Modified jets spewing sulfuric acid could haze the skies over the Arctic in a few years "for the price of a Hollywood blockbuster," as physicist David Keith of Harvard University likes to say. For a mere billion dollars a program to swathe the entire planet in a haze of sulfuric acid droplets could be ready as soon as 2020, he avers.

That's geoengineering, or "the deliberate large-scale manipulation of the planetary environment," as the U.K.'s Royal Society defines it. This hazing, which mimics the cooling effect of a volcanic eruption, is so cheap that almost any country—or any random billionaire—could afford to do it, if climate change got catastrophic enough that a crash course in cooling the planet came to seem a good idea. Already, a rogue geoengineer has tried to draw down atmospheric carbon dioxide by fertilizing the ocean with iron to promote the growth of photosynthetic plankton. That's why the U.S. National Research Council empaneled a group of scientists and other experts to take a deeper look at a variety of geoengineering options in two reports released on 10 February—after all we may need them given the rising concentration of heat-trapping atmospheric carbon dioxide, which has now touched 400 parts per million.

There are two different types of geoengineering as the examples above show: those that block sunlight to counteract global warming, dubbed solar radiation management or albedo modification, and those that remove the molecule behind much of the global warming: CO2. Managing solar radiation has considerable and so far unknown risks. For example, mimicking the cooling effect of volcanoes by spewing sulfuric acid droplets into the stratosphere would be likely to eat away at the ozone layer that protects life on Earth from damaging ultraviolet radiation; it could also lead to more of the air pollution that promotes asthma and early deaths.

Another sun-blocking concept—using aerosols to make the clouds that form over the ocean whiter in order to reflect more sunlight back to space—would not harm the ozone hole but could prove difficult in practice or impact global rainfall. Whether or not ships are already doing such marine cloud brightening by spewing aerosols from smokestacks as they travel the oceans is unclear, suggesting at best a limited understanding of the atmospheric and cloud physics involved. Plus, in the worst case, any of these technologies could be weaponized, just as mathematician John von Neumann warned back in the 1950s when geoengineering was considered as a weapon in the Cold War.

Blocking the sun is, at best, a short-term fix that does nothing to remedy the other impacts of rising levels of atmospheric CO2, such as turning ocean waters more acidic or nights that get warmer and warmer under a thickening blanket of greenhouse gases. Once used, it might also prove difficult to stop hazing the skies or brightening clouds since, without removal of the CO2 causing the problem, global warming would just be in abeyance and any pause in the fast-acting but short-lived solar blocking would result in even more rapid global warming.

As for CO2 removal, the risks are usually low, but the costs are mostly high, the National Research Council found. Saving forests, growing more trees and better soil management in agriculture are the simplest, best fixes and should go forward as part of any strategy to curb climate change, of course, and they have large potential to draw down CO2. But fertilizing oceans with iron, outfitting power plants to burn fresh plants instead of fossilized remnants and then capturing the CO2 for storage (or recycling), or even employing machines that remove CO2 directly from the air are currently too expensive or have other undesirable impacts, like changing ocean food webs or displacing food crops. Furthermore, each of these systems, or even all of them in tandem, would need to be deployed at massive scale to begin to control the extra 34 billion metric tons of CO2 added to the atmosphere each year by fossil fuel burning and other human activities.

In short, the NRC panel suggests none of this is engineering, which implies well-understood control of the planet's climate. Rather each of these techniques should be called a "climate intervention," with unknown consequences.

Still, China's fleet of coal-fired power plants continues to grow, reaching more than 800 gigawatts in 2014. And though China's coal use dipped for the first time in the 21st century last year, it may not prove to be its much-anticipated peak in coal use, which the country itself suggests will happen "around 2030." The U.S. has only just started to constrain its own coal use and India is likely to burn more and more over the course of the 21st century. Tens or even hundreds of billions of metric tons of CO2 will enter the atmosphere in the next few decades, and there are few prospects for capturing and getting rid of that carbon anytime soon with existing technologies.

In other words, geoengineering with CO2 is already happening, with uncertain but potentially catastrophic results. Studying other forms of geoengineering that may help remedy or reverse that effort seems necessary. This does not have to be jets spewing sulfuric acid over the Arctic and potentially dissolving the ozone layer. It could mean simpler fixes such as turning to nature's technologies for removing CO2—plants, ocean plankton and rock weathering—and boosting those effects.

There is clearly no substitute for cutting back on the dumping of CO2 into the atmosphere as if the skies were an open sewer—but civilization may need something to clean up that sewer a little quicker than nature does on its own. So study up, the NRC panel suggests, starting with CO2 removal techniques, receiving critical aid from new satellites to more precisely monitor the planet's overheating and the impacts of future volcanic eruptions. As NRC panel member geophysicist Marcia McNutt, editor-in-chief of Science and former head of the U.S. Geological Survey, says, "The longer we wait, the more likely it will become that we will need to deploy some forms of carbon dioxide removal to avoid the worst impacts of climate change."