【專八mini系列】溫室效應(yīng)（2/7）

Without it, scientists estimate that the average temperature on Earth would be colder by approximately 30 degrees Celsius, far too cold to sustain our current ecosystem. Second, we need to clarify the way humans contribute to the greenhouse effect. While the greenhouse effect is an essential environmental prerequisite for life on Earth, there really can be too much of a good thing. The problems begin when human activities distort and accelerate the natural process by creating more greenhouse gases in the atmosphere than are necessary to warm the planet to an ideal temperature, for example, burning natural gas, coal and oil, including gasoline for automobile engines, raises the level of carbon dioxide in the atmosphere. Some farming practices and land-use changes increase the levels of methane and nitrous oxide. Many factories produce long-lasting industrial gases that do not occur naturally, yet contribute significantly to the enhanced greenhouse effect and global warming that is currently under way.