With the development of economy， more and more greenhouse gases are being discharged into the air。 CO2 makes up about 70% of the greenhouse gases， which is considered to be one of the greatest causes of global warming。 It has harmful effects， causing the sea level to rise and many natural disasters to strike。

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So we must take effective measures to save our planet。 First， we should use more clean energy and less coal and oil。 Second， we should go to work on foot or by bus instead of driving a car if possible。 Third， we should plant more trees， because plants can turn CO2 into oxygen。 More importantly， we should form the habit of saving energy in our daily life。