Abstract: The emission scenarios were briefly introduced,which had been put forward by IPCC in the year of 2000.Then,with a simple global carbon cycle model,the atmospheric CO₂ volume fraction during the 21st century was projected.Results show that:(1)during the 21st century,atmospheric CO₂ volume fraction will increase and the increasing rate is higher than that of the 20th century.Fossil fuel source will have the dominant influence on the trend in atmospheric CO₂ volume fraction.(2)If fossil fuels were used as the main energy source,the portion of the anthropogenic carbon absorbed by the atmosphere would increase and the CO₂ volume fraction would exceed 1 000×10^-6 by the year of 2100.(3)Only can we manage to explore new energy and make the fossil fuel source intensity decrease year by year, the portion of the anthropogenic carbon absorbed by the atmosphere could decrease.If we further improve the land-use conditions,CO₂ volume fraction might decrease at the end of the 21st century.