I n order to study the reason why global warming is happening. Amounts of CO2 emission, CO2 concentration NOx emission, grain production, fish production , population and CO2 fix from 1900 to 2016 were studied. Since the industrial revolution, burning of fossil and production of CO2 and NOx increased greatly. Increased CO2 and NOx promoted the CO2 assimilation . Production of grain and fish increased. About 360 billion tone CO2 is produced by burning of much fossil . About 14.4 billion tone NOx is produced in 2015. Most of emitted CO2 is fixed by CO2 assimilation. But since developed country started NOx elimination and NP elimination at around 1975 , half of produced NOx is eliminated. Therefore emitted 360 billion tone CO2 is not fixed completely. Concentration of CO2 increased about 2 ppm . In 2016, 142 billion tone CO2 is remaining to give global warming. 142 billion tone CO2 must be reduced. We must promote CO2 assimilation by complete use of emitting NOx and NP in wast water. Fossil fuel is burning out soon. We should not spend precious fossil fuel for the elimination of NOx and NP. We must increase CO2 assimilation as much as possible.