land use changes & climate - DIGGING A LITTLE DEEPER
Land use and land use changes can significantly contribute to overall climate change. Vegetation and soils typically act as a carbon sink, storing carbon dioxide that is absorbed through photosynthesis. When the land is disturbed, the stored carbon dioxide—along with methane and nitrous oxide—is emitted, re-entering the atmosphere. Carbon dioxide, methane, and nitrous oxide are greenhouse gases, which contribute to global warming. The clearing of land can result in soil degradation, erosion, and the leaching of nutrients; which can also possibly reduce its ability to act as a carbon sink. This reduction in the ability to store carbon can result in additional carbon dioxide remaining in the atmosphere, thereby increasing the total amount of greenhouse gases.
There are two types of land use change: direct anthropogenic (human-caused) changes and indirect changes. Examples of anthropogenic changes include deforestation, reforestation and afforestation, agriculture, and urbanization. Indirect changes include those changes in climate or in carbon dioxide concentrations that force changes in vegetation. A 2002 NASA study argued that human-caused land surface changes in areas like North America, Europe, and Southeast Asia redistribute heat within the atmosphere both regionally and globally. On a global scale, carbon dioxide emissions from land use changes represent an estimated 18% of total annual emissions; one-third of that from developing countries and over 60% from the lesser developing countries.
Direct Anthropogenic Changes
The effect of land use on the climate primarily depends on the type of land cover present within an area. For example, if rainforest is removed and replaced by crops, there will be less transpiration (evaporation of water from leaves) leading to warmer temperatures in that area. On the other hand, if irrigation is used on farmland, more water is transpired and evaporated from moist soils, which cools and moistens the atmosphere. The additional transpiration can also affect levels of precipitation and cloudiness in an area.
In regions with heavy snowfall, reforestation or afforestation would cause the land to reflect less sunlight, resulting in the absorption of more heat on the land. This would, in turn, result in a net warming effect despite the removal of carbon dioxide from the atmosphere through the process of photosynthesis during the growing season. Additional reforestation could increase transpiration, leading to more water vapor in the air. In the troposphere, water vapor is considered to be the biggest greenhouse gas contributor to global warming.
Urbanization is another change in land use that can affect the climate, sometimes significantly. Local climates tend to be warmer due to the increased amount of heat released within a densely populated area. Average temperatures in city centers can increase even more due to the high density of construction materials such as pavement and roofing materials since they tend to absorb, rather than reflect, sunlight. The phenomenon of higher urban temperatures, compared to lower temperatures in the surrounding rural areas, is known as the urban heat island effect.
The main ways that changes in climate can alter land use is through higher mean annual temperatures, altered precipitation patterns, and more frequent and extreme weather events. The territories of many plant species depend largely on temperature and rainfall patterns. As climate change affects these patterns, many types of trees and vegetation are forced to shift to higher altitudes and latitudes. While greater variability in rainfall patterns can decrease overall plant growth, higher temperatures can extend growing seasons, possibly allowing for more than one cropping cycle during the same season or the expansion of agricultural land toward the higher elevations.
Changes in temperature and rainfall can also increase the risk of insect infestation outbreaks, detrimental to forests and other plants. Extreme weather events can cause significant amounts of damage to trees and other vegetation from high winds, flooding, and storm surges. Floods and storms can also alter water flows, hurting the overall health of the forest, agricultural area, or ecosystem. In other areas, changes in climate could produce droughts that greatly reduce agricultural production, limiting food supply both regionally and beyond.
Future Land Use
Forestry and land use practices hold considerable potential for counteracting the effect of greenhouse gas emissions, helping to prevent significant climate change. These practices include focusing on planting trees, preserving and properly managing forests, and changing cultivation practices to account for increased carbon storage in the soil. Such practices could make it possible to increase carbon sinks while further reducing the emission of greenhouse gases.
These forest and land use measures could reduce net carbon emissions by the equivalent of 10-20% of projected fossil fuel emissions through 2050. Efforts to increase carbon storage in U.S. forests could sequester an additional 40-80 million metric tons of carbon annually, equal to 3-5% of current annual U.S. fossil fuel emissions.