Carbon dioxide (CO2), the most important of these minor gases, is involved in a complex global cycle (see section 6.4.1). It is released from the interior of the Earth via volcanic eruptions, and by respiration, soil processes, combustion of carbon compounds and oceanic evaporation. Conversely, it is dissolved in the oceans and consumed during plant photosynthesis. Currently, there are 359 parts per million by volume (ppmv) of CO2 in the atmosphere (Schimel et al., 1995), a concentration which is continuing to rise due to anthropogenic (man-made) emissions from the burning of fossil fuels and forests. The implications of this are discussed in chapter 6.