Figure 1. Plot of monthly atmospheric  CO₂ concentrations (in parts per million by volume) from the Mauna Loa Observatory in Hawaii. A linear fit of data from the last decade shows an average increase of 1.8 ppm yr^–1 (data from Keeling and Whorf, 2005).

Figure 2. Plot of  CO₂ concentration (in parts per million by volume) as a function of time before present.  CO₂ was measured from ice bubbles retrieved from a 3623 m ice core in Vostok, Antarctica (data from Barnola et al., 2003).

Figure 3. Schematic of carbon cycle on land. Black arrows represent pre-industrial fluxes (in petagrams of carbon per year) and red arrows represent average anthropogenic fluxes for 1980s and 1990s. Values in brackets represent estimated reservoir sizes (in petagrams of carbon). Adapted from Sabine et al.) 2004a.

Figure 4. Photos of two free air  CO₂ enrichment (FACE) sites. The top photo is from a Sweetgum site in Tennessee (http://www.esd.ornl.gov/facilities/ORNL-FACE/index.html). The bottom photo is from the Aspen FACE site in Wisconsin (http://aspenface.mtu.edu/). The white circles in both photos are towers that expose the encompassed region to elevated atmospheric  CO₂ levels.

Figure 5. Schematic of ocean carbon cycle. Black arrows represent pre-industrial fluxes (in petagrams of carbon per year) and red arrows represent average anthropogenic fluxes for 1980s and 1990s. Values in brackets represent estimated reservoir sizes (in petagrams of carbon). Adapted from Sabine et al.) (2004a).

Figure 6. Schematic of global carbon cycle. Black arrows represent pre-industrial fluxes (in petagrams of carbon per year) and red arrows represent average anthropogenic fluxes for 1980s and 1990s. Values in brackets represent estimated reservoir sizes (in petagrams of carbon). Adapted from Sabine et al.) (2004a).