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.