Researchers have found that the Southern Ocean emits far more CO2 in winter than previously thought, thanks to laser satellite data that works in darkness.
This revelation redefines how we understand ocean carbon exchange and its role in climate change.
Winter Carbon Surge in the Southern Ocean
A team of scientists has found that the Southern Ocean emits far more carbon dioxide (CO2) during the long, sunless Antarctic winter than scientists once believed. According to their new research, the amount of winter CO2 released from the ocean has been underestimated by up to 40%.
The study was conducted by 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 results were published today (November 5) in Science Advances.
The Southern Ocean’s Carbon Mystery
The Southern Ocean is a critical player in Earth’s carbon system, absorbing a large share of the CO2 produced by human activity. Yet scientists have long struggled to pin down how much carbon it actually exchanges with the atmosphere, calling it the “largest source of uncertainty” in global CO2 estimates.
That uncertainty comes from a lack of direct observations. During the frigid, lightless winter months, the region is nearly impossible to study, as it is cloaked in darkness and battered by fierce storms. Conventional satellites, which rely on reflected sunlight (passive sensors) to gather ocean data, cannot operate under these conditions. As a result, researchers have had to depend on models that only partially capture what happens in winter.
Laser Eyes on a Lightless Ocean
To overcome this obstacle, the research team turned to a new approach that combined 14 years of satellite LIDAR data from the CALIPSO mission with machine learning.
Unlike passive sensors, LIDAR is an active technology that works similarly to radar but uses laser light instead of radio waves, carrying its own illumination source. This allowed scientists to collect data even in total darkness, producing the first continuous, observation-based picture of wintertime CO2 exchange across the Southern Ocean.
A 40% Underestimation Revealed
The results showed that previous estimates had missed about 40% of the ocean’s winter carbon emissions. “Our findings suggest that the Southern Ocean’s role in the global carbon cycle is more complex and dynamic than previously known,” said Prof. Kun Shi from NIGLAS.
Beyond correcting the numbers, the study changes the way scientists think about how the Southern Ocean’s carbon cycle operates. The team introduced a new “three-loop framework” that explains how different physical and biological processes govern CO2 exchange at various latitudes, offering a clearer view of the mechanisms driving this crucial part of the climate system.
The “Three-Loop” Ocean Framework
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.
Reference: “Substantially underestimated winter CO2 sources of the Southern Ocean” by Siqi Zhang, Peng Chen, Kelsey Bisson, Cédric Jamet, Paolo Di Girolamo, Davide Dionisi, Yongxiang Hu, Zhenhua Zhang, Kun Shi and Delu Pan, 5 November 2025, Science Advances.
DOI: 10.1126/sciadv.aea0024
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