Scientists are using advanced satellite-based laser technology to measure marine life and carbon beneath the ocean surface, addressing a major limitation of conventional ocean-monitoring satellites. A new review published in The Innovation examines how high-spectral-resolution lidar (HSRL) can reveal the vertical distribution of plankton and improve measurements of the ocean’s role in the global carbon cycle.
Traditional ocean-colour satellites primarily observe light reflected from the uppermost layer of seawater. However, much of the plankton supporting marine food webs and the carbon transported towards deeper waters lies beneath this visible surface layer.
The review traces the development of spaceborne lidar technology, from the accidental discovery of subsurface plankton layers by NASA’s CALIOP instrument to newer missions operated by China and Europe. The researchers argue that combining these measurements with satellite imagery and ocean-monitoring floats could provide a more complete picture of marine ecosystems.
Conventional Satellites Cannot See the Full Ocean Ecosystem
Passive ocean-colour instruments, including MODIS and PACE, measure the colour of reflected sunlight to estimate conditions such as chlorophyll concentration. However, these observations provide limited information about the deeper water column.
This creates challenges for estimating global marine carbon budgets and understanding how ocean ecosystems respond to environmental changes.
An important development came in 2006, when CALIOP was launched to study atmospheric clouds. Its observations unexpectedly revealed plankton layers beneath the ocean surface across different regions. The instrument also showed that polar plankton blooms begin much earlier in the season than previously believed.
However, CALIOP relied on elastic lidar, a technique that measures the intensity of returned laser light. Its measurements involve two unknowns: how strongly particles scatter light back towards the instrument and how quickly light is attenuated as it travels through water.
Because a single measurement cannot independently determine both quantities, researchers had to rely on assumed relationships to estimate particle concentrations.
ICESat-2, launched in 2018, introduced photon-counting technology and achieved a vertical resolution of 0.3 metres. This allowed scientists to identify finer structures within plankton layers. When combined with ocean-colour observations, its data also enabled the production of satellite-derived chlorophyll profiles.
However, the underlying measurement problem remained unresolved.
High-Spectral-Resolution Lidar Separates Scattered Light
High-spectral-resolution lidar addresses this limitation by distinguishing between light scattered by particles and light scattered by water molecules.
The technique uses the fact that particle scattering produces relatively little change in the laser’s frequency, while molecular scattering spreads the returned light across a wider range of frequencies.
An ultra-narrow optical filter, often using an iodine vapour cell, separates the returning signal into two channels: one measuring the total scattered light and another isolating the molecular component.
Since the scattering properties of water molecules are known and stable, the molecular measurement allows researchers to determine how much light is attenuated by the water. They can then calculate the particle contribution without relying on the assumptions required by conventional elastic lidar.
This makes it possible to obtain more reliable measurements of particle backscatter and the vertical structure of marine ecosystems.
China’s DQ-1 and Europe’s EarthCARE Advance Ocean Monitoring
Two satellite missions have brought high-spectral-resolution lidar measurements into operational use.
China launched the DQ-1 satellite in April 2022, carrying the Atmospheric Composition Detection Lidar (ACDL). It became the first operational spaceborne HSRL instrument. Its measurements of particle backscatter have shown agreement with observations from Biogeochemical-Argo floats.
Europe’s EarthCARE mission followed in May 2024, carrying the Atmospheric Lidar (ATLID). While China’s ACDL operates using green laser light at 532 nanometres, EarthCARE’s instrument works in the ultraviolet at 355 nanometres.
Ship-based comparisons have demonstrated the improvement offered by the technology. HSRL retrieved particle backscatter within 10% of reference measurements more than five times as frequently as conventional elastic lidar.
These developments provide researchers with more accurate information about the distribution of particles and plankton beneath the ocean surface.
Combining Satellite Data to Build an Ocean Digital Twin
The review proposes combining three complementary sources of ocean observations into what the authors describe as a “virtual constellation”.
The system would bring together:
- High-spectral-resolution lidar: Measures the vertical structure of marine particles and plankton.
- Hyperspectral imagers: Identify the composition of biological communities near the ocean surface.
- Biogeochemical-Argo floats: Collect measurements throughout the water column, profiling depths of up to 2,000 metres and providing ground-level validation.
Combining these datasets with computer models could help researchers develop an “ocean digital twin”, a digital representation of marine ecosystems and their changing conditions.
Such a system could improve predictions of how marine ecosystems respond to climate change and strengthen estimates of the carbon absorbed and transported by the oceans. These measurements could also support the carbon accounting used in national climate assessments.
Limitations and Future Satellite Missions
Despite its advantages, HSRL technology still faces several challenges.
Spaceborne lidar collects observations along a one-dimensional path, making it difficult to combine its measurements directly with the two-dimensional coverage provided by satellite imagers. Its penetration is also limited to approximately two to three optical depths, meaning that deep chlorophyll maxima in turbid waters may remain undetected.
The researchers also noted that validation efforts have largely focused on open-ocean environments. Coastal waters remain comparatively understudied, partly because Biogeochemical-Argo floats are less common in these regions.
Future missions could help address some of these gaps. Planned projects, including CALIGOLA and China’s Guanlan mission, aim to expand ocean observations through fluorescence measurements and greater penetration depths of approximately 100 to 150 metres.
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