US cities turning up the heat – which could spell trouble for climate change

City temperatures are on the rise, says a new report by US climatologists. On average, US cities have 10 more hot nights a year than 40 years ago, although rural temperatures have remained relatively constant. A separate study suggests heat redistribution from land-use changes could rival greenhouse gas contributions to climate change.

“What surprised me was the difference in the extreme temperature trends between rural and urban areas,” says Arthur DeGaetano, associate professor at Cornell University who reviewed temperature data from climate stations across the United States. “I expected maybe a 25 % increase for the urban areas compared to the rural ones. I didn’t expect a 300 % increase across the US.”

Because of urban population growth over the past four decades, particularly in major East Coast cities, there are more hot summer nights than ever, says DeGaetano. “This means that cities and the suburbs may be contributing greatly to their own heat problems,” he says. “Greenhouse gases could be a factor, but not the one and only cause. There is natural climate variability, and you tend to see higher temperatures during periods of drought.”

DeGaetano’s report, Trends in Twentieth-Century Temperature Extremes in the United States, will be published in a forthcoming Journal of Climate. A recent report found similar heating effects in Tokyo, where the city’s average temperature has risen by more than 5°C (see related story), prompting the Japanese government to encourage the use of roof gardens to combat summer heat.

A new NASA-funded study suggests land-use changes could be contributing significantly to climate change through urban sprawl, de- and re-forestation and agricultural practices that strongly affect regional surface temperatures. The study proposes a new method for comparing human-influenced agents of climate change in terms of the redistribution of heat over land and in the atmosphere.

“Our work suggests that the impacts of human-caused landcover changes on climate are at least as important, and quite possibly more important than those of carbon dioxide,” said Roger Pielke from Colorado State University, the lead author of the study. “Through landcover changes over the last 300 years, we may have already altered the climate more than would occur associated with the radiative effect of a doubling of carbon dioxide.”

Land surface can strongly influence how water and the Sun’s energy are distributed back to the atmosphere. If a rainforest is replaced with crops, there is less evaporation of water from leaves. Less transpiration leads to warmer temperatures in that area. On the other hand, if farmland is irrigated, more water is transpired and also evaporated from moist soils, which cools and moistens the atmosphere, and can affect precipitation and cloudiness.

Similarly, forests may influence the climate in more complicated ways than previously thought. For example, in regions with heavy snowfall, reforestation would cause the land to reflect less sunlight, and more heat would be absorbed, resulting in a net warming effect despite the removal of CO2 from the atmosphere through photosynthesis during the growing season. Further, reforestation could increase transpiration in an area, putting more water vapour in the air. Water vapour in the troposphere is the biggest contributor to greenhouse gas warming.

Impacts of land use changes are harder to detect because they are permanent, as opposed to effects like the El Niño phenomenon, which comes and goes. Pielke and colleagues propose a new method for measuring the impacts of both greenhouse gases and landcover changes, using a formula that quantifies the amount of heat that is redistributed from one area to another. The study has been published in a recent issue of the Philosophical Transactions of the Royal Society of London.

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