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How much impact can planting trees make on climate change? (10 minute read)

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I’m stating the obvious when I say that Antarctica has a lot of ice. There’s sea ice which freezes and melts with the seasons, ice shelves and ice tongues which float at the coast, glacial ice flowing down from mountains and the massive ice sheet which covers most of the continent. Ice dominates Antarctica’s land and sea.

What’s not so obvious is how deep the ice goes. The Ross Ice Shelf, the largest of the ice shelves, forms a cliff more than 50 metres high along its edge in the Ross Sea. But ice shelves are like icebergs – what you see above the water is only a part. On average, the Ross Ice Shelf is around 330 metres thick.

The Ross Ice Shelf seems massive, but it’s not much more than an eggshell compared to the East Antarctic Ice Sheet. On average, the ice sheet is more than two kilometres thick, and in places it is more than 4.5 kilometres thick. Ice doesn’t just dominate what we can see of Antarctica, but what we can’t see as well.

Glaciers, ice shelves and ice sheets all form when fallen snow becomes compacted. A fresh layer of fallen snow is around 90% air – it might look like a solid mass but if you try to walk on it you sink as the snow is compacted under your feet. The same process happens when more and more snow falls without melting. Eventually the weight of the new snow crushes the lower layers, removing much but not all of the air. Although the ice inside a glacier seems solid, it still contains a small amount of air trapped in the ice.

Deep in the Antarctic ice sheet, the ice is hundreds of thousands of years old. So too is the air trapped in that ice. Each air bubble is a tiny time capsule, containing a record of what the atmosphere was like hundreds of thousands of years ago. Scientists have drilled into the ice to obtain ice cores, which are transported from Antarctica back to laboratories around the world for analysis.

The Antarctic ice sheet plays an important role in our knowledge of climate change, because the tiny bubbles of air inside it have given us a record of atmospheric carbon dioxide. We only began measuring carbon dioxide in the atmosphere in the 1950s, but we have ice cores dating back 800,000 years. Records from ice cores have been crucial in helping us to understand climate change.

Any excuse for an image of Antarctica (image credit: Melanie Newfield)

Ice cores have told us that the level of carbon dioxide in the atmosphere doesn’t stay the same. At times the level rises and at times it falls. Over the last 800,000 years, the level has fallen as low as 200 parts per million – meaning that out of a million gas molecules in the atmosphere, 200 are carbon dioxide. At times the level has risen as high as 300 parts per million. But something has happened over the last century. Just after 1910, the level crossed 300 parts per million and kept climbing. By 1940, it was 310 parts per million. By 1970, the year before I was born, the level was 325 parts per million. By 2010, the level had reached 388 parts per million, and just a few years later it had reached 400.

The level hasn’t stopped rising since then. Last year, it was recorded at 420 parts per million.

A lot of science has gone into linking the rising carbon dioxide level with the burning of fossil fuel and with climate change. I’m not going to discuss that right now, although I will share some resources to help you understand and talk about how we know they are connected in upcoming newsletters. What I really want to focus on is something else.

Most climate change action is focused on reducing the amount of carbon dioxide we are pumping into the atmosphere. But we’ve already caused a huge increase in the carbon dioxide level, and it’s having an impact on us now. Can we do anything to reduce it?

The process of removing carbon dioxide from the atmosphere and locking it up somewhere it can’t contribute to climate change is known as carbon sequestration. It is a process which happens naturally when plants absorb carbon dioxide during photosynthesis and when carbon dioxide dissolves in the ocean. On land, growing plants store carbon, and long-lived plants like trees can store carbon for centuries. When plants die and decompose, some of the carbon is released back into the atmosphere, but some of it remains in the soil as organic matter. Soil storage of carbon is important – there is around 1500 gigatons of carbon in the organic matter of soils around the world. In comparison, the trees in all the world’s forests only contain 400 gigatons of carbon. In the sea, there is much more carbon sequestered than on land – around 38,000 gigatons. Some of that carbon is simply in the form of carbon dioxide dissolved in the water. But much of it is in sediments in the deep ocean, formed when marine creatures die and fall to the bottom.

Can we enhance these natural processes in some way, to draw more carbon dioxide from the atmosphere? Is it as simple as planting more trees? Or is there some way we can sequester more carbon in the ocean? Or can we use technology to mimic those natural processes, and suck carbon from the atmosphere on an industrial scale?

As you might expect, none of this turns out to be simple.

I’m going to start with trees, because they are the solution that most people think of first. It’s true that a growing tree absorbs carbon dioxide from the atmosphere and that healthy forests can be called carbon sinks, that is, they absorb more carbon dioxide from the atmosphere than they release. But that doesn’t mean that planting a lot of trees will immediately store much carbon.

For a start, it takes decades for the trees to reach anything close to their maximum carbon storage potential. Then, once they are mature, they don’t continue to absorb carbon at the same rate. Under some circumstances, forests can even release more carbon than they absorb, for example when they are attacked by pests and diseases, when they catch fire or when they are disturbed. In the longer term, natural forests are far better at storing carbon than plantations, especially if those plantation forests are going to be logged – which just releases most of the absorbed carbon dioxide back into the atmosphere.

In fact, it is far more important to protect the forests that we have than plant new ones. I’ve previously written about deforestation in the tropics, and that if tropical deforestation were a country, it would be the third largest emitter after China and the USA. But we miss the point when we focus on the tropics alone. There’s also a massive loss of boreal forest due to fire, and these fires don’t just destroy the trees but burn the soil too, releasing the carbon stored there.

If we are going to plant new trees, however, then not all planting is equal. It matters where forests are planted. Crucially, planting forest in some areas may make climate change worse. Planting trees in peat bogs results in the peat decomposing and this may release more carbon dioxide than the new trees absorb. Planting trees in savannah grassland can increase the fire risk, and fires result in a large-scale release of carbon. There is also another impact on the climate which needs to be considered in some areas. At high latitudes, where there is snow cover for long periods of time, replacing low-growing vegetation with trees can affect the amount of the sun’s energy which is absorbed and the amount which is reflected back. Dark forests absorb more heat than snowy shrubland, potentially leading to increased warming. On the other hand, replanting trees in tropical areas which have been deforested is a better prospect.

However, even in areas where forest once grew naturally but have been deforested, such as most of New Zealand, two crucial questions remain. Firstly, does the choice of tree matter? And secondly, how much of a difference can replanting trees really make?

The first question seems smaller and simpler, but it turns out it isn’t. The short answer is yes, the choice of tree matters. Which tree is the best choice depends on the growth rate of the tree, how big the tree grows, whether there is fertiliser used, how much water is available, what other species that tree can support... in short, it’s complicated. But there aren’t clear rules, such as “native species are better” or “fast-growing species are better”.

I’m curious about this question, because in New Zealand, vast areas which were once native forest are now covered with exotic plantations, most of it a single species, the Monterey pine, Pinus radiata. But this is controversial, especially with the conversion of farmland to permanent forests for carbon sequestration. The way that the New Zealand Emissions Trading Scheme works, there are financial incentives to plant Monterey pine over any other species, native or exotic, because it grows so fast. What’s not so clear is what happens long-term. There is a lot of debate and ideology, but I’m still struggling to find reliable evidence. For example, there are concerns about the fire risk of planting a lot of pine, but when I looked at this question previously, I discovered that pine is not a particularly flammable species – the real villain is gorse.

I’m going to continue looking at this question because it’s important and a hotly debated policy issue in New Zealand. But I realise that I can’t find a satisfactory answer to the question today, so I’ll move on to the other big question I had: can we really make a difference to climate change by replanting trees?

This question is also a topic of debate, this time among scientists who work on climate change. Scientific journals offer a number of different perspectives. The debate is not whether forests sequester carbon, which they certainly do, but how much difference we can make by planting more trees. Estimates of how much carbon we could sequester in natural environments vary. One publication suggested we could sequester another 205 gigatons of carbon dioxide. To put that figure in context – we emitted around 37 gigatons in 2022. However, others have suggested that the 205 gigaton figure is overestimated by a factor of 5, which would mean the maximum potential for extra carbon sequestration, for all time, is about one year’s worth of emissions. Some authors have pointed out that forests and other natural environments only reduce carbon dioxide in the atmosphere at a slow, incremental rate, and that we simply don’t have enough time or suitable land to plant our way out of the climate crisis. They see tree planting as a distraction from the more urgent need to reduce emissions from fossil fuels. Others argue that we need to do both.

Looking beyond the disagreements on the detail, it’s clear that planting trees isn’t going to solve our problem without a drastic reduction in the amount of carbon dioxide we are putting into the atmosphere. If we want to reach “net zero”, which is where we are putting no more carbon dioxide into the atmosphere than we are removing, then most of that has to come from reducing emissions, not planting trees. And frankly, “net zero” doesn’t seem like the right target when we consider how much extra carbon dioxide is already in the atmosphere, and how much climate change we are already experiencing. Can we reduce the amount of carbon dioxide in the atmosphere, back from the 420 parts per million where it’s currently sitting?

We can’t do it by planting trees, but are there technological ways that we could suck carbon from the atmosphere? I’m going to look at that question in a couple of weeks.

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