Climate change refers to long-term shifts in temperature, precipitation and other weather patterns. Scientists have documented a significant increase in the average temperature of Earth’s air and oceans over the past hundred or so years, a trend known as global warming. The release of carbon emissions into the atmosphere, mostly through industrial agriculture and burning of fossil fuels, has been a major contributor to global warming.

Climate change has warmed Earth’s average temperature by at least 1.2 degrees Celsius (2.2 degrees Fahrenheit) since scientists started measuring it in 1880. For a planet that’s over 70% water, this means warming oceans, lakes, rivers and more.

Water can store more heat than air, with the ocean storing about 90% of the excess heat produced by greenhouse gases. This rapid warming threatens aquatic ecosystems, the species that live there and the people who depend on them.

Aquatic Life Is in Hot Water

Aquatic ecosystems tend to prefer certain conditions and are sensitive to changes in temperature, salinity and pH. (pH is the measurement of how acidic or basic a solution is.) In many cases, organisms cannot survive outside of specific temperature ranges.

Heat changes the chemistry of water. Warmer water holds less dissolved oxygen, which is critical to aquatic life. Without enough dissolved oxygen, many species can die. The ocean has lost 2% of its oxygen since the 1960s due to a combination of warming temperatures and pollution. Decreases in dissolved oxygen are especially an issue for large fish, which require more oxygen. If the populations of larger fish decline, it threatens the livelihood of fishers all over the world who target and receive more money for larger fish. 

As average temperatures rise and pollution from greenhouse gas emissions increases, the oceans become more acidic. This makes it more difficult for some animals to maintain their calcium carbonate shells and skeletons, making them brittle and easy to break. Water temperature also impacts salinity, to which many aquatic species are sensitive. Freshwater systems are expected to get saltier, particularly as sea levels rise and flood river deltas and droughts leave salt behind that washes into freshwater sources.

Changes to water temperature can affect primary producers—the organisms that form the foundation of the food chain—and create a wave of change that ripples throughout the ecosystem.

Phytoplankton, the basis of many marine food webs, are on the front lines of changes to ocean temperatures. These organisms photosynthesize, creating their own food supply from the sun. Like plants, phytoplankton use chlorophyll to do this, which gives them their green color. Due to changes in phytoplankton abundance, the ocean is becoming greener as the planet warms. Scientists are still trying to understand why this is happening, but they believe it might be due to an association between warmer water and concentrations of chlorophyll. This change is more pronounced near the polar regions.

Many species experience increases in their metabolism when waters warm, which means they need more food. This is especially true for cold water species. Some species simply cannot survive the heat, which can lead to mass deaths in an area. If able, a species will migrate to new areas to seek cooler waters. In many cases, this means moving toward the poles.

Corals are particularly vulnerable to heat stress. Corals are animals, and they build reefs that serve as a habitat for many species. They are vital to both humans, who rely on coral reefs for fishing and tourism, and marine ecosystems. They have a symbiotic relationship with algae called zooxanthellae, which give many corals their color and provide nutrients to the coral. When there is excessive heat, zooxanthellae are expelled from the coral, causing the coral to lose their color, or bleach, and eventually die.

Warming waters threaten coral ecosystems further below the ocean’s surface too. National Geographic Explorer Ana Belén Yánez Suárez studies deep-sea corals. These deep-sea corals thrive in a cold, dark environment at depths of up to 6,000 meters (20,000 feet). But these corals are less frequently studied than their colorful counterparts closer to the surface. Yánez-Suárez studies deep-sea coral within the Galápagos Marine Reserve and the Isla del Coco, Costa Rica, which live in lower oxygen environments. Part of this research includes collecting high-resolution images to create 3D maps of these reefs.

As climate change expands lower oxygen environments, it is critical to understand which corals can live in these areas to assess the impact of oxygen lost in the deep-sea, Yánez-Suárez says. Using mapping tools this research also explored how the seafloor structures favor coral presence in low-oxygen waters. In her work supported by National Geographic Society/Schmidt Ocean Institute, Yánez-Suárez and her team discovered an octocoral garden living in anoxic conditions of Isla del Coco, the dominant species forming this garden is new to science.

Yánez-Suárez’s research aims to improve scientists’ understanding of cold-water coral distribution under extreme conditions, establishing a baseline to support the protection of some of the last pristine deep-sea ecosystems in the Eastern Tropical Pacific. This information will inform those in decision-making positions in the design of extensive Marine Protected Areas, helping to minimize harm to cold-water corals. By reducing cumulative stressors, increasing their chances of survival and long-term persistence.

New Species, New Areas, New Threats

Climate change is creating conditions that cause species to populate new areas with more suitable habitats. It can also cause local conditions to become more favorable for non-native species than native species.

While a species moving into new areas isn’t always a problem, some species are invasive and do substantial damage to an ecosystem. This is usually because the native species have not evolved defenses against the invasive species, or the invasive species has no natural predators in the new area. Invasive species also compete for resources with native species.

One of the most invasive species on Earth is the water hyacinth (Pontederia crassipes), originally from South America. It blocks sunlight from entering water and absorbs oxygen and nutrients, leaving less for native aquatic species. It also causes economic losses by damaging infrastructure, including clogging irrigation systems. Humans transported the water hyacinth around the world because they valued its ornamental beauty. Now, rising temperatures are expanding where water hyacinths can survive and grow.

Water hyacinth proliferation is a pressing issue in Lake Victoria, Africa’s largest lake. At times, the plant can cover 5% or more of the lake’s surface, and it has become a challenge for fishers and the ecosystem alike. However, people are working on innovative solutions to the issue. Some have proposed making water hyacinths into a bioplastic or a biofuel. Kenyan Biologist Jack Oyuji developed a feed for livestock animals, such as cattle (Bos Taurus), from the water hyacinth. Using this strategy, he is also able to hire local people, supporting local livelihoods.

Pathogens, including bacteria, may also live longer or appear in new environments because of rising water temperatures. This risks the spread of diseases, which can also impact humans. Oysters are especially concerning, because people often eat them raw, meaning they have a greater potential to spread pathogens. Vibrio is one such pathogen on the rise that can cause severe digestive distress.

New Boundaries for Fishers

Warming waters will not only impact the aquatic life and biodiversity of that ecosystem, it will also impact the humans and societies that depend on these ecosystems for their nutrition, economic stability and culture. Over three billion people depend on aquatic foods as a significant source of nutrition, especially protein. Fishers around the world depend on these ecosystems for their livelihoods.

Fishers are generally limited to the waters of their country’s exclusive economic zone (EEZ), which is the water up to about 370 kilometers (200 nautical miles) from a country’s coast. Fish, however, do not recognize political boundaries and will migrate in and out of a particular country’s EEZ as needed, especially if temperatures change. As fish leave historic fishing grounds and enter new territories, this creates the potential for economic loss and conflict.

In South America, warming waters threaten one of the largest fisheries in the world. The Peruvian anchovy (Engraulis ringens) makes up approximately 15% of the global catch, but scientists expect anchovies will lose half of their habitat off the west coast of North and South America by 2100. This fish is not only food for many people; it is also used in fish farming to feed other species, like salmon.

Waters are not warming equally around the world. Not only is the temperature increase variable, but so are the severity of the consequences. Parts of the North Atlantic, as well as the Arctic and Southern oceans are some of the fastest warming waters.

Some places will see a mix of changes—losses and new opportunities. The Chesapeake Bay is an estuary off the Atlantic Ocean that stretches north into the U.S. states of Virginia and Maryland.

Abundant and diverse populations of oysters, crabs and finfish have long sustained the people living in this area. These species were part of the diets of Indigenous people of the area for centuries. Black watermen, both unconfined and enslaved, also have history in the Chesapeake Bay. Wealthier White people lived further from the mosquito-rich bay while Black people had to turn to land close to the water. Black watermen have been relying on the Chesapeake for food and economic independence since the 1800s. 

Today, the bay produces over 227 million kilograms (500 million pounds) of seafood. Some species used for food, like the striped bass (Morone saxatilis), are under threat from warming waters. Changes in salinity have also impacted oyster reproduction. At the same time, warming waters could increase the prevalence of other plants and animals. Some shrimp species have been migrating north toward the Chesapeake in search of cooler waters.

A Needed Sea Change

Scientists, environmentalists, resource managers and others are working to better understand and restore aquatic ecosystems. These efforts can help many ecosystems adapt to climate change. While these efforts are necessary, addressing climate change by reducing fossil fuel emissions is the only lasting way to prevent continued warming. This should include both individual reductions in greenhouse gases, such as reducing car use, and larger societal change, including policies that support renewable energies.

Although waters have already warmed, it’s important to remember the resilience many ecosystems and people have shown. Continuing to build this resilience, harness innovation and prioritize our waters will be key to the future of our Earth and life’s existence on it.

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Taking the Ocean’s Temperature

How do scientists measure the temperature of the ocean? Since the ocean is both vast and deep, taking its temperature presents a challenge. One way is using robots called Argo floats. These robots drift through the ocean following the currents and can move up and down through the water. As they move, their sensors measure ocean temperatures. Approximately every 10 days, the float rises to the surface and sends the data it collected to a satellite for scientists to use. These measurements contribute to scientists’ calculations to determine the average ocean temperature.