In the 1960s Professor Bolin predicted   that the "greenhouse effect", caused by an increase in the amount of carbon dioxide (CO2) in the atmosphere, would lead to important changes in the Earth's climate. At the time his prediction were regarded as science fiction. But it is now gradually agreed that the amount of carbon dioxide in the atmosphere will double from 0.03% to 0.06% in the next 50 years and that temperatures worldwide will rise by 2degC. Although a temperature rise of 2deg may not seem significant, the local effects may be much greater: in polar regions a rise of 10deg by 2025 is expected and in Northern Europe a rise of 4deg. Indeed the first effects were expected to be felt before the end of the 20th century. But how does the "greenhouse effect" _ and why should such a tiny proportion   of CO2have such a harmful effect? When living creatures breathe out or when things are burned, CO2enters the atmosphere. Until recently all of this was absorbed by plants, which converted   it back into oxygen. However, the balance of nature has been disturbed. In power stations, in factories and in our cars, we are burning more and more fossil fuels (coal, oil and natural gas) and this produces huge quantities of CO2--18 billion tons of it enter the atmosphere every year. Added to this, the destruction of forests means that less CO2can be converted into oxygen by plants. So, the amount of CO2in the atmosphere is increasing every year. The amount of CO2in the atmosphere is increasing every year because  _  .
Carbon dioxide is produced when we breathe. Carbon dioxide is produced when we burn things. More forests are destroyed and more fossil fuels are burned. The global temperature keeps rising.