Global warming occurs when carbon dioxide, a greenhouse gas, is excessively accumulated in the atmosphere. Fortunately, about 30% of the carbon dioxide released by human activities is absorbed by the terrestrial ecosystem, and the amount is the difference between photosynthesis (absorption) and respiration (emission). The amount of photosynthesis itself is about 10 times as much as the carbon dioxide emission by human activities. This suggests that photosynthesis is the key element of the solution to climate change. Then, how much is the photosynthesis in the earth’s terrestrial ecosystem, and what is its temporal and spatial distribution like? Leading an international joint research team, Professor Ryu Youngryel’s group at SNU has developed a technology for sensing the terrestrial photosynthesis in a simple and accurate manner by means of remote sensing.

Being carbon-neutral means to make the emission of greenhouse gases (+) equal to their absorption (-) so that the total sum can be ‘zero.’ Biologists are particularly interested in the photosynthesis of plants, which can play a critical role in carbon neutrality. Photosynthesis is a process in which plants with chlorophyll utilize the light energy to produce glucose (carbohydrates) and oxygen from the water they absorb through the roots and the atmospheric carbon dioxide through the stoma of the leaves. In the process, carbon dioxide is fixed to the chloroplasts of the plant cells, and oxygen is released.

The carbon dioxide fixation* by photosynthesis is the most important element of the earth’s carbon cycle. As plants are increased, the amount of photosynthesis for fixing carbon dioxide is generally increased. That’s why huge forests are formed in many countries in the world where logging is active and the land use is frequently changed. The ‘amount of photosynthesis’ refers to the amount of organic products produced by plants by using carbon dioxide and water, the inorganic materials, and absorbing light energy. The amount of photosynthesis is dependent upon the light intensity, carbon dioxide concentration, temperature and so on.

The scientists in the world are conducting studies to measure the amount of photosynthesis in a global scale in pursuit of carbon neutrality. They are interested in the measurement of Sun-Induced Fluorescence (SIF) by using a remote platform including satellite, planes and unmanned aerial vehicle (UAV). The SIF refers to the fluorescence (red and infrared wavelengths) that is released back by the chlorophyll molecules from the light energy absorbed by plants for photosynthesis, and it is a byproduct that accounts for 1 to 2% of the absorbed energy. Since the pathway for using the light energy absorbed by chlorophyll includes SIF and photosynthesis, photosynthesis and SIF are directly related with each other in terms of the mechanism. In contrast to photosynthesis, SIF can be observed by remote sensing. Not long ago, SIF has been measured in a small scale, for example, by measuring from individual plant leaves. However, the advancement of remote sensing technology has allowed for the measurement in large areas. Nevertheless, since SIF is a tiny amount of light, it is never easy to measure the amount of SIF from plants at a high accuracy. Although spectrometers for SIF observation have been developed and the methodologies for calculating SIF have been established, the observation has rarely been conducted and so the observation data are few. Hence, it is difficult to interpret the meanings of the complicated SIF data. Then, isn’t there any way to measure the total amount of photosynthesis absorbed by the vast ecosystems of the earth?

An international joint research team, led by Professor Ryu Youngryel of the Department of Landscape Architecture and Rural Systems Engineering at the College of Agriculture and Life Sciences, has solved the problem by developing NIRvP, a novel vegetation indicator that can be used to quantify the amount of photosynthesis. Recently publishing several papers, Professor Ryu’s group found that the temporal and spatial variations of SIF are mostly described by the structure of colonies and the amount of light. Based on this finding, the group developed NIRvP.

The research group began to develop a formula, considering that the measurement in the terrestrial ecosystem through remote sensing is affected by not only vegetation but also the ground surface factors such as soil. Near-infrared (NIR) reflectance is inversely proportional to the visible light absorption rate of vegetation, which has a linear relation with the colonial structure. To eliminate the signals from things other than vegetation from the NIR signals, the normalized difference vegetation index (v) was multiplied by the NIR reflectance, resulting in ‘NIRv.’ Then, since photosynthesis is controlled by the amount of solar radiation, the resulting ‘NIRv’ was multiplied by the solar radiation (P). The finally obtained formula is ‘NIRvP.’

NIRvP is very simple, since it can be calculated by using just two channels for red wavelength and near infrared wavelength. Many space satellites are equipped with spectral cameras including these two channels. Therefore, the vegetation index can be applied to the existing satellite data that have been collected for decades. In addition, the index can be immediately applied to the satellites that are supposed to be launched, including the next-generation medium satellite No. 4 (agricultural satellite). The point of the paper is that the photosynthesis process involving complicated reactions can be accurately monitored by using NIRvP in a very easy and simple method. Measuring the amount of photosynthesis is essential in monitoring the changes of the vegetation by climate change and reducing the future carbon emission. Based on the measurement results, appropriate land use management policies may be presented to enhance photosynthesis in the terrestrial ecosystem, finding more direct and efficient ways to accomplish future carbon neutrality.