Who Moved My Oxygen?

It Took Time to Realize: Humans Can Change the Planet

It took humanity a long time to recognize that, despite being seemingly insignificant creatures compared to the vastness of Earth, we are capable of fundamentally altering the planet’s ecological systems. This realization, born from decades of scientific research, led to the development of the Planetary Boundaries concept, spearheaded by Swedish scientist Johan Rockström.

The goal of this framework is to define key environmental factors where human influence could lead to irreversible damage to Earth’s ecosystems. It also sets quantifiable thresholds that should not be exceeded for each factor. Nine planetary boundaries have been established, including greenhouse gas concentrations in the atmosphere, ocean acidification, stratospheric ozone depletion, freshwater use, atmospheric aerosol levels (tiny airborne particles), and chemical pollution from various industries.

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

The Planetary Boundaries framework has become a key tool in shaping international climate policies and regulatory processes, serving as a roadmap for countries and organizations striving to maintain Earth's ecological balance. Notable examples of its practical application include the 2015 Paris Agreement, which sets targets for reducing carbon dioxide emissions, and the 2021 European regulations restricting plastic bag use, following the recognition of plastic as a major environmental pollutant.

Planetary Boundaries 2023 – The Planetary Boundaries framework has become a key tool in shaping international climate policy and regulations. Infographic showing 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 such as lakes and springs, or in marine ecosystems like seas and oceans, the presence of dissolved oxygen is crucial for the survival of most biological systems.

A new study compiles data showing that dissolved oxygen levels in Earth's water bodies—both freshwater and saltwater—have significantly declined since 1980. This trend could have devastating consequences for marine and freshwater ecosystems. If the process accelerates, it may severely disrupt aquatic life, triggering a destructive chain reaction. Such changes would also directly impact the global food industry and human food security, as billions of people rely on fish, seaweed, and seafood for sustenance and livelihoods.

Much like on land, oxygen cycles through aquatic ecosystems, with living organisms consuming it while others produce it through photosynthesis. The problem arises when the balance between oxygen consumption and production is disrupted—that is, 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 the dead organisms, multiply, and form dense clusters that block sunlight from penetrating shallow waters, thereby reducing photosynthesis. Since photosynthesis is vital for 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 factors accelerating oxygen depletion in water. One key factor is rising water temperatures—warmer water holds less oxygen than colder water. This phenomenon can be compared to a carbonated drink: when chilled, it retains dissolved gases well, but as it warms, the gases escape more quickly into the air. Similarly, in warmer water, heat energy allows oxygen molecules to “escape” more easily. As greenhouse gas emissions continue at current levels, air and water temperatures rise, leading to a decline in dissolved oxygen levels.

Another driver of oxygen loss is the increasing density difference between water layers. This occurs because the surface layer warms much faster than deeper layers, and melting ice sheets contribute fresh water, reducing salinity in the upper ocean while exacerbating density contrasts. The upper ocean layer, rich in oxygen due to photosynthesizing algae and proximity to the atmosphere, becomes increasingly isolated. The greater the density difference, the slower the mixing of oxygenated surface waters with deeper waters. As mixing slows down, less oxygen reaches the deep ocean.

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 defined threshold that determines what constitutes a sufficient oxygen level in lakes and oceans. "The observed depletion of oxygen in Earth's aquatic systems demonstrates another planetary threshold process, one that is critical to both ecological and social systems and is also influenced by and responds to changes in other threshold conditions," the researchers write in their paper.

The researchers fear that the current list of planetary boundaries does not include one of the most important indicators of life on Earth. They propose establishing a lower threshold for oxygen levels in water, adding this measure to the list of planetary boundaries, and regularly monitoring the oceans to track changes in this area.

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.