Efforts to cool the planet, known as geoengineering, won't work if carbon dioxide levels keep rising, a new study claims. Although such projects may still prove a useful adjunct to efforts to prevent carbon dioxide emissions, they will fail as a substitute, the researchers find.
Volcanic eruptions can induce temporary cooling on a planetary scale by injecting particles and sulfur dioxide into the upper atmosphere, reflecting more sunlight. This has inspired proposals to do something similar on an industrial scale. Some supporters argue such schemes would be cheaper than rapid replacement of fossil-fuel burning industries. Others regard the idea as something to be adopted in addition to, not instead of, fast emissions reductions, balancing the gases already released.
All this matters little, however, if geoengineering won't actually work, a question Professor Tapio Schneider of the California Institute of Technology is trying to answer. In Proceedings of the National Academy of Sciences, Schneider and co-authors conclude that geoengineering could work at existing atmospheric concentrations of carbon dioxide, but would fail if those levels pass a crucial threshold.
Carbon dioxide currently influences Earth's climate by trapping radiation close to the surface that would otherwise reach the upper atmosphere. In addition, above a certain concentration carbon dioxide causes startocumulus clouds to break up. These clouds have a major cooling effect on the planet, reflecting sunlight at tropical latitudes back to space. Their removal would cause a rapid jump in temperatures of around 5ºC (9ºF). Without geoengineering, Schneider has estimated this occurs at 1,200 parts per million. Release of cooling particles would raise the threshold to around 1,700 ppm, Schneider calculates, but cannot remove it entirely.
Geoengineering efforts could potentially balance the radiative capture effects of CO2 and other greenhouse gases. However, once the threshold is crossed, cloud collapse would present a civilization-ending event that no amount of geoengineering appears to prevent. Moreover, once these clouds are gone, carbon dioxide levels need to fall a long way before the stratocumulus clouds return.
There is evidence stratocumulus catastrophes have happened in the Earth's distant past, explaining temperature spikes exceeding those predicted by climate models that don't take clouds into account.
Even centuries after the industrial revolution, carbon dioxide levels are just less than a quarter of Schneider's geoengineered tipping point. However, if emissions continue unabated we will inevitably get there eventually. Moreover, the 1,700 ppm level is only an estimate, in the subtropics it could start earlier.
Geoengineering already faces other criticisms. For example, it is likely to produce shifts in rainfall patterns, with potentially disastrous consequences in some areas. Critics also note it does nothing to address problems like ocean acidification, and may make these substantially worse. Supporters argue the planet's crisis is so serious even solutions with serious side-effects need to be adopted.
Whichever side one takes on that debate, Schneider's work suggests geoengineering is only worth considering if accompanied by measures that keep carbon dioxide levels well below where they could mess with the clouds.