Global increase in photosynthesis on land while it falls in the oceans

https://www.prensa.com/mundo/aumento-global-de-la-fotosintesis-en-tierra-mientras-cae-en-los-oceanos/

Land plants drove an increase in global photosynthesis between 2003 and 2021, partly offset by a slight decline in seaweed.

The findings, from a study published in Nature Climate Change , could inform assessments of planetary health, improve ecosystem management, and guide climate change projections and mitigation strategies, the authors said.

Photosynthetic organisms, also known as primary producers, form the base of the food chain and make most life on Earth possible. Using the sun’s energy, primary producers capture or convert carbon from the air into organic, or carbon-based, matter. However, primary producers also release carbon through a process called autotrophic respiration, similar 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 that photosynthetic organisms capture and make available to support virtually all other life in an ecosystem,” said first author Yulong Zhang, a researcher in Wenhong Li’s lab at Duke University’s Nicholas School of the Environment.

“As the foundation of food webs, net primary production determines the health of ecosystems, provides food and fiber to humans, mitigates anthropogenic carbon emissions, and helps stabilize the Earth’s climate.”

Previous research on net primary production has traditionally focused on terrestrial or oceanic ecosystems, resulting in gaps in our understanding of net primary production across the Earth and its potential implications for climate change mitigation.

For this study, the team explored annual trends and variability in global net primary production, focusing on the interaction between terrestrial and ocean ecosystems.

“When analyzing planetary health, it is necessary to consider both terrestrial and marine domains to obtain an integrated view of net primary production. 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 Professor at the Nicholas School, who supervised the research with Zhang.

Satellite observations offer ongoing insights into the photosynthesis of 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 estimate net primary production by combining greenness data with other environmental data, such as temperature, light, and nutrient variability.

Instead, the team identified an overall decline in marine net primary production, of approximately 100 million metric tons of carbon per year over the same period. The sharp declines occurred primarily in tropical and subtropical oceans, particularly the Pacific Ocean.

In short, land-based trends predominated over ocean-based ones: global net primary production increased significantly between 2003 and 2021, at a rate of 100 million metric tons of carbon per year.

To understand the potential environmental factors at play, the team analyzed variables such as light availability, air and sea surface temperatures, precipitation, and mixed layer depth, a measure of the degree of mixing in the upper ocean layer by wind, waves, and surface currents.

“The shift toward greater terrestrial primary production was primarily driven by plants at higher latitudes, where warming has extended growing seasons and created more favorable temperatures, and in temperate regions that experienced localized wetting in some areas, by forest expansion and cropland intensification,” explained Wenhong Li, a professor of Earth and Climate Sciences at the Nicholas School and co-author of the study.

Rising temperatures appeared to have the opposite effect in some ocean areas. “Rising sea surface temperatures likely reduced phytoplankton primary production in tropical and subtropical regions,” Cassar added. Warmer waters can accumulate on top of cooler waters and interfere with the mixing of nutrients essential for algal survival.

While land drove the overall increase in global primary production, the ocean primarily influenced interannual variability, especially during intense climate events such as El Niño and La Niña, the authors say.

“We observed that ocean primary production responds much more strongly to El Niño and La Niña than terrestrial 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 the reversal of the trend in ocean primary production we identified after 2015. This finding highlights the ocean’s increased sensitivity to future climate variability.”