Even with zero emissions, we may deem it necessary to reduce the atmospheric carbon concentration. Solar radiation management techniques could be employed as a stopgap solution to offset a global temperature on the rise. Such deliberate acts of climate intervention are referred to as geoengineering.

Human activity is causing climate change. By emitting greenhouse gases, we are playing a game with high stakes: we are risking sea level rises, extreme weather conditions, loss of species and ecosystems and global consequences for productivity, health, and environment. There is inertia in the global climate, and even if emissions were cut to zero immediately, climate change would continue for a long time before reaching equilibrium.

Without having planned to do so, and without understanding exactly how, we are turning the earth’s thermostat. We can take our hand from it, by eliminating greenhouse gas emissions – but can we do better? Would it be possible, with current or future understanding of its workings, to deliberately turn it back again? This idea is the foundation for the concept of geoengineering: large-scale, calculated climate intervention to mitigate the damage done.

Att stimulera molnbildning är ett sätt att påverka jordens albedo och sänka temperaturen.

Encouraging the development of clouds is one way to increase the earth’s albedo, which would lower the temperature.

There are two conceivable approaches. We could either attempt to lower the atmospheric concentration of greenhouse gases directly, or create a countereffect to their heating impact by managing solar radiation.

Capturing carbon from the air

Carbon capture can be performed in a number of ways. Carbon dioxide can be separated from the exhausts of power plants and factories (as described in detail in the recent article ”Can we pull the carbon out of the air?”).  If the flue gas comes from biomass combustion, and the carbon is permanently sequestered underground (BECCS), a net reduction of atmospheric carbon is achieved.

Another way is to enhance natural weathering processes. Silicate minerals react slowly with carbon dioxide from the air, forming carbonates, and we could speed this up by excavating and exposing vast amounts of such rock. It would of course entail a lot of mining and processing operations.

The biological pump could also be used to remove carbon. This is a natural process that exports fixed carbon from the surface of the sea to the deep sea. Atmospheric carbon is fixed into soft and hard tissue of plankton. Once they die, they begin to sink to the ocean floor where they decompose, and part of the carbon is sequestered in sediment.

This process is an important sink in the natural carbon cycle, but the effect is difficult to measure and quantify. It could be reinforced by adding limiting nutrients such as nitrogen, phosphorus and iron to the sea in order to stimulate planktonic growth and carbon fixation. It would be a relatively hazardous undertaking, though, since it would mean interfering with the complex marine ecosystem; modelling the consequences is far from trivial.

Artificial trees

Land-based capture of carbon is less perilous. Pulling it out from ambient air would be more difficult than from emission sources, because of the low concentration; nevertheless, it is far from impossible. After all, plants and trees are doing it all the time. We could plant more trees, burn them, and capture and store the carbon content. But perhaps we could also mimic nature and create artifical trees with enhanced performance? The Center for Negative Carbon Emissions are trying to develop such technology, and they claim to be able to capture carbon a thousand times more efficient than natural trees. Their concept is based on a material where ammonium ions are embedded in polystyrene, that binds carbon when it is dry and releases it again when humid. Whether the technology can be cost-efficient enough to turn the artificial trees into forests remains to be seen.

Solar radiation management

Solar radiation is also a parameter which could be regulated. Covering deserts with reflective sheets, painting roofs and roads white, switching to crops with brighter leaves or encouraging cloud formation are some of the proposed methods. All of them serve to increase the earth’s albedo, or reflecting power, so a larger portion of the incoming radiation is reflected back into space instead of raising temperature. It is a simple concept, but applying it to a large enough area would be more difficult. Such methods could still have significance to local temperatures.

In the future, it might be possible to put a mirroring or shading structure in space, between the earth and the sun. A more down-to-earth method, and probably the one that has gathered the most proponents, is to spread some kind of particles high up in the atmosphere. Sulphate aerosols are the most common suggestion, since such particles gather naturally in the stratosphere. There, they spread out to form a shielding shroud that reflects some of the solar radiation.

Volcano eruptions cool the earth

Efter stora vulkanutbrott blir jorden kallare, när aerosoler sprids i stratosfären och reflektererar undan solljus.

Large volcano eruptions cool the earth by depositing sulphate aerosols in the stratosphere. The particles act as a solar shield.

Large volcano eruptions spew out massive amounts of sulphate particles, and that they have a substantial effect on global climate is well established. The Krakatoa eruption in 1883 lowered the global average temperature more than one degree, and it took several years for it to go back to normal. The Pinatubo eruption in 1991 deposited 20 million tonnes of sulphate particles in the atmosphere, and the temperatures recorded the following year were half a degree below normal.

Such aerosols could be formed from sulphur dioxide, that could be airlifted to the proper altitude by airplanes, tethered balloons or artillery, for instance. The task amounts to moving a mass roughly corresponding to one tenth of the annual global air transport, according to estimates. It is notable that the amount of sulphur dioxide needed is small compared to what we already are emitting to the atmosphere today – approximately one percent. The difference is that current emissions stay close to the ground, soon to fall back down or be washed away by rain. At 10 km and higher, conditions are more stable, which allows the particles to spread and remain for some time.

Eventually, what goes up must come down, and even in the stratosphere the sulphate shield would have to be replenished continously. The aerosol method is not a one-time solution, but a commitment. Failure to replenish the sulphates would quickly cause the temperature to rise again. Nevertheless, it is a proven concept, it would impact temperature relatively fast, and the cost – perhaps one billion dollars per year – would hardly be discouraging compared to any other significant climate change mitigation measures.

Still, there are downsides. One of the gravest concerns is that the particles may have a detrimental effect on the ozone layer. Furthermore, adjusting the global average temperature does not compensate fully for the effects of higher carbon concentrations in the atmosphere. The vast local variations in the effects of climate change would not be accounted for, for instance, and besides temperature, precipitation and other weather conditions are affected. Carbon dioxide also causes ocean acidification.

In case of emergency or as a stopgap

Geoengineering is still a fairly unexplored field, and none of the proposed methods have been tested in large scale. The uncertainties are many. Still, we likely have the technology available to use such methods as a complement to emission controls, once their effects have been rigorously studied. Carbon capture from air is the slower and more expensive tool, but has the advantage of dealing with the underlying problem. Solar radiation management instead tries to compensate for the effects, and thus is a more blunt but quick-acting tool.

We are on our way towards a future carbon-neutral economy. Renewable energy sources, innovations and more efficient green technology is accelerating the process. But eventhough we are on the right path, the transition still takes time. It is worth keeping in mind that emission rates is not the only instrument at our disposal, even if the other options provided by geoengineering perhaps are best thought of as emergency measures and stopgap solutions, for the time being.

The article was published in November 2016.