We do not know for certain what level of greenhouse gases in the atmosphere is compatible with a stable climate and healthy oceans, but many scientists believe that stabilising at 450 parts per million volume of carbon dioxide (eqv) would give us a 50:50 probability of limiting increases in global mean temperature below 2 degrees Celcius, the threshold for ‘dangerous’ climate change.
So, there are two uncertainties:
If humans continue on the current emissions path and are unlucky and the climate response is large, amplified by feedbacks, or the ability of human and natural systems to adapt is insufficient the consequences could be dire for people and the planet. It is possible that by deploying geoengineering we may be able to forestall these consequences and to protect critically vulnerable natural ecosystems such as the arctic, or coral reefs from damage that otherwise could no longer be avoided.
Given the uncertainties and stakes involved, it is important that we conduct research to determine if any of the proposed geoengineering techniques could be employed without creating countervailing side-effects. It is far from certain that any of them could be, so it would be extremely unwise to rely on geoengineering as a ‘silver bullet’ for climate change.