Human use of fossil fuels (coal, oil, and natural gas) continues to increase as growing human populations demand more energy for transportation, heating, and manufacturing. We measure atmospheric CO₂ in units of ppm or parts per million. The rate of atmospheric increase in CO₂ is about 3 ppm per year (see Figure 9.18 in the textbook).

Web Figure 9.6.A   A high precision record of the atmospheric carbon dioxide levels measured on Mauna Loa, Hawaii. (Courtesy of NOAA, Earth System Research Laboratory: http://www.cmdl.noaa.gov/gallery2/v/gmd_figures/ccgg_figures/co2_mm_mlo.png.html)

Note the cyclic nature of the atmospheric CO₂ data, in which one oscillation cycle is exactly one year. This annual pattern reflects changes in the balance of photosynthesis (decreases atmospheric CO₂) and respiration (increases atmospheric CO₂) at a location over the course of the year. Atmospheric CO₂ tends to decrease in the spring and summer, when photosynthesis rates within an ecosystem exceed respiration rates. In contrast, atmospheric CO₂ tends to increase in the fall and winter when respiration rates exceed photosynthesis. In 2007 atmospheric CO₂ reached an average value of 384 ppm and is expected to reach 400 ppm before 2015.

Economists have good estimates of the rate of CO₂ emission globally. The U.S., European countries, China, Japan, and India are the largest sources of fossil fuel emissions.

Web Figure 9.6.B   The global distribution of the major sources of fossil fuel combustion today that leads to increases in atmospheric CO₂ levels. (Courtesy of International Panel on Climate Change: http://arch.rivm.nl/env/int/ipcc/pages_media/SRCCS-final/graphics/jpg/large/Figure%20TS-02a.jpg)

One surprising fact is that the observed rate of atmospheric CO₂ increase is actually less than the observed rate of atmospheric CO₂ increase. This is because plants on land and algae in the ocean are currently able to take up about one-half of fossil fuel emissions through enhanced photosynthesis. Scientists study how plants and ecosystems respond to elevated CO₂ using an experimental field approach called a Free Air CO₂ Enrichment (FACE) Experiment. In a FACE experiment, pipes inject CO₂ into the interior of a ringed area containing a complete ecosystem as shown below.

Web Figure 9.6.C   Four FACE rings surrounding deciduous forest trees at the Oak Ridge National Laboratory. (Courtesy of USDA, Rocky Mountain Research Station, Flagstaff Lab: http://www.rmrs.nau.edu/USAMAB/images/southern-appalacian_aerial-Aug00.jpg)

These FACE research facilities give scientists an opportunity to understand how different plant biochemical, physiological, and growth processes within the ecosystem will respond as a result of long-term exposure to elevated CO₂ levels. Since biomass production involves so much more than simply increased photosynthesis (i.e., mineral nutrients are required as well), it is doubtful that plant growth can be sustained in a linear, proportional fashion as atmospheric CO₂ levels continue to increase. The FACE studies are designed to address the question of how ecosystems will respond to future atmospheric CO₂ environments and whether the growth response level off at some future CO₂ level.

Global warming and changes in climate are anticipated effects of a rapidly increasing CO₂ levels. These are, of course, but two of the many reasons why scientists and others are concerned about the consequences of elevated atmospheric CO₂. Just how much the atmospheric CO₂ will increase is unknown. Below are estimates of the ranges, based on two plausible scenarios.

Web Figure 9.6.D   Projected changes in the atmospheric CO₂ concentrations under contrasting emission-control projections. (Courtesy of ACACIA: http://www.acacia.ucar.edu/images/co2.jpg)

In one scenario, titled “business as usual” atmospheric CO₂ levels are projected to reach 700 ppm by the end of this century. On the other hand, an aggressive global effort to curb CO₂ emission might result in an atmospheric CO₂ stabilization of 550 ppm.