Solar Geoengineering

Plan C for an Overheating Planet?

douglas macmartin is a senior research associate in Mechanical and Aerospace Engineering at Cornell University.

kate ricke is an assistant professor at the School of Global Policy and Strategy and the Scripps Institution of of Oceanography at the University of California, San Diego.

Illustrations by james steinberg

Published January 23, 2020


When people think about responding to climate change, they typically think about reducing emissions of carbon dioxide and other heat-trapping greenhouse gases. Had we started on a path to reducing these emissions 30 years ago — when the science was already clear that greenhouse emissions were on a trajectory toward big trouble — then climate change might be behind us (or at least manageable) today. Instead, three decades later, global emissions are higher than they have ever been.

The world is fast approaching a red line, the point at which greenhouse gases will have warmed the entire planet by more than 1.5 degrees Celsius (on average) above the levels of the pre-industrial era. To avoid major damage to ecosystems, suffering for hundreds of millions of people lacking the means to cope with increases in extreme weather, or the displacement associated with rising sea levels, we not only need to arrest the increase in emissions but eliminate them entirely within the next few decades.

This can only happen if we radically transform global energy infrastructure — and that just can’t happen overnight. Even if we rise to the challenge over the next quarter century (which we must), the earth will be burdened by major economic and social dislocation. Indeed, while cutting emissions is absolutely essential, it won’t be sufficient. This is our new reality in 2020.

Carbon is (almost) Forever

For many other types of pollution, once the emissions stop, the damage is quickly undone. But because CO2 remains in the atmosphere for a very long time, reaching zero emissions won’t eliminate climate change; it will just stop making the problem worse. Like a driver accelerating directly into the car in front of us, the first priority is taking our foot off the metaphorical pedal. But while necessary, that won’t be sufficient. The next step is to apply the brakes — and quickly — to lessen the impending impact. And even then, we might need airbags to avoid disaster.

So, what does a climate-scale car crash look like? In addition to the droughts, floods, heat waves and severe tropical storms that are sure to be our lot with gradual warming, every additional increment of warming increases the potential for discontinuous disasters. These include very rapid sea-level rise from a collapse of the ice covering Antarctica, methane releases from high-latitude permafrost that accelerate planetary warming through a feedback cycle or a die-out of the Amazon forest resulting in huge releases of land-based carbon to the atmosphere.

Given poorly understood and frightening prospects for rapid (and likely irreversible) changes, it is rational to explore options for drastic risk mitigation that may bring us back from the brink.

Desperate Measures for Desperate Times?

Over the past several decades, as the failure to limit emissions has become ever more apparent, there has been increasing interest in applying the brakes on global warming by removing CO2 from the atmosphere after it has been emitted. This set of ideas, collectively known as carbon dioxide removal (CDR), or negative emissions technologies, includes:

  • “Natural” methods, such as planting trees or changing agricultural practices to store more carbon in the soil.
  • Artificially fertilizing the oceans to encourage phytoplankton blooms that consume CO2 and sequester some of it in the deep ocean.
  • Chemically capturing CO2 from the air through reaction with various minerals.
  • Enhancing the rate of weathering of rocks, the natural process that will ultimately remove atmospheric CO2 over the coming millennia.

The challenge today is that, while many CDR approaches have promise, none of them currently satisfies three essential criteria.

First, carbon removal needs to be scalable. Each tree planted, for example, will absorb about one ton of CO2 over 40 years. By comparison, we are currently emitting nearly 1,300 tons of CO2per second. There are roughly a trillion more tons of CO2 in the atmosphere than there were at the dawn of the industrial revolution, and if we ramp down to zero emissions over the next 25 years, we’ll have emitted half that amount again. There simply isn’t enough available land for tree planting to make much of a dent in the problem we’ve created. There are similar scaling limitations on other carbon removal approaches as well — in particular, those that most closely mimic natural ecological processes.

Second, carbon removal needs to be reasonably economical. While planting trees might be relatively cheap, the current projected costs for more scalable approaches like direct capture of CO2 from the air are $100 or more per ton. At this price, removing just one year’s worth of our global current emissions would cost $4 trillion, about 5 percent of global GDP.

And third, carbon removal should not create local impacts that are potentially worse than climate change itself. The generation of bio-energy from plants would remove carbon from the atmosphere if the resulting CO2 were captured from the flue gas and stored underground. But deploying this approach at the scale required to have a global impact would require either a radical transformation of natural ecosystems or a massive diversion of land toward energy crops, resulting in competition for both food and water. Clearly, such unintended consequences must be factored in, even in the most dire circumstances.

While daunting, some reliance on CDR is likely essential. There is substantial public and private investment in the development of CDR technologies under way, with some promising results. Researchers have developed technologies that can remove carbon dioxide directly from the atmosphere using a diverse range of approaches — for example, by exploiting an enzyme, carbonic anhydrase, that is found in red blood cells and plays a key role in removing carbon dioxide generated during animal metabolism, or by coating the electrodes of giant batteries with specialized polymers and carbon nanotubes.

Many of these technologies represent adaptations of chemical engineering methods that have been previously applied in other contexts.

Moreover, a number of academic researchers have successfully spun off these technologies into start-up companies — such as Carbon Engineering, Climeworks and Verdox — that are piloting direct air-capture plants, proving the concept for a future in which government-mandated prices on carbon emissions (through taxes or tradable emissions credits) are high enough to make such enterprises economically viable. Indeed, there are carbon-removal approaches that, when implemented together, might avoid all three of the challenges above.

But it would be foolhardy to assume that these approaches to CO2 reduction can be relied on to fully mitigate climate risks. And it would be deeply immoral to continue to emit CO2 today on the assumption that our children and grandchildren will be able to figure out how to remove it.

We are thus left with no certain pathway to avoid the consequences of ongoing CO2 buildup. The most optimistic scenarios include both a rapid transformation of both the global energy and agricultural systems, and an enormous scale-up of “negative” emissions capacity using CDR technologies that currently do not exist. This is clearly a daunting task, both technically and politically — but one required if we are to have even reasonable odds of avoiding the consequences of significant warming.

The challenge is compounded by the fact that we don’t know precisely how rapidly the climate will warm, or how bad the impacts of that warming will be. Most people carry fire insurance on their houses despite the odds of a fire being less than 1 percent. Yet even the optimistic scenarios do not ensure that we can meet temperature targets the science says are needed to avoid the worst. We are gambling that “conventional” approaches to containing climate change will be adequate, with little reason to expect they will be.

Geoengineering to the Rescue?

In the face of this uncertainty, there is another tactic, in addition to mitigation and carbon dioxide removal, that might provide insurance against the really bad scenarios. This approach, known as “solar geoengineering” or solar radiation management (SRM), aims to reduce global warming by decreasing the energy from the sun that hits the surface of the earth. These ideas are not new; indeed, they were discussed in the mid-1960s, back when President Lyndon Johnson was first briefed on climate change. But solar geoengineering remained mostly on the fringe until 2006, when Paul Crutzen, a scientist awarded a Nobel Laureate for his work in atmospheric chemistry, suggested that it be taken seriously.

At its most basic level, solar geoengineering seeks to modify the radiation balance of Earth. When left to its own devices, the planet reaches an energy-equilibrium state, with the amount of energy received from the sun closely balanced by the amount of energy sent back into space through reflected sunlight and emissions of thermal radiation (heat).

The reason the climate is warming today is that increased greenhouse-gas concentrations in the atmosphere are making it harder for Earth’s thermal energy to escape back to space. Since the Earth is now receiving more energy than it is emitting, to reach equilibrium it must warm up (increasing thermal losses) until the input and output are back in balance.

Reducing greenhouse-gas concentrations deals with the imbalance directly by increasing the efficiency with which thermal energy is radiated into space. But reducing the amount of energy that penetrates the atmosphere could address the other side of the balance. If we could deliberately reflect as little as 1 percent of the sunlight currently hitting Earth’s surface back to space before it’s absorbed, we would cool the planet enough to counteract all the warming from our past greenhouse-gas emissions.

Just how difficult would it be to accomplish this? While 1 percent doesn’t sound like a lot, consider, for perspective, that the entire continental United States covers about 2 percent of Earth’s surface. So we cannot achieve this additional reflection by doing things like painting roofs white — there just aren’t enough roofs to make a meaningful difference. There are, however, at least two proposed approaches that might significantly influence the climate.

One such approach is to mimic the cooling effect that occurs after large volcanic eruptions, such as the 1991 eruption of Mount Pinatubo in the Philippines. On June 15 of that year, an explosive eruption tossed huge amounts of sulfur dioxide high into the atmosphere, where the gas underwent chemical reactions to produce reflective sulfate aerosols (small droplets or particles).

If the gas had been released into the troposphere (the lower atmosphere), the resulting aerosols would have fallen back to earth within weeks, with relatively little cooling effect. But higher up in the stratosphere — around 20 kilometers (12 miles) above Earth’s surface — the air is stable and dry, and the aerosolized particles float around for a year or more. These stratospheric aerosols, which were clearly visible in satellite measurements, reflected enough solar radiation back into space to decrease global temperatures by 0.3-0.5°C (roughly 0.5-0.9°F) over the following year.

If CO2 emissions continue unabated, an increasing application of SRM will be required to compensate. Future generations would thus be committed by us to maintaining the deployment practically indefinitely.

It is, in principle, possible to mimic this phenomenon — but without all the ash and local destruction of a volcanic eruption. The stratospheric-aerosol approach would cool the planet and would thus counteract many (but not all) of the impacts of carbon emissions. We don’t currently have aircraft that fly high enough with the capacity to deliver a useful payload, but these engineering challenges appear surmountable. In fact, one concern with this idea is that the direct costs might be low enough to make the idea more enticing than it should be, giving policymakers a means to rush to judgment.

Another solar geoengineering idea is to enhance the formation of reflective low clouds over the ocean. Satellite imagery reveals that, in some parts of the ocean, ships leave behind “cloud tracks” that can persist for up to a week. This phenomenon occurs when aerosolized pollution from the ship enhances the formation of cloud droplets, either creating a cloud where none previously existed or a greater amount of smaller droplets that make existing clouds “brighter.” In either case, the result is the same: more sunlight is reflected back to space. Achieving this effect, by the way, does not necessarily require adding pollutants to the air; spraying saltwater into the right type of clouds might be sufficient.

Spraying saltwater into clouds may be more benign than adding sulfate to the stratosphere, but we don’t understand the physics of cloud-aerosol interactions well enough to know how well this approach would work. Cloud brightening also comes with its own set of issues. While stratospheric aerosols spread roughly uniformly across the globe, marine clouds that can be brightened might only exist over about 10 percent of the Earth’s surface. Achieving the same global cooling effect through cloud enhancement would require much larger changes over smaller areas, resulting in potentially significant impacts on regional weather patterns.

One attempt to conduct an SRM experiment, the U.K.-funded, university-managed Stratospheric Particle Injection for Climate Engineering (SPICE) project, imploded after a public campaign and second thoughts within the scientific community awoke opposition. Another oft-cited model for an SRM experiment, the Eastern Pacific Emitted Aerosol Cloud Experiment (E-PEACE), was conducted in 2011. It released particles from ships into the marine boundary layer of the atmosphere in order to directly measure aerosol-cloud-radiation interactions using aircraft. E-PEACE’s scientific mission was to better understand clouds for reasons unrelated to geoengineering. And in light of the contentious debates around SRM today, it’s unclear whether such an experiment could be done again. A small-scale stratospheric SRM experiment led by scientists at Harvard that would use a specialized balloon to release a few kilos of material and monitor its effects on stratospheric chemistry and transport has been in the works for more than five years, but has an uncertain time line for execution.

Beyond the technical challenges of solar geoengineering, there are other significant questions to be addressed, from the details of its physical impact, to broader societal issues such as public acceptability, ethics and international relations.

For example, both cloud brightening and the introduction of stratospheric aerosols have the potential to change precipitation patterns. Climate models suggest that these precipitation changes will typically be smaller than those we will experience if we allow climate change to progress without intervention. But that might not be true in every nation or locality, and there is still considerable uncertainty in model predictions.

Consider, too, that stratospheric aerosols could delay the recovery of the ozone layer through interactions with the long-lived residual chlorine compounds (CFCs) that were phased out of use by the 1987 Montreal Protocol. Moreover, what goes up (and is heavier than air) must come down — so there may be ecological impacts as sulfate aerosols eventually return to Earth’s surface in the form of acid rain (though preliminary studies indicate the amount of acid rain would likely be a small increment over today’s background levels).

We shouldn’t have to choose between cutting emissions, developing and deploying methods to remove CO2 from the atmosphere and conducting research to better understand solar geoengineering.

All told, we simply don’t know enough today to adequately inform future policies. More research might uncover reasons why geoengineering would always be a bad idea, or might conclude that the consequences of not deploying these approaches far outweigh these concerns.

Who Decides?

More challenging still are the societal and governance questions. If deployed, solar geoengineering would affect everyone on the planet. Who would decide on the parameters, and how? Whose voices would be heard; whose interests would matter?

Just as with efforts to contain greenhouse emissions, the economics and politics of climate geoengineering will be colored by the unequal distribution of its costs and consequences. The incentives created by government policies to manage emissions containment
and set the rules for geoengineering are, however, very different.

The persistent challenge associated with reducing emissions is in compelling multiple actors to cooperate to pay the high costs of decarbonization for the diffuse benefit of many. SRM has the opposite problem, wherein its potentially very low direct costs may create incentives to exclude some stakeholders from the decision-making. Even if we understood the physical outcomes of SRM with sufficient precision, understanding its socioeconomic impacts before the fact with any certainty will be virtually impossible.

In addition to the complex trade-offs in terms of geographic equity that are bound to arise, SRM raises the prospect of intergenerational equity as well. If CO2 emissions continue unabated, an increasing application of SRM will be required to compensate. Future generations would thus be committed by us to maintaining the deployment practically indefinitely — if they ever stopped, the climate would rapidly warm to the point where it would have been without SRM.

On top of that, some of the impacts of our climate-changing emissions wouldn’t be addressed at all by solar geoengineering — think, for example, of increasing ocean acidification driven by rising rates of absorption of CO2 from the atmosphere. Despite these obvious concerns, there will doubtless be some who want to use a geoengineering option as a shortsighted excuse to delay the hard, expensive work of cutting CO2 emissions.

How could we ensure that this approach is considered only as a supplement to emissions reductions and not as a substitute? It is essential that scientific research into SRM goes hand in hand with the development of international governance capacity to make sound decisions.

In both the Paris Climate Agreement and debates among the contemporary climate science and policy communities, average global temperature change is generally used as a proxy for climate change damage. This makes the coherent comparison of outcomes with and without SRM very difficult. We know with high certainty that solar geoengineering could be used to stabilize global temperature, but regional climate changes would persist, and the excess carbon dioxide in the atmosphere would continue to drive the ocean acidification that threatens catastrophic change. In the absence of a new way to quantify the benefits and damages of geoengineering relative to an un-geoengineered counterfactual, it can seem easier to just dismiss the geoengineering option out of hand.

Return to the car-accident analogy, in which one can think of SRM as akin to air bags. It doesn’t quite deal with the underlying problem of an impending impact — in our case, of having added greenhouse gases to the atmosphere. No one would sit in their car and set off their airbag for fun. By the same token, it only makes sense to consider the side-effects of SRM in the context of climate change. But it is possible that geoengineering could reduce some of the worst effects of climate change and thus mitigate suffering, particularly for the most vulnerable in the world. Indeed, for ecosystems without capacity to adapt to rapidly changing conditions, a climate response plan that includes SRM may be the only way to avoid plant and animal extinctions.

Threading the Needle

It wouldn’t make sense to force society to choose between installing airbags in cars and enforcing speed limits. Similarly, we shouldn’t have to choose between cutting emissions, developing and deploying methods to remove CO2 from the atmosphere and conducting
research to better understand solar geoengineering. Indeed, SRM only makes sense in conjunction with cutting emissions.

Had we been working diligently to reduce our CO2 emissions over the past 30 years, perhaps we wouldn’t need to think today about additional approaches to climate change response. Even if solar geoengineering is eventually deployed to help limit the impacts of climate change, we must strive for a future in which the excess atmospheric CO2 will have been removed and SRM is no longer needed.

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