Cities were already known to retain more heat than the rural environments that surround them, but new modeling from researchers in the United Kingdom now suggests that urban areas are also more sensitive to changes in climate. Furthermore, they will experience greater increases in average temperature with rises in atmospheric carbon dioxide, and the cooling effects of night will become more of a memory than a reality.
Meanwhile, Washington, D.C. -- where Congress is debating over whether to pass a climate bill -- is getting a memorable preview of what new computer models are predicting. Last week's temperatures broke a 100-year record, and forecasters expect this June will be the hottest ever recorded in the area.
"We're getting a dramatic taste of the kind of weather we are on course to bequeath to our grandchildren," said Tom Peterson, chief climate scientist for the National Oceanic and Atmospheric Administration's National Climatic Data Center.
Urban areas produce their own environments. Vegetation is replaced with steel and Concrete. A natural breeze is reconfigured by skyscrapers and other tall buildings. Soil is covered with black asphalt and loaded with automobiles.
Each transformation contributes something more to what is known as the urban heat island, or UHI, effect, a phenomenon scientists have known about for almost 200 years.
Instead of being consumed by plants or transported away by soil moisture, much of the daytime heat directed into urban areas is absorbed by hard, impermeable surfaces that have no other way of releasing their stored heat except to re-radiate it at night. This gives residents of urban areas little relief from the summer heat long after the sun sets.
In New York City, evening air temperatures can be up to 14 degrees Fahrenheit warmer than those of rural areas that lie within roughly 60 miles, according to a study published by American Meteorological Society in 2009.
Feeling a rise of 5 degrees by 2050
This stored heat will only get worse with increased atmospheric carbon dioxide, said Mark McCarthy, lead author and a research scientist with the Climate Impacts team at the Met Office, the United Kingdom's weather service. The analysis, published in Geophysical Research Letters, was co-authored with Met Office colleagues Richard Betts, head of Climate Impacts, and Martin Best.
The research found that "urban areas are warming faster" than rural ones, in response to rising levels of carbon dioxide, said McCarthy. Their models predict that urban daytime temperatures will rise by more than 5 degrees Fahrenheit in most parts of the world when carbon dioxide levels in the atmosphere reach 645 parts per million, a figure possible as early as 2050.
Nighttime temperatures will also rise by similar numbers. However, in the Middle East, where the UHI effect is the most extreme, cities throughout the region could feel an additional 5 degrees at night.
Besides the Middle East, some of the regions whose local climate is most sensitive to urbanization, including central Asia and western Africa, are also expected to double or even triple in population by 2050, according to U.N. estimates. By that time, more than 68 percent of the world's population will reside within urban areas -- up from 50 percent in 2009.
These trends in temperature and population migration, McCarthy said, are sure to have significant consequences for human health by raising the possibility of heat-related fatalities. According to the U.S. Global Change Research Program, winter cold snaps raise death rates by 1.6 percent, while heat waves are far more lethal, pushing death rates up by 5.7 percent.