Global climate change produces many effects – warming air energizes the atmosphere and intensifies storms; warmer water expands and raises sea level; storage of more carbon dioxide in the oceans is acidifying large realms. Now it is becoming clear that another, profound result of human activities is underway: lower oxygen levels in our oceans.
The world’s oceans, coastal seas, estuaries, and many rivers and lakes are experiencing declines in dissolved oxygen. Long known as an issue associated with sewage discharges and fertilizer runoff, the problem now is exacerbated by climate change, often independent of nutrient loads, and is global in scale.
If left unchecked, this decline will result in losses of fisheries and biodiversity, poorer water quality, and knock-on effects ranging from falling tourism to reduced marine ecosystem services.
In 2015, scientists from around the world formed an IOC (Intergovernmental Oceanographic Commission)-UNESCO working group called the Global Ocean Oxygen Network or GO2NE, of which I am a member. Our goals are to raise awareness about this problem, called deoxygenation, and stimulate research and policy to understand and mitigate it.
The how and why of oxygen in water
The aquatic environment holds dissolved gases; oxygen is one of them. Add in aquatic plants, a CO2 source, nutrients and light, and the resulting photosynthesis creates biomass and oxygen. Add more nutrients, and more photosynthetic production occurs. Consumers of the plant matter also use oxygen in the process and recycle the nutrients and carbon from organic to inorganic forms. This simplified cycle describes most aquatic ecosystems.
Next, consider the nutrients running off from farmland and urban areas, and the CO2 emissions responsible for much of the global greenhouse effect. At local, regional and global scales, these inputs stimulate the growth of aquatic plants. An excess of nutrients – known as eutrophic conditions – can cause massive algal blooms. Once those algae die of off, they are decomposed by microbes that draw down the dissolved oxygen.
Now add in physics: Warmer water is less dense and holds less oxygen than cold water, and salt in seawater lowers its oxygen carrying capacity too. As the estuaries, coastal seas and oceans heat up, less oxygen is held, stratification (the separation of water into layers) intensifies, and deeper waters lose even more oxygen. This is the additional impact of global warming.
Water holding low levels of oxygen – less than two milligrams of oxygen per liter – is called hypoxic, borrowing the term from medicine where tissues are not oxygenated adequately. This level is generally deleterious for much aquatic life such as fish and shellfish. Water completely devoid of oxygen is called anoxic. Few higher organisms can make a living under these conditions.
Oxygen declines are widespread
From the waters off the Oregon coast, to the upwelling zones off Peru, to the Baltic and Black seas, Bay of Bengal, South China Sea and Gulf of Mexico, to name but a few regions, so-called dead zones are on the increase. Low oxygen areas in the deep ocean are also expanding, primarily due to warming. Those with very low oxygen are called oxygen minimum zones, or OMZs.
Worldwide, the ocean volume in a state of hypoxia, in which dissolved oxygen is at about 1.6 mg/liter, is 60.4 million cubic kilometers. Put another way, it would take close to 58 billion Empire State Buildings to contain that volume of water (or over 250 billion Wal-Mart Supercenters!) in a low-oxygen state.
Scientists have used climate change models to examine future scenarios of ocean conditions, including temperature, pH, oxygen and biological production. Despite very different assumptions in the models and different scenarios, from “business as usual” to very strong mitigation of CO2 emissions, the models project that oxygen levels will decline from 1.8 percent to 3.45 percent over the next century due to climate change.