Carbon is the fundamental building block of life, and it is continuously cycling between inorganic and organic forms. The two major parts of the cycle are photosynthesis and respiration. Photosynthetic fixation of carbon dioxide into organic form builds up the pools of organic carbon in the biosphere; then respiratory consumption of organic carbon releases carbon dioxide back to the atmosphere and ocean.

The carbon cycle is an expression of the overall activity of the Earth’s living organisms. The balance between photosynthesis and respiration controls many interrelated aspects of the Earth system, from the radiative forcing of the atmosphere to the energy available to terrestrial and aquatic food webs.

Quantifying the “breathing” of the biosphere is the key challenge in carbon cycle science. The scientific community has worked for decades to develop approaches to monitor and predict photosynthesis and respiration at the global scale. However, methods that are accurate, robust, and reliable have not yet emerged.

The figure on this page illustrates the core issue: carbon dioxide is both the substrate for photosynthesis and the product of respiration. As a result, there is no direct way to differentiate between photosynthetic capture and respiratory release of this trace gas in shared reservoirs like the atmosphere and ocean.

Two of the most promising solutions are described on the next page. Take me there >