Between 2003 and 2021, Earth’s ability to absorb carbon through photosynthesis rose, driven largely by more vigorous growth in land-based plants due to warming climates and extended growing seasons. Forests, farmlands, and other terrestrial ecosystems increased their capacity to capture carbon, while marine algae—particularly in tropical waters—experienced declines. This shift is altering the balance of global productivity, with land ecosystems strengthening and ocean ecosystems weakening.
At the core of this change is net primary production (NPP), which measures the difference between the carbon captured by photosynthetic organisms and the carbon they release through respiration. NPP reflects the energy available to support nearly all life in an ecosystem, playing a central role in food webs, biodiversity, and climate regulation. Increases in net primary production on land have been sufficient to outweigh decreases in the ocean, resulting in an overall global rise in NPP during the study period.
Researchers from Duke University analyzed six satellite-based datasets—three for terrestrial and three for marine ecosystems—covering the years 2003 to 2021. These datasets measured chlorophyll abundance (as a proxy for photosynthetic activity) and incorporated environmental factors like temperature, light, precipitation, and nutrient availability. The analysis revealed that terrestrial NPP increased by about 0.2 billion metric tons of carbon annually, especially in temperate and boreal regions, where warming and increased moisture promoted growth, as well as in areas with forest expansion and intensified agriculture. The tropics of South America were a notable exception, showing stagnation or decline.
Conversely, marine NPP declined by roughly 0.1 billion metric tons of carbon annually, with the steepest drops in tropical and subtropical regions, particularly the Pacific Ocean. Rising sea-surface temperatures were identified as a key driver, as warmer waters reduce nutrient mixing by creating stronger stratification between warm surface layers and cooler, nutrient-rich deeper layers. This nutrient limitation suppresses phytoplankton productivity.
The study also found that while land ecosystems drive long-term increases in net primary production, oceans are more sensitive to short-term climate variability. Events like El Niño and La Niña had a pronounced effect on marine productivity, with La Niña episodes after 2015 contributing to a temporary rebound in ocean NPP. This responsiveness underscores the vulnerability of marine ecosystems to climate fluctuations.
The findings highlight the intertwined roles of land and ocean in the global carbon cycle. Terrestrial gains in net primary production are helping offset ocean losses, but continued marine declines—along with tropical stagnation on land—pose risks to biodiversity, fisheries, and carbon storage capacity. This imbalance could weaken tropical food webs and reduce the ability of these regions to act as carbon sinks, potentially amplifying global warming.
The authors stress that long-term, integrated monitoring of both terrestrial and marine NPP is essential for understanding ecosystem health and informing climate mitigation strategies. Determining whether land-based gains can continue to counterbalance ocean declines remains a pressing scientific and policy question.
https://scitechdaily.com/earth-is-getting-greener-but-the-oceans-are-losing-life