For almost two decades, terrestrial vegetation – plants, forests and agriculture – has quietly outpaced its marine counterparts in the global race to capture carbon. Now, a new study published in Nature Climate Change reveals that from 2003 to 2021, land plants increased their net primary production (NPP)—the amount of carbon fixed through photosynthesis—by 0.2 billion metric tons annually. In contrast, marine phytoplankton lost about half that amount each year.
The analysis, led by Yulong Zhang and colleagues from Duke University, synthesises six independent satellite datasets—three terrestrial and three marine—to track the annual fluctuations in photosynthetic activity across Earth’s ecosystems. The results show a subtle but persistent rise in global photosynthesis, though the gains were uneven and driven by distinct regional dynamics.
“The shift toward greater primary production on land mainly stemmed from plants in higher latitudes, where warming has extended growing seasons and created more favourable temperatures,” explained Wenhong Li, a co-author of the study. Temperate zones also saw boosts from local increases in rainfall, forest expansion, and intensified agriculture. These gains more than offset stagnation in tropical South America, where productivity remained relatively flat.
Meanwhile, the oceans told a different story.
“Rising sea surface temperatures likely reduced primary production by phytoplankton in tropical and subtropical regions,” said Nicolas Cassar, senior scientist on the project. Warmer surface layers suppress the vertical mixing that replenishes nutrients in sunlit waters, leading to a decline in ocean productivity of about 0.1 billion tons of carbon per year.
And the Pacific basin was hit hardest. Despite the long-term dominance of land plants, oceans played a key role in short-term variability. “We observed that ocean primary production responds much more strongly to El Niño and La Niña than land primary production,” said Shineng Hu, another co-author. A series of La Niña events after 2015 temporarily reversed the downward trend in marine photosynthesis, underscoring the sensitivity of plankton communities to climate oscillations.
The team examined environmental drivers such as light, temperature, precipitation, and mixed-layer depth to understand the divergence between land and sea. Their conclusion: warming and, in some regions, increased rainfall favoured northern forests and fields, while the same warming deprived tropical waters of nutrients. “Whether the decline in ocean primary production will continue – and how long and to what extent increases on land can make up for those losses – remains a key unanswered question,” Zhang cautioned.
The implications go far beyond carbon accounting.
Photosynthetic organisms form the base of food webs and regulate the planet’s carbon balance. While land plants currently compensate for marine losses, the margin is slim. Continued declines in ocean productivity could unravel tropical food webs, threaten fisheries, and weaken one of Earth’s major carbon sinks: “If you’re looking at planetary health, you want to look at both terrestrial and marine domains for an integrated view of net primary production,” Cassar emphasised. The authors argue that NPP is Earth’s scoreboard for life, measuring how much solar energy is converted into organic matter that sustains ecosystems.
Many foundational studies linking land and sea productivity are now decades old. As climate patterns shift, updating this picture is urgent. “Long-term, coordinated monitoring of both land and ocean ecosystems as integrated components of Earth is essential,” Zhang concluded. For now, an energetic Northern Hemisphere is keeping the biosphere in balance. But the equilibrium is fragile, and the forces shaping it—temperature, rainfall, and nutrient dynamics—are changing rapidly.
For more information: Zhang, Y., Li, W., Sun, G. et al. Contrasting biological production trends over land and ocean. Nat. Clim. Chang. (2025). https://doi.org/10.1038/s41558-025-02375-1.
