The atmosphere is composed mainly of nitrogen and oxygen, with a small fraction of carbon dioxide (CO₂) and other trace gases. Direct measurements since the 1950s show a continuous rise in CO₂ concentration. Northern and southern hemispheres, tropics and poles, all show the same rise. The rise in the total amount of CO₂ is less than the total CO₂ due to burning of fossil fuels and tropical forest-clearing; some of the CO₂ has gone into the ocean and into temperate forest growth.

The amounts of CO₂ and some other important trace gases were nearly constant for thousands of years before human activities started pumping them into the atmosphere during the industrial revolution. We know this because we can measure the composition of air bubbles trapped in the ice sheets near the poles, and we can tell how long ago the ice was deposited. There is virtually no controversy among experts - even among those who are skeptical of human influence on temperature - that humans are making big changes in the chemistry of the atmosphere.

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Carbon Emissions: Comparison Among Nations.