The findings could inform planetary health assessments, enhance
ecosystem management, and guide climate change projections and
mitigation strategies.
Global study identifies upswing in photosynthesis driven by land, offset by oceans
The image above illustrates the annual trend in global net primary production (NPP)—or net carbon gain
Journal information: Nature Climate Change More information: Contrasting biological production trends over land and ocean, Nature Climate Change (2025).
DOI: 10.1038/s41558-025-02375-1
Terrestrial plants drove an increase in global photosynthesis between
2003 and 2021, a trend partially offset by a weak decline in
photosynthesis—the process of using sunlight to make food—among marine
algae, according to a study published in Nature Climate Change.
The rate of carbon gain after accounting for loss through respiration is called 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."
Previous research on net primary production has typically focused on
either land or ocean ecosystems, leaving gaps in our understanding of
net primary production across Earth and the potential implications for
climate mitigation.
- For this study, the team explored annual trends and variability in
global net primary production, with a focus on the interplay between
land and ocean ecosystems.
"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.
Satellite insights
- Observations from satellites offer 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.
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 the oceans: global net
primary production increased significantly between 2003 and 2021, at a
rate of 0.1 billion metric tons of carbon per year.
Environmental drivers
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."
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."
Broad implications
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."
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