Oceans lag behind forests in Earth’s carbon fight

Terrestrial vegetation has been the foundation of food webs and a crucial ally in climate regulation for eons. From 2003 to 2021, that green land engine began to outpace its blue counterpart.

A new analysis draws on six independent satellite data sets to show that land plants increased their net primary production by roughly 0.2 billion metric tons of carbon each year. In contrast, marine phytoplankton lost about half that amount.

The bottom line, write Yulong Zhang and colleagues from Duke University, is that global photosynthesis still crept upward – just not everywhere, and not for the same reasons.

Land surge in net primary production

Because satellites can detect the chlorophyll pigments that make leaves and plankton appear green, researchers can turn those brightness maps into estimates of carbon fixed by photosynthesis.

For this study, the team blended three terrestrial and three marine products, covering nearly two decades. They then traced the annual ups and downs in each realm.

The land situation stood out immediately. “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,” explained study co-author Wenhong Li.

Forests thrive, oceans decline

In addition, temperate zones benefited from “local wetting in some areas, forest expansion, and cropland intensification.” These gains more than compensated for a stubborn patch of stagnation across tropical South America.

Ocean productivity told a different story. “Rising sea surface temperatures likely reduced primary production by phytoplankton in tropical and subtropical regions,” said Nicolas Cassar, senior scientist on the project.

Warmer surface layers, he noted, form a cap that suppresses the turbulent mixing needed to replenish nutrients in sun-lit waters. The result, ocean productivity declined by roughly 0.1 billion tons of carbon per year, with the Pacific basin leading the retreat.

Ocean carbon tied to climate

Although land plants dominated the long-term trend, oceans dictated much of the year-to-year variability.

“We observed that ocean primary production responds much more strongly to El Niño and La Niña than land primary production,” said study co-author Shineng Hu.

Indeed, a trio of La Niña events after 2015 temporarily reversed the downward slope in marine photosynthesis. This underlines how tightly plankton communities are lashed to climate oscillations.

The researchers parsed several environmental drivers – light, temperatures, precipitation, and mixed-layer depth – to understand why land and sea diverged. Their conclusion was that warming (and, in some places, wetter conditions) favored northern forests and fields. Meanwhile, the same warming starved tropical waters of nutrients.

“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,” noted Zhang.

Ocean losses threaten biodiversity

Photosynthetic organisms not only fuel ecosystems; they also shape the planet’s carbon balance. Terrestrial plants, boosted by longer growing seasons and human land management, currently outweigh the losses at sea, but the margin is slim.

Should ocean productivity continue to sag, tropical food webs – already strained – could fray further, jeopardizing fisheries and weakening a major carbon sink.

As the authors caution, declines in plankton and stagnation on tropical land can weaken the foundation of tropical food webs. This will have cascading effects on biodiversity, fisheries, and local economies.

Monitoring net primary production

The synthesis highlights the need for an integrated monitoring network that treats forests and phytoplankton as parts of one global engine.

“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,” Cassar emphasized.

Many of the foundational studies that stitched land and sea together are now more than two decades old. Updating that picture is urgent as climate patterns shift.

Carbon fight shifts from ocean to land

Net primary production, the authors argue, is effectively Earth’s scoreboard for life. It registers how much solar energy is converted into the organic matter that nourishes everything else.

According to Zhang, net primary production is the amount of energy photosynthetic organisms capture and make available to support nearly all other life in an ecosystem. That metric shows if the biosphere is gaining or losing ground in sequestering atmospheric carbon.

For now, an energetic Northern Hemisphere offsets a flagging equatorial ocean. But the equilibrium is delicate, and the drivers – temperature, rainfall, nutrient mixing – are changing fast.

“Long-term, coordinated monitoring of both land and ocean ecosystems as integrated components of Earth is essential,” concluded Zhang.

The study is published in the journal Nature Climate Change.

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