Table : TABLE 1.2 AT A GLANCE: Variable Gases that Act as Greenhouse Gases

VARIABLE GAS NAME

PERCENTAGE BY VOLUME

PPM*

Water vapor (H2O)

        0 to 4

 

Carbon dioxide (CO2)

        0.0399

  399

Methane (CH4)

        0.00017

      1.7

Nitrous oxide (N2O)

        0.00003

      0.3

Ozone (O3)

        0.000004

      0.04

CFCs and HFCs**

        0.00000002

      0.0002

* ppm = parts per million. For example, 398 ppm carbon dioxide means that for every 1 million molecules of air, 398 are carbon dioxide molecules.

** Chlorofluorocarbons (CFCs) and hydrofluorocarbons (HFCs). CFCs and HFCs are manufactured by people and do not form naturally.