At a biochemical level, respiration and photosynthesis are opposite fluxes of the same general reaction.

Carbohydrate + O₂ CO₂ + H₂O + Energy

The reaction proceeds spontaneously to the right as written above, providing the energy of respiration that all organisms require. Some organisms are also able to harvest the energy in sunlight (phototrophic) or inorganic chemicals (lithotrophic) to fix CO₂ and cause flux to proceed to the left.

One can also envision this generalized reaction occuring at the global scale. Every year, during the Spring through Autumn growing season, the CO₂ concentration in the atmosphere decreases as the net flux of photosynthesis exceeds that of respiration and moves CO₂ from the atmosphere into the biosphere. Similarly, atmospheric CO₂ concentrations rise in the Winter. Thus, the carbon in the atmosphere and biosphere forms a dynamic equilibrium or steady-state called the Global Carbon Cycle [local]. Here is a concept map [local] of this cycle. Careful studies on atmospheric CO₂ concentration have revealed that in addition to the seasonal increases and decreases in CO₂ concentration, the amount of CO₂ has been steadily increasing [local] over the last century.

Increasing concentrations of CO₂ may impact our climate. Molecules with three or more atoms have vibrational frequencies (e.g. CO₂, CH₄, H₂O) that absorb infrared radiation and contribute to global warming.

Can this trend be reversed? Some think that human activity can stop the increase in atmospheric CO₂ by increasing carbon sequestration [local].