CO2 removal – feasibility questions

Does BECCS really cut it when you’re aiming to limit climate change to 2 degrees, or even just 1.5 degrees? In Nature, Phil Williamson argues the answer is “no”, and uses work by Andy Wiltshire and myself on the Planetary limits to BECCS Negative Emissions to show why.

The future is unknown for the effect of BECCS

BECCS is BioEnergy with Carbon Capture and Storage, and is included in many projections of future climate change that stay relatively low, not just those which aim to remain under 2 degrees warming.

Williamson highlights work Andy and I did, which is freely available to download from the AVOID project website, which shows that when you include the CO2 released by the land-use change, the net CO2 impact of BECCS begins to look rather different. In fact, in its worst case scenario, it can even result in more carbon being released into the atmosphere than is sequestered.

It all depends on the details of when and where the deforestation for cropland occurs, and how long the land is used for bioenergy crops. In essence, the longer land is used for bioenergy crops, the more likely it is the ‘carbon debt’ from the deforestation can be ‘paid back’. In the case of ‘abandoned agricultural land’, there is no debt to be paid – it was already cropland. But in a scenario where forest is removed to make way for BECCS, there is a considerable carbon debt to be paid back, before the BECCS actually makes a positive contribution to mitigating climate change.

However, recent work published in Science shows the real-world effect of what models have always shown: the non-carbon effects also matter. So where the figure above shows carbon emissions from large areas of forest converted to cropland and no agricultural productivity growth, adding in the non-carbon effects changes the story slightly. Because of more than half a degree of cooling from albedo effects, that scenario actually gives a small net cooling, if you’re only considering those land conversions as food crops. If they were BECCS, then probably the cooling would be even larger.

Now, I’m certainly not advocating chopping all the trees in the world down to grow BECCS and limit climate change. The biodiversity impacts would, in my opinion, be utterly unacceptable. But neither am I keen on ignoring biogeophysical effects, just because they don’t fit neatly into our carbon accounting.

Partly though, this is a failing of our science. We need clear metrics that can be used in climate change scenarios. There are ways that the biogeophysics could be fully enough represented that it could go into these sorts of policy decisions. Hopefully it won’t be long before I will be able to update you on how that aim is progressing.

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