Carbon Dioxide (CO2)

An interesting illustration of human impact on the Earth's atmosphere comes from Hawaii . Since 1958, daily measurements of carbon dioxide have been taken from the Hawaiian mountain of Mauna Loa , at 13,680 feet (4,170 m) above sea level.

In Figure 1, the solid black line is the five-year average measurement mean; the red line represents the actual monthly measurements. The average CO2 concentration has steadily increased over the last fifty years. Because carbon dioxide is relatively well mixed in the atmosphere, the measurements at Mauna Loa are a good estimate of the global average. Using ice-core measurements from Antarctica , scientists believe that today's concentration of carbon dioxide is higher than at any time over the last 650,000 years.1

Figure 1. Monthly values of atmospheric carbon dioxide concentrations at Mauna Loa Observatory , Hawaii , measured in parts per million (ppm) indicating the number of molecules of carbon dioxide per million air molecules. The black curve represents the smoothed data.2

In addition to the steady rise in carbon dioxide, the annual rise and fall of carbon dioxide in the atmosphere is also striking. Every year CO2 concentrations go through a cycle in which they slightly decrease between April and July and slightly increase in September and October. This regular pattern reveals the connection between the atmosphere and all living plant matter on Earth. Here is how it works. In spring and early summer, new growth on trees, plants, and grasses returns with the warming weather. Because plants take in carbon dioxide as they grow, CO2 levels start to decrease, and by the summertime - the peak growing season - CO2 levels have noticeably dropped. In the cooler weather of fall, plants start to lose their leaves which break down and return carbon to the atmosphere, and CO2 levels then start to increase. Some people have suggested that the annual cycle in carbon dioxide is an example of the Earth breathing. During the spring and summer, the Earth's plant life is growing and thus taking in carbon dioxide, whereas in fall and winter, plants have died back and carbon dioxide is returned to the atmosphere.

 

References

1. IPCC, Climate Change 2007: The Physical Science Basis: Contribution of Working Group 1 to theFourth Assessment Report on the Intergovernmental Panel on Climate Change, S. Solomon, et al., eds. (Cambridge: Cambridge University Press, 2007).

2. The data on Mauna Loa constitute the longest record of direct measurements of carbon dioxide in the atmosphere. Data from the Scripps Institute of Oceanography are in blue and from NOAA in red. An updated (every month) figure from NOAA can be seen at: http://www.esrl.noaa.gov/gmd/ccgg/trends/co2_data_mlo.html.