Cloud whitening (SRM)

This concept, imagined by Stephan Salter from the University of Edinburgh, consists in spraying seawater in the atmosphere to increase reflectiveness of clouds. The clouds, produced by a fleet of around 1500 unmanned ships, would reflect more radiation from the earth. However, such an operation could have unexpected -and difficult to modelize- effects on oceanic climates and streams.

Covering the deserts with white films (SRM)

According to Alvia Gaskill who proposed this solution, covering a large enough area of the earth (first candidates would be Sahara, arabic and gobi deserts) could be expected to offset some or all of the projected additional radiative forcing and global warming from 2010 to 2070. Together with tremendous costs, ecological consequences such as perturbation of the atmospheric circulation (which could result in sub-saharian monsoon perturbation) must be feared.

Space sunshade (SRM)

Basically, this concept proposed by Roger Angel (University of Arizona), involves the use of trillions of small umbrellas, that would be put in orbit an stop some sunlight from reaching the Earth. If small 1 gram, 60 cm diameter discs were used, 800 000 of them would have to be sent every… minute, for 30 … years, in order to decrease the radiation hitting our planet of 1.8%.

Stratospheric sulfate aerosols (SRM)

Inspired by the eruption of Mount Pinatubo in 1991, which projected tons of particles in the atmosphere, and noticeably cooled the global temperatures of 0.5°C, chemistry Nobel Prize 1995 Paul J. Crutzen suggested the injection of sulfur compounds in the atmosphere. But this project could perturbate water cycles, the stratospheric ozone chemistry and biological life, which make large scale experimentation somewhat unrealistic.

Ocean iron fertilization (CDR)

This method involves the seeding of ocean with iron in order to promote a phytoplankton bloom, which can remove carbon dioxide from the atmosphere. Again, several side-effects are to be expected, as well as an increased water acidification, and the creation of large zones depleted from oxygen (the more the algae ‘breath’, the less oxygen available for the other species).

Although geoengineering is a flourishing field (just try to enter it in wikipedia), many of its promises will probably come true too late (if at all) if one wants to reduce anthropogenic global warming and climate change… scientific creativity will have to find other ways to deal with climate modifications.

A first of three features, written by Richard Monastersky, explains how keeping carbon dioxide beneath dangerous levels is tougher than one previously thought. The basic question is: how much can the CO₂ concentration in the atmosphere increase before reaching a point of non-return? The pre-industrial level was roughly 250 parts per million (ppm), we are now reaching 400 ppm. It was long thought that a concentration of 450 ppm was a target to avoid, but new studies seems to indicate that a threshold of 350 ppm would have been more reasonable. This raises many questions on how to deal with the ever increasing carbon dioxide level. Studies performed at the University of Bern show that even if all CO₂ emissions stopped when its concentration is 450 ppm, it would take more than 1000 years to reach pre-industrial levels, and 3000 years would be necessary to see global temperatures slightly decrease. Other studies give even more frightening results: if CO₂ concentration reached 550 ppm (not unlikely at all) before all emission stopped, the temperatures would keep increasing for at least a century. Even if the studies are based on unperfect models, they point out the fact that climate will need a long time to recover, essentially due to the thermal inertia of the oceans (they slow up the climate warming now, they’ll delay its cooling in the future) and to the rate at which carbon sinks can absorb CO₂ from the atmosphere.

This rather pessimistic (I sould say, apocalyptic) view is counterbalanced by the two next articles, written by Nicola Jones and Oliver Morton, respectively, who discuss potential solutions to increased CO₂ concentration and global warming. At first sight the proposed ideas look weird: absorbing CO₂ into a solution of NaOH to produce sodium carbonate, that can be converted in calcium carbonate (by addition of calcium hydroxyde), and finally into pure CO₂ (that sould be stored, frozen, sent to space, etc.). Although this works on lab-scale, one may remember that, if the current emission rate is maintained, 650 gigatons of CO₂ will have to be removed from the air by 2100 in order not to reach 450 ppm… a serious scale-up will be required there! Another approach, involving geoengineering, aims at brightening clouds to increase reflexion of sunlight. Some researchers think about ships vaporizing water droplets to form clouds over the oceans, which should in turn cool earth’s atmosphere ! We’re not so far from reaching a good sci-fi novel scenario where a giant shield would be built between earth and sun, protecting us from sunlight until the global temperature has reasonably decreased…

But, if the last news are not good at all, and if we are facing a tremendous challenge, scientists are coming with ideas, trying new concepts and models, that can one day help us and the earth to survive our reckless carbon consumption. Whatever path we chose, it is certain that a compromise will have to be found between the sacrifices we’re ready to make to reduce our carbon emissions, and the consequences (at a climatic, economic, or social level) we agree to suffer. The earlier this compromise is established, the better!