If you listen to global warming alarmists for very long, you will get tired of hearing about carbon dioxide, CO2.
To put things into perspective, let us take as fact the speculation of the scientists at realclimate.org who are promoting the theory of man-made global warming.
In comment #17 of this page ("The lag between temperature and CO2"), we learn that Hansen, a NASA scientist who has been much in the news, has estimated that all the greenhouse gases put together account for 35-55% of total warming of earth's climate. CO2 is "roughly half of the total greenhouse gas change," so that brings the figure down to 17-27% of the warming.
The CO2 produced by man's activities is a fraction of the natural CO2 produced by the planet. It is hard to estimate the real impact of man-made CO2 in the atmosphere, but even using the calculations of global warming alarmists, man's contribution to global warming through CO2 is a fraction of the 17-27% already noted.
Obviously CO2 is a greenhouse gas -- it helps the atmosphere trap heat from the sun. But man's contribution to CO2 levels, even if all of the rise from the historic norm (say, 280 parts per million in the year 1000 AD up to the current level of 380 parts per million) was caused by man, is about one fourth of the total.
Man is responsible for 25% of the CO2 in the atmosphere. The CO2 in the atmosphere accounts for 17-27% of the warming, according to the most famous NASA climate scientist. Man's contribution to global warming through CO2 emissions is therefore between 4% and 9% (i.e. 25% of 17-27%).