** This blog was published in 2015. For more up-to-date information, please see:
- Q&A: Is planting trees the answer to climate change
- Briefing paper: What role can forests play in tackling climate change?
In the fourth post in our sustainability series, SSCP-DTP student Rebecca Thomas considers the role of the biosphere as a carbon sink, and assesses whether planting trees can really offset carbon emissions.
The terrestrial biosphere (land based vegetation) is often considered to have benefited from climate change over the last 50 years (an argument that is used by a number of climate skeptics). Trees, after all, need CO2 and warm temperatures to grow. Indeed, through my research I am seeing that the amount of CO2 being taken up by vegetation has been increasing over the last few decades. But this trend may not go on forever.
Increased CO2 uptake by vegetation has been fuelled by a combination of factors: the additional CO2 in the atmosphere, the changes in climate experienced by ecosystems (particularly warmer temperatures at higher latitudes) and the changes in vegetation growing in different regions of the world. This means that the terrestrial biosphere currently acts as a sink for CO2, taking up about a quarter of the anthropogenic emissions of CO2 that we release into the atmosphere each year (with the ocean taking up another quarter and the other half remaining in the atmosphere).
This may sound like great news, but this is where the question of sustainability comes in. Sustainability can be defined as something which is “able to be maintained at a certain rate or level” or more specifically “conserving an ecological balance by avoiding depletion of natural resources” (Oxford English Dictionary). So, can we rely on the terrestrial biosphere to continue taking up more and more manmade CO2?
A never-ending sink?
The two main processes involved in the exchange of CO2 between the atmosphere and the terrestrial biosphere are photosynthesis (through which plants take up CO2 from the atmosphere) and respiration (which releases CO2 back to the atmosphere). Both photosynthesis and respiration increase with rising temperatures, however while the rate of respiration continues to increase as temperature rises, photosynthesis rapidly declines above a certain point. Photosynthesis also increases with increasing atmospheric CO2, but this effect saturates at high concentrations of CO2.
Clearly then, because temperatures and CO2 concentration are expected to continue rising in the future (IPCC, 2014), the behavior that has been observed over the last few decades is unlikely to continue. Although the exact point of decline/saturation will be different for different plants and ecosystems, a number of studies have suggested that this point may be reached within the next century resulting in a decline in terrestrial biosphere CO2 uptake and potentially causing the land to become a net source of CO2 rather than a sink.
Carbon offsetting schemes: sustainable or not?
A number of years ago, planting a tree to offset your carbon emissions became popular, often amongst corporations as a way to meet their CSR targets. So let’s take a quick side-step into the sustainability of carbon offsetting schemes.
In essence, planting trees is beneficial, not only because of the additional CO2 taken up but also the support for biodiversity that forests bring. However, if we calculate the environmental cost of our activities, the benefits quickly disappear. Focusing just on the CO2 emissions and taking an average car which is driven 10,000 miles/year, emissions amount to around 5 tonnes of CO2/year. Comparing this to planting a UK native broadleaf tree, which is estimated to take up 1 tonne of carbon dioxide during its full lifetime (approximately 100 years) (www.carbonfootprint.com) means that you would need to plant 5 trees each year to offset this.
Taking the more extreme example of a long-haul flight, a return flight from London to Sydney also emits around 5 tonnes of CO2, meaning that for every long-haul flight you take you would need to plant another 5 trees (also the CO2 taken up by the tree is over its entire lifetime, whereas the flight/driving time will only amount to several hours/1 year). It’s easy to see that if everyone was to offset all of their CO2 emissions by planting trees, we would quickly run out of space in the UK, and this is certainly not sustainable.
Additionally, carbon offsetting does not take into account the other environmental impacts of driving and flying such as air pollution and radiative changes due to contrails, and doesn’t encourage more sustainable behaviour.
In short, the terrestrial biosphere isn’t going to save us from our unsustainable habits, and our habits aren’t helping the terrestrial biosphere to be sustainable. Instead of our relationship with the terrestrial biosphere being a win-win, in the long term it’s looking like a lose-lose unless we make fundamental changes to our behaviours to prevent the future projections of climate change and its impacts from becoming a reality.
Find out more
Read about the effects of biosphere feedbacks on climate change in our briefing paper
11 thoughts on “How much CO2 can trees take up?”
On the face of it, it seems unlikely that reforestation can bind up more carbon than was released from the deforestation that preceded it, let alone lock up that from profligate fossil fuel burning. The most productive agricultural lands are most likely going to be the most successful for carbon absorbing reforestation – and the least likely to be put to that use. A lot of land will remain off limits for such a use.
One way of increasing environmental and social benefits is planting trees in arid and semi arid areas. Better Globe Forestry is a company that does this in Kenya with an emphasis on providing jobs in rural areas and poor communities together with other support. The trees impact the local economy while absorbing about 500 kilos of CO2, reducing soil erosion and desertification. And because one of the tree species is similar to mahogany in characteristics it can probably help provide tropical hardwood from sustainable forestry as a future alternative to deforestation in Central African rainforests. Activities like this together with more conscientious consumption can really make an impact for the future. Here is more about how dryland tree planting may contribute to less deforestation: https://www.better-globe-trees.com/saving-central-african-rainforests-miti.php
It would need an English billion trees ie a million times a million (1000000000000) to be planted worldwide to soak up the carbon dioxide made by humans. I’m not sure whether there’s enough available land to plant them all!
Much as Geoffrey Hannam has a point I lean more towards the comment posted by Audun. My main concern is about the situation in Africa. There are a few entrepreneurs that managed to link up with corporates in the UK to plant trees for them as part of a carbon offsetting scheme, however, getting to know which ones were genuine was a real challenge. Two questions remain
1) Do these schemes still exist
2) How can they be popularised so tree farmers in Kenya can partner with manufacturers in the UK in a mutually beneficial relationship? I would really appreciate answers
You are wise to be sceptical but it is not just separating the genuine from the non-genuine. Planting trees (I’ve only been involved in planting a few million) is risky – many can fail; once planted there are all sorts of things that stop them growing – droughts, animals, insects, floods. Quite honestly farmers in Kenya don’t need any more NGOs offering them free trees – they are totally non-sustainable and those people trying to grow trees for a living are undermined by the free trees.
Independent verification helps, but is expensive. Those seeking funds should offer maps of where & when trees are planted, provide pictures of typical results and provide reasonable details of species and growth rates
My question is what happens when the trees in carbon offset schemes reach maturity? If they are left to decay naturally or are burnt for energy, then surely much or all of the carbon (as carbon dioxide) is released back into the atmosphere and there is little or no overall benefit? Am I right in thinking that the only way to avoid this is to somehow ‘lock away’ the mature wood for a long time such that there is no release of carbon dioxide to the atmosphere, but the question then arises as to how to do this? Even if the mature wood is used for wooden buildings or furniture there will eventually be some loss of the carbon (as carbon dioxide) to the atmosphere through decay or other mechanisms, plus there is the carbon dioxide released from the energy used in transport of the wood and construction using the wood? Is the only solution to store the mature wood underground like nuclear waste, but the logistics and economics of this would be daunting! Of course naturally, over eons, such underground storage has locked away the carbon from biomass as coal and oil etc.
Based on the data presented above, it takes 500 trees to consume the CO2 emitted by one average car on an ongoing basis. That’s a lot of trees. Unfortunately this thinking leads to the wrong conclusion that trees are the answer. In reality, old trees die. They rot on the forest floor. Bugs, fungi and bacteria feed on the rotting wood and release CO2 back into the atmosphere. Old growth forests are carbon neutral! Trees are only a “temporary” reservoir for carbon.
There is another fact even more frightening. Coal, that fueled the industrial revolution, was created during the carboniferous period about 300 million years ago. At that time, trees and plants were able to withstand the decay organisms. Decomposing organisms such as bacteria had not developed the enzymes necessary to digest the plant and wood polymers. Wood and plant material did not decompose, it piled up. Its carbon was not released back into the atmosphere. It became coal. That process cannot be reproduced today. Fungi that could break down those polymers started to develop toward the end of the carboniferous period. Digging up coal and burning it is a “one time” event. To take CO2 out of the atmosphere and convert it into solid, rot resistant carbon so it can be sequestered in the earth would require more energy than we’ve spent burning coal over the last 200 years. Going forward, the best course of action, is to leave the coal sequestered in the ground.
Hi Rebecca, do different tree species have different levesl of Co2 uptake over their lifetime?
I’m afraid Rebecca is no longer at the Grantham Institute. However, there is lots of information about tree planting, and what kind of trees should be planted, in this Q&A: https://www.carbonbrief.org/in-depth-qa-how-will-tree-planting-help-the-uk-meet-its-climate-goals
The Grantham Institute