Climate change

Climate change – the important role of phytoplankton and whales

Climate change – Introduction to the oceans: A story about whales and climate change by Sue Dyson and Roger McShane

In this story we will try to explain two issues relating to climate change. The second issue is one that is often discussed and the first is rarely spoken of, possibly because it is too radical to even think about!

The two issues we will discuss are:

  1. How the oxygen in Earth’s atmosphere is created and maintained;
  2. How sea plants and animals play a vital role in storing carbon from the atmosphere.

This story is going to explore the effect on climate of both the smallest life in the sea and the largest. The smallest forms of life are bacteria and plankton and the largest are jellyfish (also classified as plankton) and, of course, whales.

Bangudae petroglyphs – Korea

The motivation for this story about climate change actually came through a serendipitous event. We were reading a story about Korea and it mentioned the Bangudae petroglyphs [1] which were created some 8000 years ago and which depicted whales and whale hunting, although Korea is rarely mentioned in early histories of the hunting of whales.

The number of whales killed in those early years was minimal because boats were small and the distances travelled were not great.

However, the petroglyphs got us thinking about whale hunting and the role of whales in the oceans more generally. We also wanted to learn more about what they feed on, whether they are still hunted, how many whales are left after centuries of hunting and whether the changes caused by climate change are having an effect on their feeding and breeding areas.

We also thought it would be useful to determine the positive effects that plankton and whales as well as other sea creatures have on the climate, if any.

More systematic whale hunting was carried out in the Middle Ages by seafaring nations such as Norway, the Basques in modern day northern Spain and southern France, Iceland and the Japanese. Whale oil was being used to light lamps, and corsets and hoop skirts were made from whale bones. But the number of whales killed was still relatively small and remained so until later.

Whaling became a huge industry worldwide in the 18th Century and on into the 19th century where they were hunted by some countries for their flesh but were also hunted even more for their oil which, prior to the availability of petrochemicals, was needed to oil the machines of the emerging Industrial Revolution.

Nantucket had become the home of whaling in the early 18th Century due to its proximity to the Atlantic Ocean, although Iceland was also used as a base for whale hunting by the Basques, Danes and French at various times.

By the mid-1700s whales were difficult to find in the northern Atlantic and ships needed to go further afield to find their prey.

At the other end of the world, whaling was popular in the new colonies such as Van Diemen’s Land (now the state of Tasmania). Whales were even hunted in the Derwent River that flows through the city of Hobart. This activity was documented by artist William Duke who painted a number of scenes of whaling near the city. The whales were so numerous that recreational boats were warned to keep close to the shore so that they were not attacked. You can read about this here.

Herman Melville published his semi-fictional account of the perils of whaling in 1851 [2] after visiting the island of Nantucket which lies off the eastern coast of Canada.

Here he met the former captain, George Pollard Jr, of the whaling ship Essex that had been sunk in the southern Pacific Ocean by an attack from a single very large whale.

Melville had also hunted whales in the 1840s for a number of years, so had good knowledge of their habits when he wrote the book. He didn’t sell many copies at the time, suggesting that whaling wasn’t a big issue at that stage. It was also a very dense book with allusions to Greek mythology, biblical characters and Shakespearean character formation.

However, in the 20th Century it became one of the largest selling books in the world and certainly a very famous book.

As we will see later in this story, whales were starting to be killed in industrial quantities in the 19th Century at the same time as massive amounts of carbon dioxide began to be pumped into the atmosphere by the new factories that were being established in Europe, Britain and the United States.

Whales were starting to be killed in industrial quantities in the 19th Century at the same time as massive amounts of carbon dioxide began to be pumped into the atmosphere by the new factories that were being established in Europe, Britain and the United States

And this leads us to an explanation of why we are concerned about the continued hunting of whales [3] and why restoring their numbers to pre-18th Century numbers would benefit the inhabitants of this planet.

First, we will discuss what they eat and why this effects the climate.

Exploring the role of phytoplankton

Let’s start with the plankton. Scientists normally divide plankton according to a difficult division between plants and animals because these life forms are hovering on the edge between the two.

The first in this category are phytoplankton which are regarded as plants. The word phytos is Greek for plant, hence the name. These are small cell-like plants that derive their food from the water and atmosphere using the sun’s energy.

Phytoplankton are tiny plants that derive their food from the sun, water and atmosphere via the process of photosynthesis

The food that they create through the photosynthesis process consists of simple molecules of sugar [4] (glucose). In order to do this the plant must “assemble” the right components (called reactants) to make sugar.

The reactants required are carbon dioxide (of which there is more than enough in the atmosphere), water (in the case of phytoplankton they get this from the ocean they are living in) and sunlight (which is why phytoplankton are found near the surface of the ocean where the light from the sun can penetrate and provide the energy the plants need).

The chemical reaction that occurs can be written as follows.

Carbon dioxide + Water + Sunlight ———> Glucose + Oxygen

The chemical formula is:

6CO2 + H2O ———> C6H12O6 + O2

On the right-hand side of the equation is glucose which provides the phytoplankton with energy [5] and oxygen which is released back to the atmosphere, thus helping to maintain the amount of oxygen that we need to breathe!

As a side note, the top part of the ocean is known by scientists as the epipelagic zone, being the section of the top of the ocean where the light from the sun contains enough energy to allow photosynthesis to occur.

Climate change Oceans
Ocean layers: Source

It has been estimated that phytoplankton are responsible for producing over half of the oxygen in the atmosphere (some scientists think it is as high as 70%). But also notice that the glucose contains 6 atoms of carbon which it has taken out of the atmosphere thus playing a vital role in reducing atmospheric carbon.

Phytoplankton are responsible for producing between 50% and 70% of the oxygen in the atmosphere as well as sequestering atmospheric carbon in their tissues.

This is a hidden issue that is rarely discussed within the topic of climate change. The level of oxygen in the atmosphere is dependent on these tiny plants (it is estimated that there can be a million phytoplankton in a single drop of water) along with kelp and algae.

However, the oceans are becoming warmer every year and phytoplankton need a stable temperature to survive.

Tiny phytoplankton: Source Wikipedia

Now to the scary bit! Scientific reports suggest that the level of phytoplankton in the oceans has decreased [6] by up to 40% in the last fifty years – this could be catastrophic as it means that there is less oxygen being produced that we absolutely need in order to breathe!

But it also means that there is less food for all the other sea creatures and hence less food for whales, which as we are about to explain, carry out another important function relating to our climate.

Anyway, after they do all this hard work, the phytoplankton get eaten by other sea creatures such as zooplankton (the second type of plankton, which includes the ubiquitous krill and jellyfish) which, in turn, get eaten by larger creatures such as whales.

The contribution of whales to climate change

We need to provide some context as to why we are so interested in whales. There are two reasons. The first is that they are fascinating, intelligent creatures. The second is that they play an important role in reducing the effects of climate change.

To understand why, it is useful to understand a little about these huge creatures – the largest there has ever been on Earth. The largest are blue whales which can weigh between 130,000 kilograms (286,000 pounds) and 200,000 kilograms (440,000 pounds). To put this weight in perspective, just the tongue of a blue whale weighs as much as an elephant which is the largest land creature [7]!

The tongue of a blue whale weighs as much as an elephant

Blue whale versus human size
Blue whale versus human size comparison

In the 20th century, it has been estimated that some 350,000 of these massive creatures were killed – and this figure is just for blue whales. Given that there are approximately 45 different species of whales ranging in size from the rare pygmy whales right up to the blue whales, the total number killed must have been enormous.

What has this got to do with climate change you might ask? Put simply, whales are very, very good at sequestering (extracting and storing) carbon. For every kilogram of carbon they hold in their bodies, there is less carbon going into our atmosphere and contributing to global warming.

We often read about the role of forests in sequestering carbon and how we must preserve and extend them. However, the role of phytoplankton and sea creatures such as whales in the oceans is rarely discussed.

Rough estimates are that 8 billion metric tonnes of carbon are added to the atmosphere from forests each year due to burning and removal of trees from areas such as the Amazon Basin. The good news is that world-wide, some 16 billion metric tons of carbon are absorbed by forests. That sounds pretty good! More carbon is being absorbed by trees than is being added to the atmosphere by trees.

The situation, however, is not all that good when other factors relating to climate change are taken into account. For example, as the Arctic ice melts, carbon is released from exposed soil where it has been previously trapped. Other major sources of carbon emissions currently adding to the amount of carbon in the atmosphere include cement production and the burning of fossil fuels such as coal, oil and natural gas.

The burning of fossil fuels accounts for approximately 97% of all carbon emitted as well as a high proportion of the methane (which is even more of a problem than carbon dioxide).

The burning of fossil fuels accounts for approximately 97% of all carbon emitted.

Because of this uncontrolled (in many cases) burning of fossil fuels we have to carefully nurture soils, plants and animals that are capable of extracting and storing carbon to reduce the amount of carbon dioxide in our fragile atmosphere.

Obviously, from the figure provided above, forests that are left to grow are very good at sequestering carbon, being responsible for 16 billion tons a year.

Water in the oceans is also capable of directly absorbing carbon dioxide and it is estimated that 67 billion tons has been absorbed between 1992 and 2018 [8]. This, unfortunately, is because there is now so much carbon dioxide in the atmosphere that it is easier for the oceans to absorb it!

A similar study [9] has confirmed the role of the oceans in sequestering carbon, reporting that approximately 30% of the CO2 produced by human activities is absorbed by oceans (otherwise global warming would have become more intense).

30% of the CO2 produced by human activities is absorbed by oceans

The other good news is that all of the sea plants and sea creatures are also very good at extracting carbon from the atmosphere and storing it in their cells, thus reducing the climate change effects. When these plants and creatures die (whether they be simple phytoplankton or massive whales), they sink to the bottom of the ocean and the carbon is stored there for a very long time.

Now, back to whales. One estimate [10] is that the average whale can store 30,000 kilograms of carbon in its body. As mentioned earlier, when the whale dies it sinks to the bottom of the ocean taking the carbon with it, hence sequestering it for a very long time. When land animals die they release carbon directly into the atmosphere.

Now let’s consider the role of whales versus the promise of an alternative called Direct Air Capture (DAC) where carbon dioxide is removed from the atmosphere and stored underground or converted to other products.

Put very simply, DAC is currently very expensive as a process [11] and it also uses a lot of energy for each kilogram of carbon captured and stored or converted, whereas whales provide this entire, vital climate change service free of charge.

The lesson here is to do everything possible to increase the number of whales in the oceans and to stop the decline of the phytoplankton and sea animals in the whales’ food chain, so that the number of whales in the ocean increases. It is estimated that, in the case of the massive blue whales, only 3% of the 18th Century number remains!

The importance of this was emphasised in a paper by Holmen [12] who pointed out that:

The carbon content present in the ocean is 50 times that in the atmosphere and 20 times that in the soil.

Therefore we need the whales and other creatures to store as much of the atmospheric carbon as possible so that it doesn’t reach even more dangerous levels!

Unfortunately, the amount of carbon dioxide, methane and nitrous oxide being pumped into the air by the coal, oil and gas industries as well as the cars and trucks we use, means that the oceans will continue to absorb more heat from the atmosphere and hence decrease the number of phytoplankton which feed the krill which are then consumed by whales.


While there is a lot of money being poured into carbon capture in soils through the planting of trees and the reservation of forested areas, the amount of carbon retained is small compared with the amount that could be sequestered by dramatically increasing the phytoplankton and whale populations in the oceans.

Taking measures to increase these populations also has the added (and necessary) benefit of increasing the amount of oxygen being added to the atmosphere and reducing the amount of carbon in the atmosphere that is a primary cause of climate change.


[1] Petroglyphs are paintings which have been etched into rocks.

[2] Melville, H. (1851) Moby-Dick; or, The Whale.

[3] According to the Journal of Marine and Island Cultures, South Korea and other countries still harvest whales that have been caught in commercial fishing nets using the “excuse” that it is bycatch.

[4] The term “sugar” is a very broad term used to describe carbohydrate compounds of which one of the simplest is glucose, which can be extracted from common vegetables such as corn, peas and carrots. Sugar is also found in fruit in the form of fructose, milk in the form of lactose, and malt in the form of maltose.

[5] You will have heard of the term “sugar hit” which implies that taking in sugar provides a hit of energy, which is why athletes eat a carbohydrate diet prior to an event.

[6] J P. Gattuso et al., Science, July 2015, Vol 349 Issue 6243.

[7] The largest elephants are the African elephants, and they can weigh up to around 5,500 kilograms.

[8] Watson, A. J. et al (2020) Revised estimates of ocean-atmosphere CO2 flux are consistent with ocean carbon inventory, Nature Communications.

[9] La Quere et al (2014). Global carbon budget 2013. Earth System Science Data. Vol 6: pages 235–263.

[10] International Monetary Fund (December 2019), Nature’s Solution to Climate Change. IMF Finance and Development Unit. VOL. 56No. 4.

[11] Up to almost $1,000 per tonne of CO2 stored according to Adlen, E and Hepburn, C (2019) 10 Carbon Capture methods compared: costs, scalability, permanence, cleanness.

[12] Holmen, K. (2000). The global carbon cycle. International Geophysics 72: 282–321.

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