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Scientists are working to understand the role oceans play in the Earth’s climate.

Washington — Pollution, greenhouse gas emissions and commercial fishing are changing oceans — bodies of water so vast that they were once thought to be impervious to human activity.

Scientists are working to understand more about the integral role the oceans play in Earth’s climate.

Josh Willis, an oceanographer at NASA’s Jet Propulsion Laboratory in California, said evidence for the three phenomena is “hard to refute.” The amount of carbon dioxide in the atmosphere is increasing. Average temperatures are increasing in the lower atmosphere (near Earth’s surface) and in the surface ocean. And global average sea level is rising at a rate faster than at any time since the end of the last ice age.

Rapid changes in the chemistry of seawater are negatively affecting ocean ecosystems, which were already stressed from overfishing, according to Willis.

“How bad will it get? That depends,” he said, “on how much carbon dioxide humans add to the climate.”

THE GREAT OCEAN CONVEYOR BELT

Earth’s five oceans are connected. Water circulates throughout the entire ocean along a great oceanic conveyor belt, where surface water sinks and spreads throughout the deep ocean and into every ocean basin around the world. Scientists call this the “overturning” or “thermohaline” circulation because water’s temperature (thermo) and salinity (haline) drive the water’s movement around the globe.

Water separates into layers based on density and these layers rarely mix. Warm water rises, cold or salty water sinks. Warm water flows at the surface, cold water flows beneath. Even in the tropics, deep water is practically freezing. Water expands as it warms up, so sea level will rise as the oceans heat up.

In the North Atlantic, the overturning circulation keeps atmospheric temperatures warmer than they would be otherwise. Surface water, driven by the thermohaline circulation, transports heat from the South Atlantic to the North Atlantic, warming the surrounding continents.

As the Earth warms, one possibility is that the ice sheet in Greenland will melt rapidly due to ever warmer temperatures from southern waters. If so, Greenland could dump large quantities of fresh water into the ocean, stopping or even reversing the overturning circulation. Some evidence exists that something similar occurred thousands of years ago, as the last ice age was ending. Many scientists agree that a sudden shutdown is unlikely to occur today.

However, most climate models suggest that the overturning circulation will slow, Willis said, but scientists are unsure how much and how quickly. A slowing of the overturning circulation in the North Atlantic will affect the climate in Europe, North Africa and possibly the United States. Average temperatures in Europe would continue to rise but would rise less quickly as circulation slows, and rainfall in North Africa and hurricanes in the United States could be affected as well.

The ocean absorbs more than 90 percent of heat added to the atmosphere, Willis said. If not for the ocean, global warming would be far more rapid ― an increase in atmospheric temperature that takes hundreds of years would take decades if Earth had no oceans.

The ocean also absorbs about one-fourth of the carbon dioxide released by human activity. This carbon dioxide flows from the atmosphere into the ocean, where it reacts with the seawater and makes it more acidic.

In the last 200 years, the ocean has become about 26 percent more acidic, according to the 2013 Intergovernmental Panel on Climate Change Fifth Assessment Report. This is largely due to the increased carbon dioxide humans have emitted into the atmosphere.

Fifty years ago scientists thought the idea that humans could change the chemistry of the ocean absurd. Today scientists know otherwise.

Ocean acidification makes the ocean less habitable. The increased acidity corrodes the shells and skeletons of many marine organisms and damages coral reefs. But the impacts of ocean acidification are still not well understood. Scientists are working to understand how changes in acidity work their way through ecosystems.

FEEDBACK CYCLES

“Sea surface temperatures have a big impact on local climate,” Willis said. Warm water evaporates into the atmosphere and falls to the land as rain. Regional ocean surface temperature cycles at regular intervals. For example, during a cool phase of the Pacific Decadal Oscillation ― a long-lived pattern of climate variability ― the surface water near California is slightly cooler than usual, causing less rain in California.

In what scientists call a positive-feedback cycle, the ocean can amplify changes in climate. Human activities emit greenhouse gases, trapping heat and warming the atmosphere. As temperatures rise, more water evaporates from the ocean. A warmer atmosphere holds more water, and because water vapor is a potent greenhouse gas, the atmospheric temperature rises even more.

Theoretically, with more water vapor in the atmosphere, heavier rains and bigger storms could pound the planet. In some places, evidence is growing to suggest that heavy downpours have already become more extreme. But the biggest changes in rainfall will happen in the latter half of the 21st century, if carbon dioxide emissions do not slow down, Willis said.

“What we can say for certain,” he said, “is that the oceans are warming, sea level is rising and the planet will continue to warm for many decades to come.”