Earth is warming, and much of the uptick on our planetary thermostat is caused by human activity. Though many leaders persist in denying this reality, intense natural disasters and accelerated extinction rates are canaries in the coal mine for anthropogenic climate change, among many other worrisome trends.
Ideally, the response to this clear threat to civilization would be to reduce greenhouse gas emissions from the consumption of fossil fuels, which is the major culprit behind human-driven climate change. That's why world leaders convened in Paris to hash out a gameplan for capping the rise in global mean temperature rise to 2°C above the preindustrial average. But President Donald Trump's recent decision to pull the United States out of the Paris Agreement, signed in 2016, underscores a growing demand for a "plan b" to confront climate change, in case nations fail to get their act together in time to prevent catastrophic temperature increases.
One controversial solution is climate geoengineering, the notion that we could halt or reverse the warming of Earth with technological intervention. These concepts have historically fallen into two camps: Solar radiation management and greenhouse gas removal.
The solar approach aims to boost Earth's albedo, or reflectivity, by introducing reflective aerosols to stratosphere (as one example). These compounds would bounce sunlight back into space, thereby cooling the planet, almost like an Earth-scale version of painting your roof white. Greenhouse gas removal, meanwhile, would focus on scrubbing warming gases, especially carbon dioxide, from the atmosphere using a range of techniques.
The unknown side effects of humans tampering with complex planetary systems demands a cautious approach, lest our good intentions lead us to some kind of Snowpiercer-style dystopia. But if we do nothing to rein in our ongoing accidental geoengineering of the climate, we may well end up in a Mad Max dystopia, so it's worth considering all our options. This includes a "cocktail" combination of several approaches, an idea outlined by a Geophysical Research Letters study published in July.
"We could do SAI [stratospheric sulfate aerosol increase] and a slow ramp up of CDR [carbon dioxide removal]," study co-author Govindasamy Bala, a climate scientist at the Indian Institute of Science, told me in an email.
"Once CDR pulls out enough carbon dioxide from the atmosphere, SAI could be ramped down," he said. "This is one plausible scenario of climate engineering."
Another scenario explored in Bala's study melds together solar radiation management with the concept of cirrus cloud thinning (CCT), which has recently become a flashy new kid on the climate intervention block.
The CCT method, like solar radiation management, involves the careful release of compounds into the atmosphere to cool the planet. But instead of blocking shortwave radiation from outer space, this technique aims to figuratively throw open a window in the skies for longwave radiation to escape Earth.
Cirrus clouds, which are those wispy rippled structures that form at high altitudes over 20,000 feet, are something of a nephological anomaly. Lower thick clouds tend to reflect more sunlight (shortwave radiation) than they absorb heat (longwave radiation), leading to a cooling effect, but cirrus clouds do the opposite.
As a result, manually tweaking these clouds so that they dissipate faster could allow more heat to leave Earth, without throwing off the water cycle to the extreme projected with traditional geoengineering methods. This thinning could be theoretically accomplished by seeding cirrus clouds with particles that disrupt ice crystal formation (nucleation), leading to reduced cirrus cloud cover.
"With the caveat that there are still significant uncertainties associated with ice nucleation in cirrus clouds and its representation in climate models, cirrus seeding appears to represent a powerful [climate engineering model] with reduced side effects," concluded a team led by Trude Storelvmo, a climate scientist at Yale University, in a 2013 Philosophical Transactions of the Royal Society study that modeled the impact of cirrus cloud seeding.
Follow-up studies have warned that the CCT strategy could backfire to create more cirrus clouds, or that it might have no impact at all. Given the persistent mysteries of cirrus cloud formation and dynamics, a lot more research needs to be done before it can be considered a viable solution to our warming planet.
To that point, some scientists have fretted that the debate over geoengineering is inherently damaging because it attracts focus away from climate change mitigation strategies with its allure of an easy out. Ultimately, the best case scenario is that humans change our current unsustainable behavior swiftly enough to avoid resorting to any of these ingenious but risky plans to hack the global climate.
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