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Land Use

NCAR research demonstrates that croplands in the United States and other countries in the midlatitudes tend to cool temperatures more dramatically than forests, even though forests are shadier. The reason: croplands are lighter in color than forests, and therefore they reflect more sunlight. You’ve probably noticed that when you walk across a paved parking lot on a summer day, the air around you is a lot warmer than when you walk through an open field. That’s because pavement absorbs more heat than open ground. The difference is one example of how the land surface can have an impact on the air above it. Scientists are now working to measure and better understand the effects plants, soil, microorganisms, and the urban environment can have on our climate.

A relatively new research area, known as biogeoscience, looks at the interaction between living organisms and the physical world, including the atmosphere. Many climate researchers are turning their attention to plants, because plants affect the cycling of energy and water between land and atmosphere and the mix of chemicals in the atmosphere.

NCAR research demonstrates that croplands in the United States and other countries in the midlatitudes tend to cool temperatures more dramatically than forests, even though forests are shadier. The reason: croplands are lighter in color than forests, and therefore they reflect more sunlight into space instead of absorbing that energy. By one estimate, present-day farmland is cooling U.S. temperatures in July by as much as 1°C to 2°C (1.8°F to 3.6°F).

Plants also have profound impacts on chemicals in the atmosphere, such as carbon dioxide—a greenhouse gas associated with global warming. Trees can offset human-related emissions of carbon dioxide by absorbing the gas. But when forests are cleared to make room for development, a large amount of carbon dioxide is released back into the atmosphere, which may accelerate warming.

Researchers are gradually unraveling other interactions between atmospheric chemicals and the biosphere. For example, NCAR scientists have found that increased levels of atmospheric carbon dioxide are likely to have a detrimental impact on biologically diverse coral reefs. As carbon dioxide builds up in the atmosphere, more of it is dissolved into the ocean. This increases ocean acidity and lowers concentrations of the carbonate ion, a building block of calcium carbonate that corals and other organisms use to grow their skeletons and build up the reefs.

As the coral health declines, more algae grow on the reefs. Interestingly, the algae may take in carbon dioxide, thereby slightly reducing the amount of the gas in the atmosphere.

Soil also plays a role in climate. Moist soil can feed showers and thunderstorms, whereas dry soil may aggravate drought conditions. When suburban residents water their lawns, they may actually be cooling the air somewhat and creating more humidity, because the water will evaporate. Soil acts as a reservoir of carbon dioxide as well, absorbing it as plants degrade and eventually releasing it back into the air. A team of NCAR researchers is developing a computer model for soil activity to determine its role in the global carbon cycle.

As you probably have experienced firsthand, large cities are warmer than rural areas. One of the primary reasons for this is that asphalt and concrete absorb heat much more efficiently than do trees and grass. Because urban areas are projected to expand throughout the world in coming decades, this urban heat-island effect can have important impacts on regional climate. In addition to higher temperatures, large cities can also spawn localized storms. The cause has to do with the clash of warm air over the city and cooler air over the suburbs.

Changes in land cover are not only a modern occurrence. Paleoclimate researchers study reflections of prehistoric climate in the proxy climate record of fossils, ice bubbles, and other materials and reconstruct Earth’s prehistoric orbit, ocean circulation, and land masses. They use computers to model these factors in the same way their colleagues model future climate.

NCAR researchers are examining how the paleoclimate affected vegetation and, in turn, how vegetation affected climate. In a trial run, they have simulated conditions in North Africa 6,000 years ago, when the now-arid region was comparatively fertile because of intense monsoons. They found that a greener North Africa, which had darker and more loamy soils than the sands of today’s Sahara Desert, helped fuel the monsoons for two reasons. The vegetation and darker soil absorbed sunlight (increasing ambient heat and providing more energy for the storms). And the soil collected a considerable amount of moisture (leading to local evaporation and providing potential storms with water vapor).