It has been well established that our emissions of greenhouse gases are changing the Earth’s climate and that in order to avoid future warming and ocean acidification, fossil fuel use will need to be limited. There is a sort of “Plan B,” however—the intentional manipulation of the climate, known as “geoengineering.” Some forms of geoengineering could be done relatively easily, while others seem more like “terraforming” schemes out of sci-fi novels.

As part of its latest report, the Intergovernmental Panel on Climate Change (IPCC) includes a review of research into the two main categories of geoengineering: the artificial removal of CO 2 from the atmosphere and the artificial reduction of sunlight reaching the Earth. There’s nothing yet about the practical feasibility of any techniques—that data may appear in sections of the report that have not yet been released—but the paper nicely describes how various schemes actually work.

Taking off some blankets

The removal of carbon dioxide from the atmosphere could take many forms, some of which have much more potential than others. The first is biological. The regrowth of forests that have been cleared does have an effect, but it can only remove about as much carbon as the forests released when we cut them down. Soil provides another place to stuff some atmospheric carbon, though. By adopting soil-carbon-enhancing farming techniques, restoring some low-quality agricultural land, and producing and mixing in biochar, soils could make a bigger difference than reforestation over the twenty-first century.

Another popular idea is to enlist the help of phytoplankton. These photosynthetic plankton take in dissolved carbon dioxide from the surface ocean and can sink to the deeps when they die. If they make it to the deep ocean or seafloor, the carbon inside them won’t be seeing the atmosphere again anytime soon. Stimulating the growth of these organisms by providing them with limiting nutrients (like iron) should, in theory, crank up this “biological pump” that pulls CO 2 out of the atmosphere.

Experiments testing this approach, unfortunately, have yielded mixed results. A phytoplankton bloom doesn’t do you any good, for example, if the zooplankton that eat them bloom as well, consuming (and respiring) all that carbon before it can sink. Some experiments haven’t seen much carbon making it down to deeper water, while others have had better luck. But even model simulations using unrealistic scenarios (fertilizing the entire ocean, for example) estimate a total effect similar to what soil could potentially provide.

Carbon dioxide removal could also take the form of juicing another natural process—the chemical breakdown of rocks. Some minerals react with the CO 2 dissolved in rainwater, turning it into bicarbonate in the process. By pumping concentrated CO 2 into the right rocks or by crushing those rocks and adding them to agricultural soil, that reaction could be accelerated.

Together, these techniques could make a dent in atmospheric CO 2 , but not much more than that. Ultimately, we'd need a way to directly remove carbon dioxide from the air, especially if fossil fuel use remains significant over the coming decades. No suitable technology for turning atmospheric CO 2 into a storable fluid or solid has yet been developed. And as a study we recently covered illustrates, whatever technology we develop might have to operate for centuries.

Designer sunglasses

If carbon dioxide removal is a long-term proposition, taking decades to have an impact, reflecting some incoming sunlight back into space is a short-term alternative. One way to do this is to take a page out of the volcano playbook, boosting the amount of aerosol particles in the stratosphere. This is how large volcanic eruptions cool the Earth for a couple of years. But the short-lived influence that eruptions have is indicative of the problem with this technique—it will quickly dissipate if not maintained.

That’s not such a big deal when the amount of cooling it’s providing is small, but if it was counteracting a couple of degrees of warming, we wouldn’t want to feel the full force of that temperature rise suddenly hit us over just a few years.

The aerosol of choice would likely be sulfur dioxide—the same stuff that causes acid rain when it comes out of a smokestack down here in the troposphere. Injecting a spray of the stuff up into the stratosphere (maybe through pipes held up by blimps, for example) would slightly reduce the amount of sunlight coming through to warm the Earth. Assuming the concentration of greenhouse gases in the atmosphere continues to rise, we’d need to emit the equivalent of a Mt. Pinatubo eruption each year before the end of the century in order to cancel out the warming.

Those added aerosols could also subtly change the character of the light coming through by slightly increasing scattering. Phenomena that make sunlight less direct and more diffuse can boost photosynthesis, and since CO 2 would still be high, plants would lose less water through their leaves. It’s not clear how these two responses would interact with the water cycle. While more diffuse light could be beneficial to photosynthesis, it would also lower the efficiency of solar panels, which rely on direct light.

There are other known side effects. Adding aerosols could push the recovery of the hole in the ozone layer back by 30 to 70 years, for example. Altering the temperature profile of the atmosphere (by cooling the surface but leaving the stronger greenhouse effect) would probably lead to a decrease in precipitation. Daily temperature swings could change since greenhouse gases would still be doing their thing after the Sun goes down. And there would be regional differences in how this all played out.

A different approach would be to seed low-level clouds over the ocean with the aim of making them more reflective. This introduces different kinds of side effects, partly from the more localized nature of the technique (you would only affect clouds near the seeding locations) and partly from the complex processes within clouds.

Regardless of the specific technique used to reduce incoming sunlight, it would do nothing to remedy the problem of ocean acidification, since the CO 2 in the atmosphere would remain. However, the IPCC report states that it could “counteract a portion of global warming effects (on temperature, sea ice, and precipitation) due to high concentrations of anthropogenic greenhouse gases.”

“But,” it goes on to say, “the level of understanding about [reducing incoming solar radiation] is low, and it is difficult to assess feasibility and efficacy because of remaining uncertainties in important climate processes and the interactions among those processes.”

This theme runs throughout the report’s discussion of geoengineering. The research is still in the early stages, and there is quite a lot we don’t yet know about these techniques. Their effectiveness is uncertain, their side effects are very uncertain, and the practicality of these ideas is extremely uncertain. A great deal more research will be required before we’ll have reliable guidance on what could be done and how it would best be implemented. And that guidance may very well be needed.