While many scientists and policy makers have focused only on how heat-trapping gases like carbon dioxide are altering our global climate, a new NASA-funded study points to the importance of also including human-caused land-use changes as a major factor contributing to climate change.

Land surface changes, like urban sprawl, deforestation and reforestation, and agricultural and irrigation practices strongly affect regional surface temperatures, precipitation and larger-scale atmospheric circulation. The study argues that human-caused land surface changes in places like North America, Europe, and southeast Asia, redistribute heat regionally and globally within the atmosphere and may actually have a greater impact on climate than that due to anthropogenic greenhouse gases combined.

These simulations are examples of how global land-use changed from 1700, 1900, 1970, and 1990. The human-disturbed landscape includes intensive cropland (red), and marginal cropland used for grazing (pink). Other landscape includes, for example, tropical evergreen and deciduous forest (dark green), savanna (light green), grassland and steppe (yellow), open shrubland (maroon), temperate deciduous forest (blue), temperate needleleaf evergreen forest (light yellow), and hot desert (orange). Of particular importance in this paper is the expansion of the cropland and grazed land between 1700 and 1900. Credit: from Klein Goldewijk, K., 2001: Estimating global land use change over the past 300 years: The HYDE Database. Global Biogeochemical Cycles, 15, 417-433

The study also proposes a new method for comparing different human-influenced agents of climate change in terms of the redistribution of heat over land and in the atmosphere. Using a single unit of measurement may open the door to future work that more accurately represents human-caused climate change.

"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, Sr., an atmospheric scientist at Colorado State University, Fort Collins, Colo., and 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." If carbon dioxide (CO2) emissions continue at current rates, atmospheric CO2 concentrations are expected to double by 2050. Land surface changes will also continue to occur.

Types of land surface strongly influence how the Sun's energy is distributed back to the atmosphere. For example, if a rainforest is removed and replaced with crops, there is less transpiration, or 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 or afforestation 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 vapor in the air. Water vapor in the troposphere is the biggest contributor to greenhouse gas warming.

The effect of landscape change on thunderstorms in the central Great Plains of the U.S. is illustrated in these two figures. In the top figure, the current mixed landscape of grasslands, shrubs, and crops are specified in a computer model of weather on a specific May day for the Texas/Oklahoma panhandle area. A deep thunderstorm was simulated in the model, which actually occurred on the day of the computer simulation. In the bottom figure, the same large scale weather was prescribed, but the landscape was replaced with the natural short grass prairie of this region. With the reduction in vegetation in the natural landscape (and thus less transpiration), there is less water vapor to fuel thunderstorms. As a result, only a shallow line of cumulus clouds was simulated. The green shading in the figure represents the most humid air in the model atmosphere. (from Pielke et al 1997). Credit: Conrad Ziegler, National Severe Storms Laboratory

Local land surface changes can also influence the atmosphere in far-reaching ways, much like regional warming of tropical eastern and central Pacific Ocean waters known as El Nino. El Nino events create moist rising air, thunderstorms and cumulus clouds, which in turn alter atmospheric circulations that export heat, moisture, and energy to higher latitudes. Tropical land surface changes should be expected to play a greater role on global climate than El Nino, given that thunderstorms prefer to form over land, and the fact that the large area of tropical land-use changes far exceeds the relatively small area of water responsible for El Nino. Impacts of land use changes are harder to detect because they are permanent, as opposed to El Nino, which comes and goes.

Pielke Sr., and colleagues propose a new method for measuring the impacts of both greenhouse gases and landcover changes by using a formula that quantifies all the various anthropogenic climate change factors in terms of the amount of heat that is redistributed from one area to another. This heat redistribution is stated in terms of watts per meter squared, or the amount of heat associated with a square meter area. For example, if a flashlight generated heat of one watt that covers a square meter, then the heat energy emitted would be one watt per meter squared.

By using a measure based on the spatial redistribution of heat to quantify the different human influences on climate, including landcover changes and greenhouse gases, the researchers hope to achieve a more accurate portrayal of all of the anthropogenic influences on climate change in future research.

The paper was published in a recent issue of the Philosophical Transactions of the Royal Society of London. The research was funded by grants from NASA and the National Science Foundation.