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The amount of carbon dioxide in the atmosphere has been increasing by 0.4 per cent a year because of the use of fossil fuels such as oil, gas, and coal. The cutting of tropical forests has also been a contributing factor in the carbon cycle. Other gases that contribute to the greenhouse effect, such as methane and halocarbons, are increasing even faster. The net effect of these increases could be a worldwide rise in temperature, estimated at 2° to 6° С (4° to 11 0 F) over the next 100 years. Warming of this magnitude would alter climates throughout the world, affect crop production, and cause sea levels to rise significantly. If this happened, millions of people would be badly affected by flooding.