Negative Emissions: Not Seeing the Forest for the Trees

Jul 11, 2017 | Negative emissions

1 June 2017

Amidst all the discussion – often very technical discussion – of emissions trajectories and carbon budgets, one simple fact is often under-appreciated: reducing greenhouse gas emissions will, on its own, not be sufficient to keep temperature rise below 2°C. We will, in addition, have to invest in technologies that remove greenhouse gases directly from the atmosphere: so-called ‘negative emissions’ technologies.

Stabilisation of global temperature will require stabilisation of atmospheric greenhouse gas concentrations. This will, in turn, require zero net greenhouse gas emissions: to prevent further accumulation of greenhouse gases, it is clearly necessary to remove them at the same rate (at least) as they are being emitted.

Given that some economic sectors are more difficult to decarbonise than others – certain industrial processes, transport and heating come to mind – and are likely to remain net emitters for some time to come, the implication is that other sectors will have to take up the slack and not only decarbonise but become net sinks.

Negative emissions are needed to counter-balance the positive, difficult-to-reduce emissions from industrial and other sources. Credit: Phys.org

The extent to which this will be necessary will depend on our success at what might be termed ‘standard mitigation’ – the bread-and-butter energy efficiency, renewable energy and fuel-switch activities that we already understand well and just need to scale-up and mainstream across the economy.

‘Just’ is, of course, relative: overcoming the financial, political, behavioural and technical barriers to decarbonisation isn’t so much a moonshot as a Mars-shot. But it’s do-able, it’s do-able at low cost and it’s compatible with a range of other policy objectives – industrial innovation, energy security, job creation, clean air – that reinforce the imperative.

Included under the ‘standard mitigation’ banner is carbon capture and storage (CCS). CCS isn’t as ‘standard’ as it should be: two decades and 17 million tonnes of CO2 after the Norwegian Sleipner project started capturing carbon dioxide from a natural gas facility and injecting it 1 km beneath the sea bed, there are still only 15 large-scale CCS projects worldwide.

But it’s standard in the sense that it reduces greenhouse gas emissions: it makes emissions ‘less positive’ and potentially even zero from a particular source, but it doesn’t remove gases that have already been released into the atmosphere, which is the hallmark of a negative emissions – or Greenhouse Gas Removal (GGR) – technology.

GGR technologies come in a variety of flavours, some more developed than others. The scientific and economic aspects of some – such as ocean fertilization, enhanced silicate weathering and biochar sequestration – are currently the subject of active but still largely fundamental academic research.

Bioenergy with carbon capture and storage (BECCS) is attracting a lot of headlines (and surreptitious insertion into a lot of abatement models) but is also encountering significant push-back from groups concerned about possible displacement of food crops – quite an achievement for a technology that has yet to be rolled out on any meaningful scale.

Direct Air Capture (DAC) – ‘artificial trees’ in the media portrayal – is expensive (financially, as well as in terms of energy and water demand) and, like all other GGR approaches, must deal with the disposal question: having captured CO2 from the air, where is it to be stored? But cost-reducing advances in catalyst chemistry and the flexibility of being able to locate air scrubbers anywhere (away from population centres and near to underground disposal sites, for example) are powerful selling points of DAC and its prospects look good.

Only last week, the world’s first commercial DAC plant commenced operation near Zurich in Switzerland, utilising low-grade waste heat from a municipal waste recovery facility to supply CO2 to nearby commercial greenhouses growing tomatoes and cucumbers. An initiative on this scale – removing just 900 tonnes of CO2 per year – and at a cost per tonne of CO2 roughly 100 times greater than polluters need to pay under the EU Emissions Trading Scheme is clearly not by itself going to change the world. But it could very possibly set in train a virtuous circle of learning, cost reduction and risk mitigation that does indeed change the world.

The Hinwil DAC plant, near Zurich. Credit: Climeworks

Which brings us to trees. Why build artificial trees when real trees offer a proof-of-concept mitigation technology millions of years in the making? The world’s forests store more than twice the carbon in the atmosphere and absorb about 30% of annual anthropogenic CO2 emissions. The carbon sequestration potential of afforestation and reforestation – and their underlying soil substrate – is significant, even before other positives such as biodiversity, soil stabilisation and rural livelihoods benefits are considered.

It’s estimated that large-scale reforestation of formerly agricultural areas would lead to an increase of 215 Gt in terrestrial carbon content and to a reduction of 85 ppm in atmospheric CO2 concentration by the year 2100 (for comparison, the current concentration, as of June 2017, is 409 ppm). This could be achieved at lower cost than most other mitigation measures – and, indeed, would occur naturally where abandoned farm land is left to its own devices.

Conflicts with agriculture and water usage are, of course, real barriers to the expansion of forest cover, as are the weak governance and fragmented institutional frameworks associated with the forestry sector in many countries. The impermanence (‘reversibility’) of biologically-stored carbon is a physical reality, although often overlooked is the fact that removal of forest does not necessarily result in commensurate emissions: the timber in buildings and furniture continues to store carbon just as effectively as a living tree.

Nonetheless, one can’t help but conclude that forestry is too often overlooked as an attractive negative emissions approach – ironically, perhaps precisely because of its low-tech, long-established pedigree.

Facing the stark reality that even immediate and complete decarbonisation of the global electricity sector will be insufficient to limit temperature increases to 2°C, the importance of negative emissions technologies cannot be overstated. In terms of rapid scale-up potential, forestry currently stands primus inter pares among such technologies.