Everyday wisdom tells us it’s much better to avoid a problem than to try to fix it afterward. That’s one reason cutting greenhouse emissions is by far the preferred option for limiting climate change. 

Yet society has dragged its collective heels on climate action for decades, and it’s unclear whether the world will achieve the roughly 50% emission cuts this decade that the Intergovernmental Panel on Climate Change (IPCC) deems essential to avert the worst consequences of a human-warmed planet.

Enter the notion of geoengineering. Often referred to as “climate intervention” or “climate-altering technologies,” geoengineering refers to the idea of messing with the climate system that humans have already been messing up – this time in an effort to turn back the clock and restabilize the climate. 

As recently as the 1950s, when weather modification was on the ascent, there was a hyper-optimistic sense that humans might someday manage the climate too, as if twiddling knobs on a newfangled TV set. Famed researcher John von Neumann pondered the idea of using dark-colored material to hasten snowmelt and warm the local climate: “What power over our environment, over all nature, is implied!”

The environmental consciousness that took root in the 1970s led to broad skepticism about global-scale technological fixes. Even as global temperature (and climate awareness) rose steadily in the 1980s and 1990s, geoengineering was little discussed. The script flipped in 2006, when eminent atmospheric chemist Paul Crutzen wrote a journal op-ed arguing that society might have to resort to blocking sunlight artificially in order to avoid catastrophic climate change. 

Such a dramatic step, Crutzen wrote, “should not be used to justify inadequate climate policies, but merely to create a possibility to combat potentially drastic climate heating.”

Crutzen’s essay paved the way for climate policy makers to evaluate geoengineering with new seriousness. A stream of reports since then, including a pair of 2015 studies from the U.S. National Academies, have examined how climate intervention might be done and how diplomacy could keep various options in reserve until they’ve been thoroughly vetted. 

It’s now clear that some types of geoengineering are more fraught with peril than others, yet there remain huge questions across the board on cost, governance, and moral hazard. 

What are the main types of geoengineering?

There are two broad categories of climate intervention: 

  • Carbon dioxide removal (CDR) – pulling CO2 out of the air
  • Solar radiation management (SRM) – reflecting sunlight to cool the planet

Carbon dioxide removal is the more direct and less risky of the two groupings, and it’s the one that would address the root of the problem: too much CO2. The most commonly cited modes of CDR are afforestation (creating new forests) and reforestation; both make intuitive sense, given the power of photosynthesis to take up carbon dioxide. 

Other ways to enhance natural CO2 uptake by the landscape include sequestering carbon in the soil, such as with no-till agriculture, and restoring wetlands. There’s also the idea of capturing carbon dioxide – either as a byproduct from a power station or directly from the air – and storing it underground. If CO2 is grabbed from a power station that derives energy from plants, then you have what’s called bioenergy with carbon capture and storage, or BECCS

Even more speculative are the notions of using vast quantities of minerals to react with and take up CO2 (enhanced chemical weathering) and to make the ocean more alkaline and thus enhance its CO2 uptake (a type of marine engineering). 

One little-publicized aspect of the IPCC’s 2018 report on keeping global warming at or near 1.5°C above preindustrial levels is that the panel found it’s too late for emission cuts alone to do the trick. At least some CDR would be needed in all of the IPCC’s projected pathways that yield a solid 2/3 chance of meeting the 1.5°C goal. That said, as the panel noted, the various types of CDR “vary widely in terms of maturity, potentials, costs, risks, co-benefits and trade-offs.” 

As for solar radiation management, it can include uncomplicated fixes such as highly reflective roofs. However, in order to divert enough sunlight to rival the scale of human-produced warming, you’d need a whole lot of roofs. When it comes to global-scale SRM, the most commonly discussed mode is stratospheric aerosol injection: using aircraft to spray mammoth amounts of sulfate aerosols into the atmosphere more than 10 miles above the ground. At these stratospheric heights, the typically placid conditions would allow sulfates to linger for long periods, reflecting and absorbing sunlight. 

Another approach would be to inject ice crystals into cirrus clouds in order to make them thinner and reduce their collective warming impact. Still another option: spraying saltwater from below into marine stratocumulus clouds, which collectively cool the climate, to make them brighter and thus more reflective.

Blocking sunlight would be an impressively nimble but still-dangerous end run around the core problem of too much carbon dioxide in the air. In particular, stratospheric aerosol injection presents huge risks and side effects (see below). So much so that IPCC refused to factor SRM into its 2018 analysis, citing “large uncertainties and knowledge gaps” – and also the fact that SRM wouldn’t fix the acidification now occurring as oceans take up ever more CO2 from the air. 

Could these approaches be scaled up enough to make a real difference?

The idea behind stratospheric aerosol injection is quite straightforward, and it’s already been tested by nature itself. The 1991 eruption of Mount Pinatubo belched enough sulfur dioxide into the stratosphere (around 17 million tons) to cool the planet’s climate by several tenths of a degree Fahrenheit for a couple of years. A series of stratospheric sulfate-injecting flights could in theory tamp down a large part of human-produced warming within a few years. 

Apart from the ethics and risks of geoengineering injection flights, they would be relatively affordable by the standards of other massive global endeavors. One 2020 study estimated that each degree Celsius of avoided warming would cost about $18 billion USD per year.

Because every bit of carbon removal helps, the CDR approach could be more modular than most SRM proposals, adopted nation by nation or region by region. The IPCC report suggests it could be both practical and necessary to use CDR as a supplement to emission cuts. However, the benefits of CDR would likely take longer to achieve than for SRM – after all, growing a forest takes more time than spraying aerosols into the air – and any technology to capture CO2 directly from the atmosphere could be enormously pricey. 

What could possibly go wrong?

Plenty, it turns out, especially with SRM. One reason that human-produced climate change is so insidious is that the greenhouse gases added to the air are trapping heat that would otherwise be escaping to space, around the clock and all around the globe. 

Blocking sunlight, by contrast, can work only where the sun is shining. The overlay of both effects would lead to a host of complications. Even if the planet’s average temperature were brought back to preindustrial levels, for example, that cooling wouldn’t be spread equally across the globe: the poles would still be warmer, yet the tropics would be cooler, than in preindustrial times. Tropical precipitation would drop, and deficient monsoons in Africa and East Asia could become a major risk.

Moreover, starting SRM would be a no-turning-back option until long after major emission cuts had been carried out. Any sudden cessation of SRM would lead to breakneck warming that could imperil many species, among other catastrophic possibilities. Even a gradual SRM ramp down would have its challenges, though the IPCC did conclude in 2021 that a gradual phase-out of SRM combined with emission reductions and CDR could avoid the harms of a rapid cessation.

CDR, as a rule, would be much less prone to dangerous outcomes. However, many of the more complex CDR strategies haven’t yet been tested at scale. There’s also a weird catch: because the planet’s oceans and vegetation have been increasing their absorption of CO2, in line with the growing atmospheric concentration, any attempt to remove CO2 from the air would most likely also lead to a trimming back of the natural absorption rate. Large-scale CDR efforts would have to factor in this natural drag on CO2 removal, which could be as high as 10% of the total removed, according to the IPCC.

Of course, as with any massive attempt at a technological fix, there’s always the chance that unanticipated consequences could spring up.

What’s to keep countries or companies from experimenting with geoengineering?

International law has surprisingly few constraints on geoengineering. The UN Convention on Biological Diversity added non-binding language in 2010 that urged research prior to any large-scale geoengineering efforts that could affect biodiversity. In the wake of an aborted 2007 attempt by the U.S. firm Planktos to dump 100 metric tons of iron near the Galapagos, the London Convention/London Protocol on marine pollution adopted a resolution on marine geoengineering in 2013 that effectively halted commercial attempts at ocean fertilization to absorb CO2.

The increased societal focus since 2018 on the need for major emission cuts may have swung the pendulum away from geoengineering for now. Even university-based research has faced increasing scrutiny. A longstanding attempt by a Harvard-led consortium to learn more about stratospheric aerosol injection on a modest scale – by deploying less than five pounds of aerosols about 13 miles high that would cover the equivalent of about eight city blocks – had an instrument-testing launch in Sweden postponed in 2021 after protests on multiple fronts.

It’s possible that any future comprehensive global attempt to regulate geoengineering could find a home in the 1992 UN Framework Convention on Climate Change, according to a 2019 analysis by the Carnegie Climate Geoengineering Governance Initiative (C2G2). The report also warned that “the ungoverned deployment of these technologies poses potentially critical environmental and geopolitical risks that now demand urgent consideration, before events overtake us.”

What next?

Increasingly sophisticated climate models now are analyzing various geoengineering strategies in new detail, and the IPCC is serving as the de facto global lead on analyzing the findings. Various models now handle both CDR and SRM in more nuanced ways, helping to better quantify time frames and identify both desired and unwanted effects. Thus far the individual  approaches have been modeled mostly in isolation, rather than evaluating how multiple types of geoengineering might interact with each other and with the environment.

Much of the recent modeling was discussed in the Working Group I volume (physical science) from the IPCC’s Sixth Assessment Report, released in August 2021. More analysis will follow in the next two volumes, especially in the findings of Working Group III (mitigation of climate change), to be released in spring 2022.

There’s still no global mechanism parallel to the IPCC for coming up with a comprehensive governance scheme that would oversee any physical experiments or actual attempts at larger-scale geoengineering. Likewise, there is little consensus on the wisdom of geoengineering across nongovernmental organizations, noted the C2G2. And researchers have only begun to grapple with assessing the role of equity in geoengineering – including who would benefit from it the most and who might be harmed the most. 

The ultimate Catch-22 for geoengineering may be the fear, perhaps quite legitimate, that the very research into technology, governance, and infrastructure crucial for any chance of success would make geoengineering itself seem inevitable. For the time being, at least, the world appears to be in a face-off between a sense that geoengineering might someday be needed and a fervent desire to avoid the known, unknown, and unknowable implications of that approach.

Bob Henson

Bob Henson is a meteorologist and journalist based in Boulder, Colorado. He has written on weather and climate for the National Center for Atmospheric Research, Weather Underground, and many freelance...