The uptake of ocean carbon dioxide by marine biological activity (biological carbon pump※1) is known to play an important role in controlling atmospheric carbon dioxide concentrations, but there is still considerable uncertainty in estimates of the biological pump.
A detailed calculation of the dissolved oxygen budget※2 in the upper ocean using observational data reveals the global spatial distribution of the biological pump and estimates the total amount to be 74 billion tons of carbon per year.
The results suggest that previous estimates may have been overestimates. Continued attention should be paid to how the biological pump will change in the future as climate change continues.
Dr. Ryohei Yamaguchi and Dr. Shinya Kouketsu of the Japan Agency for Marine-Earth Science and Technology (JAMSTEC; President, Hiroyuki Yamato) have revealed the spatial distribution and total amount of annual ocean carbon uptake by biological activity by carefully calculating the dissolved oxygen budget in the upper ocean using observational data on dissolved oxygen and the latest knowledge on ocean circulation and mixing from recent studies.
Figure 1. Schematics of the global carbon cycle (left) and the biological carbon pump (left). In the left panel, Numbers of each system and numbers of arrows indicate their carbon stocks (unit is billion tons of carbon) and carbon fluxes (unit is billion tons of carbon per year), respectively (reference) . Black and white numbers and red numbers show values in the preindustrial state and anthropogenic changes, respectively.
As a vast reservoir of carbon, the ocean plays an important role in shaping today's climate by absorbing carbon dioxide from the atmosphere and sequestering it for long periods of time (Figure 1, left). One of the mechanisms by which the ocean absorbs and sequesters carbon dioxide is the "biological carbon pump", a mechanism mediated by biological activity in the upper ocean (Figure 1, right). The processes of net carbon fixation (photosynthesis and respiration) in the euphotic zone and the subsequent transport of fixed carbon to deeper layers are complex and varied, making it difficult to directly measure the amount of carbon transported along the various pathways. There are still large uncertainties in estimates of where and how much carbon dioxide is absorbed by the ocean through the biological pump.
Using the relationship that phytoplankton produce oxygen at an almost constant ratio to the amount of carbon dioxide they consume during photosynthesis, this research has succeeded in estimating the global amount of carbon dioxide taken up by the biological pump from the distribution of net oxygen production by organisms obtained from the calculation of the upper ocean dissolved oxygen budget. The results show that the actual ocean carbon uptake by the biological pump (7.4 billion tons of carbon per year) is less than previously estimated (13 billion tons of carbon per year). The spatial distribution of the biological pump, revealed for the first time in this study from the global dissolved oxygen budget, indicates the relative importance of the biological pump for ocean carbon uptake at high latitudes and in the tropics.
These results will not only deepen our fundamental understanding of the ocean carbon cycle, but will also improve the performance of Earth System Models※3 used to predict future climate. It is hoped that further improvements to the observing system and the associated increase in data will lead to a more detailed understanding of climate change-induced changes in the biological pump and various processes that make up the pump.
The results will be published in Communications Earth & Environment on December 16. This work was supported by JSPS Grant-in-Aid for Scientific Research JP24H0222 and JP22H05207.
Ryohei Yamaguchi1, Shinya Kouketsu1,2, Naohiro Kosugi3, Masao Ishii3
Biological carbon pump
One of the mechanisms by which the ocean absorbs carbon dioxide. Carbon dioxide dissolved from the atmosphere is fixed by photosynthesis by phytoplankton in the upper ocean, and the fixed carbon is then transported to deeper layers by various routes, resulting in long-term carbon storage in the deep ocean.
Dissolved oxygen budget
To investigate the causes of changes in dissolved oxygen concentration by decomposing the contributions of each of the processes driving the change (e.g. advection, diffusion, photosynthesis, respiration, etc.).
Earth System Model
A numerical model used to predict future climate using a supercomputer. The fluid motion and ecology (including the carbon cycle) in each subsystem (atmosphere, oceans and land) are modelled, and the model is used to investigate the mechanism and future changes of the whole Earth system, which is a combination of these subsystems.
IPCC Sixth Assessment report, working group 1, Chapter 5, Global Carbon and other Biogeochemical Cycles and Feedbacks, https://www.ipcc.ch/report/ar6/wg1/chapter/chapter-5/
For this study
Ryohei Yamaguchi, Researcher, Research Institute for Global Change (RIGC), Global Ocean Observation Research Center(GOORC), Physical and Chemical Oceanography Research Group, JAMSTEC
For press release