The way humans alter the Earth's surface may be a major cause of climate change. NASA scientists say the way land is used is probably just as important as greenhouse gas emissions, and changes in the surface of the land in the tropics may have a greater influence on climate than El Nino.
Urban sprawl, the destruction and planting of forests, farming and irrigation all have a big effect on regional surface temperatures and the amount of rain. Land use changes alter the climate because different types of surface affect the distribution of the Sun's energy. Replacing a rainforest with crops means there is less evaporation from plant leaves, which leads to warmer temperatures. Irrigating farmland increases this evaporation, and there is also more evaporation from moist soils, which cools and moistens the atmosphere and can alter precipitation. Planting or replacing trees in forests that get heavy snowfall means there is less reflected sunlight and more heat absorption, which causes global warming, despite the trees? use of CO2. At sea, periodic El Nino disturbances in the Pacific create moist rising air and thunderstorms, which can affect the weather thousands of miles away.
Researcher Roger Pielke says, "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. Through landcover changes over the last 300 years, we may have already altered the climate more than would occur with the radiative effect of a doubling of carbon dioxide."
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