Atmospheric CO2 Level and Global Climate System Implications
The flux of carbon among terrestrial, aquatic, and atmospheric pools is at least partially controlled by photosynthetic processes. The relative carbon pool sizes have major effects on global climate system. The CO2 flux among land, sea, and atmosphere has, since the industrial revolution, been disturbed by a rapid injection of carbon dioxide to the atmosphere from the burning of fossil fuels and from the clearing of forests. Both the fossil fuel and biomass carbon are relatively long-lived pools with long residence times. However, human activity has volatilized this carbon originally fixed by photosynthesis into the atmosphere. At the beginning of the industrial revolution, atmospheric CO2 level was approximately 260 ppmv. Since then, input of CO2 has increased this concentration to approximately 372 ppmv in 2002.
Only approximately 40–50% of the total CO2 injected into the atmosphere remains there. What happens to the rest is a subject of current research and not completely understood. Most likely, the ocean is a large sink for this excess carbon. The other potential major sink is the terrestrial biota. Terrestrial ecosystems, especially forests, do have the ability to store some carbon through the photosynthetic production of biomass. However, whether this storage is greater than the contribution of carbon through respiration is not clear in many terrestrial systems.
A complete mechanistic understanding of the global carbon cycle has yet to be achieved, and this will be an area of active scientific endeavor for many years. However, there is very little disagreement that atmospheric CO2 level has increased dramatically during the past century and that if atmospheric CO2 concentration continue to increase at the present rate, it will exceed 400 ppmv by 2030. In view of our dependence on fossil fuels and the growing body of evidence linking atmospheric CO2 level and global climate system, understanding the fate of excess atmospheric carbon is one of the most pressing scientific research needs.
In addition to potentially changing the global climate system, rising atmospheric CO2 concentration directly impact the photosynthetic process and may increase the primary production of photosynthetic organisms in some systems. Satellite imagery and remote-sensing information for the past two decades indicate as much as a 6% increase in total production of terrestrial systems, primarily in the tropics. Smaller scale manipulative experiments, such as free air CO2 enrichment (FACE) studies, have yielded more complex results. Most FACE studies have observed an initial increase in productivity but then a stabilization in productivity over time. The mechanisms behind these observations are not clear, but it seems likely that some other limitation besides CO2 is eventually expressed in most terrestrial systems. The processes and feedbacks of carbon cycling, of which photosynthesis is a major component, are very active areas of research.
The photosynthetic process is literally the most important energy transfer on Earth. The fixation of carbon by photosynthetic organisms provides the high-energy molecules needed to sustain nearly all organisms on Earth. Photosynthesis is a two-stage process: (i) harnessing of light energy to produce reducing power (NADPH) and energy (ATP) and (ii) use of the reducing energy to fix organic molecules. We usually associate photosynthesis with the reduction of atmospheric CO2 to organic sugars.
However, the energy generated during the light reactions of photosynthesis can be used to reduce other compounds (e.g., nitrogen and sulfur). Photosynthetic reactions have, in part, controlled the relative carbon pool sizes on Earth. Human activities, especially fossil fuel consumption and biomass burning, have led to an injection of carbon as CO2 into the atmosphere. This rapid increase in carbon in the atmosphere has the potential to dramatically alter future Earth systems function and climate. A complete mechanistic understanding of global carbon cycling including photosynthetic processes is imperative.