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Strengthening the carbon sink

The terrestrial biosphere is a natural way to sequester carbon emissions. What if we could strengthen this sink? Watch Dr Iain Hartley explain more.
My name is Iain Hartley. What I’d like to talk to you about today is about the terrestrial carbon sink, and the role it plays helping us in the fight against climate change. First of all, I want to talk about our emissions, and where they actually end up. So each year, we release about 9 billion tonnes of carbon in the form of carbon dioxide, the greenhouse gas, into the atmosphere, and that’s through the process of burning fossil fuels. In addition, land use change, such as the deforestation, releases a further one billion tonnes. But it’s what happens to that carbon that’s quite important.
So although it’s all literally released into the atmosphere, less than half, maybe 40% or 45% actually stays in the atmosphere. About a quarter is taken up by the oceans, which sounds really beneficial, but it causes negative impacts, such as acidification, and that’s a really serious issue that you’ll hear about more next week. The remaining third is taken up by the terrestrial biosphere, and in this context, forests are particularly important. So in places like the tropics, understory forests take up a lot of carbon, and that roughly balances out the deforestation taking place in those regions. And then in temperate regions, we have a net carbon uptake by the forests. So this is because they’re recovering from past disturbance.
And the key question is whether this past disturbance and this recovery could actually be enhanced to allow the forest to take up even more carbon in the future. The terrestrial biosphere is taking up a really important amount of our emissions. The question, therefore, becomes why has this carbon been taken up, and could we increase it in the future to help with the fight against climate change? Well, one of the main reasons, or one of the key reasons why the carbon has been taken up is actually, some of these forests are recovering from past disturbance.
So in temperate regions, where we had huge clearance, historically, for agricultural land, now that the land is now being used more intensively, we actually need slightly less ag land fall our agricultural needs. And that’s allowing us to abandon some agricultural land, which then goes through the process of ecological succession. And that means that we go back to the vegetation we would have otherwise had there naturally. So the shrub lands and forests. And as we go through that process, carbon storage tends to increase. So plant biomass increases substantially. In addition, some of the land that’s been abandoned has been actively afforested. So we now have more managed forest than we did in the past.
Now, we haven’t, though, increased our harvest rate proportionally, and therefore, we now have a greater amount of our forests that are in a relatively old state. So the average age of the trees in our manage forests is greater than it was in the 20th century, and that means those forests also store more carbon. So although there are lots of reasons why the ecosystems are currently taking up carbon, this recovery from past disturbance, and greater age of our managed forests is potentially a key reason. And the question then becomes can we maintain this carbon uptake, or even increase it in these temperate regions. In the temperate regions, we have net carbon uptake.
So the key is either maintaining that, or enhancing that. In the tropics, we’re roughly carbon neutral because deforestation is balancing out the carbon that’s been taken up by the entire forests. So the key probably becomes to reduce deforestation rates, and government initiatives or initiatives like the United Nations Framework on Climate Change Red Programme, Reducing Emissions from Deforestation and Forest Degradation programme, may help us in that regard. So making payments to countries to maintain their forest to reduce carbon dioxide emissions. And there are examples of success stories, such as in China, where there was, a few decades ago, net carbon release, but through afforestation, the biosphere in China is now taking up carbon.
Whether that can be repeated more globally depends on how much we need our land for agriculture, for biofuels, and for other pursuits. So can we actually put more land back into forests? The final thing I want to emphasise is it’s not as simple as just planting trees anywhere if we want to maximise carbon storage in the terrestrial biosphere. Even in this type of forest, with really huge trees like the one behind me, there’s more organic matter, and more carbon below my feet in the soils than there is in the trees above my head. So we have to also manage the carbon stocks in the soils as well as in the trees.
So if you afforest a really degraded area where the soils are low in carbon, you can increase carbon storage in the soils, and also gain in terms of greater plant biomass. Whereas if we look at schemes in the past, for example, where the UK uplands have been afforested, those wet, organic, rich soils that the trees been planted on first have to be drained with large amounts of carbon being released to the atmosphere. Then we end up having to wait for decades for that carbon to be regained, and in some cases, the carbon is never regained. So ultimately, carbon becomes lost in these situations. So it’s not as simple as just planting trees anywhere.
We have to think carefully about where those trees get planted. In a temperate region, we have net carbon uptake, and maintaining or enhancing that is our challenge. The tropics remain about carbon neutral, but if we could reduce rates of deforestation, these areas would also become a carbon sink. But there are lots of different demands on land, in particular, for agriculture to feed the now 7 and 1/2 billion people on the planet, and maybe also for biofuels as well. So whether we can maximise carbon storage in the terrestrial biosphere could be a big challenge as we move through the course of the 21st century.

So far this week, we’ve looked at sustainable agriculture and improving food security in the wake of climate change. Over the next three steps we’ll learn about how to mitigate climate change by strengthening the terrestrial biosphere.

The terrestrial biosphere encompasses all land vegetation, including the world’s forests. It is important to strengthen the terrestrial biosphere because photosynthesis results in an exchange of atmospheric carbon dioxide for oxygen. This means it is a natural way to sequester anthropogenic carbon emissions.

In 2015, the Paris agreement pledged to keep carbon emissions within the capability of the terrestrial biosphere’s natural sequestration potential. This means we shouldn’t be putting more carbon into the atmosphere than the biosphere can remove. Another key part was a pledge to increase forest stocks across the world and do everything possible to strengthen natural carbon sinks.

In this video, Dr Iain Hartley explores the terrestrial biosphere sink further and the puzzle of the ‘missing carbon sink’.

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