Environment: Climate, increasing ocean desertification due to climate change

In over two decades, nutrient-poor and low-biodiversity areas in the global oceans have nearly doubled, going from 2.4 to 4.5 due to desertification. This phenomenon results in severe nutrient deficiency and could have a significant impact on ocean health and the global climate, as shown by an international study conducted by the ENEA Climate Models and Services Laboratory in collaboration with the ISMAR-Cnr Institute of Marine Sciences and the Chinese State Key Laboratory of Satellite Ocean Environment Dynamics ^[1] (SOED). The study, published in the scientific journal 'Geophysical Research Letters, focuses on analysing changes in phytoplankton, the set of microorganisms that form the base of the marine food chain (they are the food of zooplankton, fish and other organisms) and play a vital role in mitigating climate change by removing atmospheric carbon dioxide (CO2) through photosynthesis.

“This phenomenon is very evident in the North Pacific Ocean where it is expanding at a rate of 70,000 km² per year. But desertification is increasingly impacting ocean regions, particularly in tropical and subtropical areas, where nutrient loss can have significant impacts on productivity and biological diversity. This happens because of global warming, which causes warm, less dense surface water to remain on top preventing mixing with colder, nutrient-rich deep  waters. Reduced mixing can lead to less ‘food’ reaching the surface to support phytoplankton growth and, consequently, impact the entire food chain”, explained Chiara Volta, ENEA researcher at the Climate Models and Services Laboratory.

The study also shows that the amount of chlorophyll, a key indicator of phytoplankton health and abundance, is decreasing. Higher chlorophyll concentrations indicate a greater abundance of phytoplankton.

 “However, according to the study, this decline may not indicate a reduction in the phytoplankton population, but it could signify that these organisms are adapting to new new growth conditions, like increased temperature or changes in nutrient availability, which are influenced by climate change” pointed out the ENEA researcher.

To carry out this study, researchers examined time series of chlorophyll and phytoplankton satellite data between 1998 and 2022 in Earth's five major oceanic vortices (subtropical gyres) located in the North and South Atlantic, North and South Pacific, and Indian Ocean. These are systems of currents characterized by anticyclonic water movement that develop between the Equator and subtropical areas of high pressure, and whose formation is the result of a complex interplay of winds, Earth's rotation, and landmass distribution.

 “Over the past two decades, many satellite studies have documented that ocean warming has led to an expansion of these ocean systems and a consequent decrease in chlorophyll, raising serious concerns about potential impacts on global climate and the health of our oceans. However, our results show that, despite the decrease in chlorophyll observed in the nutrient-poor subtropical gyres, phytoplankton biomass has remained relatively stable over time. Considering that, by their nature, satellite data are limited to observing the ocean surface, the next steps will be to study the changes in the phytoplankton community along the water column and quantify their impact on ocean productivity at regional and global scales”, concluded Chiara Volta.

Figure 1. Global mean distribution of chlorophyll concentration in the oceans, derived from satellite observations. Areas in blue indicate low chlorophyll concentration, while those in green correspond to high concentrations

[1] The State Key Laboratory of Satellite Ocean Environment Dynamics (SOED) is an advanced research laboratory located at the Second Institute of Oceanography (SIO), which operates under the Ministry of Natural Resources of China, in Hangzhou.