 20 in buildings without. The researchers
assessed how quickly and accurately students performed an addition and
subtraction test and a test that asked for the color of a written word,
rather than the word itself. During the heat wave, the students without
air conditioning got about 6 percent fewer correct answers on the math
problems and 10 percent fewer on the color problems than the students
with air conditioning.
Heat may even increase the risk of
stillbirth. Researchers with the National Institute of Child Health and
Human Development in Bethesda, Md., analyzed weather data and more than
223,000 U.S. births from 2002 to 2008. During the warm months of the
year, a 1 degree C increase in temperature during the week before birth
was associated with about four additional stillbirths per 10,000 births, the researchers reported in June 2017 in
Environmental Health Perspectives.
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In July 2014, a worker in Las Vegas cools off with a wet cloth during a break from her outdoor advertising job. Hot days are expected to become more common. AP Photo/John Locher
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As heat gets vicious, it threatens to
disrupt the fabric of society. Extreme heat — beyond a wet-bulb
temperature of 35° C — could become more regular in South Asia and the
Persian Gulf,
rendering parts of those areas uninhabitable, according to studies in the August 2017
Science Advances (
SN: 9/2/17, p. 10) and the February 2016
Nature Climate Change.
It’s not hard to imagine that there will be profound societal and
political instability “in a world where tens of millions of people have
to move and are looking for cooler places to live,” says Howard Frumkin,
a physician epidemiologist specializing in environmental health at the
University of Washington.
Emerald cities
Fifty-four
percent of the world’s population — and around 80 percent of U.S.
residents — live in urban areas. Cities are where some action to combat
heat can be taken now, says Brian Stone Jr., an environmental planner
and member of the Urban Climate Lab at Georgia Tech in Atlanta. “If
we’re waiting for the national government to signal it’s time to do
this, we’re going to wait too long,” he says. “We are well into a world
that’s been altered by climate change.”
Heat thrives in cities.
All of the nonreflective roofs, walls, roads and other surfaces absorb
and retain heat during the day. Waste heat, emitted from air
conditioners and vehicles, concentrates in cities too. Together, these
factors contribute to what’s called
an urban heat island,
an amplification of heat that occurs within cities. On average, a city
with at least a million residents can be 1 to 3 degrees C hotter than
surrounding areas. At night, the temperature differences widen. Cities
may be as much as 12 degrees C hotter than surrounding areas in the
evening hours, because cities release built-up heat back out among
buildings and avenues.
Hotlanta
These Landsat
satellite images show urban Atlanta on September 28, 2000. The core
urban area is at the center of the images. The left side shows areas of
vegetation (green), bare ground (brown) and roads and dense development
(gray). The heat map on the right shows the areas of densest development
also have the hottest land surface temperatures (red), near 30 degrees
Celsius. The areas of heaviest vegetation are the coolest (yellow) due
to evaporation of water and shade.
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| Marit Jentoft-Nilsen/NASA |
City planners can rid their locales
of some of this heat with several strategies. One is to plant more trees
to create shade for residents and structures. Trees also lower the air
temperature by transferring water from the soil through the tree to the
air. The surrounding air is cooled as the water changes from a liquid to
a vapor. The process is “much like the way sweating works for our
bodies,” says George Ban-Weiss, an environmental engineer at the
University of Southern California in Los Angeles.
Another strategy
is to reduce the amount of sunlight that city surfaces absorb by using
“cool” materials on exposed surfaces. The best known are cool roofs,
which “reflect more sunlight than usual,” says Ronnen Levinson of
Lawrence Berkeley National Laboratory in Berkeley, Calif., who studies
cool surfaces and urban heat islands. In general, to make a surface
cool, you make it lighter, with coatings or other light-colored
materials. For example, a white roof that reflects 80 percent of the
sun’s light on a typical summer afternoon will stay about 31 degrees C
cooler than a gray roof that reflects only 20 percent.
Cool Top
A light roof (top) reflects more sunlight and can thus be dozens of degrees cooler than a dark roof.
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Giving buildings cool-surface makeovers
counters the urban heat island effect and reduces the temperature inside
a building. “In disadvantaged communities, people simply may not have
air conditioning to help them ride out hot summers,” Levinson says.
Cooling off the insides of buildings is “where I think the greatest
potential benefits are for improving human comfort and health,” he says.
Stone
has estimated how many heat-related deaths could be avoided by reducing
urban heat island effects. In 2016, he and colleagues produced a report
for the city of Louisville, Ky., that analyzed the impact of adding
450,000 trees, converting 168 square kilometers of surfaces to cool
materials and more. The researchers estimated that areas of the city
could reduce average summertime temperatures by as much as 1.7 degrees C
or more. And based on the 53 deaths Stone attributed to the city’s
unusually warm summer of 2012, there could be 11 fewer deaths from heat,
a reduction of 21 percent. “When we get a big heat wave,” Stone says,
“that could really translate into hundreds of lives.”
Many cities
in the United States and abroad are working on tempering their urban
heat islands with a variety of strategies, including programs to install
cool roofs or plant more trees. The city of Los Angeles now requires
that new or replaced roofs for homes and other residential buildings
meet a solar reflectance index value — a measure of a materials’ ability
to stay cool in the sun between zero (black surface) and 100 (white) —
of at least 75 for flatter roofs and 16 for steeper ones. Through a
provision in
California’s building energy efficiency code,
cities throughout the state have been converting flat, commercial
roofs, like those on big-box stores, to light-colored cool roofs when a
new topper is needed.
Sun-day In The Park
The cooler hues of Central Park jump out of this otherwise red-hot map of New York City heat on a summer day. The built areas of the city are around 10 degrees Fahrenheit higher than tree-filled parts of the park.
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New York City has
planted a million new trees since 2007
and committed additional funds to adding even more to streets and
parks. The city also has coated 0.62 square kilometers of roof surfaces
white since 2009. The city of Ahmedabad, India, where about 25 percent
of the residents live in slum communities, announced a heat action plan
in 2017 that includes a cool roofs initiative to paint or otherwise
convert at least 500 slum household roofs and to improve the
reflectivity of roofs on government buildings and schools.
Measures
that tackle the urban heat island effect also make cities more energy
efficient (by reducing the cooling needs inside buildings) and more
comfortable (by shading city residents). Individual cities need to
implement strategies that make sense for their landscapes, their water
resources, their usual climate and their populations, Ban-Weiss says.
But
ameliorating urban heat can only do so much. There will still need to
be a worldwide push to reduce emissions of greenhouse gases. Ban-Weiss
and colleagues estimated how much cool roofs could counter warming from
climate change in Southern California. Assuming that greenhouse gas
emissions continue to increase, the widespread adoption of cool roofs in
the Los Angeles metropolitan area
would offset some of the warming expected by midcentury, the team reported in 2016 in
Environmental Research Letters. But by the end of the century, Ban-Weiss says, the cool roof benefits “become mostly dwarfed by climate change.”
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