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A collaborative research team has discovered that the Southern Ocean releases substantially more carbon dioxide (CO₂) during the dark austral winter than previously thought. Their new study reveals that this winter outgassing has been underestimated by up to 40%.

The team comprises researchers from the Second Institute of Oceanography, Ministry of Natural Resources (SIO-MNR), and the Nanjing Institute of Geography and Limnology (NIGLAS) of the Chinese Academy of Sciences. Their findings were in Science Advances.

The Southern Ocean plays a pivotal role in the global carbon cycle, absorbing a significant portion of human-caused CO₂. However, it remains the "largest source of uncertainty" in global CO₂ flux estimates.

This uncertainty stems from a critical data gap. Shrouded in polar darkness and battered by harsh weather, the region becomes an "observational black box." Traditional satellites, which rely on sunlight (passive sensors) to measure ocean properties, are unusable during this period, forcing scientists to depend on incomplete models.

To address this challenge, the team adopted an innovative methodology, integrating 14 years of data from satellite LIDAR (on the CALIPSO mission) with machine learning.

Unlike passive sensors, LIDAR is an active sensor that operates like radar but uses a laser, carrying its own light source. This enabled the researchers to "see" in the dark and provide the first-ever year-round, observation-based assessment of these critical winter fluxes.

The study confirmed a 40% underestimation of the winter CO₂ source. "Our findings suggest that the Southern Ocean's role in the global carbon cycle is more complex and dynamic than previously known," said Prof. Shi Kun from NIGLAS.

The findings do not just revise the numbers; they reshape the fundamental understanding of how the Southern Ocean's carbon cycle functions. The team proposed a new "three-loop framework" to mechanistically explain the different processes controlling CO₂ exchange at different latitudes.

In the Antarctic Loop (south of 60°S), CO₂ exchange is dominated by physical processes such as sea ice dynamics and salinity. In the Polar Front Loop (45°S–60°S), a complex interplay between atmospheric CO₂ and biological activity (chlorophyll) was identified. In the Subpolar Loop (north of 45°S), CO₂ exchange is primarily controlled by sea surface temperature.

Correcting this gap has implications for the global carbon budget, which serves as the basis for climate models used by bodies like the Intergovernmental Panel on Climate Change (IPCC) to project future climate scenarios.

This work underscores the innovative application of active remote sensing in global climate studies.