Land use and land cover affect the global climate system through bio-geophysical, biogeochemical, and energy exchange processes. Variations in these processes due to land-use and land-cover change affect the climate. There are several natural processes that cause the exchange of greenhouse gasses such as CO2 from the Earth's surface to the atmosphere, such as photosynthesis, respiration, and évapotranspiration.
Changing the land use of an area can also change the amount of heat transferred between the Earth's surface and the atmosphere. The nature of the Earth's surface is also important whether the surface reflects or absorbs heat. Reflectivity of the land's surface determines how much of the Sun's energy is absorbed and available as heat. Vegetation transpiration and surface hydrology determine how the heat will be spent—whether it is converted to latent heat (absorbed or radiated heat at constant temperature) or sensible heat (heat stored as a result of an increase in temperature).
There are also physical reactions transpiring at the Earth's surface. Both vegetation and land structure determine the surface roughness, which directly determines air momentum and heat transport. According to the U.S. Global Change Research Program (USGCRP) land-use and land-cover change studies are also important because large-scale vegetation biomass and forest cover provide a critical component to the carbon cycle. Understanding how land-use change affects this biomass and how the subsequent changes in the carbon cycle affect global warming—the linkages and feedbacks between land-use and land-cover change, climate change forcings, and other related human and environmental components—is critical in order to slow global warming today and in the future.
The USGCRP also believes: "Research that examines historic, current, and future land-use and land-cover change, their drivers, feedbacks to climate, and environmental, social, economic, and human health consequences is therefore of utmost importance and often requires interagency and intergovernmental cooperation."
One of USGCRP's examples of a multiagency effort is their Congo Basin Forest Partnership, which focuses on conserving the second-largest tropical rain forest in the world in equatorial Africa. They are currently using satellite data to map forest extent, determine habitat fragmentation, and enforce conservation laws in order to minimize greenhouse gas emissions as a result of deforestation.
Another project they were directly involved in was the North America Land Cover Summit that was held in September 2006, which was designed to gather information and encourage collaboration among private institutions and government agencies in order to advance the development and application of land-cover information in Mexico, Canada, and the United States.
In 2008, they were involved in research dealing with potential land-use and land-cover change patterns, dynamics, and drivers; understanding the mutual effects and feedback between climate variability and land use/land cover; and forecasting environmental, social, economic, and human health consequences. They used satellite imagery to accomplish this.
Even though the buildup of CO2 in the atmosphere is one of the best-known anthropogenic contributions to global warming, variations in land use and surface cover are also significant impacts that affect climate change. According to the Union of Concerned Scientists (UCS), activities such as urban sprawl, agriculture, deforestation, and other human influences have altered and fragmented the natural landscape enough that it has changed the global distribution of CO2 and has changed the Earth's energy balance, which can affect the climate on multiple scales. Scientists at UCS have determined that even relatively small changes in land use affecting only 100 square miles of urban development or deforestation can change the local rainfall pattern enough that it can trigger climate disruptions.
According to a study conducted by Eugenia Kalnay and Ming Cai of the University of Maryland, land-use changes are responsible for more of the rise in global temperatures than scientists previously had thought.
According to Kalnay, who is a professor of meteorology, "Our estimates are that land-use changes in the United States since the 1960s resulted in a rise of more than 0.2°F (0.1°C) in the mean surface temperature, and estimate twice as high as those of previous studies. These findings for the United States suggest that land-use changes may account for between one-third and one-half of the observed surface global warming. The larger effect found in this study is likely because our method covers all changes in land use. Previous methods for estimating the impact of land-use change relied on measures of light at night—that only provide an indication of the effects of urbanization, but not of other changes in land use."
According to research conducted at the University of Guelph, land-use modifications for urbanization and agriculture have affected climate change data more than was previously thought. When land is changed from forests to seasonal crops or from natural land cover to urban environments, regional climate systems can be altered. Clear-cut hillsides, for instance, are much warmer than natural forested hillsides. In areas of heavy urbanization they become "heat islands" because so much heat energy is generated from industry, transportation, cars, and asphalt's absorption of solar energy. Deforestation, urbanization, and agricultural practices also add CO2 to the atmosphere.
The National Center for Atmospheric Research (NCAR) in Boulder, Colorado, has completed simulations of 21st-century climate that show that human-produced changes in land cover could produce additional warming in the Amazon region comparable to that caused by greenhouse gases. The results also showed that changes in land use in some midlatitude areas could counteract some greenhouse warming. NCAR has concluded from their simulations that including land cover as a variable in their computer modeling of climate change is important in order to get a true reflection of the future.
Johannes Feddema, of the University of Kansas, who led the research included for the first time variables representing not only the atmospheric and oceanic components of global warming but also changes in land cover as a result of deforestation, urbanization, agriculture, and other human activities.
In their simulations, deforestation added 3.6°F (2°C) or more to surface temperatures in the Amazon by 2100. In addition, it also showed a cooling in the nearby Pacific and Atlantic waters with a weakening of the large-scale Hadley circulation. Converting midlatitude forests and grasses to croplands had a cooling effect because crops reflect more sunlight and release more moisture to the air. The research team found that converting more land to agriculture tends to counteract global warming up to 50 percent across parts of North America, Europe, and Asia. The boreal forests in Canada and Russia, however, add to regional warming as they migrate northward.
According to Feddema, "The choices humans make about future land use could have a significant impact on regional and seasonal climates. The purpose of our project is to include human processes more directly in global climate models. This is the first step."
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