The Intergovernmental Panel on Climate Change is preparing a report on keeping global warming below 1.5 °C. How the panel chooses to deal with the option of solar geoengineering will test the integrity of scientific climate policy advice.
The landmark Paris Agreement on climate change was one giant leap for climate policy, but one small step for climate protection. For the first time the world's countries agreed a universal, legally binding climate deal. This is an enormous achievement, and one that would have been unthinkable just a few years ago. But the small print of the Paris Agreement highlights the growing inconsistency between climate talks, decisions and actions1. The parties to the United Nations Framework Convention on Climate Change (UNFCCC) made emissions pledges that would result in roughly 3 °C of warming2, but they set an aspirational temperature target (1.5 °C) that is achievable only through emissions cuts so drastic that they are politically infeasible3. The UNFCCC then outsourced this dilemma to the Intergovernmental Panel on Climate Change (IPCC) by inviting a special report that discusses options for keeping warming below 1.5 °C.
As such, the Paris Agreement is set to test the integrity of scientific climate policy advice like never before4. There are no easy options for achieving the 1.5 °C target — this is climate's new inconvenient truth — and there is a possibility that it is achievable only through solar geoengineering. Here we argue that the IPCC must neither duck the controversy of solar geoengineering, nor quietly build it into climate scenarios as it did in its previous assessment report with direct removal of carbon from the atmosphere, also termed negative emissions. Instead it must candidly report the facts around 1.5 °C, however uncomfortable they may be, and lead a critical, evidence-based conversation around solar geoengineering and our climate future.
The IPCC has been in a similarly tough position before and it did not acquit itself well. After publication of the Fourth Assessment Report in 2007, climate policymakers asked for more mitigation scenarios that could keep projected warming below 2 °C. Faced with the problem that integrated assessment models had very few credible scenarios that achieved 2 °C with emissions cuts alone, climate economists implemented a conceptual innovation in their scenarios: direct carbon dioxide removal from the atmosphere. As a result, in the IPCC's Fifth Assessment Report, the scenarios that held temperatures below the 2 °C threshold projected that the carbon quotas associated with 2 °C would be overshot, and then carbon dioxide removal would be used to draw the excess carbon back out of the atmosphere.
One problem is that the scale of carbon removal assumed in the IPCC models, estimated at roughly 500 gigatonnes (Gt) of CO2, is vast5. The disconnect between policymakers and modellers is also concerning: policymakers never explicitly requested that researchers should make use of carbon dioxide removal in the Fifth Assessment Report, nor have they shown great interest in how to deliver it in practice, even though there are serious questions over feasibility and potential consequences for land use, agriculture and biodiversity6. Conversely, climate economists did not appear to question the socio-political feasibility of carbon dioxide removal. This working relationship can be described as a co-production of irresponsibility1.
Now, in the wake of the Paris Agreement, there is a risk that these mistakes will be repeated and magnified. The IPCC is preparing a report on the 1.5 °C temperature target, even though it is probably too late to achieve it by simply controlling emissions. Calculations indicate that humans could only release about 200 Gt more CO2, whereas collectively we are emitting about 40 Gt per year7. Global CO2 emissions would need to peak almost immediately and then start shrinking rapidly. Although more doctrinaire environmentalists point out that this is technically possible8, there is no hope of making it politically feasible.
Not without geoengineering
The world will probably have only two choices if it wants to stay below 1.5 °C of warming. It must either deploy carbon dioxide removal on an enormous scale or use solar geoengineering. This would involve blocking out a small amount of the incoming sunlight, for example by spraying millions of tonnes of tiny reflective particles (such as sulfate aerosols) into the upper atmosphere where they would scatter and reflect away some solar energy like a thin global sunshade.
“It would be very risky to bet the planet on humanity's ability to decarbonize immediately and rapidly.”
The idea of using solar geoengineering is receiving growing attention because it is the only known way to quickly slow, stop or even reverse global temperature rises9,10. This means solar geoengineering has a unique potential for reducing some of the climate risks to which Earth is committed from the greenhouse gases that have already been released11,12. However, there are still large uncertainties around the possible risks and benefits13. To rely on solar geoengineering would be very risky, just as it would be to rely on vast amounts of unproven carbon dioxide removal14, or to ignore both options and bet the planet on humanity's ability to decarbonize immediately and rapidly.
How the IPCC addresses solar geoengineering — both in the special report on 1.5 °C and then in the upcoming broader Sixth Assessment Report — represents both an opportunity and a trap for climate policy advice. There are two ways to get it wrong and one way to get it right.
The first option is inclusion by stealth. When the pressure rises to generate warming scenarios that policymakers would like to see, the IPCC could be tempted to repeat the carbon dioxide removal fudge in a new guise and employ solar geoengineering as a deus ex machina that allows for the generation of scenarios in which 1.5 °C is achieved. In this case, solar geoengineering would be used to shave off the temperature peak that will result from excess emissions; temperatures could be held constant while the world overshoots the carbon budget for 1.5 °C and then slowly claws its way back under the temperature ceiling using carbon dioxide removal. This approach should be rejected immediately. If solar geoengineering is ever to become a component of the collective, canonical vision of our climate future that the IPCC's scenarios represent, then it needs to be put there on the back of a broad discussion. It must not be imported through the back door by modellers who feel pressured to come up with a scientific foundation for a political temperature target.
The second option is for the IPCC to ignore solar geoengineering. There are some early signs that this might be the approach that the special report on 1.5 °C is heading for15. Although ignoring solar geoengineering avoids some controversy and may be the safest option, it would also be ill advised. After all, it is being discussed by concerned climatologists and exasperated environmentalists because, enormous direct carbon dioxide removal from the atmosphere aside, solar geoengineering is probably the only known, plausible way to stay below 1.5 °C of warming — if it could be made to work, that is. It is the IPCC's duty to report this to political decision makers.
Option three, and the only sensible option in our view, is for the IPCC to use its reports — alongside other efforts from researchers and non-governmental organizations — to help spark a critical conversation around solar geoengineering. In this way the 1.5 °C report becomes a chance to open up a new kind of conversation about macro climate policy: a negotiation of the relative roles that mitigation, adaptation, carbon dioxide removal and solar geoengineering can play in managing climate risks.
Face up to the task
This is the only sensible option for the IPCC. Sadly, making the 1.5 °C target through emissions cuts alone no longer seems possible. To base future climate scenarios on an unproven promise of solar geoengineering would be irresponsible, just as it has been for carbon dioxide removal. The temptation to steer clear of the controversies surrounding solar geoengineering will be large, but must be resisted. What we need now is a global, open and science-based discussion on the risks of passing the 1.5 °C limit versus the risks of using solar geoengineering to stay below it.
Of course, if it turns out that solar geoengineering is the only realistic way to avoid 1.5 °C that does not mean that we will have to do it. We still need a much larger evidence base to make an informed decision, and a much broader dialogue to make an inclusive decision. But to fulfil its responsibilities as the world's climate change advisory body, the IPCC must deal with solar geoengineering head on.
The opinions expressed in this piece are those of the authors alone.
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