Detecting Climate-Carbon Feedbacks: Next-Generation Approach for Space-Based Integration of OCS, CO₂, and SIF

Abstract

Photosynthesis is a keystone process for the Earth system. The emergence of photosynthesis transformed Earth’s geologic, geochemical, and biologic evolution, and today, virtually all life on Earth depends on this process as a direct or indirect food source. Photosynthesis controls a fundamental link between the global carbon, water, and energy cycles, which underlies central scientific mysteries of the Earth system. In particular, post-industrial growth in global photosynthesis is responsible for one of the largest and most uncertain feedbacks to anthropogenic climate change. Despite its importance, photosynthesis cannot be measured directly at scales larger than the leaf. Historically, measurements of CO₂ gas exchange are suitable for leaf chambers, but at larger scales, this technique is confounded by CO₂ emissions from soils. Theories of global photosynthesis are largely in the realm of computer simulations. Thus, measurement technology limits our ability to pursue questions that are essential to understanding the processes governing the Earth system and impacting our future. To confront this key scientific challenge, the workshop "Next-Generation Approach for Detecting Climate–Carbon Feedbacks: Space-Based Integration of OCS, CO₂, and SIF" assembled a multi-disciplinary team to conceive a new integrated technique for measuring photosynthesis at regional to global scales. The participants merged perspectives from the fields of ecology, biogeochemistry, atmospheric chemistry, and space science to focus on how a rapidly emerging technique with carbonyl sulfide sensors (OCS or COS) could be integrated with existing CO₂ and satellite observations of solar-induced chlorophyll fluorescence (SIF) platforms. The workshop discussions leveraged recent findings from atmospheric OCS observations and plant gas exchange studies that reveal a robust relationship between regional variation in photosynthesis and atmospheric variation in OCS. Plant leaves consume atmospheric OCS gas through a one-way hydration sink, which is controlled by stomatal conductance that is also a primary control on photosynthesis. Atmospheric OCS observations, such as the satellite detection of a massive depletion in OCS over the Amazon, can then provide a measurement-based estimate of photosynthesis. These OCS findings were analyzed within the context of recent breakthroughs from spaceborne SIF analysis. SIF platforms record the electromagnetic energy released from plant leaves during photosynthesis. Strong correlations between SIF and photosynthesis suggest an alternative means of assessing global photosynthesis from space. The key result of this workshop is that these alternative methods fill critical, yet different, methodological gaps, suggesting the need for a unified, space-based, photosynthesis observation platform. First, the highly complementary temporal and spatial scales of SIF analysis provide instantaneous, spatially resolved data and OCS provides spatially and temporally integrated data. Second, the independent photosynthesis processes that need to be constrained include the biochemical SIF constraint and stomatal conductance OCS constraint. Third, the Amazon basin is identified as an ideal domain where the temporally integrated OCS analysis could confront cloud contamination problems of alternative approaches. These outcomes has been used to develop a roadmap for near-, mid-, and long-term activities to achieve this vision for a unified global photosynthesis observing system. Proof-of-concept studies, including an airborne field experiment in the Amazon and an observing system simulation experiment, will provide critical evidence for the proposed satellite observations. The workshop team will collaborate on perspective articles in diverse disciplinary journals and develop a research coordinating network to communicate this new approach to the broad community of scientists and technologists who would be impacted by enabling a large-scale understanding of global photosynthesis.