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 new study published in Nature Climate Change on Aug. 1. The findings could inform planetary health assessments, enhance ecosystem management, and guide climate change projections and mitigation strategies.
Photosynthetic organisms — also known as primary producers — form the base of the food chain, making most life on Earth possible. Using energy from the sun, primary producers fix, or convert, carbon from the air into organic, or carbon-based, matter. But primary producers also release carbon through a process called autotrophic respiration, which is somewhat akin to breathing. 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 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."
