Wood – carbon neutral fuel, or climate red herring?

One regular cause of friction in our house is the lighting. My wife would happily light up the entire house even when sitting in just one room, whereas I’m all for the easy wins when it comes to saving energy so walk around turning them off. After all our years together we’ve settled into a workable routine: she complains, and I carry on anyway.

Now, though, I’m afraid we’re about to get a whole new reason to bicker. Because she also likes to fire up the stove when friends are coming round, even if it’s not cold. And I sympathise: few things are more welcoming than the entrancing cosiness of a log fire. So where’s the harm, especially if – as is frequently claimed – wood is a renewable energy source?

The theory is that, in burning logs, we’re simply releasing what would have gone into the atmosphere anyway as that tree rotted. So as long as a new tree is planted to take the place of the one crackling in our stove, there’s no net increase in CO2 emissions, and our flickering fire is carbon neutral. If only…

The truth, as I’d have realised long ago if I’d stopped loading the stove long enough to think about it, is that not all the carbon in a fallen tree is released as CO2. Rather, as a tree rots and is munched ever smaller by hordes of grateful organisms, some carbon enters the food chain. Yet more is leached by rain deep into the soil where, with the pressure of millennia, it gradually turns into the very fossil fuels we’re so keen on burning today. Leave a tree to decay naturally and a reasonable proportion of its carbon will be buried safely in the ground – until we start digging it back out, of course.

Even if the carbon emissions floating up my chimney did precisely match the carbon given off by a rotting tree, that doesn’t take into account the impact of clearing that tree on the surrounding soil: by letting more sunlight warm the ground, organisms are able to consume all the deeper, thereby releasing yet more sequestered carbon – in some cases, up to twice as much as is stored in the tree itself.

For every log you burn in your fire at home, a similar amount of carbon dioxide is being released from the forest floor outside.

The Hidden Life of Trees, Peter Wohlleben

Besides, for wood to be even close to carbon neutral, a burnt tree would actually have to emit less carbon than its rotting equivalent, to compensate for the emissions linked to extraction, processing and transportation.

And all this assumes I’m restricting myself to windfall from a natural forest. If my burnt tree is to be replaced with a commercial plantation, there are the emissions from crop fertilisation as well, to say nothing of the fact that commercial plantations are far less effective at storing carbon: older trees generate far more biomass, but the average plantation is harvested long before it can reach carbon neutrality.

Nor does this even touch on the loss of diversity that comes from replacing natural forests with commercial plantations – assuming the felled land is even used for timber. Nor does it account for the devastation to the soil. Or the air pollution that comes from burning wood that, though not CO2-based, is another driver of climate change.

Wood smoke is thick with tiny particulates known as PM2.5, linked to heart attacks, strokes, cancer, dementia and a host of other ailments. One study suggests that domestic wood burning produces more than twice as much as all the PM2.5 emissions from road traffic.

Comparative emission factors for domestic consumption of fuels implied by energy content of fuel (DEFRA)

These figures include the use of open fires and less efficient, older wood stoves, and obviously the majority of stove emissions fly up the flue (to contribute to air pollution elsewhere). But the figures make the point: if I’m to light the fire for our guests, perhaps I should be handing out face masks as well.

But what if burning wood is for more than just aesthetics? What if lighting the stove allows me to turn off my oil boiler? The problem then is one of timing: biomass emits its carbon in a flash, and more of it per unit of energy than gas, oil or even coal. Meanwhile trees grow slowly; it would require decades of growth (from woodland that wouldn’t otherwise have existed) to compensate for what I’ve thrown on the fire. And since we’ve only a few years left to reduce the CO2 in the atmosphere if we’re to avoid the worst effects of global warming, we might arguably be better off trickling CO2 from our oil boiler than gushing CO2 from our log stove. This ‘carbon payback’ issue is just one reason why the UK’s rush to biomass is viewed with concern.

The obvious, then, holds true: while the world is desperately planting trees and fighting to halt the devastation of the rainforests, the more we burn wood, the more CO2 will end up in the atmosphere. In an ideal world, every tree could live and die in peace because in the short term – and that’s all we have – burning wood is not climate neutral. Telling myself otherwise is like telling myself it’s okay to fly because I’m offsetting. The aim is to reduce, not excuse.

I’ll therefore be offering all our guests not a roaring fire, but a choice of home-knitted, locally-grown, hemp jumpers. And it’ll be so much easier to break that news to my wife if the lights are off.

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