Carbon Offset Tree Planting Calculator: How Many Trees Does It Actually Take?

Tree planting to combat climate change is a popular phenomenon. The UK is planning its biggest tree planting programme in 50 years.¹ A restoration project in the Democratic Republic of Congo recently planted its millionth new tree.² Meanwhile, China continues to plant new trees this year following the afforestation of 6.77 million hectares in 2020.³ The effect these efforts have can be measured with a carbon offset tree planting calculator.

What is a carbon offset tree planting calculator?

A carbon offset tree planting calculator helps people to calculate their carbon footprint. It shows how many trees they need to plant to offset the carbon emissions they cause.⁴ Some calculators even show what type of trees someone needs to plant to ‘cancel out’ their emissions.⁵ This is because different tree species absorb varying quantities of carbon dioxide (CO2). 

But, planting trees alone cannot solve climate change. We should not merely be trying to offset our emissions. We need to stop producing CO2 in the first place. A carbon offset tree planting calculator is useful to encourage people to consider their emissions and to plant more trees. However, they should not distract us from the principal problem: fossil fuels.

How do trees offset carbon dioxide emissions?

Trees and all plants absorb carbon dioxide during photosynthesis. They take in the gas directly from the air. Using water from the ground and energy from sunlight, they convert the carbon into glucose. Glucose is a type of sugar that plants need to grow.⁶ 

In the process of photosynthesis, plants store carbon in their biomass and release oxygen. Trees, in particular, sequester large quantities of carbon in their leaves, branches and trunk. They also help the surrounding soil to hold the element. Therefore, the carbon is kept out of the atmosphere in the wood of a tree until it is cut down and burned or left to rot.⁷ When this happens, the carbon is released and enters the atmosphere once more.⁸ Consequently, deforestation is responsible for about 10 per cent of emissions worldwide.⁹

Limitations of tree planting and the existing carbon footprint

Therefore, planting more trees reduces the amount of CO2 in the air. It is “one of the biggest and cheapest ways of taking CO2 out of the atmosphere to tackle the climate crisis”, according to scientists.¹⁰ But it has its limitations. We cannot prevent climate change while continuing to burn fossil fuels, like coal, oil and natural gas. The carbon footprint is simply too high. Global emissions rose by 62 per cent between 1990 and 2019.¹¹ They are set to rise by a further 1.5 billion tonnes in 2021.¹²

Moreover, trees take a long time to grow. Studies have found that as they age, most species absorb more carbon. Therefore, larger and older trees tend to soak up and store more carbon than smaller and younger saplings.¹³ This highlights an issue with tree planting as a solution to global warming. It will take decades for the trees that we plant today to remove significant quantities of carbon from the atmosphere.

This does not mean that we should not be planting more trees. On the contrary, it is a vital tool in the fight against climate change. CO2 can remain in the atmosphere for up to 1,000 years.¹⁴ We need trees to remove this greenhouse gas. However, it is extremely important that we also concentrate on protecting and enhancing our existing forests. Forests, land and other natural ecosystems can get us at least a quarter of the way to limiting global warming to 1.5 °C. But, for this to happen, we must stop deforestation and manage them well.¹⁵  

Which trees offset the most carbon?

As mentioned, older and, therefore, larger trees tend to absorb more carbon than their younger counterparts.¹⁶ These findings are based on the growth measurements of 674,046 trees from 403 tree species across six continents.¹⁷ As the trees grow and add more biomass, they continuously store more carbon.¹⁸

On the other hand, young forests sequester more carbon per year than old-growth forests. Whilst this may seem contradictory, it makes sense when considering the number of trees in old-growth forest. This type of forest may have older and bigger carbon-absorbing trees, but it has fewer of them. Younger trees can grow much closer together. They may absorb less carbon, however, they do so more quickly than mature trees. As such, when considering individual trees, older and larger trees offset the most carbon. But, forests of young trees actually absorb more carbon than old-growth woodland.¹⁹

Tree species

When looking at which tree species absorb the most carbon, there are several factors to consider. For instance, how quickly does a species grow? Fast-growing trees store the most carbon in their first few decades. This may be useful for immediate carbon offsetting. Alternatively, long-lived trees can store carbon for centuries. As such, the climate change benefit is maximised over a longer period.²⁰ 

Considering the size of a tree’s crown and leaves is also important. A broader span and wider leaves will result in maximum photosynthesis. In addition, it is very important to plant indigenous tree species. They will thrive in their native soil and subsequently absorb more carbon. They will also support more local wildlife.²¹ For example, a single native UK oak tree can harbour 284 different species of insect. Meanwhile, a non-native horse chestnut can support just four.²²

Coniferous trees

An accurate carbon offset tree planting calculator will bear these factors in mind. It should recommend which species of tree to plant, based on how much CO2 it will absorb. Many pine species sequester large quantities of carbon, particularly the tallest varieties. For example, Ponderosa pines can grow to be 61 metres tall.²³ Likewise, the Douglas fir grows to between 12 and 21 metres tall.²⁴ They are both significant carbon sinks.²⁵

Deciduous trees

With regards to deciduous trees, oak is the genus with the most carbon-absorbing species. Their large canopies accelerate photosynthesis. They also have dense wood, which is ideal for storing carbon. Other large species, such as the common horse-chestnut, the London plane and American sweetgum, also soak up lots of CO2.²⁶

Once again, it is important to plant the right trees in the right places. A UK native broadleaf tree takes up an estimated one tonne of CO2 during its lifetime of 100 years.²⁷ However, it is unlikely to grow as well in a non-native environment. It would therefore absorb less carbon.

Can we really offset carbon by planting enough trees?

A carbon offset tree planting calculator could work

One study found that planting trees over an area the size of the United States could erase nearly 100 years of carbon emissions.²⁸ It would involve planting over one trillion trees in total.²⁹ Excluding existing forests, agricultural and urban areas, the researchers concluded that there was room for an extra 0.9 billion hectares of canopy cover. Planting trees over this area could store 205 gigatonnes of carbon.³⁰

Issues with tree planting calculators

However, there are issues with this approach. Trees can take a century to reach maturity. It can take decades for them to absorb significant amounts of carbon. They require large amounts of space to absorb sufficient carbon to offset emissions. The right species must also be planted in the right places for them to prosper. Otherwise, they can have adverse impacts on soil, water flows, water quality, recreation, biodiversity and air quality.³¹

An individual may be able to offset their carbon footprint by using a carbon offset tree planting calculator. However, it would require a huge number of trees to be planted every year. For example, the typical American produces 14.95 tonnes of CO2 every year.³² A mature tree captures on average about 22 kilograms of CO2 per year through photosynthesis.³³ Every American would need to plant an incredible 618 seedlings every year to offset their carbon footprint. This is both unfeasible and unrealistic. 

Ultimately, we need to reduce emissions and remove CO2 from the atmosphere immediately. Therefore, it is dangerous to think that a carbon offset tree planting calculator alone will solve our problems. Planting trees whilst continuing to cut down forests and burn fossil fuels is not a sustainable solution. Climate change will continue until we stop burning fossil fuels and cutting down our existing forests.

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