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. More analysis via Scientific American Blog Network.