There are approximately three quarters of a ton of termites (白蚁) for every person on Earth. It now turns out that these critters may be helping to alter the climate and affect man's life. For years scientists have been saying that carbon dioxide sent into the atmosphere by the burning of fuels might lead to the rise in temperatures of the whole earth, owing to the greenhouse effect in which the gas prs the escape of heat into outer space. Now an international team of researchers has discovered that termites generate more than twice the carbon dioxide that fuel burning does. And this production, which comes from the insects' eating and consuming of different kinds of vegetables and wood, has risen greatly. This is chiefly because man has cut so many trees in order to open more land and thus has provided large quantities of food for the insects in the form. of tiny pieces of wood. The output from the burning of fuels is of greater concern, however, because it is continuing to rise steadily and so far no effective measures have been taken to reduce the amount of carbon elements in this way. Production by termites, on the other hand, is not likely to increase much more. From the passage we know that' the greenhouse effect' is an effect in which ______ .