When trying to understand the big-picture effects of climate change, no subject is more important than the oceans and their role.

Water, as endowed Associate Professor of Marine Science Jason Adolf points out, has the ability to absorb heat without significantly changing temperatures. And because the oceans hold 97% of Earth’s water, they are the largest absorber of the Earth’s excess heat.

“Of the excess heat that is accumulated at the surface because of the greenhouse effect, the oceans have absorbed 93% of it. So we have to ask the question, what is that doing to the ocean?” says Adolf. “When we’re trying to understand how climate change is going to—and does—affect the oceans, we have to do that under the caveat that we know very little about the oceans to begin with.”

Adolf, who studies harmful algal blooms, says we are already seeing some of the effects of climate change on the oceans. As water warms, it expands, causing sea levels to rise. That excess heat absorption also affects ocean circulation. And as the oceans absorb more carbon dioxide to cool the Earth, carbonic acid increases, causing the pH to acidify. In many cases, these changes to ocean physics and chemistry translate to negative impacts on ocean life.

“Oceans are losing oxygen,” says Adolf. “In terms of harmful algae blooms, the combination of existing nutrient pollution, and climate change increasing the temperature of the ocean and extending the growing season, is creating larger windows of opportunity for harmful toxic species to grow in the environment and contaminate shellfish and fish resources.”

Assistant Professor of Biology Keith Dunton, who studies fish species native to the Mid-Atlantic, says fish are already being affected by temperature changes alone.

Because oxygen levels are affected by warming oceans, fish essentially have to work harder and increase their rate of breathing in order to extract enough oxygen out of the water to survive, says Dunton. It’s theorized that because of this, fish species will begin decreasing in size over time.

“Gills can only extract so much oxygen from the water to support a body size,” says Dunton. “If our oceans get deoxygenated because of warmer temperatures—warmer water has less oxygen than colder water—the fish will meet a critical maximum, and they will not be able to grow larger than a certain size because their body cannot extract the oxygen for it.”

Because of factors such as this, many fish and shellfish species are migrating toward the poles to survive. Dunton says this is a major cause of concern not only for biologists, but also for the individuals and local economies that rely on the shifting industry. As fish species move north, fisherman will need to travel farther distances to attain the same catch. And as species such as the American lobster and Atlantic cod move farther north, there is a possibility these fisheries will cross international boundaries and be lost entirely for U.S. fishermen.

Compounding the problem: as fish species from southern states move north into regions that are cool enough for them but have not yet experienced an outmigration of other fish, the range of the species contracts, says Dunton.

“What we get is fish coming to New Jersey have to suddenly move up farther; then we get an influx of southern fish—species that we hadn’t seen before,” he says. “This is contracting their overall range … This eventually leads to what we say will enable harvesting to extinction, because the habitat the fish have gets smaller and smaller and smaller, and then eventually they’re stuck in this isolated island in the ocean based on temperature.”

Next, read Part V of this series: Lay of the Land.