(The image above illustrates the annual trend in global net primary production (NPP) — or net carbon gain by photosynthetic organisms on Earth — from 2003 to 2021. Credit: Yulong Zhang, et al, 2025)
From 2003 to 2021, Earth’s ability to absorb carbon through photosynthesis increased—mostly thanks to land plants growing more vigorously in warming climates.
While forests and farmland expanded their role in capturing carbon, ocean algae began to struggle, especially in tropical waters. This shift is changing the balance of life on Earth, with land becoming more productive while marine ecosystems weaken.
Photosynthesis on the Rise: Plants Lead the Charge
Between 2003 and 2021, photosynthesis around the world increased, largely due to the growing activity of land-based plants. However, this gain was slightly reduced by a mild decrease in photosynthesis among marine algae, according to a new study published August 1 in Nature Climate Change. Researchers say the findings could help shape efforts to assess the planet’s health, manage ecosystems more effectively, and develop better strategies for predicting and addressing climate change.
Photosynthesis is driven by organisms known as primary producers, which form the foundation of the food chain and support nearly all life on Earth. These organisms use sunlight to turn carbon dioxide from the atmosphere into organic matter. But in addition to capturing carbon, they also release some of it back through a process called autotrophic respiration (similar to breathing). The difference between the carbon absorbed and the carbon released is known as net primary production.
“Net primary production measures the amount of energy photosynthetic organisms capture and make available to support nearly all other life in an ecosystem,” said first author Yulong Zhang, a research scientist in the lab of Wenhong Li at Duke University’s Nicholas School of the Environment. “As the foundation of food webs, net primary production determines ecosystem health, provides food and fibers for humans, mitigates anthropogenic carbon emissions, and helps to stabilize Earth’s climate.”
Global Perspective: Land and Ocean Together
Past studies on net primary production have often focused on land or ocean ecosystems separately. As a result, scientists have lacked a complete picture of how carbon is processed across the entire planet, and how this affects efforts to slow climate change.
In this new research, the team examined yearly trends and shifts in global net primary production, paying close attention to how changes on land relate to those in the ocean.
“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. The pioneering studies that first combined terrestrial and marine primary production have not been substantially updated in over two decades,” said co-author Nicolas Cassar, Lee Hill Snowdon Bass Chair at the Nicholas School, who jointly oversaw the research with Zhang.
Watching Earth Breathe: Satellite Technology
Observations from satellites offer a continuous perspective on photosynthesis by plants and marine algae called phytoplankton. Specifically, specialized satellite instruments measure surface greenness, which represents the abundance of a green pigment called chlorophyll produced by photosynthetic life. Computer models then estimate net primary production by combining greenness data with other environmental data, such as temperature, light, and nutrient variability.
The authors of the new study used six different satellite-based datasets on net primary production — three for land and three for oceans — for the years from 2003 to 2021. Using statistical methods, they analyzed annual changes in net primary production for land and, separately, for the ocean.
They found a significant increase in terrestrial net primary production, at a rate of 0.2 billion metric tons of carbon per year between 2003 and 2021. The trend was widespread from temperate to boreal, or high-latitude, areas, with a notable exception in the tropics of South America.
Ocean Declines: A Subtle Carbon Shift
By contrast, the team identified an overall decline in marine net primary production of about 0.1 billion metric tons of carbon per year for the same time period. Strong declines mainly occurred in tropical and subtropical oceans, particularly in the Pacific Ocean.
All told, trends on land dominated those of oceans: Global net primary production increased significantly between 2003 and 2021, at a rate of 0.1 billion metric tons of carbon per year.
Climate Clues: What’s Driving the Change?
To understand the potential environmental factors at play, the team analyzed variables such as light availability, air and sea-surface temperature, precipitation, and mixed layer depth — a measure that reflects the extent of mixing in the ocean’s top layer by wind, waves, and surface currents.
“The shift toward greater primary production on land mainly stemmed from plants in higher latitudes, where warming has extended growing seasons and created more favorable temperatures, and in temperate regions that experienced local wetting in some areas, forest expansion and cropland intensification,” said Wenhong Li, a professor of earth and climate sciences at the Nicholas School and a co-author on the study.
Warming temperatures appeared to have an opposite effect in some ocean areas.
“Rising sea surface temperatures likely reduced primary production by phytoplankton in tropical and subtropical regions,” Cassar added. “Warmer waters can layer atop cooler waters and interfere with the mixing of nutrients essential to algal survival.”
El Niño’s Influence: Oceans React Faster
Although land drove the overall increase in global primary production, the ocean primarily influenced year-to-year variability, especially during strong climate events such as El Niño and La Niña, the authors found.
“We observed that ocean primary production responds much more strongly to El Niño and La Niña than land primary production,” said co-author Shineng Hu, an assistant professor of climate dynamics at the Nicholas School. “A series of La Niña events was partly responsible for a trend reversal in ocean primary production that we identified after 2015. This finding highlights the ocean’s greater sensitivity to future climate variability.”
Big Picture: What This Means for Earth
The study points to the important role of terrestrial ecosystems in offsetting declines in net primary production among marine phytoplankton, according to the authors.
But they added that declines in net primary production in tropical and subtropical oceans, coupled with stagnation on land in the tropics, can weaken the foundation of tropical food webs, with cascading effects on biodiversity, fisheries, and local economies. Over time, these disruptions could also compromise the ability of tropical regions to function as effective carbon sinks, potentially intensifying the impacts of climate warming.
“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 with major implications for gauging the health of all living things, and for guiding climate change mitigation,” Zhang said. “Long-term, coordinated monitoring of both land and ocean ecosystems as integrated components of Earth is essential.”
Ren Ecosystem