Climate intervention: a possible hope in the face of humanity’s biggest problem

29 Aug 2022 Hamish Johnston

Hamish Johnston reviews Pandora’s Toolbox: the Hopes and Hazards of Climate Intervention by Wake Smith

Unknown outcomes Climate-intervention methods could provide hope or hazards. (Courtesy: iStock/fergregory)

The rapid reduction of greenhouse-gas emissions to net zero is the only practical way to halt climate change. But thanks to two centuries of burning fossil fuels, we have created a warmer climate that will endure for generations. As a result, humanity will be faced with an important decision: do we live on a hot planet with all the problems that brings, or do we intervene to try to cool things down?

Pandora’s Toolbox: the Hopes and Hazards of Climate Intervention, by the US academic and former aerospace executive Wake Smith, looks at how we might try to cool the Earth. In doing so he has written an expansive scientific, technological, economic, sociological and moral exploration of the climate challenges we face.

Although the scope of his book is very broad, Smith’s goal is to make the case for a rapid expansion of research on how we could cool the planet by stratospheric aerosol injection (SAI). In principle, this method would create a “veil” of chemicals in the atmosphere that would reflect some sunlight back into space. However, doing so is controversial for reasons that Smith covers in forensic detail.

One obvious reason for caution is that altering the chemical makeup of the atmosphere is what got us into this climate mess, and some worry that further tinkering could make things worse. Another important issue is moral hazard – if we can cool the Earth by spraying chemicals into the stratosphere, then why should we bother cutting our greenhouse-gas emissions?

Smith begins Pandora’s Toolbox by highlighting the dangers of global warming and pointing out that the greatest uncertainty in the future is how humans will respond to the challenges of addressing climate change. Even if we do manage to meet the Paris Agreement and get to net-zero emissions shortly after 2050, Smith warns, the excess carbon dioxide that is already in the atmosphere will endure for centuries or even millennia. This means that temperatures will not quickly return to pre-industrial levels. Worse still, icecaps will continue to melt and the oceans will continue to expand, and therefore sea levels will rise well into the next century and beyond.

Smith argues that if future generations want to improve the climate in their lifetimes, they will have to resort to climate interventions to cool the planet – in fact, he predicts that they will demand them.

Remove and reduce

The book is based on a course on climate-change intervention that Smith teaches at Yale University, and it looks at two broad strategies for reducing temperatures in the short term. One is to remove carbon dioxide from the atmosphere, and the other is to reduce the amount of solar energy Earth receives from the Sun.

Planting trees is one option for removing carbon dioxide. However, for the levels needed this would require vast amounts of land, and forests reach a saturation point in carbon absorption after about 50 years. A solution is to harvest the wood – or other biomass crops – and burn it to generate energy while capturing the carbon dioxide produced and pumping it underground, where it would remain for a very long time.

Smith looks at other removal strategies such as making biochar, which involves the partial recovery of elemental carbon from biomass and then using that carbon to enrich soils. Increasing the uptake of carbon by the oceans and coastal areas is also discussed along with the enhanced weathering of rocks, which locks up carbon in carbonate materials. He also considers carbon capture and storage directly from the air.

Smith’s conclusion about carbon-removal schemes is that they will have to be done “in a big way and for a long time”. As he points out: “We will need to perfect those tools and, more crucially, we will need to organize the world to pay the trillions of dollars required to deploy them year in and year out for decades to come.”

Treating the symptom

Unlike cutting emissions or capturing carbon, SAI will not stop or reverse climate change. However, Smith believes that it could be a useful and relatively inexpensive way to deal with the main symptom of climate change: rising temperatures.

For several years, Smith and colleagues have been looking into the practicalities of sending material 20 km up into the stratosphere, where it would cool the Earth by reflecting sunlight back into space. One way of doing this is to disperse tiny droplets of sulphuric acid, which we know will work because such droplets are responsible for the cooling effects seen after large volcanic eruptions. In 1991, for example, sulphurous effluent from Mount Pinatubo in the Philippines cooled the Northern Hemisphere by about 0.5 °C.

Smith has calculated that SAI could be done using several hundred specially designed aircraft. Running constantly, he reckons that such a fleet would reduce Earth’s temperature by 2 °C within one year. What’s more, such a programme would not be that expensive, costing between about $7bn and $70bn per year to run (at 2020 prices). He claims that the size of such an operation is manageable – pointing out that more than 40 companies in the US have revenues greater than $70bn. Indeed, he says that an SAI programme would be much cheaper than just about any other climate intervention technique – costing about $5 per head of global population.

Smith adds that there is more than enough precursor sulphur dioxide available to run such a programme, and although we do not have suitable aircraft today, creating a fleet should not be a technological problem.

Unlike atmospheric carbon dioxide, sulphuric acid is expected to endure about 18 months in the atmosphere. So, Smith argues that if we are not happy with the effects of SAI, we could stop them relatively quickly.

Global effects need global co-operation

The biggest challenge according to Smith is the governance of an SAI programme. He argues that it would have to be a global initiative and ideally would have the consent of all the people on the planet. However, because of the relatively low cost, it would be possible for a major power to run an SAI programme unilaterally, or with the help of allies. This would have worldwide implications because once dispersed, SAI material would move over much of the world so its effects could not be confined locally – at least in our current understanding.

Indeed, Smith admits that there is much that we do not understand about SAI, and that will not change until we do much more in the field. In the meantime, he believes we should think of SAI as a “fire extinguisher” that we may have to use to dampen down temperatures in the future.

When I first picked up Pandora’s Toolbox I was expecting a comprehensive treatment of SAI and Smith delivers on this – writing in precise, thoughtful and sometimes wry prose that is easy and enjoyable to read. What I did not expect was an exploration of the science, economics, politics and psychology of climate change. Smith includes this exposition to justify the need for more research into SAI. However, the narrative that he presents about the imminent threat of climate change and the challenges of addressing it stands alone as an excellent introduction to the most important issue facing humanity.2022 Cambridge University Press 401pp £20hb

Hamish Johnston is an online editor of Physics World

from physicsworld.com   7/9/2022