• Scientists believe tanker planes could spray sulfur dioxide aerosol in the upper atmosphere over Earth’s poles to help them refreeze.
  • The temperatures of Earth’s poles could be cooled by about 2 degrees Celsius, the study says, costing about $11 billion annually.
  • But this proposal comes with risks, and it’s not a final solution to climate change—just a Band-Aid.
Faced with the growing threat of a warming world, scientists continue to concoct new concepts for stabilizing Earth’s temperatures. The latest entrant is a plan published in Environmental Research Communications that calls for high-altitude tanker planes to spray aerosol particles over Earth’s subpolar regions. Once in place, they would help deflect incoming solar radiation.

These stratospheric aerosol injections (SAI), as the study calls them, offer “a prospective climate intervention that would seek to abate global warming by slightly increasing the reflectiveness of the Earth’s upper atmosphere.”

The concept of SAI is nothing new, but this proposal calls for aerosols to be targeted at the places where melting ice can most dramatically lead to sea-level rise across the entire planet. With greater warming at the Earth’s poles over time, the study calls for “injections of sulfur dioxide (SO2)… which will oxidize into sulfuric acid (H2SO4) and coagulate into liquid supercooled aerosols after a month in the stratosphere.”

The expectation is that this could lower temperatures at the poles by 2 degrees Celsius. That’s enough cooling to help refreeze the polar extremes and bring average temperatures back in line with what they were before the industrial era. The harmful effects of climate change, such as extreme weather and massive flooding, could possibly be slowed.

“Subpolar deployment would quickly envelope the poles as well and could arrest or reverse ice and permafrost melt at high latitudes. This would yield global benefit by retarding sea level rise,” writes lead author Wake Smith in the study. Smith is a lecturer at Yale College and senior fellow at the Harvard Kennedy School. Unlike previous aerosol plans, which would encompass the entire Earth, the one Smith proposes focuses just on latitudes beyond 60 degrees north and south (think north of Anchorage, Alaska, and south of Patagonia).

Related video: DNA | Global Warming causing excess heat, rain and cold on earth

DNA | Global Warming causing excess heat, rain and cold on earth

“Because the tropopause is considerably lower at high latitudes, aerosols or their precursors would not need to be lofted as high, reducing the engineering challenges relative to a global deployment,” Smith writes.

In order to work, the study determined the aerosol would need to be released at a minimum altitude of 13 kilometers, or 43,000 feet above Earth’s surface—a lower altitude than required elsewhere in the world by other SAI plans. To offset the greatest amount of solar radiation, a specialized fleet of around 125 aircraft would work during the extra-long spring and early summer days of one hemisphere before traveling to the other pole to perform the same tasks.

There’s just one hitch: releasing the necessary payload of aerosol at that altitude is beyond the capability of any current tanker aircraft. The researchers narrowed the potential aircraft down to the Boeing KC-135R, the Boeing KC-46A Pegasus, the Airbus A330 MRTT, the Boeing KC-10, and the Airbus A340F—but all would still need to be modified for the mission. Ideally, a purpose-built “lofter” plane designed by the researchers would be used, called the SAIL-43K, which is based on the SAIL-01 design.

By focusing effort on the poles, the plan would reduce costs radically over other SAI plans, according to the study, with an annual price tag of just $11 billion. That’s significantly less than the projected annual cost of $36 billion to lower the surface temperature by the same degree over the entire Earth instead.

Of course, a less generous disbursement of aerosol also helps with the obvious concerns about dumping particles into the atmosphere. Will the aerosols have an unexpected ecological impact on Earth, if the delivery could lead to further depletion of the ozone? “There is widespread and sensible trepidation about deploying aerosols to cool the planet,” Smith says, “but if the risk-benefit equation were to pay off anywhere, it would be at the poles.” This plan would decrease the risks, with less than one percent of the human population living within those areas.

A polar bear swims beside a melting iceberg near the Arctic Circle in Hudson Bay, Canada.

A polar bear swims beside a melting iceberg near the Arctic Circle in Hudson Bay, Canada.© Paul Souders – Getty Images

The concept of stratospheric aerosol injections comes with a host of unknowns. Along with devising the best aerosol to use—the most recent study sticks with a commonly considered sulfur-based aerosols—scientists aren’t certain of the best delivery method or how long the aerosols will provide benefit once released. Could we eventually see giant SO2 rockets that release aerosols over other continents to slow down global warming, like in the futuristic geoengineering-themed novel Termination Shock by Neil Stephenson? Maybe not, but this could be the first step.

Plus, this SAI proposal is by no means the ultimate solution to climate change. “Game changing though this could be in a rapidly warming world, stratospheric aerosol injections merely treat a symptom of climate change but not the underlying disease,” Smith says. “It’s aspirin, not penicillin. It’s not a substitute for decarbonization.”

While it’s still preliminary in concept, the study authors believe the idea warrants exploration. “Given its apparent feasibility and low cost,” they write, “this scenario deserves further attention.”


A fleet of roughly 125 tankers could return subpolar regions close to pre-industrial temperatures.
Sade Agard

Plane flying over the poles.Paul Campbell/iStock

Scientists presented a controversial plan whereby high-flying jets similar to a modified American KC-135R could spray microscopic aerosol particles into the atmosphere to refreeze the North and South Poles. The recently published research in IOP’s Environmental Research Communications suggests that the method would be feasible and fairly cheap.

The plan, which falls under a controversial climate intervention technology called Stratospheric aerosol injection (SAI), would target only subpolar regions- instead of having a global intervention that most other SIA operations assume, highlights the paper.

The SAIL-43K: A step up from ‘hand-me-down’ military air-to-air refueling tankers

Scientists want to spray sulphur dioxide into the atmosphere to refreeze the poles
The SAIL-43K- an efficient candidate for the subpolar mission

The researchers argue that ‘hand-me-down’ (reused) military air-to-air refueling tankers such as the aged KC-135 and the A330 MMRT don’t have enough payload at the required altitudes- even when modified. They, therefore, submit the SAIL-43K as an efficient candidate for the subpolar mission.

A fleet of 125 of these SAIL-43K tankers would release a cloud of microscopic sulfur dioxide particles, which at an altitude of 43,000 feet (13 kilometers) and latitude of 60 degrees north and south- could loft a payload sufficient to cool the regions by two degrees per year. Areas such as Anchorage, southern Alaska, and the southern tip of Patagonia- could return close to their pre-industrial average temperatures.

Upon slowly drifting towards the poles by ferrying on high-altitude winds, the particles would slightly shade the Earth’s surface beneath.

An operation would be equivalent to more than two days of global commercial air traffic

Still, the paper highlights that such an operation would be equivalent to more than two days of global commercial air traffic in 2021, or about two-thirds of the annual flights departing New York’s Kennedy Airport.

The SIA operation would tap into the healthy number of pre-existing commercial airfields in the Northern Hemisphere that could serve as operational bases for a polar SAI operation. Anchorage, for example, has three runways longer than 10,600 feet, and whilst located at 61.2°N latitude—this would be close enough for the purpose.

For the southern hemisphere, it’s a little more complicated. 60 degrees of the south pole touches nowhere on land and is inhabitable. The closest significant airfields are in Chile and Argentina at the southern tip of Patagonia. As sub-optimal bases here may be relative to the 60°S target, the researchers reveal they will have to serve.

Additionally, the paper highlights that the ground infrastructure for any pre-existing base would need to be greatly enhanced to accommodate the program.

Costs are less than one-third of alternative climate responses aiming to cool to the same 2°C extent

According to the paper, the costs of the subpolar SIA program are estimated at 11 billion dollars annually. This is less than one-third the cost of cooling the entire planet by the same 2°C extent proposed by other climate responses such as mitigation, adaptation, or carbon capture and sequestration.

Nevertheless, to compare the newly proposed SIA operation with the alternatives mentioned above would be like comparing apples and oranges, admits the researchers.

A subpolar mission using highly controversial technology

Whilst subpolar in nature, using the airspace of no more than a dozen countries, it still stands that the program is controversial.

The governance and legitimacy challenges that would confront such a program include not knowing the unintended consequences of releasing sulfur particles into the atmosphere, such as reducing crop yields.

A short while ago, a similar SIA plan in Sweden by a Harvard research project had to be abandoned due to environmental protests. In this case, scientists proposed using balloons to release the particle and further consolidate the need for international agreement.

Still, the researchers behind the newly proposed program argue that their SAI program would entail deployment directly overhead of far less than 1% of the world’s population and nearly none of its agriculture. Given its apparent feasibility and low cost, this scenario deserves further attention.

Regardless of the outcome, the current study still provides a boost in understanding the costs, benefits, and risks of such climate intervention measures at latitudes of thousands of feet. Who knows, perhaps such tools could prove helpful in saving the cryosphere near the poles and delaying global sea level rise.

After all, the poles are warming several times faster than the global average. It was only last year that we saw record-breaking heatwaves reported in both the Arctic and the Antarctic.

Study abstract:

Stratospheric aerosol injection (SAI) is a prospective climate intervention technology that would seek to abate climate change by deflecting back into space a small fraction of the incoming solar radiation. While most consideration given to SAI assumes a global intervention, this paper considers an alternative scenario whereby SAI might be deployed only in the subpolar regions. Subpolar deployment would quickly envelope the poles as well and could arrest or reverse ice and permafrost melt at high latitudes. This would yield global benefit by retarding sea level rise. Given that effective SAI deployment could be achieved at much lower altitudes in these regions than would be required in the tropics, it is commonly assumed that subpolar deployment would present fewer aeronautical challenges. An SAI deployment intended to reduce average surface temperatures in both the Arctic and Antarctic regions by 2 °C is deemed here to be feasible at relatively low cost with conventional technologies. However, we do not find that such a deployment could be undertaken with a small fleet of pre-existing aircraft, nor that relegating such a program to these sparsely populated regions would obviate the myriad governance challenges that would confront any such deployment. Nevertheless, given its feasibility and potential global benefit, the prospect of subpolar-focused SAI warrants greater attention.


Sulphate pollution killed hundreds of thousands of Africans. A plan to use sulphur to fight climate change risks the same

Crutzen is, as you would expect, a brilliant man. He was one of the atmospheric chemists who worked out how high-level ozone is formed and destroyed. He knows more than almost anyone about the impacts of pollutants in the atmosphere. This is what makes his omission so odd.

This month, he published an essay in the journal Climatic Change. He argues that the world’s response to climate change has so far been “grossly disappointing”. Stabilising carbon dioxide concentrations in the atmosphere, he asserts, requires a global reduction in emissions of between 60% and 80%. But at the moment “this looks like a pious wish”. So, he proposes, we must start considering the alternatives, by which he means re-engineering the atmosphere in order to cool the earth.

He suggests we use either giant guns or balloons to inject sulphur into the stratosphere, 10km or more above the surface of the earth. Sulphur dioxide at that height turns into tiny particles – or aerosols – of sulphate. These reflect sunlight back into space, counteracting the warming caused by manmade climate change.

One of the crueller paradoxes of climate change is that it is being accelerated by reducing certain kinds of pollution. Filthy factories cause acid rain and ill health, but they also help to shield us from the sun, by filling the air with particles. As we have started to clean some of them up, we have exposed ourselves to more solar radiation. One model suggests that a complete removal of these pollutants from the atmosphere could increase the world’s temperature by 0.8C.

The virtue of Crutzen’s scheme is that sulphate particles released so far above the surface of the earth stay airborne for much longer than they do at lower altitudes. In order to compensate for a doubling of carbon dioxide concentrations (which could happen this century), he calculates that we would need to fire some 5m tonnes of sulphur into the stratosphere every year. This corresponds to roughly 10% of the sulphate currently entering the atmosphere.

Crutzen recognises that there are problems. The sulphate particles would slightly reduce the thickness of the ozone layer. They would cause some whitening of the sky. Most dangerously, his scheme could be used by governments to help justify their failure to cut carbon emissions: if the atmosphere could one day be fixed by some heavy artillery and a few technicians, why bother to impose unpopular policies?

His paper has already caused plenty of controversy. Other scientists have pointed out that even if rising carbon dioxide levels did not cause global warming, they would still be an ecological disaster. For example, one study shows that as the gas dissolves in seawater, by 2050 the oceans could become too acid for shells to form, obliterating much of the plankton on which the marine ecosystem depends. In Crutzen’s scheme, the carbon dioxide levels are not diminished.

It would also be necessary to keep firing sulphur into the sky for hundreds of years. The scheme would be extremely expensive, so it is hard to imagine that governments would sustain it through all the economic and political crises likely to take place in that time. But what I find puzzling is this: that by far the most damaging impact of sulphate pollution hasn’t even been mentioned – by him or, as far as I can discover, any of his critics.

In 2002 the Journal of Climate published an astonishing proposition: that the great droughts which had devastated the Sahel region of Africa had been caused in part by sulphate pollution in Europe and North America. Our smoke, the paper suggested, was partly responsible for the famines that killed hundreds of thousands of people in the 1970s and 1980s.

By reducing the size of the droplets in clouds, thereby making them more reflective, the sulphate particles lowered the temperature of the sea’s surface in the northern hemisphere. The result was to shift the intertropical convergence zone southwards. This zone is an area close to the equator in which moist air rises and condenses into rain. The Sahel, which covers countries such as Ethiopia, Sudan, Chad, Niger, Burkina Faso and Senegal, is at the northern limits of the zone. As the rain belt was pushed south, those countries dried up. As a result of the clean air acts, between 1970 and 1996 sulphur emissions in the US fell by 39%. This appears to have helped the North Atlantic to warm, allowing the rains to return to the Sahel in the 1990s.

Since then, several studies – published in the Proceedings of the National Academy of Sciences, Geophysical Research Letters and the Journal of Geophysical Research – have confirmed these findings. They show that the 40% reduction in rainfall in the Sahel, which has “few if any parallels in the 20th-century record anywhere on Earth”, is explicable only when natural variations are assisted by sulphate aerosols. We killed those people.

I cannot say whether or not Crutzen’s scheme would have a similar outcome. It is true that he proposes to use less sulphur than the industrialised nations pumped into the atmosphere, but does this matter if the reflective effect is just as great? Another paper I have read lists seven indirect impacts of aerosols on the climate system. Which, if any, will be dominant? What will their effects on rainfall be?

Crutzen suggests that in order to keep the particles airborne for as long as possible they should be released “near the tropical upward branch of the stratospheric circulation system”. Does this mean that they will not be evenly distributed around the world? If so, will they shift weather systems around as our uneven patterns of pollution have done? I don’t know the answers, but I am staggered by the fact that the questions are not even being asked.

I am not suggesting that they have been deliberately overlooked. It seems more likely that they have been forgotten for a familiar reason: that this disaster took place in Africa. Would we have neglected them if the famines had happened in Europe? The story of industrialisation is like The Picture of Dorian Gray. While the rich nations have enjoyed perennial youth, the cost of their debaucheries – slavery, theft, colonialism, sulphur pollution, climate change – is visited on another continent, where the forgotten picture becomes ever uglier.

The only responsible way to tackle climate change is to reduce the amount of climate-changing gases we emit. To make this possible, we must suppress the political and economic costs of the necessary cut. I think I have shown how this can be done – you will have to judge for yourself when my book is published. But what is surely clear is that there is no uncomplicated short cut. By re-engineering the planet’s systems we could risk invoking as great a catastrophe as the one we are trying to prevent.

· George Monbiot’s book Heat: How to Stop the Planet Burning is published next month by Penguin



Scientist: Inject Sulfur into Air to Battle Global Warming

The June 12, 1991 eruption at Mount Pinatubo taken from the east side of Clark Air Base. (Image credit: USGS/Dave Harlow)

One way to curb global warming is to purposely shoot sulfur into the atmosphere, a scientist suggested today.

The burning of fossil fuels releases carbon dioxide, a greenhouse gas, into the atmosphere. It also releases sulfur that cools the planet by reflecting solar radiation away from Earth.

Most researchers say the warming effect has been winning in recent decades.

Injecting sulfur into the second atmospheric layer closest to Earth would reflect more sunlight back to space and offset greenhouse gas warming, according to Nobel laureate Paul Crutzen from the Max Planck Institute for Chemistry in Germany and the Scripps Institution of Oceanography, University of California at San Diego.

Crutzen suggests carrying sulfur into the atmosphere via balloons and using artillery guns to release it, where the particles would stay for up to two years. The results could be seen in six months.

Nature does something like this naturally.

When Mount Pinatubo erupted in the Philippines in1991, millions of tons of sulfur was injected into the atmosphere, enhancing reflectivity and cooling the Earth’s surface by an average of 0.9 degrees Fahrenheit in the year following the eruption.

“Given the grossly disappointing international political response to the required greenhouse gas emissions, … research on the feasibility and environmental consequences of climate engineering of the kind presented in this paper, which might need to be deployed in future, should not be tabooed,” Crutzen said.

This proposal is detailed in the August issue of the journal Climatic Change.

Other Offbeat Solutions

More to Explore


Sulfur Dioxide Effects on Health

The Halema’uma’u plume in Kilauea Crater at Hawai’i Volcanoes NP contains extremely high levels of sulfur dioxide, about 500-1,000 tones/day. The NPS sulfur dioxide advisory program alerts the public and park staff if air quality conditions reach unhealthy levels.  Visitors viewing the plume coming off lava lak Halema’uma’u.