A recent study sheds light on why Earth's oceans have been losing oxygen in recent decades. Globally, scientists can measure oxygen levels in the ocean and evidence points to a 2% decrease in oxygen levels in the past 50 years.

Although this decrease in ocean oxygen levels seems small, there is a ripple effect that can alter ecosystems. This, combined with estimates that oxygen levels will continue to decrease by one to seven percent by 2100, leaves scientists concerned about what lies ahead.

The decrease we see in our ocean's oxygen will stress ecosystems and alter habits of fish and mammals. For example, larger fish that require more oxygen will be more limited in the areas they can go to eat and reproduce.

What Is Causing Oxygen Levels To Decrease?

A number of factors are likely influencing the decrease in oxygen levels in oceans. However, the key culprits are likely an increase in fertilizer use and a warming planet.

Humanity has drastically increased the amount of fertilizer we use to produce ever higher crop yields. However, when that fertilizer runs off into our waterways it eventually makes its way to the ocean. When that happens, microbial communities and algae all of a sudden receive a massive delivery of food. Bacteria and algae colonies exponentially increase to feed on the newly delivered food and when that happens they respire, same as humans. As in, they take in oxygen and release carbon dioxide, removing dissolved oxygen from the ocean.

You have likely heard of dead zones in the Gulf of Mexico for instance. This is a scenario where enough nutrients are brought to the Gulf of Mexico that you get algae and bacteria blooms, plummeting the available oxygen in the water. This, in turn, kills nearby fish that require oxygen to breathe. In case you missed it, we just had one of the largest Gulf of Mexico dead zones in history.

NASA, Mississippi River sediment plume into the Gulf of Mexico

The second culprit to declining oxygen levels is a warming planet. This one is fairly straight forward. Basic principals of chemistry and physics dictate that colder liquid can hold more dissolved gas than a warmer liquid. You can witness this by opening two Coke cans, then leaving one out on your counter and the other in the fridge. Come back the next day and I bet the one in the fridge has more dissolved carbon dioxide (carbonation).

History Is A Glimpse Into The Future

Has something like this ever happened in the past? Have oxygen levels in oceans declined and if so, what were the effects? To answer the question of oceans declining oxygen levels, professor Jeremy Owens from Florida State University had to step back 94 million years ago.

Approximately 94 million years ago there was a well documented Oceanic Anoxic Event (OAE), whereby large swaths of oceans were devoid of oxygen (anoxic). These anoxic events are a key source of oil and gas around the world. This is because when the ocean is anoxic, bacteria cannot break down organic carbon as it falls to the ocean floor. Thus, the organic carbon is preserved, buried, heated, and forms hydrocarbons.

The study finds a period of 50,000 years before the anoxic event where oceans saw gradually declining oxygen levels. In fact, the rates of declining oxygen are similar to the rates we see today, providing these events as good analogs into Earth's future.

However, what triggered these anoxic events in the past, as clearly humans weren't around to spread fertilizer in the oceans? The deoxygenation period was coincident with a noticeable increase in volcanic activity. As you may know, volcanoes emit large amounts of carbon dioxide during an eruption. This would have the same effect as burning fossil fuels today, an increase in global temperatures. On top of that, large volcanic eruptions can send volcanic ash and dust around the globe, providing nutrients to oceans in the process.

Hence, studying these oceanic anoxic events provide useful insights into how our planet will react, specifically in terms of ocean oxygen levels. While we clearly will not see the effect of this in our lifetime, it will certainly impact future generations.