Without forests, Earth would be uninhabitable. Forests provide water storage, dictate weather patterns and, critically, act as the planet’s lungs by absorbing carbon dioxide and releasing oxygen into the atmosphere through photosynthesis. Some of the carbon dioxide absorbed by the trees in this process, however, remains stored in their leaves, trunks and roots. While scientists have long understood the mechanics of this process, what has remained a mystery, and the subject of many conflicting estimates, is precisely how much carbon is stored in the biomass of trees.
Knowing the exact amount of carbon stored in the world’s forests is important for a number of reasons. Two of the most important are (1) measuring emissions caused by global deforestation and (2) designing appropriate compensation for REDD+ programs. The most commonly cited figure for global emissions due to deforestation has been 20%. But this is only an estimate that is, in turn, based on a combination of other estimates about global rates of deforestation and global carbon storage in forests. Obviously, clarifying either of the underlying assumptions will help provide additional accuracy in estimates of the carbon emissions due to deforestation.
A new study out of the Woods Hole Oceanographic Institute takes a step towards providing that greater accuracy. Using Light Detection and Ranging (LiDAR) technology, researchers have created the first “wall to wall” map of carbon storage in global tropical forests. Their work helps to advance understanding of how deforestation contributes to global emissions as well, and it highlights the importance that LiDAR and other remotely sensed data will play in a global REDD+ program.
According to the study, the world’s tropical forests contain 228.7 PgC stored in their biomass. Putting this number in perspective, the sum total of carbon released from the combustion of fossil fuels since 1850 is around 212 PgC. There is no risk of rapidly releasing all of this stored carbon; these numbers merely highlight the importance of tropical forests in the Earth’s carbon cycle by highlighting their immense capacity for carbon storage.
The Americas’ tropical rainforests, including the Amazon, store 51.4% of this carbon. Africa’s forests store 28.4% of the total, and Indonesia’s store 18.4%. Not all forests store carbon equally, however; in most of the Americas, including the Amazon, most of the carbon is stored in “thick forest,” but in Africa and Asia, scrublands, shrublands and savannas store almost as much carbon as forests do. This discovery has important implications for how REDD+ programs are structured. Current REDD+ programs tend to favor forest-based conservation; the protection of other types of vegetation such as scrubland or peat lands is more difficult. This new study suggests that in Africa and Asia, at least, REDD+ must seriously consider the role of non-forest lands in sequestering carbon in biomass.
If we combine these new estimates of tropical carbon storage with several different projections of deforestation rates, we are left with a range of possible values for how much deforestation contributes to global carbon emissions. The authors of this study report a figure of 1.14 PgC/yr, or about 13% of global emissions. While this number is lower than previously suggested, it remains very significant and indicates that addressing deforestation is an important component in the fight to lower emissions.
Understanding that this is an important part of lowering global emissions, the study thus emphasizes the importance of LiDAR and remotely sensed data in a REDD+ program; if we can accurately understand the amount of carbon stored in various types of biomass, we can structure more effective emissions reduction programs. In this study, LiDAR was used to generate a 3-D model of the forest canopy that, when combined with field-testing, infrared photography and other spectrographic imaging, can remotely generate the biological information necessary to measure carbon content of a forest. The technology does this by beaming a laser from a plane or satellite at the ground. This laser is then reflected back to the plane at different times by everything it hits (leaves, limbs, trunks and eventually the ground). By measuring the time between each returning beam, the height of each of these objects can be measured. Modern LiDAR systems send almost one million pulses per minute, which allows for the measurement of both the height and the shape of the canopy. When combined with infrared photography, in which different types of trees show up as different colors, scientists can determine the size, species and density of trees in a forest. All of this information is necessary to more accurately measure carbon content. Historically, this measurement required a trained forester to walk through the forest to gather data — a process that was both expensive and time consuming. LiDAR allows the same data to be gathered with a fraction of the time and money.
Remote sensing is not without flaws, though. Much of the process is dependent upon having clear skies — which can be a challenge in the tropics — and having enough flight paths over a given area, so complete coverage of the tropics is still not possible. The study mitigates this issue by combining remote sensing with field work (for verification) and the largest forest model to date to generate its unprecedented maps. Combining all three, at this scale, is a major step forward and will help advance the use of remote sensing for global forest protection.
Tropical forests play an immense role in combating global warming; they currently sequester more carbon than has been released from the combustion of fossil fuels since 1850. Ensuring that this carbon remains in the forests through REDD+ programs is therefore crucial to lowering emissions, and accurate measurements of forest carbon content make these programs even more feasible. Remotely sensed data will not only make these measurements more accurate, but also make REDD+ programs available to those countries and communities that could not previously afford to initiate them. While this information is still not cheap, accessibility and accuracy increase as the field advances, making REDD+ an ever more likely success.