There is now no doubt that the world is getting warmer. Data from both the Southern and Northern Hemispheres shows an upward trend in average temperatures over the 1 hundred years. At the moment most scientists are not 2 to stick their necks out and 3 a cause to this trend. It is possible that it represents a natural climatic change caused by a 4 increase in the sun’s 5 output. But these same scientists are 6 aware that recent temperature changes are at the limit of known natural 7 it seems increasingly likely that the 8 is an enhanced greenhouse effect. The basic theory of the greenhouse effect is quite simple. The earth's atmosphere consists 9 of oxygen and nitrogen, but there are small 10 of various “greenhouse” gases—notably carbon dioxide, water vapor, and methane—which play a very important role in maintaining the planet's “ 11 balance”. As their name suggests these gases have a similar effect to 12 in a greenhouse: they let heat from the sun 13 , and keep it in. The atmosphere is 14 transparent to the visible solar radiation, which warms the earth (ground or oceans) on which it falls. 15 it has been heated, the earth is warmer than space and 16 energy in the form of invisible long-wave infrared radiation. This 17 for nighttime cooling. Greenhouse gases absorb some of this long-wave radiation, and 18 it in the lower atmosphere. The gases therefore act rather like a blanket by preventing some infrared radiation from leaving the earth-atmosphere system. 19 certain limits, the more greenhouse gases present, the more infra-red radiation will become 20 , and the higher the surface temperature of the earth.