Five extreme ways to beat climate change

From virtual volcanos to space mirrors, science is exploring new ways to fend off global warming
David Adam
The Guardian, Wednesday 4 November 2009
Mimics the cooling effects of a volcanic eruption by spraying shiny sulphur compounds into the high atmosphere. Relatively cheap and easy to do, though it would require non-stop effort as the chemicals gradually fall back to Earth. Possible side effects include changes to global water cycle and rainfall. Would not stop CO2 build up and subsequent effects including ocean acidification.
Plausibility: 7/10
Ocean fertilisation
Dump iron into the sea to boost plankton growth and soak up carbon dioxide from the atmosphere. Hard to do on a significant scale and could contravene international laws on ocean dumping. Doubts about how deep the plankton would sink have raised questions on how permanent a solution it would be.
Plausibility: 5/10
Cloud whitening
Fleets of sailing ships strung across the world's oceans could spray seawater into the sky to evaporate and leave behind shiny salt crystals to brighten clouds, which would then reflect sunlight back into space. Could be turned off at any time, but might interfere with wind and rain patterns. Would not address ocean acidification.
Plausibility: 6/10
Space mirrors
Seemingly straight out of Hollywood; a giant sunshade in space could block the sun. More likely to be a collection of millions or even trillions of small mirrors rather than a giant orbiting parasol. Very expensive and would require some serious rocket building to enable so many launches, which could cause problems for the ozone layer.
Plausibility: 2/10
Carbon swallowing
Artificial trees could soak up carbon dioxide from the air using a chemical process. Technically possible but very expensive on a meaningful scale. Captured carbon would still need to be disposed of. One of the few options that could turn back the clock and reduce carbon levels in the atmosphere. Plausibility: 4/10