The Geoengineering Question

The idea of geoengineering is scary. There is a reason why the trope of human hubris creating unintended consequences is ubiquitous in literature, and humanity trying to actively manage global temperatures seems like a cautionary tale waiting to be written.

But for better or worse we are already geoengineering the planet today. Our emissions of greenhouse gases have significantly changed the composition of our atmosphere, increasing CO2 concentrations by 50% compared to preindustrial levels. We’ve emitted massive amounts of sulfur to the lower atmosphere as a byproduct of burning fossil fuels, which has served to mask around a third of the warming the world would otherwise have experienced (and at the horrific cost of millions of lives per year due to outdoor air pollution).

With growing impacts from climate change and an apparent acceleration of the rate of warming in recent years (albeit in line with what our models project), more prominent voices like Dr. James Hansen have become more open to purposeful cooling of the planet through reducing the amount of energy it receives from the sun.

In this piece I’ll try to take a balanced look at the issue; the potential pros and cons of considering geoengineering as part of our toolkit to reduce the impacts of climate change, as well as the potential unintended consequences it might have both to the climate and to our ability to effectively address climate change. My own position on the issue remains relatively unchanged: its worth doing more research to have it as a break-glass-in-case-of-emergency option for halting warming, but large-scale deployments today are a bad idea.

What is geoengineering?

Geoengineering broadly refers to any deliberate large-scale intervention in the Earth's natural systems to counteract climate change. In practice, most of the conversation around geoengineering has focused on approaches that reduce the amount of incoming solar radiation absorbed by the Earth’s surface – called solar radiation management (SRM). Note that carbon dioxide removal (CDR) used to be conflated with geoengineering, but is more accurately seen as part of mitigation today.

SRM approaches include stratospheric aerosol injections, marine cloud brightening, surface albedo adjustments (surface mirrors, ice, etc.), and space shades among other proposed options. This article will discuss the most commonly advocated version of SRM – stratospheric aerosol injection – but many of the issues brought up would apply to any SRM approach.

Stratospheric aerosol injection involves putting aerosols – generally sulfur dioxide – into stratosphere (the upper part of the atmosphere) where they will reflect incoming sunlight back to space, cooling the surface. This is a mechanism that we know works, because large volcanic eruptions that put sulfur dioxide into the stratosphere cause cooling for a few years after they erupt. Similarly, our emissions of sulfur dioxide into the troposphere (lower atmosphere) today mask a portion of the warming the world would otherwise have experienced.

Putting aerosols in the stratosphere is much more effective at cooling the planet – more than 100x more effective per ton of sulfur – than putting aerosols in the troposphere. This is because aerosols in the troposphere are very short lived, falling back to the surface in a matter of days, while aerosols in the stratosphere will stay on the order of a year or two. Aerosols are also more localized in the troposphere (vs better mixed in the stratosphere, at least within the hemisphere), which produces more regional cooling effects and is more prone to saturation dynamics (e.g. tropospheric adding aerosols to a region like China where there is already a lot of aerosols gives diminishing returns for cooling).

Due to the need to add much less aerosols to the stratosphere for the same cooling effect, the potential human health impacts of stratospheric aerosol injection are orders of magnitude smaller than the current catastrophic air pollution problems caused by the unintentional tropospheric aerosol geoengineering experiment we are running today. If society does decide to continue to add sulfur dioxide to the atmosphere, it would make a lot more sense to have it in the stratosphere rather than than the troposphere.

Adding sulfur dioxide to the stratosphere is also likely to be relatively inexpensive – at least to start. Global temperatures could potentially be stalled at current levels for only a few billion dollars a year today, though the costs would increase over time as more and more sulfur injections are required to counterbalance the continued accumulation of atmosphere CO2.

This leads to the fundamental issue of geoengineering: it is not a solution to climate change. Rather, it is a bandaid that treats the symptoms of climate change rather than addressing the underlying cause: our greenhouse gas emissions from burning fossil fuels and land use change.

The warming the world has experienced to-date is primarily caused by our emissions of CO2. CO2 has an extremely long atmospheric residence time – about half of what we emit today is still in the atmosphere after 100 years, and it takes on the order of 400,000 years for it to be fully removed. Furthermore, the warming from CO2 persists long after concentrations decline, given the inertia of the Earth’s oceans. Even if we can get CO2 emissions to zero and atmospheric concentrations fall, the planet does not cool down for many centuries to come.

There is no world where we can continue to emit CO2 and “solve” climate change through geoengineering (as Futurama presciently noted 22 years ago in the video below). Rather, at best it can help mask impacts while we rapidly cut emissions, and at worst it might be a way for us to kick the proverbial can down the road and leave future generations a crippling carbon debt.

The case for considering geoengineering

The case for geoengineering is relatively straightforward: the world is on track to overshoot its most ambitious climate targets; even if we start rapid emissions reductions tomorrow we will very likely pass 1.5C by the early 2030s (and would pass 2C in the early 2050s under current policies).

If we could use SRM mask the effects of climate change while we reduce emissions – and eventually reverse overshoot through large-scale CO2 removal – we could reduce damage to society and the natural world. This is illustrated in the figure below, which shows the potential for a temporary deployment of SRM on top of aggressive emissions cuts to minimize peak warming.

Figure 2 from After Geoengineering by Holly Jean Buck.

We know that SRM would work to cool the planet; after all, we have plenty of examples of volcanoes doing this naturally. The relatively short-lived nature of stratospheric aerosols (falling out of the atmosphere in a few years) means that any deployment of SRM could be easily adjusted if it results in more or less cooling than initially expected.

We are already doing SRM today by putting vast amounts of sulfur dioxide into the troposphere – with devastating impacts on our air quality. It makes sense to consider replacing these tropospheric aerosols with a tiny fraction of the amount of sulfur in the stratosphere to achieve the same cooling effect and save millions of lives. In this approach (suggested by David Keith among others), we would only deploy SRM today to compensate for the additional warming associated with the declining aerosol emissions as we phase out fossil fuels.

SRM also acts very quickly once started. Having the ability to rapidly cool the climate when needed could be an important hedge against surprises. For example, suppose we discover that there is a high likelihood of a particularly dangerous climate tipping point this century when global temperatures pass a particular threshold – say, a shutdown of the AMOC, or the loss of much of the Amazon rainforest. In this case it might be too late to avoid these impacts by stopping emissions, and SRM could buy the world time to cool temperatures back down by removing CO2 from the atmosphere.

The case against geoengineering

Geoengineering is an attractive solution in an ideal policy world where we can ensure that emissions are reduced rapidly and the deployment of SRM is time-bounded while we get emissions to zero and use CDR to reverse temperature overshoot.

Unfortunately for us, we don’t live in an ideal policy world.

Rather than being synergistic with a world of rapid emissions reductions (and later CDR deployment), there is a real worry that deploying SRM to mask the effects of warming will deter mitigation and leave an ever greater carbon debt on future generations.

Actually solving climate change will be expensive (though likely less than some have suggested). The world will need to fundamentally transform its energy system in the decades to come to shift from using fossil fuels to clean energy, while protecting the enhancing natural carbon sinks. In a world where we routinely discount the future, it may be appealing to spend a few billion dollars a year to mask the problem for now rather than actually solving it.

Unfortunately, if SRM slows down mitigation it leaves a larger and larger carbon debt for future generations. Getting emissions to zero does not reverse warming, it only stops it. Any warming masked by SRM needs to be “undone” with permanent carbon dioxide removal if we ever want to be able to stop deploying SRM without rapidly rebounding to the conditions it was masking (the so-called “termination shock”).

This turns out to be extremely expensive; even if we succeed in getting permanent carbon removal down to $100/ton (from ~$500/ton today) it would cost approximately $22 trillion to remove enough CO2 from the atmosphere to cool the planet down by 0.1C. In other words, every 0.1C of warming that is being masked by SRM is a $22 trillion debt passed down to future generations if they ever want to stop deploying SRM without triggering renewed warming.

In addition to concerns about mitigation deterrence, there are also some remaining uncertainties about the consequences of large-scale SRM deployment. While volcanoes provide a useful analogue here, they are short-lived compared to continuous deployment anticipated by most SRM advocates. The impacts of continued SRM on precipitation patterns, crop yields, and other climate variables remain areas of active research with teams of researchers using earth system models to better constrain uncertainties. There are also some impacts of CO2 emissions – such as ocean acidification – that would not be even temporarily masked by SRM.

The geopolitics of SRM are also potentially quite gnarly. Given the relatively low cost, any individual country (or even rogue billionaire) could undertake SRM against the wishes of other countries. Given the remaining uncertainties in climate response, there is a real risk that those who undertake SRM might end up “owning the weather”.

For example, in the recent speculative fiction book Termination Shock author Neil Stephenson speculates what might happen if the Indian monsoon fails to arrive after some countries start geoengineering. Even though it may well be natural variability, it would be fiendishly difficult to prove that unusual changes to precipitation or other climate conditions was not influenced by the decision to start SRM.

A path forward?

There are no easy answers to the geoengineering question. For better or worse, we are already geoengineering the planet today through our emissions of greenhouse gases and aerosols.

My main concern with proposals to deploy SRM today is one of mitigation deterrence; I could see technologists who see things in terms of ideal systems making decisions on quick, temporary fixes that undermine the messy real-world process of mitigating emissions. At the same time, I’m also cognizant that not treating the symptoms of climate change through something like SRM in the hopes that more suffering would speed up mitigation is arguably morally reprehensible in its own way.

In the short term I think we should continue research on the topic, with an aim of having it as a break-glass-in-case-of-emergency option to deal with unexpected climate surprises. I’m also intrigued but still unconvinced by suggestions of using stratospheric aerosol injections as a way to compensate for reductions in tropospheric aerosols due to decarbonization.

But regardless of one’s thoughts on the wisdom of ever deploying it, we should be absolutely clear that geoengineering is not a solution to climate change, and cannot serve as an alternative to rapid emissions reductions.