Sudden cut in pollution in 2020 meant less shade from sun and was ‘substantial’ factor in record surface temperatures in 2023, study finds
The slashing of pollution from shipping in 2020 led to a big “termination shock” that is estimated have pushed the rate of global heating to double the long-term average, according to research.
Until 2020, global shipping used dirty, high-sulphur fuels that produced air pollution. The pollution particles blocked sunlight and helped form more clouds, thereby curbing global heating. But new regulations at the start of 2020 slashed the sulphur content of fuels by more than 80%.
Researchers had been testing a sprayer that could one day be used to push a salty mist skyward, cooling the Earth. Officials stopped the work, citing health questions.
The public debate over whether we should consider intentionally altering the climate system is heating up, as the dangers of climate instability rise and more groups look to study technologies that could cool the planet.
Such interventions, commonly known as solar geoengineering, may include releasing sulfur dioxide in the stratosphere to cast away more sunlight, or spraying salt particles along coastlines to create denser, more reflective marine clouds.
The growing interest in studying the potential of these tools, particularly through small-scale outdoor experiments, has triggered corresponding calls to shut down the research field, or at least to restrict it more tightly. But such rules would halt or hinder scientific exploration of technologies that could save lives and ease suffering as global warming accelerates—and they might also be far harder to define and implement than their proponents appreciate.
Earlier this month, Tennessee’s governor signed into lawa bill banning the “intentional injection, release, or dispersion” of chemicals into the atmosphere for the “express purpose of affecting temperature, weather, or the intensity of the sunlight.” The legislation seems to have been primarily motivated by debunked conspiracy theories about chemtrails.
Meanwhile, at the March meeting of the United Nations Environmental Agency, a bloc of African nations called for a resolution that would establish a moratorium, if not a ban, on all geoengineering activities, including outdoor tests. Mexican officials have also proposed restrictions on experiments within their boundaries.
To be clear, I’m not a disinterested observer but a climate researcher focused on solar geoengineering and coordinating international modeling studies on the issue. As I stated in a letter I coauthored last year, I believe that it’s important to conduct more research on these technologies because it might significantly reduce certain climatic risks.
This doesn’t mean I support unilateral efforts today, or forging ahead in this space without broader societal engagement and consent. But some of these proposed restrictions on solar geoengineering leave vague what would constitute an acceptable, “small” test as opposed to an unacceptable “intervention.” Such vagueness is problematic, and its potential consequences would have far more reach than the well-intentioned proponents of regulation might wish for.
Consider the “intentional” standard of the Tennessee bill. While it is true that the intentionality of any such effort matters, defining it is tough. If knowing that an activity will affect the atmosphere is enough for it to be considered geoengineering, even driving a car—since you know its emissions warm up the climate—could fall under the banner. Or, to pick an example operating on a much larger scale, a utility might run afoul of the bill, since operating a power plant produces both carbon dioxide that warms up the planet and sulfur dioxide pollution that can exert a cooling effect.
Indeed, a single coal-fired plant can pump out more than 40,000 tons of the latter gas a year, dwarfing the few kilograms proposed for some stratospheric experiments. That includes the Harvard project recently scrapped in light of concerns from environmental and Indigenous groups.
Of course, one might say that in all those other cases, the climate-altering impact of emissions is only a side effect of another activity (going somewhere, producing energy, having fun). But then, outdoor tests of solar geoengineering can be framed as efforts to gain further knowledge for societal or scientific benefit. More stringent regulations suggest that, of all intentional activities, it is those focused on knowledge-seeking that need to be subjected to the highest scrutiny—while joyrides, international flights, or bitcoin mining are all fine.
There could be similar challenges even with more modest proposals to require greater transparency around geoengineering research. In a submission to federal officials in March, a group of scholars suggested, among other sensible updates, that any group proposing to conduct outdoor research on weather modification anywhere in the world should have to notify the National Oceanic and Atmospheric Administration in advance.
But creating a standard that would require notifications from anyone, anywhere who “foreseeably or intentionally seeks to cause effects within the United States” could be taken to mean that nations can’t modify any kind of emissions (or convert forests to farmland) before consulting with other countries. For instance, in 2020, the International Maritime Organization introduced rules that cut sulfate emissions from the shipping sector by more than 80%, all at once. The benefits for air quality and human health are pretty clear, but research also suggested that the change would unmask additional global warming, because such pollution can reflect away sunlight either directly or by producing clouds. Would this qualify?
It is worth noting that both those clamoring for more regulations and those bristling to just go out and “do something” claim to have, as their guiding principle, a genuine concern for the climate and human welfare. But again, this does not necessarily justify a “Ban first—ask questions later” approach, just as it doesn’t justify “Do something first—ask permission later.”
Those demanding bans are right in saying that there are risks in geoengineering. Those include potential side effects in certain parts of the world—possibilities that need to be better studied—as well as vexing questions about how the technology could be fairly and responsibly governed in a fractured world that’s full of competing interests.
The more recent entrance of venture-backed companies into the field, selling dubious cooling credits or playing up their “proprietary particles,” certainly isn’t helping its reputation with a public that’s rightly wary of how profit motives could influence the use of technologies with the power to alter the entire planet’s climate. Nor is the risk that rogue actors will take it upon themselves to carry out these sorts of interventions.
But burdensome regulation isn’t guaranteed to deter bad actors. If anything, they’ll just go work in the shadows. It is, however, a surefire way to discourage responsible researchers from engaging in the field.
All those concerned about “meddling with the climate” should be in favor of open, public, science-informed strategies to talk more, not less, about geoengineering, and to foster transparent research across disciplines. And yes, this will include not just “harmless” modeling studies but also outdoor tests to understand the feasibility of such approaches and narrow down uncertainties. There’s really no way around that.
In environmental sciences, tests involving dispersing substances are already performed for manyotherreasons, as long as they’re deemed safe by some reasonable standard. Similar experiments aimed at better understanding solar geoengineering should not be treated differently just because some people (but certainly not all of them) object on moral or environmental grounds. In fact, we should forcefully defend such experiments both because freedom of research is a worthy principle and because more information leads to better decision-making.
At the same time, scientists can’t ignore all the concerns and fears of the general public. We need to build more trust around solar geoengineering research and confidence in researchers. And we must encourage people to consider the issue from multipleperspectives and in relation to the rising risks of climate change.
This can be done, in part, through thoughtful scientific oversight efforts that aim to steer research toward beneficial outcomes by fostering transparency, international collaborations, and public engagement without imposing excessive burdens and blanket prohibitions.
Yes, this issue is complicated. Solar geoengineering may present risks and unknowns, and it raises profound, sometimes uncomfortable questions about humanity’s role in nature.
But we also know for sure that we are the cause of climate change—and that it is exacerbating the dangers of heat waves, wildfires, flooding, famines, and storms that will inflict human suffering on staggering scales. If there are possible interventions that could limit that death and destruction, we have an obligation to evaluate them carefully, and to weigh any trade-offs with open and informed minds.
Daniele Visioni is a climate scientist and assistant professor at Cornell University.
This article is from The Spark, MIT Technology Review’s weekly climate newsletter. To receive it in your inbox every Wednesday, sign up here.
Usually when we talk about climate change, the focus is squarely on the role that greenhouse-gas emissions play in driving up global temperatures, and rightly so. But another important, less-known phenomenon is also heating up the planet: reductions in other types of pollution.
In particular, the world’s power plants, factories, and ships are pumping much less sulfur dioxide into the air, thanks to an increasingly strictset of globalpollution regulations. Sulfur dioxide creates aerosol particles in the atmosphere that can directly reflect sunlight back into space or act as the “condensation nuclei” around which cloud droplets form. More or thicker clouds, in turn, also cast away more sunlight. So when we clean up pollution, we also ease this cooling effect.
Before we go any further, let me stress: cutting air pollution is smart public policy that has unequivocally saved lives and prevented terrible suffering.
The fine particulate matter produced by burning coal, gas, wood, and other biomatter is responsible for millions of premature deaths every year through cardiovascular disease, respiratory illnesses, and various forms of cancer, studies consistently show. Sulfur dioxide causes asthma and other respiratory problems, contributes to acid rain, and depletes the protective ozone layer.
Air pollution is killing millions of people per year.
It would be a vastly better outcome for humanity if we could cut it rapidly, despite the resulting warming impact on the climate. https://t.co/JYWAWwVtiG
But as the world rapidly warms, it’s critical to understand the impact of pollution-fighting regulations on the global thermostat as well. Scientists have baked the drop-off of this cooling effect into net warming projections for the coming decades, but they’re also striving to obtain a clearer picture of just how big a role declining pollution will play.
A new study found that reductions in emissions of sulfur dioxide and other pollutants are responsible for about 38%, as a middle estimate, of the increased “radiative forcing” observed on the planet between 2001 and 2019.
An increase in radiative forcing means that more energy is entering the atmosphere than leaving it, as Kerry Emanuel, a professor of atmospheric science at MIT, lays out in a handy explainer here. As that balance has shifted in recent decades, the difference has been absorbed by the oceans and atmosphere, which is what is warming up the planet.
The remainder of the increase is “mainly” attributable to continued rising emissions of heat-trapping greenhouse gases, says Øivind Hodnebrog, a researcher at the Center for International Climate and Environment Research in Norway and lead author of the paper, which relied on climate models, sea-surface temperature readings, and satellite observations.
The study underscores the fact that as carbon dioxide, methane, and other gases continue to drive up temperatures, parallel reductions in air pollution are revealing more of that additional warming, says Zeke Hausfather, a scientist at the independent research organization Berkeley Earth. And it’s happening at a point when, by most accounts, global warming is about to begin accelerating or has already started to do so. (There’s ongoing debate over whether researchers can yet detect that acceleration and whether the world is now warming faster than researchers had expected.)
Because of the cutoff date, the study did not capture a more recent contributor to these trends. Starting in 2020, under new regulations from the International Maritime Organization, commercial shipping vessels have also had to steeply reduce the sulfur content in fuels. Studies have already detected a decrease in the formation of “ship tracks,” or the lines of clouds that often form above busy shipping routes.
Again, this is a good thing in the most important way: maritime pollution alone is responsible for tens of thousands of early deaths every year. But even so, I have seen and heard of suggestions that perhaps we should slow down or alter the implementation of some of these pollution policies, given the declining cooling effect.
A 2013 study explored one way to potentially balance the harms and benefits. The researchers simulated a scenario in which the maritime industry would be required to use very low-sulfur fuels around coastlines, where the pollution has the biggest effect on mortality and health. But then the vessels would double the fuel’s sulfur content when crossing the open ocean.
In that hypothetical world, the cooling effect was a bit stronger and premature deaths declined by 69% with respect to figures at the time, delivering a considerable public health improvement. But notably, under a scenario in which low-sulfur fuels were required across the board, mortality declined by 96%, a difference of more than 13,000 preventable deaths every year.
Now that the rules are in place and the industry is running on low-sulfur fuels, intentionally reintroducing pollution over the oceans would be a far more controversial matter.
While society basically accepted for well over a century that ships were inadvertently emitting sulfur dioxide into the air, flipping those emissions back on for the purpose of easing global warming would amount to a form of solar geoengineering, a deliberate effort to tweak the climate system.
Many think such planetary interventions are far too powerful and unpredictable for us to muck around with. And to be sure, this particular approach would be one of the more ineffective, dangerous, and expensive ways to carry out solar geoengineering, if the world ever decided it should be done at all. The far more commonly studied concept is emitting sulfur dioxide high in the stratosphere, where it would persist for longer and, as a bonus, not be inhaled by humans.
On an episode of the Energy vs. Climate podcast last fall, David Keith, a professor at the University of Chicago who has closely studied the topic, said that it may be possible to slowly implement solar geoengineering in the stratosphere as a means of balancing out the reduced cooling occurring from sulfur dioxide emissions in the troposphere.
“The kind of solar geoengineering ideas that people are talking about seriously would be a thin wedge that would, for example, start replacing what was happening with the added warming we have from unmasking the aerosol cooling from shipping,” he said.
Positioning the use of solar geoengineering as a means of merely replacing a cruder form that the world was shutting down offers a somewhat different mental framing for the concept—though certainly not one that would address all the deep concerns and fierce criticisms.
Now read the rest of The Spark
Read more from MIT Technology Review’s archive:
Back in 2018, I wrote a piece about the maritime rules that were then in the works and the likelihood that they would fuel additional global warming, noting that we were “about to kill a massive, unintentional” experiment in solar geoengineering.
Another thing
Speaking of the concerns about solar geoengineering, late last week I published a deep dive into Harvard’s unsuccessful, decade-long effort to launch a high-altitude balloon to conduct a tiny experiment in the stratosphere. I asked a handful of people who were involved in the project or followed it closely for their insights into what unfolded, the lessons that can be drawn from the episode—and their thoughts on what it means for geoengineering research moving forward.
Keeping up with Climate
Yup, as the industry predicted (and common sense would suggest), this week’s solar eclipse dramatically cut solar power production across North America. But for the most part, grid operators were able to manage their systems smoothly, minus a few price spikes, thanks in part to a steady buildout of battery banks and the availability of other sources like natural gas and hydropower. (Heatmap)
There’s been a pile-up of bad news for Tesla in recent days. First, the company badly missed analyst expectations for vehicle deliveries during the first quarter. Then, Reuters reported that the EV giant has canceled plans for a low-cost, mass-market car. That may have something to do with the move to “prioritize the development of a robotaxi,” which the Wall Street Journal then wrote about. Over on X, Elon Musk denied the Reuters story, sort of—posting that “Reuters is lying (again).” But there’s a growing sense that his transformation into a “far-right activist” is exacting an increasingly high cost on his personal and business brands. (Wall Street Journal)
In a landmark ruling this week, the European Court of Human Rights determined that by not taking adequate steps to address the dangers of climate change, including increasingly severe heat waves that put the elderly at particular risk, Switzerland had violated the human rights of a group of older Swiss women who had brought a case against the country. Legal experts say the ruling creates a precedent that could unleash many similar cases across Europe. (The Guardian)
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