In 2013, the absolute atmospheric concentration of greenhouse gases were 396 parts per million (ppm) for carbon dioxide (CO2), 1,824 parts per billion (ppb) for methane (CH4), and 325.9 ppb for nitrous oxide (N2O). This information comes from the Greenhouse Gas Bulletin issued by the World Meteorological Organization. As shown in the first chart at right, the concentration of each has grown significantly since 1750. The concentrations in 2013 were record highs for all 3. (Some of the charts do not start at zero, to better show the changes that have occurred.)
(Click on chart for larger view.)
Notice how the carbon dioxide and methane lines zigzag up and down each year. This pattern comes from the seasonal change between summer and winter in the northern hemisphere. Plants absorb or emit carbon dioxide and methane differently during summer and winter, and it affects the amounts of these gases in the atmosphere. So much of the earth’s land surface is in the northern hemisphere that the seasonal pattern there outweighs the one in the southern hemisphere. If you want to see a NASA animated simulation of these effects for carbon dioxide, follow this link to:
The second chart at right shows the radiative forcing from the most important greenhouse gases from 1750 to 2013. (The chart does not start at zero, to better show the change that has occurred). Light blue is carbon dioxide, light green is methane, purple is nitrous oxide, yellow is CFC-12 (also known as Freon), the dark blue band is CFC-11, and the red band is for 15 other GHGs combined. It is easy to see that carbon dioxide produces by far the most forcing, followed by methane.
I have provided a third chart at right that shows what percentage of the atmospheric concentration of carbon dioxide, methane, and nitrous oxide have been added to the atmosphere since 1750: about 30% of the carbon dioxide, 60% of the methane, and 17% of the nitrous oxide. In looking at the chart, don’t overlook the fact that the concentration of carbon dioxide is in parts per million, whereas the concentrations of methane and nitrous oxide are in parts per billion.
Methane and Nitrous Oxide have an atmospheric concentrations so much lower than does CO2 – how can they have radiative forcings that can even show up on the same graph as CO2? The answer is that, gram-for-gram, they have much stronger radiative forcing effects.
Carbon dioxide occurs naturally in large amounts. Even so, human activity emits so much that it is overwhelming the preindustrial balance. Methane and nitrous oxide are not uncommon, but in nature they occur in much smaller amounts than does carbon dioxide, and we emit less, also.
World Meteorological Organization Global Atmosphere Watch. 2014. WMO Greenhouse Gas Bulletin. http://www.wmo.int/pages/prog/arep/gaw/ghg/GHGbulletin.html.