Controversial spraying method aims to curb global warming

A fleet of 100 planes making 4,000 worldwide missions per year could help save the world from climate change. Also, it may be relatively cheap. That's the conclusion of a new peer-reviewed study in Environmental Research Letters.

It's the stuff of science fiction. Planes spraying tiny sulphate particulates into the lower stratosphere, around 60,000 feet up. The idea is to help shield the Earth from just enough sunlight to help keep temperatures low.

The researchers examined how practical and costly a hypothetical solar geoengineering project would be beginning 15 years from now. The aim would be to half the temperature increase caused by heat-trapping greenhouse gases.

This method would mimic what large volcanoes do. In 1991, Mount Pinatubo erupted in the Philippines. It was the second largest eruption of the 20th century, according to the United States Geological Survey (USGS).

In total, the eruption injected 20 million tons of sulfur dioxide aerosols into the stratosphere. USGS said the Earth's lower atmosphere temperature dropped by approximately 1-degree Fahrenheit. The effect only lasted a couple of years because the sulfates eventually fell to Earth.

Although controversial, some think that trying to mimic the impacts of a volcano eruption is a viable way to control global warming. This proposed type of climate geoengineering is called stratospheric aerosol injection (SAI). Theoretically if done at scale — and sustained — the impact can be large. The 1-degree temperature drop which accompanied Mount Pinatubo's eruption is equal to about half of the human-caused warming Earth has experienced since the Industrial Revolution began.

Dr. Gernot Wagner from Harvard University is an author of the paper. He said their study shows this type of geoengineering "... would be technically possible strictly from an engineering perspective. It would also be remarkably inexpensive, at an average of around $2 to 2.5 billion per year over the first 15 years."

But to reach that point, the study said an entirely new aircraft needs to be developed. Partly because missions would need to be conducted at nearly double the cruising altitude of commercial airplanes. The study's co-author, Wake Smith explained, "No existing aircraft has the combination of altitude and payload capabilities required."

So, the team investigated what it would cost to develop an aircraft they dub the SAI Lofter (SAIL). They say its fuselage would have a stubby design and the wing area — as well as the thrust — would need to be twice as large. In total, the team estimates the development cost for the airframe to be $2 billion and $350 million to modify existing engines.

In their hypothetical plan, the fleet would start with eight planes in the first year and rise to just under 100 within 15 years. In year one, there would be 4,000 missions, increasing to just over 60,000 per year by year 15. As you can see, this would need to be a sustained and escalating effort.

As one may imagine, a concept like this comes with a lot of controversy. Like treating a fever with aspirin, this type of engineering only treats the symptoms, it does not fix the root cause of the warming: Escalating levels of heat trapping greenhouse gases produced by the burning of fossil fuels.

The American Meteorological Society (AMS) expressed concerns that the possibility of seemingly quick and inexpensive fixes will distract the public and policymakers from addressing the underlying problems and developing adaptation strategies. And if for whatever reason the aerosol missions stopped, within a few years the temperatures would shoot up at breakneck pace. A pace that would likely be too fast for humanity to adjust.

The AMS official policy statement regarding this type of geoengineering begins with a warning, "Reflecting sunlight would likely reduce Earth's average temperature but could also change global circulation patterns with potentially serious consequences such as changing storm tracks and precipitation patterns."

In other words, the atmosphere is complex. Any band-aid fix is bound to have unintended consequences and possibly cause a new set of problems. The AMS goes on to say results of reflecting sunlight "would almost certainly not be the same for all nations and peoples, thus raising legal, ethical, diplomatic and national security concerns." One region may become a desert, while others become flooded out.

And if we learn to control SAI to tailor a favorable result, there's the concern it may be used for the disproportionate benefit of one nation over another. In a 2017 study in the publication Nature Communications, the authors warn their work "... reemphasizes the perils of unilateral geoengineering, which might prove attractive to individual actors due to a greater controllability of local climate responses, but with inherent additional risk elsewhere."

But perhaps the greatest reason to be skeptical of aerosol solar sunlight management is that it's not a silver bullet. As carbon dioxide continues to increase, the oceans are becoming increasingly acidic. According to NOAA, ocean acidification can cascade through the ocean food chain, reducing the ability of shell fish and reef-building corals to produce their skeletons. Injecting aerosols into the stratosphere simply limits sunshine, it does not tackle the underlying carbon dioxide build up. The ocean would continue to acidify.

Despite the potential drawbacks, the AMS does recognize — even with aggressive mitigation — we can't avoid some dangerous consequences of climate change already baked into the system. Plus, the scale of human adaptation is limited. Therefore, they urge caution and continued research.

The AMS policy statement closes with: "Geoengineering will not substitute for either aggressive mitigation or proactive adaptation, but it could contribute to a comprehensive risk management strategy to slow climate change and alleviate some of its negative impacts. The potential to help society cope with climate change and the risks of adverse consequences imply a need for adequate research, appropriate regulation and transparent deliberation."