Injecting sulphuric acid into the stratosphere could help to counter the effects of global warming, but should humanity take that step? Academics David Keith and Mike Hulme debate
David Keith is Gordon McKay professor of applied physics at the Harvard School of Engineering and Applied Sciences
Scientific evidence suggests that we could reduce some of the effects of the accumulation of greenhouse-gases (GHGs) in the atmosphere by adding particles of sulphur into the stratosphere. Such a move would, similar to the impact of volcanic eruptions, increase the Earth’s reflectivity, deflecting more of the sun’s rays and resulting in a cooling that has the potential to restore the climate, in most places, to that of pre-industrial times.
We could, for example, roughly halve the rate of manmade climate change by the end of 2070 if we begin introducing sulphuric acid into the stratosphere in small amounts in 2020, and slowly increase the amount over the next 50 years. In 2070, we would be putting close to 1 million tonnes of sulphur into the stratosphere a year, which equates to just 2% of the sulphur currently in the lower atmosphere caused by burning fossil fuels. The next step would be to decrease injections of sulphur down to zero over the following 50 years.
Taking such action would considerably slow climate change, giving us more time to adapt, as well as reduce the impact of climate change during that time – decreasing risk of crop failures, for example. Currently, the whole of our built infrastructure is designed to sustain industries that emit carbon dioxide and CO2 has a very long footprint; if we halted global emissions today, CO2 already in the atmosphere would continue to have an impact on our climate in 1,000 years. It’s not possible for us to magically make the CO2 problem go away.
Climate engineering is not a new idea; it’s been around since the 1960s. But a taboo emerged around it because of a fear that it would lessen efforts to cut GHG emissions. That fear was well founded, but the consequences of climate change are too severe and what we know about this technology is promising enough that it is worth studying and discussing.
I’m not advocating we begin sulphur injection tomorrow – we don’t know enough – but we do have sufficient knowledge to start debating its use and officials from governments, including those from the UK, the US, India and China are engaging with the subject.
A fundamental misconception about sulphur injection is that if you start you must do it forever and that it will increase the acidity of the oceans. This perspective assumes that we are going to use this technology as a substitute for cutting emissions, but this is completely implausible. No credible expert believes we can keep emitting CO2 forever and use geoengineering technologies as a way to offset the impact.
With regards to the effect on the oceans, putting sulphuric acid into the stratosphere essentially does nothing directly to alter the acidity of seas. There is, however, a direct link between adding CO2 to the atmosphere and acidification. The connection between sulphur injection and acidification is made only if we adopt a policy for using the technology while continuing to put more carbon into the atmosphere.
Another crucial misconception is to do with certainty; that while we can assess specific risks in subscale tests, we cannot test the technology at full scale. It’s true that we can never know precisely how pumping sulphuric acid into the atmosphere will work, but neither can we predict the exact climate response of human-caused CO2 emissions. Carbon is one of many things in our atmosphere that are changing the climate, so it is impossible to tell exactly what change was caused by CO2 in the recent decades, and the same would be true if we undertook geoengineering.
The most difficult issue, however, is not the science or technology, which is available and relatively cheap, but about how we govern it. How do we build international consensus on how to study, develop and, potentially, manage sulphur injection technologies? If there is no global coordination, and different countries take different approaches, the consequences could be horrific. There’s no magic answer, but that remains true of many other new technologies. We urgently need to be thinking of how to build institutions that are capable of making rational decisions about it.
Mike Hulme is professor of climate and culture in the School of Social Science and Public Policy at King’s College London
Solar climate engineering, through sulphate technologies, is not an effective way of reducing future climate-related loss and damage for two main reasons. First, is that the welfare “bads” we’re trying to minimise – crop failures due to drought and damages caused by sea surges, for example – relate predominantly to regional and local weather, and these are poorly correlated to global temperatures.
Using solar climate engineering to create a global thermostat may allow us to reduce average temperatures worldwide and compensate, in part, for local heat accumulation caused by GHGs, but these globally averaged quantities are not what cause climate-related loss and damage. It is regional and local weather that does this – from droughts in the US to floods in Pakistan.
Any call to implement climate engineering to reduce the risks of GHG-fuelled global warming must establish what I call the “core claim”. Advocates must be able to convince all interested parties that the weather damages of a natural + GHGs + solar-engineered climate will be substantially less than those caused by a natural + GHGs climate.
This presents a significant challenge and one that distinguishes solar climate engineering from other forms of manipulative technology, such as pharmaceuticals. In the case of solar climate engineering there is no option that parallels double-blind clinical trials to test a new drug – such claims can only be tested through simulation models. So what do these models that we become dependent on show? Well, that at regional scales, solar climate engineering leads to variable and contrasting changes in local weather, especially rainfall.
Then there is still the question of the veracity of numerical simulation models; are they credible? I’ve spent nearly half my occupational life assessing climate models’ ability to simulate regional and local effects, comparing empirical observations with simulations, and I don’t have great faith in their ability to predict rainfall at regional weather scales. In September 2013, an IPCC report concluded: “At regional scales precipitation is not simulated well.”
Solar engineering at the weather scales that matter most for people is not a win-win technology. At regional scales the danger is that deploying this technology is like a game of Russian roulette. Climate models will not be able to offer us the accuracy necessary to establish what the consequences of the technology are to human life or to defend against claims of legal liability in relation to local weather events once sulphur is introduced into the stratosphere.
This brings me to my second argument, which is about whose hand will be on the thermostat. Deciding when to implement the thermostat, agreeing what the setting should be and how it should be securely governed, would demand an unprecedented and simply unobtainable degree of trust and cooperation between nations. All of the affected agents need representation in any decisions made and over any regulatory bodies established.
After more than 20 years of the UN framework convention on climate change, which can’t be described as a roaring success, it’s extremely optimistic to expect a novel system of global governance can be invented and sustained over the time necessary for solar climate engineering to be effective. Even more so when we recognise the extra geopolitical antagonism that solar climate engineering would bring about. There will be nations that will claim any damaging weather extreme that has affected their country was caused by sulphur. The potential for liability and counter-liability claims is endless.
The world has banned and heavily regulated some areas of science and technology and I believe there is a similar argument to be made for a ban on solar climate engineering, certainly of field trials. Solar climate engineering opens up a nightmare prospect of politically motivated weather designs and, potentially, legal, economic or even military conflict to the world.
* The views in this article were presented at a debate on geoengineering hosted by the Institute for Science and Ethics at Oxford Martin School.
