THE ASSESSMENT of the science of climate change released by the IPCC in August looked at various future-emission scenarios. In every one the best estimate for the difference in temperature between the mid-19th and the mid-21st centuries was above 1.5°C. Only two scenarios, both with close to net-zero emissions by mid-century and net-negative ones thereafter, had best estimates lower than 2°C for the final two decades of the century.
What the report did not say was how the world would pay for the net-negative decades needed to provide a climate both stable and within the Paris temperature limits. In a net-zero economy, it is possible to imagine “polluter-pays” schemes under which emitters pay for the negative emissions that cancel out their activity. They might conceivably evolve from emission-trading schemes like the EU’s. But once you need to remove more CO2 than is being put in an insufficient number of polluters makes things trickier.
Some companies, including Microsoft and Ikea, plan to go net-negative at their own expense. Microsoft aims to remove from the atmosphere an amount of carbon equivalent to all the emissions it has been responsible for over its corporate life. But it is hardly likely that all big emitters will volunteer to behave so admirably.
An imaginative group of academics writing in the journal Nature recently described a polluter-pays scheme which might force companies to become at least somewhat carbon-negative. Well before net zero is reached, emissions would start to attract a “carbon-removal obligation” requiring later removal. To incentivise companies to act promptly, the obligations would grow over time: leave a tonne up in the air long enough and you would have to pull two tonnes out. It is an ingenious idea. But it would do nothing to remove emissions made before the scheme began.
It is hard to imagine negative emissions on a scale large enough to make up for overshooting the carbon budget being paid for by anyone except governments. Any agreement on how that should be done would surely reflect some version of “common but differentiated responsibilities”: countries disproportionately responsible for using up the carbon budget in the first place would be asked to take on most of the burden of dealing with the overshoot.In a world where rich countries had already invested heavily in helping poorer ones reduce emissions, that added expense might be borne willingly. But it is also possible that it would not.
What is more, in a world where some, presumably richer, countries have enough negative-emissions capability to get to net zero and beyond, other countries—either poorer or just more bloody-minded—might abandon further emissions reduction, gambling that the countries with negative-emissions capability would rather increase their use of that capability than see the CO2 level continue to rise.
They might win that gamble; they might not. It is all too easy to imagine a world where most people in poor countries continue to think that climate change is not their fault and someone else should sort it out, while people in richer countries don’t want to shoulder the full burden of doing so. A stand-off develops in which the overshoot is never sucked down and temperatures eventually stabilise at a level considerably higher than 2°C over pre-industrial levels—if they stabilise at all.
The alternative to this game of chicken would be to meet the Paris temperature limits through other means. Solar geoengineering is an approach to the climate that concentrates not on making it easier for the Earth to cool itself, as removing carbon dioxide does, but stopping it from warming so much in the first place. The way of doing this which has been studied in most detail, albeit almost entirely in models, is to create a thin layer of reflective particles in the stratosphere that reflects sunlight back into space before it warms the surface. Studies suggest that, for good or ill, such a scheme could be implemented by a relatively small fleet of purpose-built aircraft.
These ideas have, for understandable reasons, long been marginalised in policy circles. They reek of hubris and the potential for unintended consequences. And if, by providing another way of limiting temperature rises, they reduce the perceived urgency of curbing emissions, they might weaken or even stall the commitment to emissions reduction.
That would be disastrous. Though it has the opposite effect on temperature, the cooling offered by solar geoengineering is not, in physical terms, an exact counter of the warming created by greenhouse gases. One difference is seen in the world’s hydrological cycle; solar geoengineering tends to suppress precipitation. The more the world came to rely on it, the more this and other mismatches would become harmful. This, alongside the fact that it does nothing about ocean acidification, is a strong part of the reason why researchers looking seriously at solar geoengineering never advocate using it instead of mitigation. Those not turned off by it completely see it as a possible complement to mitigation, not as an alternative.
In this it is very like negative emissions—one reason they are sometimes called “carbon geoengineering”. The way a solar-geoengineering capability might reduce the incentive to continue with emissions reduction is similar to the way existing negative-emissions capacity might work. And in both cases that reduced ambition might well come about even if the proposed geoengineering did not.
A crucial difference between the two, though, is the cost of instigating them. It seems likely that putting a veil into the atmosphere would be comparatively cheap (in the tens of billions of dollars a year, perhaps even less), though the externalities not included in the cost might be high. Another difference is that negative emissions would affect the climate in more or less the same way whoever undertook them; the world’s carbon-dioxide level is what it is. Different sorts of solar geoengineering, though, would have distinctly different regional effects.
Design for living
If the world as a whole were able to design a solar-geoengineering scheme, studies suggest it could come up with one that provided climate benefits to almost everyone and serious problems to almost nobody. A plausible way to use such an optimal scheme might be to offset the temperature rise caused by overshooting the carbon budget. That would provide a sort of breathing space to allow the excess CO2 to be drawn down more slowly and cheaply than would be necessary if its warming effects were actually being felt. A comparatively gentle acceleration of chemical weathering, rather than a huge investment in DAC or other schemes, might prove sufficient. As those measures slowly wore the overshoot down the solar geoengineering could be reduced. When the CO2 level was low enough it would be phased out completely, leaving behind a stable climate.
But the world as a whole might not get to choose the wisest path. A small group of countries, or even a single large one, could undertake a solar-geoengineering scheme unilaterally. Such a club would be likely to optimise its own interests rather than those of the world. It might, for example, choose to cool some places to a degree that, because of geoengineering’s effects on the water cycle, risked desiccating others. That might lead to resentment, rancour or reprisals.
Nor would any given set of solar geoengineers be sure to get things their own way. The late Marty Weitzman, a pioneering economist of climate change, pointed out that though emission reductions (and by implication negative emissions), because of their cost, have a free-rider problem, solar geoengineering, because it looks cheap, has a “free-driver” problem. Where free-rider problems lead to underprovision (countries are unwilling to commit to cutting emissions when others do not, as 26 years of COPs has made clear) free-driver problems may lead to overprovision. Solar geoengineering looks cheap enough that countries wanting more of it can provide it unilaterally, whatever others think.
This implies that, other things being equal, the level of solar geoengineering would be a trade-off between the amount that the most enthusiastic countries wanted and the amount that the least enthusiastic would endure before taking decisive action, quite possibly through force of arms. Another game of chicken.
The difficulties involved in negative emissions and solar geoengineering are very different, and the second looks much more unfamiliar and threatening than the first. But in both cases the problem is fundamentally similar—and closely related to the contradiction between global temperature goals and national emissions-reduction commitments at the heart of the Paris agreement. The world has needs which can be agreed on; but the decisions necessary to meet those needs are made by countries which differ in their interests, their capabilities and the amount of persuasion they can exert or resist. The responsibilities may be common; the differentiation is endless.
Glasgow will show the degree to which national governments are able to deliver progress in terms of the increased ambition of their NDCs, the realisation of long-promised transfers of money from north to south, and workable rules for the parts of the Paris agreement which remain sketchy. The progress is unlikely to be spectacular, reinforcing the point that managing global issues like the climate purely through national lenses is impractical. But coming up with an alternative in which individual countries will allow the interests of others to be placed above their own is implausible. And if the world is to stabilise the climate at an acceptable level, something will have to give.
The climate crisis was brought about as part of the creation of a new world economy. Coming through it successfully may well require a new world politics. Failing to do so will be calamitous in terms of politics and economics alike.■
Correction (November 8th 2021): This article originally mischaracterised Ikea’s carbon-negative ambitions. We regret the error
Full contents of this special report
The agenda for the COP 26 summit: Stabilising the climate
What the Paris agreement of 2015 meant: The state of play
How Asia is crucial in the battle against climate change: The Asian century’s emissions
The economics of the climate: Economics and energy
Why the world needs negative emissions: Negative emissions
The case for geo-engineering: Veils and ignorance*
Stabilising the climate The search for stability
This article appeared in the Special report section of the print edition under the headline "Governing the atmosphere"