As waterbodies lose oxygen, are we breaching a potential planetary boundary? (2024)

The Earth is in crisis — a reality made increasingly clear by intensifying heat, drought and storms, worsening pollution, land stripped of forests, and the ramping up of the sixth great extinction. But some scientists say we’re failing to prioritize a critical Earth change that is accelerating in the modern world and that helped drive past global extinction events: aquatic deoxygenation.

In 2023, scientists suggested that anoxia, the lack of dissolved oxygen in a water body, played an important role in ocean ecosystem disruption and extinctions during the Triassic-Jurassic mass extinction around 200 million years ago, while others say it contributed to the “great dying” of the Permian extinction 50 million years earlier, which wiped out 90% of all marine species.

Today, human-caused hypoxia (reduced oxygen levels), due increasingly to synthetic agricultural fertilizer pollution and climate change, annually helps create vast dead zones in the Gulf of Mexico, Baltic Sea, East China Sea, and estuaries, lakes and streams around the globe.

Now, a new perspective piece in Nature Ecology & Evolution seeks to add aquatic deoxygenation of both fresh- and saltwater ecosystems to the planetary boundary framework, a theory currently used to define the safe operating limits for Earth’s natural systems.

Natural systems out of balance

In 2009, a team of international scientists proposed the planetary boundary framework, a theory defining nine interconnected biophysical and biochemical processes: natural systems that regulate Earth’s stability and resilience and make life as we know it possible. The researchers also tried to identify metrics for the human stressors reshaping and degrading those natural processes today.

According to the theory, each of the nine processes needs to stay within certain limits, referred to as “boundaries,” to maintain “the safe operating space” for humanity. But human activities over the past 150 years have pushed hard up against these boundaries, and toward dangerous Earth system tipping points — thresholds that, if violated, could threaten life.

The nine currently identified boundaries include climate change, biosphere integrity (incorporating biodiversity loss), ocean acidification, stratospheric ozone depletion, atmospheric aerosol pollution, freshwater change, biogeochemical flows of nitrogen and phosphorus, land-system change, and release of novel entities (including pollution by tens of thousands of synthetic chemicals).

A 2015 study, published by many of the same authors, found that humanity was already operating outside the safe operating space for four of the boundaries: climate change, biosphere integrity, land-system change, and biogeochemical flows. In 2023, this was further updated by the same core team who reassessed all nine boundaries, adding more control variables and sub-boundaries. They found that humanity had transgressed the safe operating space for six boundaries, adding novel entities and freshwater change.

The argument for a 10th boundary

In July 2024, a separate team of scientists, none of them involved in the original planetary boundaries research, argued in Nature Ecology & Evolution that aquatic deoxygenation should be adopted as an additional, 10th boundary.

Deoxygenation in both freshwater and marine ecosystems is critically impacting the integrity of Earth’s ecological and social systems today, the authors wrote, adding that the planet’s aquatic habitats are approaching critical oxygen thresholds “at rates comparable to other planetary boundary processes.” Furthermore, aquatic oxygenation isn’t an independent process, but “regulates and responds to ongoing changes in other planetary boundary processes.”

Lead author Kevin Rose told Mongabay in an interview that when the planetary boundary theory was proposed in 2009, there wasn’t a lot of compiled data on dissolved oxygen. However, he said, the growth of scientific knowledge and understanding of the issue of deoxygenation, and its “implications for habitability, sustainability of freshwaters, coastal systems and the open oceans,” warrants its inclusion in the planetary boundary framework now as a 10th boundary.

“In aquatic environments, dissolved oxygen concentrations have largely been declining for, in some cases, decades to centuries,” Rose said. “There’s a number of factors driving this, but one of the things that’s come out in the last few years is the scale at which we’re losing dissolved oxygen and aquatic ecosystems, as well as its implications for [interacting with and impacting] a lot of the other planetary boundaries.”

The authors wrote that dissolved oxygen concentrations have “rapidly and substantially declined across both freshwater and marine habitats, ranging from small ponds to large lakes and reservoirs, rivers, inland seas, estuaries, and areas of the coastal and open ocean.”

They note that lakes and reservoirs have experienced oxygen losses of 5.5% and 18.6% respectively since 1980, and that the global ocean overall has experienced a 2% loss in oxygen since 1960. While marine deoxygenation appears lower than that of freshwater, this loss is “far more geographically and volumetrically extensive,” according to the authors.

Rose noted that oxygen availability in aquatic environments regulates, and is regulated by, other planetary boundaries through a “give and take” process. For instance, an interplay between the climate change boundary and the aquatic deoxygenation boundary takes place when anoxic forms of respiration take over in deoxygenated marine and freshwater environments, which can produce methane. “That [results in] an important feedback to climate change, because methane is a really potent greenhouse gas,” Rose said.

Deoxygenation doesn’t only interact with the climate change boundary. The study authors wrote that it also interacts in a major way with the land-system change, biogeochemical flows, and biosphere integrity boundaries. For example, increased biogeochemical flows of nitrogen and phosphorous, due largely to fertilizer runoff, contribute to red tides and falling oxygen levels in the world’s estuaries, and the resulting biodiversity declines occurring in these dead zones.

An evolving theory

Johan Rockström, director of the Potsdam Institute for Climate Impact Research and one of the originators of the planetary boundary frameworks, said in a recent Mongabay that the planetary boundary theory is robust and continues to evolve.

In the future, he said, he believes each planetary boundary could include up to four control variables — features linked to and influencing a particular boundary — to better emulate the complexity of Earth systems.

While not specifically commenting on the new study led by Rose, Rockström also noted that the ocean planetary boundary currently only has one quantified control variable — ocean acidification — which he said is inadequate for describing marine complexities.

“That is scientifically and operationally very unsatisfying,” Rockström told Mongabay. For example, “We presently have no control variable for the biology in the ocean. So, there is a quest to find a twin control variable that can capture biology there — all phytoplankton, zooplankton, the food webs and the nutrient cycling in the ocean, so we’ll not only be able to [evaluate] the heat and chemistry of the ocean as we do today, but changes in biology too.

“Having multiple control variables for each boundary not only shows us the complexity of the Earth system,” Rockström concluded, “but also [gives] us multiple ways of measuring the planetary boundary process.”

It therefore seems reasonable that aquatic deoxygenation could eventually figure into planetary boundary refinements, possibly as an additional control variable for the ocean planetary boundary, which is currently described as the ocean acidification boundary.

Lan Wang-Erlandsson, a researcher at the planetary boundaries research lab at the Stockholm Resilience Centre, who was not involved in the new paper led by Rose, said she believes that previous planetary boundaries assessments already indirectly include aquatic deoxygenation, but that it could be considered as a control variable for other boundaries.

“I think the original scope of the nine planetary boundaries should be able to capture key transgressions driven by deoxygenation, at least indirectly. Especially as the specific variable used to represent the boundaries can be (and have been) revised to keep up with scientific advances,” Wang-Erlandsson told Mongabay in an email.

On the other hand, Wang-Erlandsson said she doesn’t have an opinion as to whether aquatic deoxygenation should be considered its own planetary boundary.

“Regardless, I think the issues raised by the authors are urgent and important,” Wang-Erlandsson said, adding she finds “their suggestions for future research valuable and relevant for better understanding Earth system resilience and sustainability.”

Banner image: A sea turtle in Ningaloo Reef, Australia. Image by Emilie Ledwidge / Ocean Image Bank.

Warmer, oxygen-poor waters threaten world’s ‘most heavily exploited’ fish

Citations:

Richardson, K., Steffen, W., Lucht, W., Bendtsen, J., Cornell, S. E., Donges, J. F., … Rockström, J. (2023). Earth beyond six of nine planetary boundaries. Science Advances, 9. doi:10.1126/sciadv.adh2458

Rockström, J., Steffen, W., Noone, K., Persson, Å., Chapin, F. S., Lambin, E. F., … Foley, J. A. (2009). A safe operating space for humanity. Nature, 461(7263), 472-475. doi:10.1038/461472a

Rose, K. C., Ferrer, E. M., Carpenter, S. R., Crowe, S. A., Donelan, S. C., Garçon, V. C., … Breitburg, D. (2024). Aquatic deoxygenation as a planetary boundary and key regulator of earth system stability. Nature Ecology & Evolution, 8(8), 1400-1406. doi:10.1038/s41559-024-02448-y

Steffen, W., Richardson, K., Rockström, J., Cornell, S. E., Fetzer, I., Bennett, E. M., … Sörlin, S. (2015). Planetary boundaries: Guiding human development on a changing planet. Science, 347(6223), 1259855. doi:10.1126/science.1259855