Using remote sensing to reduce uncertainties in the global carbon budget: The potential of radiation acquired in middle infrared wavelengths

The concentration of carbon dioxide (CO2) in the atmosphere is increasing, but is still less than expected given our current knowledge of carbon sources and sinks. It has been hypothesised that the terrestrial biosphere in general and tropical forests in particular are acting as major sinks for atmospheric carbon. A possible mechanism for these sinks is the rapid regeneration of forests following the abandonment of previously cleared land. To quantify the amount of carbon sequestered by regenerating tropical forests requires data on their location and, since it is known that young regenerating forests grow faster and sequester more carbon than do old regenerating forests, data on their age. The biophysical properties of regenerating forests change with age and are different from those of the surrounding mature forests. Certain key biophysical properties can be detected, at least in principle, using satellite sensor data recorded in red and near infrared (NIR) wavelengths. However, while the relationship between red and NIR radiation and biophysical properties is generally strong in temperate forests, it has proved to be relatively weak for tropical forests. To overcome the problems of using red and NIR radiation in studies of tropical forests alternative approaches must be sought and one such approach includes the use of remotely sensed data acquired at other wavelengths (e.g., middle infrared (MIR) wavelengths (3.0–5.0 μm). Recent research suggests that biophysical properties may be related strongly to middle infrared (MIR) radiation. This review outlines the important role that remote sensing can play in quantifying the carbon cycle and the reasons for a strong relationship between MIR radiation and the biophysical properties of tropical forests. It is concluded that remote sensing in MIR wavelengths could have an important part to play in enhancing our understanding of the processes that result in the current concentrations of atmospheric CO2.