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What is a Carbon Sink?
Blog...What is a Carbon Sink?

What is a Carbon Sink?

Net zero trajectory
grass with flowers and forest in the distance
What are carbon sinks? Why are they so important? And why are they increasingly under threat?
grass with flowers and forest in the distance

You might not have heard of carbon sinks before, but they play a vital role in the capture of excess carbon dioxide (CO2) from our atmosphere. The Earth's natural carbon sinks (ie. our oceans, forests and soil) absorb around a half of the excess CO2 that we produce, preventing global temperatures from rising even further.

However, they're under threat from human activity, and some of our carbon sinks may even become carbon sources if we don't do more to stop their destruction. 

👉 In this article we'll explore what carbon sinks are, why they're so important, and why they're increasingly under threat.

What is a carbon sink?

In very simple terms a carbon sink (also referred to as a carbon pool) is something that absorbs more carbon than it releases. It refers to a process known as carbon sequestration - a natural or artificial process by which carbon from the atmosphere is removed from the atmosphere and held in solid or liquid form.

Natural sources include plants, forests, the ocean, and soil, but it can also be artificial (ie. man made) - for example landfill sites and carbon capture and storage facilities. 

Forest full of green trees

An important part of the carbon cycle

Carbon sinks (ie. carbon pools) store carbon for an indefinite period of time, which means that they are actively removing CO2 from our atmosphere. This function forms a very important part of the carbon cycle and helps to balance out levels of CO2 in our atmosphere.

👉 To understand why carbon sinks are so important, let's start with a quick overview of what the carbon cycle is and how it works.

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How does the carbon cycle work?

Carbon is the backbone to life on earth. We're made up of organic carbon atoms, we eat food made from carbon atoms, and our societies are built on carbon. In fact carbon is the fourth most abundant element in the universe. So what happens to all this carbon? 

The Earth and its atmosphere are what's called a closed system when it comes to carbon atoms - this means that the Earth doesn't lose or gain carbon atoms to outer space, instead the carbon atoms are constantly moving about within this closed system.

The carbon cycle is the name given to this natural process - carbon atoms continually travel from the Earth's atmosphere to Earth and then back into the atmosphere again. It's nature's way of reusing carbon atoms again and again.

👉 The carbon cycle consists of two balancing processes - carbon is released by carbon sources and absorbed by carbon sinks, to create a natural carbon equilibrium. 

image of planet Earth taken from outer space

Carbon sources

Carbon sources are sources that release carbon into the atmosphere.

Every time we breathe for example we release carbon dioxide into the atmosphere, when vegetation and microbes decompose they also release CO2, volcanoes also continually expel CO2 even when they're not erupting - there are natural carbon sources everywhere. 

Carbon sinks

The carbon cycle aims to balance this out, and carbon sinks work to create a natural equilibrium. Carbon sinks such as the ocean, our forests, and soil, capture and store carbon through carbon sequestration. 

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Why is the carbon cycle out of balance?

The carbon cycle creates a natural carbon equilibrium, maintaining the Earth's temperature and keeping it stable. Unfortunately human activities have upended the system.

The Industrial revolution marked the start of our heavy reliance on fossil fuels such as coal, natural gas and petroleum. This might have given us technological and societal advancement, but it came at a cost - burning fossil fuels is one of the primary driving forces of climate change and global warming.
Industrial power plant

What happens to this excess carbon?

All the excess CO2 that we're producing through human activity has to go somewhere, so where does it go? 

Around half of the excess CO2 produced through human activity is absorbed by the Earth's carbon sinks. However, the remainder of this excess carbon is trapped in the Earth's atmosphere. This is the driving force behind global warming - the excess carbon traps the heat which warms the planet and drives climate change. 

Rising temperatures are having catastrophic consequences on our planet, with increasingly extreme and frequent weather events, as well as changes to wider weather patterns which threaten our delicate ecosystems. The effects of climate change are threatening the very existence of life on Earth. 

👉 To read more about the carbon cycle, check out Greenly's article on the topic.

What are the primary carbon sinks?

Now that we understand the important role that carbon sinks play in helping to absorb excess carbon dioxide, let's take a closer look at the different types of natural carbon sinks.


👉 The ocean is the world's largest carbon sink - it absorbs 25% of all CO2 and captures 90% of the excess heat generated by carbon emissions.

There are two methods by which carbon is absorbed by the Earth's oceans.

The first is the simple reaction of carbon dioxide in seawater - the chemicals react, dissolving the CO2 and producing carbonic acid in its place. 

The second way that the Earth's oceans capture and store CO2 is via photosynthesis by plant-like organisms such as plankton and algae. These organisms absorb CO2 and release oxygen back into the atmosphere. A large part of this captured carbon eventually makes its way back into the atmosphere, however some of the carbon will sink down into the deep ocean as dead organic material, faeces etc. As this matter decomposes CO2 is released into the ocean, which dissolves the CO2.

ocean water with the sunset reflected on it


Forest ecosystems and plants sequester carbon by capturing carbon dioxide in the atmosphere and transforming it into organic matter through photosynthesis.

👉 This trapped carbon is then stored in biomass, deadwood, organic litter, and in soil.

rainforest full with trees and vegetation


We've already talked about how plants and trees absorb carbon for use as part of the process of photosynthesis.

The carbon that is captured by the plant tissue goes on to either be eaten by animals, or added to the soil when the plant dies. The soil then stores this trapped carbon as soil organic matter (SOM). This SOM is made up of a mix of carbon compounds, microbes, and carbon associated minerals. Carbon can be stored in the soils millennia this way depending on conditions such as climate, water levels, vegetation etc. 

👉 Peatlands are another soil-based carbon sink worthy of mention. Made up of wetland landscapes that consist of waterlogged organic soil and matter, peatlands are able to capture and store large amounts of CO2. Plants growing in this environment capture CO2 and store the carbon as biomass, however, because the peatland is a wet environment, when the plants die they're not able to fully decompose and release CO2 into the atmosphere, making peatland a powerful carbon storage source.

small plant growing from the soil

Decreasing capacity of carbon sinks

Scientists are concerned that the capacity of our natural sinks will decrease over time.

In 1959 for example carbon sinks removed about 60% of CO2 produced through human activity, however, this has dropped to just 55% today. 

So what's the reason for this decreased ability to capture and store CO2? Well, there are actually many different reasons for this.


Forests are one of our most important carbon sinks, they absorb around 30% of carbon emissions released from the burning of fossil fuels, and will normally store more carbon than they produce. However, human activity is threatening these invaluable carbon sinks, and risks accelerating the effects of climate change even further. 

The Amazon is the world's largest tropical forest and an incredibly important carbon sink - it's often described as the Earth's lungs.

However, deforestation to make way for agricultural land and the chopping down of trees by loggers is damaging the tropical forest's ability to absorb carbon. In fact, researchers are worried that the damage being caused is so severe that instead of being a carbon sink, the Amazon rainforest could end up a source of CO2 emissions as early as the next decade.

logging operation with lots of chopped trees

Global warming

Global warming is a direct result of human activity and it is driving climate change. Across the world we're seeing more extreme weather events such as flooding, tropical storms, and drought - the increased intensity and frequency of such events can be linked all the way back to the increased levels of greenhouse gases in the atmosphere. 

Global warming is also having an effect on the ability of carbon sinks to absorb carbon dioxide. Take forests and vegetation for example, increased temperatures and drought are killing our plants and forests, which decreases their ability to absorb carbon dioxide.

And it's not just forests that are affected - the ability of oceans to absorb carbon dioxide are also under threat from rising global temperatures. Cooler water is able to absorb more carbon dioxide, which means that as the average temperatures of our oceans continue to rise, their ability to act as a carbon sink will diminish.

Industrial farming

Industrial farming practices are another harmful human activity that is having an impact on the ability of the Earth's soil to capture and store carbon. Not only does agriculture contribute as much as 26% of all greenhouse gas emissions, it is also affecting the ability of soil to absorb and store carbon dioxide. 

Healthy soil is a mix of microbes and carbon, without these elements it becomes dry earth, incapable of providing life to other plants and vegetation.

👉 Unfortunately intensive farming practices are harming the health of soil by depleting the soil of vital nutrients and releasing carbon into the atmosphere through practices of tilling (the practice of churning the soil to expose deeper layers for crop planting).

tractor in a field, surrounded by industrial farming equipment

Artificial carbon sinks

Many have questioned whether artificial carbon sinks could provide a solution to global warming - but what exactly are artificial carbon sinks and how do they work? 

Artificial carbon sinks rely on technology that captures carbon dioxide from the atmosphere and then either stores or repurposes the carbon (also known as carbon capture, utilisation and storage). They're increasingly important because the Earth's natural carbon sinks are under threat and are unable to absorb all of the excess carbon dioxide that we're releasing into the atmosphere.

So what are some examples of artificial carbon sinks and how is the carbon stored?

Direct Air Capture (DAC)

Direct Air Capture technology works by capturing carbon dioxide directly from the atmosphere.

👉 In very simple terms, DAC is a collector system based on a series of fans. It works by pulling air from the atmosphere and subjecting it to a series of chemical processes that allow the carbon dioxide to be separated.

The air captured carbon dioxide can then be used for other purposes - for example it can be used in manufacturing processes, or stored - for example the carbon may be stored in rocks or pumped to the depths of the ocean where it's injected into the ocean floor.

Industrial waste carbon capture

Industrial waste carbon capture is the process whereby carbon dioxide that is a by-product of different industrial processes (eg. the production of cement, or the production of fossil fuels) is captured before it's able to be released into the atmosphere.

This then allows the stored carbon dioxide to be repurpose for another use.

power plant releasing pollution into the air

The problem with carbon capture, utilization, and storage

The problem with artificial carbon sinks is that it's experienced a number of difficulties, preventing the technology from really taking off.

The issues are numerous and complex, but it includes considerations such as poorly established revenue streams resulting in poor business cases, unproven technology and concepts, insufficient policy to allow the technology to thrive, and complexities of project coordination and inadequate infrastructure. 

👉 However, we may start to see some progress within the CCUS market. The urgent need to find solutions to climate change is driving investment and innovation within this area. This renewed interest could result in technological innovation and the upscaling of projects.

What can we do to help improve the efficiency of carbon sinks

In addition to investing in the development of effective artificial carbon sinks, we can also do more to protect and restore the Earth's existing natural carbon sinks.

Sustainable forestry and ecosystem restoration

The Earth's forests absorb as much as 2.6 billion tonnes of carbon dioxide each year, but they're increasingly under threat from human activity.

Therefore, if we want to protect this vital carbon sink, more needs to be done to reduce their destruction. This means the use of sustainable forestry practices that protect and preserve existing forests, alongside efforts of reforestation to restore areas that have already been lost.

people planting trees in the soil

Reducing emissions

Our oceans' capacity to capture and store carbon dioxide is decreasing as a direct result of warming ocean temperatures, which are caused by excess carbon dioxide (it's a bit of a catch 22!).

Therefore, it's essential that societies across the world take steps to decarbonise and eliminate their reliance on harmful fossil fuels (the most harmful man-made carbon source). Unless we're able to prevent further global temperature rises the ocean's capacity to act as a carbon sink will diminish.

Reduce our use of plastics

Plastic pollution in our oceans is incredibly harmful for organisms such as plankton and algae (two microscopic organisms that absorb more carbon than all of our plants and trees combined!)

👉 Microplastics have been proven to affect their ability to absorb carbon dioxide, and so it's essential that we stop releasing harmful plastics into our oceans and take efforts to clean up the existing pollution.

lots of plastic trash

Sustainable farming 

Sustainable agricultural practices are important if we're going to be able to prevent the deterioration of our soil health and improve the ability of the Earth's soil to act as a carbon sink.

👉 This involves a shift from intensive, industrial scale farming towards a more sustainable regenerative practice that relies less on harmful chemicals, and prioritises working alongside nature to produce healthy nutritious crops.

Climate change mitigation and carbon sinks

Carbon sinks are a vital part of the carbon cycle and are crucial when it comes to preventing further rises in our global average temperature and the rapid deterioration of our climate. However, today's carbon sinks may become tomorrow's carbon sources if we don't do more to protect these vital systems. 

This is why it's imperative that governments, companies, and individuals across the world take action to reduce our reliance on harmful fossil fuels, and transition to a more sustainable way of existence - one that protects our natural carbon sinks and respects the Earth's natural processes.

At Greenly we can help you to assess your company's carbon footprint, and then give you the tools you need to cut down on emissions. Why not request a free demo with one of our experts - no obligation or commitment required. 

If reading this article has inspired you to consider your company's own carbon footprint, Greenly can help. Learn more about Greenly's carbon management platform here.

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