MOST land use changes occurring in the continental United States reduce vegetative cover and raise regional surface temperatures, according to a new study by scientists at Purdue University, the University of Maryland and the University of Colorado-Boulder.
The study, which will appear in the Royal Meteorological Society's International Journal of Climatology, found that almost any change that makes land cover less "green" contributes to warming.
However, a less obvious finding is that the conversion of any land to agricultural use results in cooling - even land that was previously forested.
This suggests that local and regional strategies such as creating green spaces and buffer zones in and around urban areas could be a tool to use in addressing climate change.
It is also further evidence that land use should be better incorporated into computer models projecting future climate conditions, said Purdue doctoral student Souleymane Fall, the article's lead author.
"What we highlight here is that a significant trend, particularly the warming trend in terms of temperatures, can also be partially explained by land use change," said Dev Niyogi, a Purdue earth and atmospheric sciences and agronomy professor and the Indiana state climatologist.
Niyogi and Fall said the idea that land use helps drive climate change has been poorly understood compared to factors such as greenhouse gas emissions.
"People realise that land use cover also is an important force - and not only at the local but also at the regional scale," said Fall, whose doctoral research focuses on the impacts of land surface properties on near-surface temperature trends.
The researchers used higher-resolution temperature data than previous studies, meaning the data were more detailed, Niyogi said.
They also employed dynamic data on land use changes from 1992 to 2001 that were derived from satellite imagery.
Among the study's findings:
- In general, the greener the land cover, the cooler the surface temperature.
Conversion to agricultural use results in cooling, while conversion from agricultural use generally results in warming. Deforestation generally results in warming, with the exception of a shift from forest to agriculture. No clear picture emerged from the impact of planting or seeding new forests. Urbanisation and conversion to bare soils have the largest warming impacts. In general, land use conversion often results in more warming than cooling.
The study took an approach called observation minus re-analysis.
Through this process, the researchers used temperature data from local ground observations, observation and computer modeling, Geographic Information Systems and statistical methods.
They were able to separate the effects of land use or cover from greenhouse warming and isolate the impact from each land use or cover type.
The more detailed data provided a clearer picture of the effects of land surface properties on near-surface temperature trends.
"We found that most land use changes, especially urbanisation, result in warming," said Eugenia Kalnay, University of Maryland professor of atmospheric and oceanic sciences and one of the study's co-authors.
"A clear exception is conversion of land from other uses to agriculture, which produces relative cooling, presumably because of increased evaporation."
While the effects of greenhouses gases like carbon dioxide are clear, Kalnay said the study does suggest that land use also needs to be considered carefully.
Another study co-author, Roger Pielke Sr, a senior research scientist in atmospheric and oceanic sciences at the University of Colorado-Boulder, said the results indicate that "unless these landscape effects are properly considered, the role of greenhouse warming in increasing surface temperatures will be significantly overstated".
More to climate change
Until recently, most scientists have viewed human-induced changes (warming) in climate as primarily the result of increasing concentrations of carbon dioxide and other greenhouse gases.
However, the current paper is the latest of a number of studies in recent years to shed light on the climate impact of land cover change.
These finding don't negate the effects of greenhouse gases like carbon dioxide, according to Kalnay.
"I think that greenhouse warming is incredibly important, but land use should not be neglected. It clearly contributes to warming, especially in urban and arid areas," she said.
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