New study shows global upswing in photosynthesis driven by land but offset by oceans
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It’s one of the most pivotal processes on the planet. Photosynthesis – the removal of carbon from the atmosphere to produce glucose and oxygen – supports the respiration of plants and animals worldwide, helps to sequester carbon and forms the foundation of most ecosystems.
While the process is carried out virtually planet-wide, a new study – published in Nature Climate Change – has shown how photosynthesis is increasing globally. Such an uptick can be explained by plants on land driving the increase between 2003 and 2021 – yet, this trend is partially offset by a decline in photosynthesis within the world’s oceans.
Photosynthetic organisms – also known as primary producers – form the base of the food chain, allowing most life on Earth to exist. Using the sun’s energy, these primary producers convert carbon from the air into carbon-based matter. However, primary producers also release carbon through a process known as autotrophic respiration, similar to breathing.
After accounting for the loss of carbon through respiration, net primary production can be established.
‘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 fibres for humans, mitigates anthropogenic carbon emissions and helps to stabilise Earth’s climate.’
Previously, research on net primary production has focused on either land or ocean ecosystems, but not both. However, in this new study, researchers 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,’ said co-author Nicolas Cassar, Lee Hill Snowdon Bass Chair at the Nicholas School. ‘The pioneering studies that first combined terrestrial and marine primary production have not been substantially updated in over two decades,’
To come to their conclusions, researchers utilised observations from satellites, allowing a continuous perspective of photosynthesis by plants and marine algae. Specialised satellites were able to measure surface greenness, which represents the quantity of a green pigment called chlorophyll produced by photosynthetic organisms. From this data, computer models can estimate primary production.
Authors of the new study used six different satellite-based data sets – three for the ocean, and three for land – for a time period between 2003 and 2021. Using statistical methods, they then analysed annual changes in net primary production for land and the ocean.
In doing so, 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 an exception in the tropics of South America.
In contrast, however, there was a marked overall decline in marine net primary production, amounting to approximately 0.1 billion metric tons of carbon per year during the same time period. Strong declines mainly occurred in tropical and subtropical oceans, particularly in the Pacific Ocean.
Overall, the trends on land dominated those of the oceans. Global net primary production increased significantly between 2003 and 2021, at a rate of 0.1 million metric tons of carbon annually.
What is driving the uptick?
As professor of earth and climate sciences at the Nicholas School and a co-author on the study, Wenhong Li explains, the trend on land mainly stemmed from plants at higher latitudes. Here, warming has extended growing seasons and created more favourable temperatures for plants. In temperate regions, local wetting , forest expansion and cropland intensification also occurred.
However, these warming temperatures are having 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.’
As such, it is evident that the ocean has a much greater sensitivity to future changes to the planet’s climate.
But what do these changes spell for the future of our oceans and terrestrial land? For Zhang, the new study has highlighted just how necessary it is to undertake research which considers them both in tandem.
‘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.’
