A study released earlier this year found that we may soon see a mass extinction of ocean life.

That's alarming, since oceans comprise nearly 70 percent of Earth, provide habitat for more 200,000 known species (and potentially millions more unknown ones) and are integral to all known life on our planet, as well as climate and weather patterns.

The World Wildlife Fund estimates the total value of the ocean's assets at around $24 trillion and said if the ocean were measured as an economy, it would have an annual gross domestic product of $2.5 trillion — the seventh largest economy in the world.

In the United States alone, what the government calls "the ocean economy" — six economic sectors that depend on the ocean and Great Lakes — contributed more than $282 billion to the U.S. GDP and provided more than 2.8 million jobs in 2011, according to the National Oceanic and Atmospheric Administration (NOAA).

Here are five of the biggest challenges the world's oceans face — and how humans can tackle them.

—By Robert Ferris, science reporter, CNBC.com
Posted 20 November 2015

Around 90 percent of the world's fisheries are either "fully exploited, overexploited or have collapsed," according to the Monterey Bay Aquarium. More than twice as many fishing boats than fishery stocks can sustain are in the water. "There are simply too many boats chasing a dwindling number of fish," said an aquarium spokesperson.

Organizations such as the Monterey Bay Aquarium publish updated "Seafood Watch" guides that advise consumers on which kinds of fish are being overfished, but these approaches require that customers care enough about the issue to make use of them.

Other approaches toward the regulation of fishing are being implemented, such as cutting fishing seasons short, setting limits on the number of fish a boat can take in a season, or suspending fishing in overharvested areas until populations rebound.

Roughly 80 percent of the pollution in the ocean comes from land, according to NOAA.

Pollution from fertilizers, as well as other sources, is creating huge "dead zones" near coastal areas. The chemicals drive down the oxygen content, killing off sea life in the area.

Plastics and other solid garbage in the ocean also create all kinds of problems for sea life. A study published earlier this year estimates that millions of tons of plastic are landing in the ocean annually — enough to litter every foot of coastline in the world with five plastic grocery bags. In recent years there has been a rise in the use of "microbeads" — the tiny grains found in some soaps and personal-care products.

But even large plastics corrode and break down into smaller and smaller pieces. These pieces can become sponges for toxic chemicals, and they collect in the organs of sea life — including fish that end up on dinner tables. Even large whales can die from the plastic that slowly amasses in their bellies.

Rising acid levels in the water have been connected to carbon pollution from factories, vehicles, homes and other human sources.

Acidity levels in the ocean have risen by about 30 percent since the Industrial Revolution, according to NOAA — the largest driver of change to ocean chemistry over the last 50 million years.

Researchers believe the increasing acidity is due to the carbon dioxide in the atmosphere, mostly from the burning of fossil fuels.

It has been thought to be one of the forces contributing to coral reef bleaching mentioned earlier, though recent research has challenged that.

But acidifcation has other effects.

Recent research has shown that carbon dioxide can cause the shells of small sea animals to disintegrate. The smallest of these, such as Pteropods, form an important food source for sea life — and ultimately, at least 1 billion humans who rely on seafood for their primary source of protein.

It has also been connected to the increase of toxic algal blooms, which can infect fish and sea life and render some types of seafood too dangerous to eat.

About 20 percent of the world's of its coral reefs are thought to be damaged beyond repair, and more are further threatened.

Roughly half of the reefs in the United States and the Freely Associated States (Mashall Islands, Micronesia, Palau) are in "poor" or "fair" condition, and have been declining at alarming rates.

This is an ecological catastrophe: Coral reefs are ecosystems that support thousands of known species of fish, invertebrates and algae, and scientists estimate there are 1-8 million more undiscovered species living around them. They are home to one-fourth of all marine species and are crucial to many local economies — especially fishing and tourism.

An ongoing survey by reinsurance firm XL Catlin estimates that $30 billion, and the livelihoods of 500 million people, are at risk if the reefs continue their decline. Reefs also form natural barriers that protect coastlines against storms — of the $29.8 billion global net benefit of coral reefs, coastal protection coral reefs provide accounts for $9 billion.

Warming waters are blamed, along with dynamite fishing, pollution and the acidification of ocean water.

Efforts to replenish coral reefs are under way, such as NOAA-led efforts in Florida and the U.S. Virgin Islands, but the pace at which the reefs are disappearing is far higher than the pace at which they are growing back.

The world's mangrove forests have perhaps not received the attention given to coral reefs, but they are dying off or being destroyed, and some watchers are worried about what that may bring.

Mangroves are forests of saltwater-tolerant plant species that occur in tropical environments close to the equator. They exist all over the the world — from Asia to south Florida. They are hubs for biodiversity, and they protect land — and the people who live on it — from storms and erosion.

Rising sea levels could also drown many of the world's mangrove forests, according to some research, though other studies have suggested that mangroves might help fight against sea level rise. Sea levels have risen more than 2.5 inches since 1993, according to NASA.

Thousands of acres of mangrove forest have been cleared for the construction of pools for fish farming, especially for shrimp, according to the American Museum of Natural History. Coastal development, agriculture and even charcoal and timber harvesting are all contributing to mangrove destruction.

A 1998 report from NOAA estimated that intact mangrove forests bought far more economic value than the shrimp farms the forests are cleared for.

Some groups — such as in Florida's everglades, for example — are replanting mangroves.