Humanity has managed to stabilize its carbon emissions, but they have yet to start trending downwards. It looks increasingly probable that we're going to emit enough to commit to at least 1.5° C of warming—and we need to act quickly to avert going past 2° C. This failure to get our emissions in order may force us to consider alternatives such as pulling carbon dioxide out of the air or geoengineering to reduce the amount of incoming sunlight.

Of the two, geoengineering comes with the longest list of unknowns, with a recent report from the National Academies of Science saying, "Scientific understanding of many aspects of solar geoengineering technologies remains limited, including how they could affect weather extremes, agriculture, natural ecosystems, or human health."

So, some Silicon Valley types naturally decided to go ahead and launch a startup company that would offer geoengineering for a fee. The company claims to offer warming offsets despite the considerable unknowns regarding geoengineering. And it's even worse than that sounds; based on an article in MIT Technology Review, the company has already started launching balloons to the stratosphere, despite not being capable of determining whether they're actually deploying their payload.

Engineer the stratosphere?

Geoengineering is generally defined as manipulating the environment in a way that alters the climate. Given that definition, our widespread burning of fossil fuels is a form of geoengineering. But, in the face of our steadily warming climate, most references to geoengineering are now focused on ways of counteracting that warming. While a number of possible techniques have been considered, the most practical approach appears to be lofting reflective particles into the stratosphere to reduce the amount of sunlight Earth receives.

The general concept has already been validated by volcanoes, which can send sulfur dioxide into the stratosphere and trigger cooling in the years following an eruption. For example, the largest eruption of last century (Mount Pinatubo) cooled the planet for roughly three years before the sulfur dioxide it placed in the stratosphere drifted downwards and then came out of the atmosphere in rain.

Sulfur dioxide is cheap, and we have the technology needed to carry it to the stratosphere without the need for an eruption, so that may be an appealing alternative to the many expensive downstream impacts of climate change. The "may" largely comes from the extensive unknowns involved in pursuing it. Everything from plants to solar panels relies on sunlight reaching Earth. And, while we know the approach works, we still don't know the details well enough to assign a specific cooling value for a given amount of sulfur dioxide. That sulfur dioxide also forms sulfuric acid when exposed to water, which can create environmental impacts if deployed at the levels needed to alter the climate. Finally, relying on geoengineering commits us to continue with it for as long as it takes for atmospheric carbon to come back down to manageable levels.

For all those reasons, the scientific community has been very hesitant about the idea. The National Academies report mentioned above suggests that there are so many unknowns that any research we do on geoengineering should be designed so that it doesn't make it easier to go ahead and pursue it. "Deliberate outdoor experiments that involve releasing substances into the atmosphere should be considered only when they can provide critical observations that cannot be provided by laboratory study, modeling, or experiments of opportunity—such as volcanic eruptions," the report's authors concluded. "Outdoor experiments should be subject to appropriate governance including permitting and impact assessments."