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| Impact of Human Activity | The impact of human activity on global weather and long term climate change |
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Atmospheric Composition
The explosive growth of world population and industry over the last
200 years has led to increases in the concentration of
"greenhouse" gases. The long term effects of these gases, CO2 in particular, is
still unclear but it is believed that they will result in an overall
warming of the Earth's atmosphere.
In October 1984, an area of marked ozone depletion was observed in the lower
stratosphere over, and extending beyond, Antarctica.
It is estimated that chlorofluorocarbons (CFCs) were, until the recent implementation of the Montreal Protocol, accumulating in the atmosphere five times faster than they were being destroyed by ultraviolet radiation and that it is these man made gases that were the cause of the ozone hole. It is also estimated that this cut in levels of CFC emmission back to pre-1970 levels will not eliminate the Antarctic ozone hole for at least 50 years due to the slow global circulation and slow reaction rate of CFCs with ozone.
Mass deforestation Forests store large amounts of CO2, buffering the CO2 in the atmosphere. The carbon retained in the Amazon basin is equivalent to at least 20% of the entire atmospheric CO2. Destruction of this would release about four fifths of this to the atmosphere. Half of this would dissolve in the oceans but the other half would add to the 16% increase already observed this century, accelerating world temperature increases. A further effect of tropical rainforest destruction would be to reduce the natural production of nitrous oxide (NO). Tropical forests and their soils produce up to one half of the world's NO which helps to destroy stratospheric ozone. Any increase in stratospheric ozone would warm the stratosphere but lower global surface temperatures.
Dense tropical forests also have a great effect on the hydrological cycle through evapotranspiration and the reduction of surface runoff. With dense foliage, about a third of the rain falling on the forest never reached the ground, being re-evaporated off the leaves. Locally, deforestation results in:
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All of these work to degrade existing primary and secondary tropical rainforests into savanna
The deforestation of the Amazonian reainforests is being tracked by satellites and the extent of the deforestation can be seen here:
Deforestation in the closed forest zone of the Brazilian Amazon Basin in 1988 was estimated from Landsat Thematic Mapper data. The map shows the total area deforested as of 1988. The orginal data were analyzed and digitized in a geographic information system at 1:250,000 scale as shown in the inset. For display purposes the data have been summed into grid cells of 16 km by 16 km, and represented as a density (precentage of the cell deforested).
Computer models have been designed to simulate Amazonian deforestation and indicate that the deforestation of a typical rainforest (air temperature 27oC, mean monthly rainfall of 220 mm) and subsequent degradation to savanna would result in:
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