Marybeth Collins
Between 2003 and 2021, Earth saw a net gain in photosynthesis primarily driven by land plants, while marine phytoplankton productivity declined, according to a new study published August 1 in Nature Climate Change. The findings suggest that terrestrial ecosystems are becoming increasingly critical to global carbon uptake—but this shift may mask deeper vulnerabilities in the planet’s climate system.
Net primary production (NPP)—the amount of carbon plants and algae store after respiration—is a key indicator of ecosystem health and carbon cycle stability. Duke University researchers analyzed 19 years of satellite-based data across land and ocean, revealing an annual global increase of 0.1 billion metric tons of captured carbon, largely due to high-latitude forest growth and longer growing seasons.
“Net primary production determines ecosystem health, provides food and fibers for humans, mitigates anthropogenic carbon emissions, and helps to stabilize Earth’s climate,” said Yulong Zhang, lead author and research scientist at Duke’s Nicholas School of the Environment.
Rising Land Gains, Sinking Marine Losses
The study shows:
- Terrestrial NPP increased by 0.2 billion metric tons of carbon per year, especially in boreal and temperate zones like Canada, Siberia, and parts of Europe.
- Marine NPP declined by 0.1 billion metric tons of carbon per year, most notably in tropical Pacific and Indian Ocean regions, due to reduced nutrient mixing from ocean stratification.
These trends were tracked using multiple satellite datasets that measured chlorophyll activity, surface greenness, sea surface temperatures, and precipitation variability.
El Niño’s Outsized Influence
While land productivity has generally trended upward, ocean productivity proved more volatile. The decline in marine photosynthesis has been closely tied to El Niño and La Niña events, which shift trade winds and water column mixing patterns. These shifts are increasingly frequent and severe under global warming.
“The ocean’s primary production responds much more strongly to El Niño and La Niña than land ecosystems,” said Shineng Hu, co-author and assistant professor at Duke. “A series of La Niña events helped reverse marine declines briefly after 2015, but the long-term trend remains downward.”
Why It Matters
According to the Global Carbon Project and IPCC AR6 reports, oceans currently absorb about 25% of anthropogenic CO₂ emissions. A continued decline in marine primary productivity would weaken this vital sink, increase atmospheric CO₂ concentrations, and destabilize tropical food chains.
At the same time, while expanding forests help offset emissions, their future is also uncertain. Deforestation, wildfires, pest outbreaks, and drought-induced diebacks threaten the durability of terrestrial carbon sinks. In 2023 alone, the Amazon rainforest lost nearly 5,000 km² of canopy—its highest rate in five years.
Moreover, a 2024 study in Science Advances warned that boreal forest gains might slow as Arctic warming accelerates permafrost thaw and soil carbon loss.
Integrated Monitoring Urged
Zhang and his co-authors stress that monitoring Earth’s productivity requires a joined-up approach, covering both land and ocean ecosystems.
“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 said.
Looking Ahead
This new Earth system imbalance—forest productivity up, ocean productivity down—should influence how nations model carbon budgets and design climate policies. It reinforces the need for:
- Strong forest preservation and reforestation
- Reductions in nutrient runoff that further degrade ocean ecosystems
- Improved long-term satellite monitoring (like NASA’s upcoming PACE mission)