The ‘Forest Pump’ – how timber works wonders for carbon storage and substitution

In an extract from his new book, Timber! How Wood Can Save the World from Climate Breakdown, former Labour MEP Paul Brannen explains why timber really is a wonder material when it comes to carbon storage and substitution – and why cutting down trees can be a good thing when it’s done at the right time.

We have been building houses from wood for thousands of years. Most European cities still have a few timber-framed buildings dating back 300 years or more. In the UK, York is a good example. Here you will find The Shambles – a street of medieval timber-framed buildings with a strong Harry Potter feel to them. When these buildings were erected, they had only one purpose – that of providing shelter.

The Shambles: a street of medieval timber buildings still standing in York.

The Shambles: a collection of medieval timber buildings still standing in York.

However, they were also safely storing the carbon that the timber had sequestrated (soaked up) when it was growing as a tree in the forest. Trees absorb carbon dioxide (CO2) and release oxygen (O2) – holding onto the carbon that was in the carbon dioxide. For every dry tonne of manufactured timber around 1.8 tonnes of CO2 is removed from the atmosphere.

Chopping the tree down does not release the carbon, it remains stored in the wood. However, if we burn the wood, we do release the carbon into the atmosphere, where it joins up with oxygen to form CO2 – the very gas whose effects we are trying to mitigate, as its relentless increase is causing the climate to change.

Greensted Church in Essex, England, has a nave constructed from large split oak tree trunks that have been dated to between 1063 and 1108 – that is over 1,000 years of carbon storage. Westminster Hall, part of the Palace of Westminster, has an oak hammer-beam roof commissioned in 1393 – over 600 years of carbon storage.

If we build with wood, we safely store the carbon in the built environment – potentially for hundreds of years. At the same time, back in the forest, we replant the felled trees with new saplings – often three saplings per felled tree – to ensure the process is circular and hence sustainable. In sustainably managed forests, new trees are regenerated to replace trees that are harvested so that there is no net loss of forest carbon.

When to fell a tree (from a carbon storage perspective)

When a tree is felled, it can no longer absorb carbon, but the wood continues to hold the carbon it has previously sequestered. However, it should be noted that a growing tree cannot go on absorbing carbon forever. Older trees start to slow down on absorption before stopping altogether and dying, whereupon the rotting trees begin to release their carbon back into the atmosphere as methane (CH4), which is also a greenhouse gas – one 80 times more potent than CO2, albeit with a shorter lifespan.

Graph showing carbon capture ability of trees as they age

A graph showing the carbon-capture capability of trees as they age; © Carbon Neutral, Australia

While it may seem a shame to fell a tree in its prime, from a climate perspective this is the ideal time to do so – before growth and hence carbon absorption slows and stops. (Farmers do the same as foresters – they harvest their crops at the optimum point in their life cycle.) We can then move the maximum amount of carbon stored in the timber into wooden products in the built environment, for example, house frames, floor-boards, beams and roof joists.

Why would we not want to maximize the potential of the built environment to safely store as much carbon as possible? Every existing building in Europe could safely store between 2‒6 tonnes of carbon in wood products, including wood fibre insulation.

The ‘forest pump’

The often-cited paper ‘Buildings as a global carbon sink’ (Churkina et al) has promoted the idea of the ‘forest pump’. We grow trees in the forest and they sequester carbon. We then fell the trees. We turn the trees into wooden building products. These products store carbon in the built environment for 50, 75, perhaps 100 years. Meanwhile we replant new trees – saplings – and they start the carbon sequestration process all over again, and will in time produce more timber for use in the built environment, and so on.

There is no question that the built environment’s ability to store carbon is massive. For instance, a 2021 report from ASN Bank and Climate Cleanup stated that there was the potential in the Netherlands to use the projected one million new homes required before 2030 to store 50MtC. This is a quarter of the country’s annual emissions. The USA is also waking up to wood’s carbon-storing ability. A 2023 report from the Rocky Mountain Institute recognized that the construction of new homes in the USA resulted in 50MtC of emissions annually.

The ceiling of Westminster Hall; © Thomas Erskine

The ceiling of Westminster Hall; © Thomas Erskine

However, it went on to argue that the very same homes could store significantly more carbon than their construction generated if carbon-storing materials were deployed in their construction, helping the US reach its climate targets quickly and efficiently. The Institute referred to this opportunity as “low-hanging fruit”.

Timber as a substitute for other materials

Timber’s ability safely to store carbon in the built environment is therefore a great asset in the battle against climate change. But the good wood news does not end there, because wood has another valuable attribute: the ability to substitute for other materials that are significantly more carbon-intensive. This may seem a statement of the obvious – when we build with timber, we do not use concrete – but it is often missed when carbon emissions are calculated for building work.

If you build your block of flats out of wood rather than concrete, you will replace the concrete, a material that was made by burning fossil fuels, with a material that is essentially the opposite. Wood is a safe store of carbon that has been taken out of the atmosphere, not a material whose manufacture has put more CO2 into the atmosphere. Recent studies on the impact of substitution suggest that on average one tonne of CO2 stored in wood represents 1.2 tonnes of CO2 avoided through the replacement of conventional materials. This is the “low-hanging fruit” referred to in the Rocky Mountain Institute report.

The timber retrofit and extension at the Technique Building, London, achieved an overall 1,709-tonne reduction in carbon compared to a reinforced concrete and steel alternative.

The timber retrofit and extension at the Technique Building, London, achieved an overall 1,709-tonne reduction in carbon compared to a reinforced concrete and steel alternative.

Yes, wood has a relatively small amount of embodied carbon as a result of being transported from the forest, cut in the sawmill, then transported to the construction site: i.e. fossil fuels were burned in the process of turning it from a tree into a timber roof beam for a house.

However, that roof beam could have been concrete and the beneficial figure from a climate perspective is the difference between the CO2 emitted to make a wooden beam and the CO2 emitted to make a concrete beam. This figure, this difference, is a measure of the substitution effect – a figure as valuable in our endeavours to get to net zero as the storage figure.

Scale this up from city to city, country to country, continent to continent and we can drive down carbon emissions from the built environment via this substitution role – yet more low-hanging fruit. Calculations made by Michael Ramage at the University of Cambridge have shown that erecting a 300m2 four-storey wooden student residence in the city generated 126 tonnes of CO2 emissions. If it had been made with concrete the tally would have risen to 310 tonnes. If steel had been used emissions would have topped 498 tonnes. There is the equivalent of 540 tonnes of CO2 stored in the building’s wood, resulting in a long-term subtraction of CO2 from the atmosphere.

To sum up, we have two issues at play and they are both equally important:
1. Wood’s ability to continue storing the carbon that the tree sequestered in the forest as timber in the built environment;
2. Wood’s ability to prevent the carbon emissions which would have happened if we had used concrete and steel, brick and block, instead of timber – wood’s substitution impact.

As researchers at the Potsdam Institute have put it: “By employing bio-based materials, technologies and construction assemblies with high carbon storage capacity and low embodied emissions, we can create a durable human-made global carbon pool while simultaneously reducing CO2 emissions associated with building sector activities.”

Timber! How Wood Can Save the World from Climate Breakdown by Paul Brannen will be published by Agenda Publishing on 27 June 2024.

The book will be launched at the UK Timber Design Conference on 26 June. Tickets are now available.