On Friday 10 May 2013 the  concentration of carbon dioxide in the atmosphere, measured at the Mauna Loa Observatory in Hawaii, breached 400 parts per million – the highest it has been in millions of years.  Projections from the World Bank suggest that the global average temperature could rise by up to 4 degrees over preindustrial levels by 2100 if current pledges to cut emissions are not met, with a 20% chance of exceeding 4 degrees even if they are. This would put us in a world very much unlike the one we currently enjoy, and which allowed civilisation to rise and flourish.

The Copenhagen Accord accepted 2 degrees of warming as a ‘safe’ limit based on scientific evidence of the time. Recent research suggests that to have a 50% chance of staying under 2 degrees, global emissions must peak by 2020 at the latest, followed by 5% reduction every year thereafter.  In 2011-12, global emissions grew by 3%, and even the global financial crisis – and its effects on economic activity and production – only reduced emissions by 1.4%. There is also evidence to suggest that a 2 degree rise over preindustrial levels would actually be more damaging than previously predicted, so we may well need to peak and decarbonise even faster.

Various trends make such deep emissions reduction unlikely.  There is a lack of global political consensus on the most appropriate strategies to reduce emissions, and we are locking ourselves into a long-lived energy system fuelled mostly by coal, oil and gas. At the same time global population continues to rise, as does the expectation of a higher standard of living – especially in rapidly developing countries pursuing a perfectly equitable growth agenda.

These points lead us to an uncomfortable conclusion: we are already at risk of failing to meet a target that is itself inadequate to avoid dangerous climate change. Against this backdrop of increasingly challenging news, what are our options?

In addition to reducing carbon emissions at a truly heroic pace – something we are still not doing – there is increasing interest in removing CO₂ that is already in the atmosphere, by way of large-scale deployment of so-called negative emissions technologies.  Negative emissions technologies take advantage of the carbon flows between the atmosphere, oceans, land and biosphere.  In much the same way as we have artificially accelerated the flows into the atmosphere, we could also accelerate flows in other directions. For example, a tree is a natural negative emissions technology. A tree draws CO₂ from the atmosphere to grow, and unless the tree burns or rots, the carbon within the tree is stored away from the atmosphere. Afforestation – the conversion of land into forest – would draw a certain amount of carbon from the atmosphere, but it would be slow-acting, and it is questionable whether there is enough land available for tree-planting to make a significant difference to global CO₂ concentrations.

Bioenergy with carbon capture and storage (BECCS) is a good example of negative emissions using existing technologies. BECCS uses plants for fuel, but captures the carbon that they have already absorbed from the atmosphere. Given that we are pursuing bioenergy and carbon capture and storage anyway, combining the two seems sensible, but again there is a limit to its potential. Capturing CO₂ from burning existing biomass that would otherwise have been untouched would do nothing to reduce atmospheric CO₂, so BECCS is limited by the supply of sustainable biomass.

Another option is a process known as direct air capture, the origin of the ‘artificial tree’ image, where CO₂ would be removed from ambient air with chemical processes. One benefit is that it could be deployed anywhere with no fundamental constraints on its scale, though given the very low relative concentration of CO₂ in the atmosphere – 400 parts per million is 0.04% – this process would likely be expensive and energy intensive.

But consider a scenario where we do have a viable and cheap way of achieving negative emissions. This introduces a political challenge: if we know we can effectively remove CO₂ from the atmosphere will we be less inclined to cut emissions in the first place?  This is what economists describe as a moral hazard, and is a valid issue to be addressed. Negative emissions technologies are not an alternative to deep cuts in emissions. The infrastructure and investment required to remove enough CO₂ to solve the climate problem would be enormous, and sufficient deployment would take time. As such, negative emissions should only be considered as an additional measure to minimise the risk of dangerous climate change.

Negative emissions technologies are not yet technically ready for large-scale deployment. In general, they are expensive, energy intensive and slow to act. There is a perceived risk that their very existence may discourage action on mitigation, and it is far from clear how well they would even work.

But in a world where we are failing to do the mitigation we know to be necessary, and where 400 parts per million might prove to be a milestone on the journey to a 4-degree warmer world, we must question whether we can afford to ignore anything that could potentially help relieve our climate predicament, including negative emissions. And if we are to understand which forms of negative emissions can be economically, environmentally and socially sustainable, then research needs to happen now to help us understand what our options are.