Chris Stark, Committee on Climate Change: Introduction to greenhouse gas removal methods

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Chris Stark: Hi again, everyone. Joining you virtually this time, but nice to see you all and we'll have a chance to speak later on. I'm going to talk to you today about one of the most important topics for net zero, and that's greenhouse gas removals. So today, we're going to do an introduction into how we can actually remove a greenhouse gas like carbon dioxide from the earth's atmosphere, and how we can then store that carbon safely so it doesn't cause climate change.

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Throughout this talk, I'll be a referring to methods that remove carbon dioxide from the air. There are other greenhouse gases but there's less of them in the atmosphere, so they're more difficult to remove.

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So for the whole of this talk, we're talking about carbon dioxide and carbon. So, I'm going to run through six of the main greenhouse gas removal methods. I'm going to talk about their status and their potential, and try and explain some of their pros and cons. Now the pros and the cons I'll cover are examples. They're not supposed to be comprehensive, so we can always cover more detailed questions in the Q&A. There are other methods, and I want to be clear about that. The other methods, though, are more speculative at the moment, and what I mean by that is that we need to do work to allow us to understand how they could be used at scale, and especially to ensure that the risks associated with those other methods can be managed properly. Because of all that, we felt it was important to understand what you thought as an assembly about the main contenders for greenhouse gas removals. So I'm only going to focus on six methods today.

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So let's move on to slide two. The first method I want to talk about is growing forests and woodland, and probably this is the simplest of the removal methods. When trees grow, they naturally absorb carbon dioxide from the atmosphere. It takes a forest about 10 years to ramp up to the maximum amount of carbon it can store. After between 20 and 100 years, the forest reaches what we call 'maturity' when the amount of carbon it absorbs starts to slow down. Now, we can grow more forests and woodlands and we can also manage forests better to maximise the amount of carbon they store. So, a forrester can remove some of the young tree growth to make room for other trees to grow more successfully, and that allows us to increase the overall amount of carbon that's stored in that forest.

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Moving on to slide three. If that's what could be done with forests and woodlands, what are their potential advantages and disadvantages of doing greenhouse gas removals in that way? Well, the costs are low. We understand very well indeed the potential to store carbon through that route, and we think it's possible to store a lot of carbon dioxide and lock that away in forests for a long time. But some of the cons are that it does take time for trees to grow and to mature, at least 10 years. The land that we use could be used for other activities, and especially growing food. So Jim is actually going to talk to you about what the group looking at food and farming and land use thought about and discussed in Birmingham a bit later on. It could also have a negative impact on what we call biodiversity. So nature is another way of looking at that. So if we grow the wrong trees in the wrong places., that can have a damaging impact on nature.

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Let's move on to slide four. The second method I want to talk about is peatlands and wetlands. Now, just like forest, this is another natural way of doing greenhouse gas removals. Peatlands and wetlands are the damp, wet, natural habitats that you find all around the UK. These are places that naturally store a lot of carbon. But interestingly, if they dry out, they can actually release that carbon back out into the atmosphere. So if we want to use these habitats as a way of removing greenhouse gases from the atmosphere, then we need to keep them wet. And often that means restoring those peatlands and wetlands to what they had been before agricultural practices deliberately dried them out to create farmland.

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Moving on to slide five. So, some of the potential advantages and disadvantages of doing greenhouse gas removals in this way. We know a lot about how to restore peatlands and wetlands. It's a quick way to store carbon and it can be relatively low cost, and there are a set of other wider environmental benefits that come from having these restored natural habitats. On the cons side, the disadvantages are that we understand less about whether we can safely store carbon in these places for a long time, and especially when climate change may warm the planet. So we run the risk of releasing some of that carbon again in the future. It uses up lands that may be used for something else, especially agricultural land, farming. There's only limited potential, so there's only some areas of the UK that are suitable to be restored in that way.

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So moving on to slide six. The next method I'm going to talk about is enhancing the storage of carbon in soil. Again, as with forests and with peatlands and wetlands, this is a natural way to store carbon. How much carbon we can store in the soil is affected by a few things, and especially how we farm on that soil. So, firstly, how often we disturb the soil, and especially ploughing it with machines. Secondly, how much organic material is in the soil? Now, organic material is things that were once living like plants, but are now dead. So those of you who have gardens might have a compost heap where you put your garden waste. If you let that decay a bit and then dig it back into the soil, that's the organic material I'm talking about. If we want the soil to be able to store more carbon, we need to disturb it less and ensure it has more organic material in it, and that's really about changing farming practices. So changing the way that crops are grown is one way to do it, reducing how often we plough the soil with machines, using natural fertilisers rather than the artificial chemical fertilisers that are often used.

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Moving on then to slide seven. These are some of the potential advantages and disadvantages of storing carbon in the soil in that way. It is a very well understood method so we can get cracking on that now, and it does have a low cost for those that do it, and there are other wider benefits, especially to the environment if we treat the soils in that way. On the disadvantages side, it does require a change to modern recent farming practice. Farmers might need some support to make that kind of change, and it can reduce the amount of food that could be produced from a given area of lands. The yield of that land.

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Slide eight, is the simplest of these, really. Something I'm sure you will be familiar with, and that's using wood when we build and construct buildings. Very simply, this is a method where we grow a tree, as it grows it absorbs the carbon from the atmosphere, we then cut that tree down for timber. Hopefully, we plant a new tree to replace it when we do that, and we use the timber in the construction of a house or another building. And that building is a safe store of carbon for often many hundreds of years, at least until the house is knocked down or the timber is burned. So it's a good way of storing carbon if we can use more wood in the building of a house.

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Moving on to slide nine. So why might using wooden construction be a good idea, why might it not be a good idea? Well, wood has been something we've understood and used for centuries in constructing buildings. It's a relatively low cost way to store carbon and it reduces our carbon emissions by replacing other, much more carbon intensive materials, like concrete and brick. On the disadvantage side, it does require a change to the modern building practices that are typically used. It might mean that some very tall buildings aren't possible. So you can't use timber for some of the tallest buildings, and this scale of this as a method is limited by, of course, the availability of the wood to supply it from sustainable sources.

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Moving on to slide 10. Now, this is one of the most promising of the technological options, bioenergy with carbon capture and storage. Also one of the options that many people are quite concerned about. So I'm going to spend a bit of time on this one. Bioenergy with carbon capture and storage involves a number of processes, some of which you've already discussed at the assembly. I'm going to talk you through the stages of it. So first, what we call the 'biomass' is growing. Biomass just means a plant, so it grows and it absorbs carbon as it does so. In this example, biomass is a tree. So we grow a forest, some smaller trees might be removed by the forester to allow more successful trees to grow even larger. This is a byproduct of good forest management, and sometimes we call that forest thinnings, and those thinnings are a supply of biomass which can then be used to produce energy. So in the simplest example, we burn that wood to generate electricity. That's very helpful, but it also produces carbon dioxide, so we need to capture as much of that carbon dioxide as we can, and we do that through an industrial process which is represented here with a big grey tower with some clouds above it. We haven't yet found a way to capture 100%, or all of the carbon dioxide that's produced when we burn wood like this, when we burn biomass like this, so some will escape into the atmosphere. That's one of the criticisms of this approach, but we do know how to capture most of it. So in the future, there might be technology that improves the amount that we can capture to even higher rates of the carbon dioxide. The carbon dioxide that is captured then needs to be stored somewhere, so we can pipe that to a safe storage destination underground. In the UK, some of the best locations are where we once found oil and gas in the sea, in the North Sea, for example. It goes underground through a relatively quick geological process to effectively turn that back into rock. So it should be quite a low risk of leaking when it's when it's under the sea.

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Let's move on to slide 11. So the main advantage of this is that we can capture and we can store a large amount of carbon dioxide emissions. It combines technologies that we understand, and there is lots and lots of storage space for that carbon in the UK, especially, in the depleted, the old oil and gas fields that we have in the seas around the UK. On the disadvantage side, though, it's likely to have high costs, at least initially, and we need land to grow the biomass, so that uses land that could be used for other activities. We also need to ensure and be clear that the carbon that is stored is in a place that is secure and won't be released.

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Let's then move on to slide 12. The final method that I'm going to talk to you about today is direct air capture with carbon storage, and this is one of the newest technologies for greenhouse gas removal. Here, a machine actually sucks the air and passes it over a carbon absorbing material or a chemical which then separates that carbon dioxide from the atmosphere, taking it out of the atmosphere and reducing the impact that it has on climate change. This is an interesting approach because we don't actually need to grow a plant to capture the carbon. Going through that process creates a supply of carbon dioxide which we then need to transport to save storage underground just as we did with the previous method.

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So moving on then to slide 13. And these are some of the potential advantages and disadvantages of doing greenhouse gas removals in this way. We can capture and store a significant amount of carbon dioxide emissions doing this. There's lots of storage space in the UK, in particularly in the old oil and gas fields, and there are a variety of technical options that are being developed to try and do this in the future. Disadvantages are that it's still very new and therefore quite an experimental technology. It might require a very large amount of low carbon electricity to plug these things in to make sure they work, and current costs and current projections for costs are still very high. Though, of course, they might fall as the technology improves. So that concludes my introduction to some of the greenhouse gas removal methods. I hope I could explain what's often quite a tricky set of concepts, and I'll obviously be available later to answer questions on it as best I can. For now, thanks very much.

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