Ocean & Sea Life: Climate Change Impacts Beneath the Surface

9 minute read

Updated on: 12 Aug 2020

Even though our planet is called Earth, 71% of its surface is covered by water. For most of our history, the ocean has been regarded as a nearly inexhaustible resource, so large that we could never affect it. This has led us to mistreat the ocean, filling it with waste and sewage. Combined with the effects of climate change, these represent a serious threat to life in the ocean.

You probably don’t live underwater, and maybe you can imagine a diet without fish. So why should you care?

The ocean absorbs 90% of global warming

It takes much more heat energy to warm 1kg of water by 1°C than it takes to warm 1kg of air by 1°C. In scientific terms, this means water has a very high specific heat capacity.

Image of Specific Heat Capacity

Image: Specific Heat Capacity

The ocean has absorbed over 90% of the heat energy gained by the planet over the last 50 years. If this energy had been absorbed by the air in the lowest 10km of our atmosphere, we would have seen a temperature increase of around 36°C already!

Instead, all that energy has warmed the ocean. Most warming has occurred in the top 75m of the ocean surface, which gained 0.4-0.5°C in 40 years. This is much less than 36°C because of the water’s heat capacity.

But that’s not all! The ocean also stores almost 25% of the carbon we emit into the atmosphere. This storage is driven by life within the ocean, and by CO₂ dissolving in seawater.

Storing both energy and CO₂ allows the ocean to keep Earth’s climate stable despite human activities. This is known as buffering climate change.

Although predicting these values is very difficult, it is estimated that the ocean will absorb and store 9% (+/- 8%) less carbon in the 2090s, compared to the 1990s, if we continue to increase CO₂ emissions.

Let’s look at an example of the effects these have.

Why are we losing coral reefs?

Coral reefs are home to hundreds of thousands, if not millions, of species. This makes them some of the most biodiverse places on Earth.

Just like trees in rainforests, corals give reefs their structure. Corals are invertebrate animals that allow algae to live inside them. In return, algae pay their rent by providing corals with food. This is called symbiosis.

Image of Coral reefs are full of life

Image: Coral reefs are full of life

Coral reefs directly support around 500 million people across the world in several ways:

  1. They provide homes for a quarter of all fish species. Although they only produce 2-5% of global fishery catches annually, fish associated with reefs contribute 25% of the total fish catch in developing countries.
  2. They protect people from floods and storms by breaking waves.
  3. They provide income to at least 94 countries by attracting tourism, contributing significantly to many of their economies.

While buffering by the ocean may be good for us on land, corals are taking the hit underwater. When stressed by temperature increases, algae produce molecules that can damage their coral hosts. This causes the corals to get rid of the algae.

As well as providing food, algae are responsible for the corals’ colour. Corals without algae, therefore, turn white and starve. This is called coral bleaching.

Image of Coral Bleaching

Image: Coral Bleaching

Since the 1870s, we have lost around half of the live coral in the ocean! Severe bleaching events, where entire areas of coral are bleached, used to occur once every 25 years. Now they happen every 6 years.

We have already reached 1.0°C warming. Coral reefs are so sensitive that if temperatures reach 1.5°C above pre-industrial levels, it is predicted that 70 to 90% of them will be lost. If we reach 2°C, nearly all coral reefs will be lost.

This is all made worse by an issue known as ocean acidification: when CO₂ reacts chemically with water (H₂O), it results in an increase in the number of hydrogen ions (H⁺) that are found in the water. This makes the ocean more acidic, like a lemon, although not as much. Higher acidity slows the chemical reactions that corals need to build their hard skeletons, making them even more vulnerable.

The challenges that marine species face

Normally, marine species are more widely distributed than land species and can travel further distances more easily. This means that complete extinctions in the ocean are rarer and harder to keep track of. However, there are several ways in which population reductions or movement in species are felt throughout an entire ecosystem.

Let’s look at an example of how the abundance of a species can matter to an ecosystem:

Image of The Importance of Sea Otters

Image: The Importance of Sea Otters

Southern sea otters are very important in kelp forests because they eat sea urchins, thus keeping their population in check. However, sea otters are endangered. When sea otter populations are low, urchins are left to overgraze the kelp, and the kelp forest as a whole eventually disappears.

The consequences of warming on interactions between species

Deeper water, and water close to the poles, is colder. Therefore, many species are moving towards them. Since the 1950s, we have seen the area in which species live move by up to 50 kilometres per decade!

However, some species are less able to move, and the size of the area they can live in has decreased. This is because regions become too warm or experience stronger heatwaves. As we saw in the previous chapter, higher temperatures at earlier times of the year can alter species’ life cycles and food availability.

Changes to where and when species are found in the ocean don’t necessarily happen at the same time or speed for different species. As a result, some species that would normally interact fail to find each other because they look in the wrong place or at the wrong time. This is called mismatching.

Image of Mismatching Species

Image: Mismatching Species

Disruptions to species interactions change the arrangement of marine ecosystems. Predator populations shrink: when they mismatch with their prey or when their prey declines they have less food to eat, and the dominant species in the ecosystem can change.

Let’s look at one example of how temperatures affecting food availability for one species can have effects on many other species along a food chain:

Image of Impacts Spread Along the Food Chain

Image: Impacts Spread Along the Food Chain

What other threats does the ocean face?

The ocean is threatened by many other human activities which combine to make the situation worse.

We continue to use the ocean to dump waste, including sewage, industrial chemicals, fertilizers from agriculture, and around 8 million tonnes of plastic every year.

Still, the biggest threat facing life in the ocean today is exploitation through fisheries. To learn more about this, you can check out our chapter on Fish Farming in the Agriculture course.

Image of Impacts on the Essential Biodiversity Variables

Image: Impacts on the Essential Biodiversity Variables

Essential Biodiversity Variables are important factors for describing the state of ecosystems, such as whether populations are stable, and aspects like the length and complexity of food chains. Fisheries cause 29% of changes to these variables in marine systems, while climate change accounts for 16% and pollution for 15% of the changes.

Does this mean you shouldn’t worry about the effects of climate change on the ocean?

No, you should worry! As you saw with coral reefs, the impact of climate change is worsening, and as important ecosystems disappear, so does the habitat for many species.

The vastness of the ocean has fascinated us for hundreds of years. Let’s take care of it like it has taken care of us!

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