A global deal to combat climate change lurches toward reality in Lima, Peru, this week—and yet any politically feasible agreement will be insufficient to restrain continued warming of global average temperatures, perhaps uncomfortably high. Although recent pledges by China, the 28 countries of the European Union and the U.S. are the first signs of the possibility of restraining the endless growth of greenhouse gas pollution on a long-term basis, atmospheric concentrations of carbon dioxide have crossed the threshold of 400 parts per million—and will reach 450 ppm in less than two decades at present growth rates. The estimated one trillion metric tons of carbon the atmosphere can absorb could be burned through in even less time, particularly if India, as it develops, picks up where China leaves off by burning coal without any attempt to capture the CO2 before the greenhouse gas spews from smokestacks.
The world may find itself in need of another alternative, such as geoengineering, if catastrophic climate change begins to manifest, whether in the form of even more deadly heat waves, more crop-killing droughts, more rapid rises in sea level or accelerating warming as natural stores of carbon—such as the ocean’s methane hydrates—melt down, releasing yet more greenhouse gases to drive yet more climate change.
So maybe the answer is to genetically soup up plants so they can pull more CO2 out of the air and then bury them at the sea bottom? Or give the planet a giant sunshade, whether in the form of more clouds or a haze of light-reflecting sulfur bits floating in the stratosphere? "In a crisis the temptation will be to use the quick fix of geoengineering," argued economist Scott Barrett of Columbia University at a research symposium on CO2 capture technologies this spring.
If civilization continues, the unplanned, undirected geoengineering of the climate via burning fossil fuels—whether coal in a power plant or oil sludge in a massive container ship steaming across the Pacific—then perhaps nations will need to plan for a directed attempt at geoengineering or the "deliberate, large-scale manipulation of the planetary environment" as the U.K.'s Royal Society defines it. Still, scientists are starting to agree that geoengineering will prove insufficient for solving climate change.
To understand this it helps to think of two distinct flavors of climate engineering: those that reduce greenhouse gases and those that block sunlight to keep the planet cool. The various sun-blocking schemes could be fast and cheap, like a fleet of airplanes spewing sulfur particles in the stratosphere to mimic the cooling effects of volcanic eruptions or an armada of ships brightening clouds by increasing the number of water droplets within them. On the other side, carbon removal schemes are slow and expensive, such as big air filters to suck CO2 out of the sky and bury it, turn it into fuel or otherwise keep it from trapping heat. Or the natural processes of rock weathering and plant growth that over geologic time constrain climate change could be sped up. The Intergovernmental Panel on Climate Change in its most recent comprehensive report suggested that one member of this set of ideas—burning plants paired with CO2 capture and burial, aka bioenergy with carbon and capture, or BECCS—might prove vital to restrain global warming. And the U.S. Department of Agriculture provided a $91-million loan guarantee in October to a company—Cool Planet—looking to build a kind of BECCS facility in Louisiana to make biofuels and biochar, a carbon-rich residual ash that can be used to improve soil fertility, keeping the carbon out of the atmosphere.
But neither flavor of geoengineering can serve as a solution to climate change. As outlined in the December Philosophical Transactions of the Royal Society A, sun-blocking schemes require continual refreshing and, at best, only buy time for real solutions, such as cutting down on the amount of CO2 piling up in the atmosphere as a result of fossil fuel burning, while failing to account for other impacts such as the increasing acidity of the oceans. And CO2 removal schemes could find themselves in a continuous game of catch-up with the world's voluminous output of greenhouse gases—an ever-more onerous burden if the world did nothing to restrain global warming pollution.
Geoengineering could play a role in coping with some of the impacts of climate change, perhaps used to cool off the rapidly warming Arctic and save summertime sea ice. Or "these strategies might be used throughout the period required to replace fossil fuel burning with globally distributed clean energy and even be continued while CO2 concentrations remain too high," as atmospheric scientists put it in an overview of the Philosophical Transactions issue. Small-scale tests of such techniques are therefore warranted to assess the real risks, such as unexpected chemical reactions with the existing mix of atmospheric gases, for example. Of course, it took massive emissions of CO2 to detect human-caused global warming, suggesting small-scale tests may not reveal much. And even at a miniscule scale engineering the climate remains a radical step with consequences for both the climate and civilization that cannot be predicted in advance.
There is no technological fix for global warming other than the hard work of transforming a global energy system that relies on burning fossil fuels into one that relies on energy sources—the sun, Earth's heat, fission or, maybe some decade, fusion—that do not use the atmosphere as a dump. The fact that geoengineering cannot suffice is good news because it means that a viable form of climate engineering cannot undercut the urgency of making that switch. No form of climate engineering can solve global warming at present. To think so is science fiction.