Who Moved My Oxygen?

It took humanity a long time to recognize that, despite our apparent insignificance compared to the vastness of Earth, we are capable of fundamentally altering the planet’s ecological systems. This realization, shaped by decades of scientific research, led to the development of the Planetary Boundaries framework, pioneered by Swedish scientist Johan Rockström.

This framework aims to define key environmental thresholds where human activity could cause irreversible damage to Earth’s ecosystems. It identifies nine critical planetary boundaries, including greenhouse gas concentrations in the atmosphere, ocean acidification, stratospheric ozone depletion, freshwater use, atmospheric aerosol levels (tiny airborne particles), and chemical pollution from industrial activities. Each boundary has quantifiable limits that should not be exceeded to maintain a stable and habitable planet.

Now, a new perspective paper calls on scientists, environmental advocates, and policymakers to recognize a tenth planetary boundary: the depletion of oxygen in Earth’s water systems.

The Planetary Boundaries framework has become a cornerstone in shaping global climate policies and regulations, serving as a roadmap for countries and organizations in their efforts to maintain Earth's ecological balance. Notable applications of this framework include the 2015 Paris Agreement, which set targets for reducing carbon dioxide emissions, and the 2021 European regulations on single-use plastic, acknowledging plastic pollution as a major environmental threat.

Planetary Boundaries 2023 – The framework has become a key tool in shaping international climate policy and regulations. This infographic illustrates the different boundaries and their current status as of 2023.| Credit: Azote for Stockholm Resilience Centre, based on analysis in Richardson et al. 2023.

The Tenth Threshold: Oxygen in Water

Many aquatic organisms depend on oxygen, just as it is essential for life on land. Whether in freshwater environments like lakes and springs or in marine ecosystems such as seas and oceans, dissolved oxygen is crucial for the survival of most biological systems.

A new study compiles data showing a significant decline in dissolved oxygen levels in both freshwater and marine environments since 1980. This trend could have devastating consequences for marine and freshwater ecosystems. If the process accelerates, it may severely disrupt marine life, triggering a cascading chain reaction with wide-reaching impacts. These changes would also directly affect the global food industry and human food security, as billions of people depend on fish, seaweed, and other seafood for sustenance and livelihoods.

Much like on land, oxygen cycles through aquatic ecosystems, with some living organisms consuming it while others produce it through photosynthesis. The problem arises when this delicate balance between oxygen consumption and production  is disrupted—when oxygen is depleted faster than it can be replenished.

In low-oxygen conditions, oxygen-dependent bacteria die, leading to the collapse of larger organisms that rely on them. Meanwhile, anaerobic bacteria—those that thrive without oxygen—feed on decaying matter, multiply rapidly, and form dense clusters that block sunlight from reaching shallow waters, further reducing photosynthesis. Since photosynthesis is a primary source of oxygen production, this process creates a self-reinforcing feedback loop, accelerating oxygen depletion even further.

Without dissolved oxygen, most biological systems would cease to exist. A seal swimming among seaweed. | Shutterstock, Jonas Gruhlke.

Why Are the Oceans Losing Oxygen?

The study highlights several key factors accelerating oxygen depletion in the oceans. One major contributor is rising water temperatures—warmer water holds less oxygen than colder water. This effect can be compared to a carbonated drink: when chilled, it retains dissolved gases well, but as it warms, the gases escape more easily into the air. Similarly, in warming oceans, heat energy enables oxygen molecules to “escape” more readily. As greenhouse gas emissions persist, both air and water temperatures continue to rise, leading to a decline in dissolved oxygen levels.

Another major driver of oxygen loss is the increasing density difference between water layers. The ocean’s surface layer warms much faster than deeper layers, and melting ice sheets introduce freshwater, reducing salinity in the upper ocean while intensifying density contrasts. The upper ocean layer—rich in oxygen due to photosynthesizing algae and its proximity to the atmosphere—becomes increasingly isolated. The greater the density difference, the slower the mixing of oxygenated surface waters with deeper waters. As this mixing slows, less oxygen reaches the deep ocean, exacerbating oxygen depletion in deeper marine ecosystems.

The study identifies multiple causes of oceanic oxygen loss. Regions in the oceans with severe oxygen depletion. | Source: Benway et al. 2019

What Can Be Done?

Currently, there is no established threshold defining what constitutes a sufficient oxygen level in lakes and oceans. “The observed deoxygenation of the Earth’s freshwater and marine ecosystems represents an additional planetary boundary process that is critical to the integrity of Earth’s ecological and social systems, and both regulates and responds to ongoing changes in other planetary boundary processes,“ the researchers write in their paper.

They warn that the current framework of planetary boundaries overlooks one of the most fundamental indicators of life on Earth. To address this gap, they propose establishing a lower threshold for oxygen levels in water, integrating this measure into the planetary boundaries framework, and implementing regular ocean monitoring to track oxygen depletion trends.

Additionally, they call for limiting other activities that contribute to oxygen depletion, such as releasing untreated sewage into the sea, which leads to increased oxygen consumption as organic materials in the waste decompose, as well as disturbing the seabed due to fishing or deep-sea mining—both of which release substances from the seabed that stimulate oxygen-consuming bacterial activity, thereby increasing oxygen consumption and further depleting oxygen levels in marine ecosystems.