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Human interference had altered the surface of the earth long before the present era (Thomas, 1956). The first major change started about 7000 years ago when man developed agriculture. This led to systematic changing of forested areas to fields and pastures.
Agriculture and lumbering have undoubtedly led to mesoscale climatic changes, but these are poorly documented, although one can make some approximate guesses at their magnitude. In many instances secondary changes have been more far-reaching. After the clearing, wind and water erosion have washed or blown the top soil away. Bare rock has become exposed, and now far more extreme temperatures and lower humidities prevail where once the even-tempered mesoclimate of the forest dominated. Stretches of Anatolia, the Spanish plateau, and some slopes of the Italian Apennines are silent witnesses to this development.
But by far the most alarming development has been the substitution of rocklike, well-compacted, impermeable surfaces for vegetated soil, a development that is the natural consequence of urbanization. Square kilometer after square kilometer has yielded to the bulldozer and has been converted to buildings, highways, and parking lots. Reservoirs and irrigation also have become important.
The scientists examined satellite measurements of sulfuric acid droplets called aerosols in the upper atmosphere, most of which come from volcanic eruptions. The particles can reflect sunlight back into space, the researchers said, perhaps reducing the warming effects climate scientists say are driven mainly by release of heat-trapping carbon dioxide into the atmosphere by humans. The study, led by Susan Solomon, an atmospheric scientist who recently retired from NOAA, showed that such particles have been gradually increasing over the past decade. That was enough to cool the Earth's atmosphere by 0.1 watts per meter squared between 2000 and 2010. But while it might have blunted the effects of global warming, it would not be enough to overcome the trend; carbon dioxide emissions warmed the planet during the same period by 0.3 watts per meter squared.