What You’ll Learn In This Module

• How are climate change and the ocean linked?
• What effect is climate change having on the oceans?
• What is blue carbon?
• What is the “ocean pump”?
• How much heat does the ocean absorb?
• What are hypoxia or dead zones?
• What are the effects of the ocean getting warmer?
• What are phytoplankton and why are they important?
• Why are the oceans becoming more acidic and what are the consequences?
• What are the effects of rising sea levels?

How are climate change and the ocean linked?

Climate change and the ocean are inseparably linked with the ocean being in the frontline of limiting climate change.

1. The oceans absorb heat and distribute nutrients to a rich marine life.

The ocean covers 70% of the world’s surface, and by volume makes up over 95% of the earth’s inhabitable space.

It’s made up of the five smaller oceans: the Atlantic, Pacific, Indian, Arctic and Southern oceans, which form a huge mass of water.   

The oceans absorb heat from the sun and from the atmosphere. They are a giant heat reservoir and their high heat capacity means that they heat up and cool down very slowly.

Strong currents, such as the gulf stream, keep the waters constantly in motion and control the distribution of heat, nutrients, freshwater and life in the seas.

Swells of water form far out to sea, breaking as waves when they reach the coastlines of the land.

The number of species of marine life living in these oceans is much larger than the number of species living on the land.

2. The oceans play a key role in regulating weather.

Water vapour rising from the ocean surface forms clouds and generates the storms that rain on the land.

With climate change, this process tends to accelerate, leading to more frequent extreme weather events such as floods, droughts, or cyclones.

Much of the water falling as rain, snow or ice sinks into the soil and runs into rivers and groundwater.

It ends up returning to the ocean in a cycle known as the water cycle.

3. The ocean absorbs carbon.

The oceans act as the world’s largest carbon sink, absorbing carbon from the atmosphere and storing it in the deep ocean.

It’s estimated that the oceans absorb almost one third of the global carbon dioxide being emitted annually.

What effect is climate change having on the oceans?

The oceans of the world are already experiencing the impacts of climate change. These include:

• Increases in water temperature
• A decline in oxygen production.
• Changes in the pattern of rain and more extreme weather events.
• Increased coastal erosion.
• The creation of hypoxic or dead zones
• New marine diseases
• Rises in sea level
• A reduction or even extinction of marine species
• Declines in fish stocks
• Coral bleaching
• Changes in the usual pattern of marine life migration along the seasons
• The migration of fish and other species to new places, crossing national borders.

These effects not only threaten the marine world, but also the many benefits and services that the oceans provide to mankind.

These include:

• Providing the primary source of protein – fish – for a billion people in the world.
• Producing oxygen that we breathe.
• Regulating the weather.
• Capturing and storing carbon.
• Treating wastewater.
• Cultural services such as tourism, recreation and natural beauty.
• Facilitating the study of marine life and the application of techniques to human problems.

The quality of these services depends on how well we protect the oceans.

Restoring the health and strength of marine ecoystems, of mangroves, seagrass meadows, corals, kelp forests, fisheries, and of essentially all ocean life will help the ocean cope with these changes.

While at the same time, we need to decrease the greenhouse gas emissions that we are producing to net zero.

What is blue carbon?

Blue carbon is carbon from the atmosphere that is drawn down and captured by the world’s oceans, coastal and other water ecoystems.

The world’s oceans and coastal ecosystems provide several methods for the long term sequestration of carbon from the atmosphere.

The carbon is stored in the form of biomass and sediments, for example, in mangroves, tidal marshes, seagrass meadows and in the deep sea.

What is the “ocean pump”?

The ocean pump comprises two parts: 

(i) a biological process which transfers carbon from the surface of the water to the seabed via the food chain, and 

(ii) a physical process which carries carbon to the deep ocean because of water circulation.

These two processes together are how the ocean captures and stores carbon as a carbon sink.

The poles are important for carbon storage because, in high latitudes, the colder water stores carbon dioxide more easily.

In the areas around the two poles, more dense water (which is usually colder or saltier) flows towards the deep sea, dragging down dissolved carbon.

The two processes of the ocean pump work to a different time scale.

• The physical process works on a very slow and steady clock. It’s affected in the long term. Like a tanker sailing off-course, once disturbed, is very difficult to correct.
• The biological process, based on the food chain, is sensitive to environmental conditions and can be destabilized, re-emitting carbon into the atmosphere. But it is easier to influence. It relies, for example, on the health of the plankton’s ecosystem. Also, healthy coastal ecosystems such as mangroves, seagrass beds and salt marshes are very important. These are super-sinks, storing about 10 times the amount of carbon than continental forests.

Although difficult to measure, it’s estimated that the ocean can hold 50 times more carbon dioxide than the atmosphere.

Certainly, it is a precious carbon store for the planet.

How much heat does the ocean absorb?

The ocean absorbs around 90% of the heat generated from global warming.

Climate change is increasing the temperatures in the oceans, melting ice and causing rises in sea levels.

Although the effect is slow, it can be detected as far down as 700 meters below sea level and it has now reached the deep sea in the polar regions.

This in turn is altering the paths of the currents and affecting the distribution of nutrients.

Much of the carbon dioxide released into the atmosphere by human activity has been absorbed by the oceans increasing its acidity by nearly 30% over the past two centuries.

All of this affects the life and health of marine species close to the shore and in deep ocean ecosystems.

They are facing shifting food supplies, changing habitats, coastal erosion, rising water levels and more extreme weather events.

What are hypoxia or dead zones?

Hypoxia is when oxygen levels in water are too low to sustain life. Reduced oxygen levels lead to biological deserts.

The term ‘dead zone’ is often used for hypoxia.

Dead zones occur naturally but are made larger and more serious by warmer water and other disruptions to water ecosystems resulting from climate change.

Often they start with an overgrowth of harmful algae blooms in the water. When the algae die, it sinks to the bottom of the ocean and decomposes, leading to oxygen depletion.

Surplus nutrients that run off the land and into waterways are the primary cause of the increase in dead zones.

Historically, sea floors have taken millennia to recover from dead zones.

Because of human activity and rising temperatures, dead zones currently make up 10% of the world’s ocean surface area. And this figure is rising.

What are the effects of the ocean getting warmer?

The ocean has been warming since the industrial revolution, with a number of consequences:

Hot water takes up a larger amount of space than cold water, so as the oceans warm, the global sea level is rising.

Added to this is an increase in the volume of water coming from melting glaciers and ice on land.

The warmer the water at the surface of the ocean, the more it heats the air and increases the amount of water evaporating into the atmosphere.

This increases the intensity of both rainfall and droughts in some parts of the world.

The increase of heat in the ocean, along with the increase in the atmosphere, is also impacting circulation in the atmosphere, increasing the intensity of hurricanes and other severe weather events.

The warming of surface waters reduces the ability of the ocean to absorb carbon dioxide.

It is also changing the behavior of marine wildlife, some of whom migrate towards the poles or to new areas.

Other species cannot move so easily. They must adapt to the warmer and more acidic environment, or die.

The abundance of several cold water species of fish is reducing.

Some types of coral bleach and die when their supply of algae is interrupted. 

The more ocean ecosystems are stressed and challenged by climate change, the more they become depleted and the less they can adapt.

What are phytoplankton and why are they important?

Plankton are microscopic organisms, including viruses and bacteria, that flow with the ocean currents and make up over 95% of marine biomass.

Biodiversity is huge, with thousands of different species of algae and planktonic animals.

A part of this plankton is called phytoplankton, or microalgae. They absorb carbon dioxide and, like plants, produce oxygen through a photosynthesis process.

While the oceans absorb carbon dioxide from the air mainly through the gas being dissolved into surface water, around 10% of the carbon that the oceans absorb is through phytoplankton photosynthesizing.

They store the carbon and release the oxygen into the air. 

When they die, the plankton sink to the bottom of the sea where they can form thick layers, which over thousands of years can develop into oil or gas.

The sediments of phytoplankton with calcareous shells can form into limestone.

Plankton is hugely important.

They form the first link in the marine food chain and are vital in feeding all fish and the global food chain.

They also act as a carbon pump and oxygen producer.

However, they are increasingly being affected themselves by climate change. A serious concern is how vulnerable they may be to future changes in our climate.   

Why are the oceans becoming more acidic and what are the consequences?

It is estimated that about one-third of the total carbon dioxide produced by humans since the start of the industrial revolution has been absorbed by the ocean, increasing the acidity of the water by 26%.

This is 10 times faster than any other period during the last 55 million years.

Dissolved carbon dioxide reacts with water to produce carbonic acid. Carbon acid can in turn break into Bicarbonate and hydrogen. Bicarbonate can in turn break into Carbonate and hydrogen.

It’s the concentration of hydrogen that gives the acidity. The higher the concentration of hydrogen, the lower the pH of the water and the higher its acidity.

In the oceans before the industrial revolution, carbon dioxide, bicarbonate and carbonate were found in stable proportions. Now the amount of carbon dioxide and bicarbonate has increased and the amount of carbonate has decreased.

This means there has been an increase in hydrogen in the water.

Acidification is an ongoing process. The pH is becoming more acidic, although water is not an acidic substance, which has a pH lower than 7.

The pH of the ocean is currently about 8.1, lower than in the past.

The effects of this acidification are not well understood. Some experiments have shown that certain types of phytoplankton develop defects when living in an acidic environment.

Some species, such as shellfish, are especially sensitive to acidification.

It has been estimated that coral ecosystems in tropical areas will disappear by 2050.

The global fishing zones are also shifting, with particular consequences for some countries highly dependent on this industry.

What are the effects of rising sea levels?

Since 1992, the global sea level has risen by 3 millimetres per year and this rate is increasing.

Erosion and flooding disturb marine life in coastal areas.

The deterioration of natural environments in turn weakens their ability to adapt to the other changes associated with climate change.

Some species may migrate, others die, leading to a decrease in biodiversity.

This is especially the case in coastal habitats such as mangroves and seagrass belts.

These are important breeding areas, as well as carbon capture zones.

Mangroves help to keep crumbling soil on the coast, preventing coastal erosion.

They are natural barriers to water currents and are a sheltered habitat for young species of fish.

Want to know more about the role of wetlands in carbon capture? Read about the soil and climate change.