28 May 2010
Warm summer nights could become baking summer nights in cities such as London, Beijing, and São Paulo, since continued pressures from urbanization and global warming will boost the “urban heat island” effect and hike up nighttime temperature lows, a new study suggests. Unless cities adapt, the poor in rapidly growing megacities of the developing world could suffer the brunt of this effect, the researchers argue.
Researchers have known for nearly 200 years about the “urban heat island” effect. Buildings and roads in cities soak up a lot of heat from the sun, tall buildings can block cooling winds, and the cities use a lot of energy—all of which makes urban areas hotter than nearby rural areas.
But until now it hasn’t been clear how patterns of daily temperature highs and lows of cities around the world might change in decades to come, as greenhouse gases—especially carbon dioxide, or CO2—build up and global warming intensifies. “There’s been very little research on climate change projections within urban environments,” says Mark McCarthy, a climate scientist at the United Kingdom’s Met Office and lead author of the study.
McCarthy and colleagues report on the first global climate simulations that estimate how much cities might heat up in the future, from a combination of global warming, the urban heat island effect, and people’s energy use.
For their study, they used a global climate model developed by the Met Office Hadley Centre, known as HadAM3. The model included simulations of urban areas, and also energy use within those cities, which produces waste heat that boosts cities’ temperatures.
“The impact of this waste heat on a global scale is very small,” McCarthy says. “But it is very important at the scale of cities, where it can have a significant impact on urban climates.”
The study—co-authored by Martin Best and Richard Betts, also of the Met Office—considered the effect of doubling CO2 levels, from 323 parts per million, what they were in the mid-twentieth century, to 645 parts per million, which could be achieved as early as 2050 in a business-as-usual scenario. The global warming from doubling CO2 levels would boost temperatures—both the daytime highs and the nighttime lows—by about 3 degrees Celsius (5.4 degrees Fahrenheit). On top of this warming, the urban heat island effect would turn up the thermostat yet another notch, this effect being more severe at night.
The authors simulated two different levels of energy use within cities—high and low.
In the scenario with high energy, the waste heat pushed the mercury much higher. “In some regions, the increased energy use has as great an impact on the frequency of extreme events as the impact of doubled CO2 concentrations,” McCarthy says.
One of the main impacts would be that city dwellers would have to contend with hotter summer nights, the study found, in which the cities of the future don’t cool down as much at night as they do today. This could have big health impacts, since during hot summers, people need relief during cooler nights, to help them recover from the heat of the day.
“Even in wealthy countries with very high air conditioning use, heat is still one of the greatest weather-related causes of mortality,” McCarthy says. In Europe, an extreme heat wave in 2003, which has been linked with global warming, caused 20,000 to 35,000 excess deaths.
Half the world’s population already live in urban areas, and most of the growth in urban populations in coming decades will likely be in developing countries across Africa and Asia. “The ever-increasing urban population will greatly increase the number of people exposed to this risk,” McCarthy says.
“Those without respite from the heat, and those who are most vulnerable physiologically—including the elderly or people of ill health—could be most heavily affected by the extra heat,” McCarthy says.
In the study’s simulation of London, for example, the heating from higher levels of CO2 alone caused the number of hot nights to double. But the number of hot nights doubled again for the high energy use experiment. Other cities—such as Beijing, China, and São Paulo, Brazil—would respond in a similar way, the study found.
The authors use a technical definition for which nights are rated as “hot.” In essence, they count those with nighttime lows way above the norm – i.e. that “exceed the 99th percentile of the Tmin temperature distribution”– for the surrounding non-urban areas. McCarthy and colleagues found that the climates of semi-arid countries—especially in a band running from West Africa, through the Middle East, Pakistan, and India, and into eastern Asia—are most sensitive to the additional heating from urbanization. For the high energy use scenario, in the Middle East, the urban effects boosted nighttime temperatures by about 3 more degrees Celsius (5.4 degrees Fahrenheit), while nights in South Asian, West African, and the Western U.S. cities would be about 2 degrees Celsius (3.6 degrees Fahrenheit) warmer.
These areas include many of the places expecting the biggest increases in their urban populations, compounding the impact on people—unless those countries take appropriate measures to adapt to higher temperatures.
–Mason Inman, contributing science writer