In the previous posting "Two Routes to the Need for Geoengineering" I might have given the impression that there is a doubt as to whether or not there is a need for geoengineering. However, if the life which has arisen on the Earth is to survive there is very little doubt about this need. The only real question concerns the timescale within which the need exists.
To see this let me say a few words about the 'environmental crisis'. It is easy to make the mistake of believing that before humans had a significant impact on the Earth that the Earth was in some kind of 'static' state. According to this view, the Earth (and life on Earth in particular) was in a perfectly 'happy' and 'stable' state until humans started having a significant impact on the Earth; when this impact arose then humans 'disturbed this static state' by 'destabilising' the systems and cycles of the biosphere. The first thing to appreciate is that there is actually no such thing as a 'static' Earth. The Earth is an evolving and ageing whole. When the Earth was in its 'youthful' stage of evolution life spread out over the entire planet and was almost effortlessly maintaining the conditions of the Earth to keep them favourable for its continued existence. The Earth aged out of its youthful stage a long time ago. Life on Earth is now in a stage in which it is in peril (see Sir James Lovelock on this - Gaia, The Ages of Gaia, The Revenge of Gaia). In the future the ageing process will have reached the stage in which the Earth is unable to sustain any life. The youthful exuberance of a young Earth gives way to decay and death.
The only way in which the Earth as a whole can escape such a death is if humans somehow find a way to move the Earth to a more youthful part of the universe (via very advanced technology); and it is hard to take such a possibility seriously. It is more plausible that the life which has arisen on the Earth could survive by leaving the Earth and travelling to a distant part of the universe via human technology - a highly advanced spaceship - in a Noah's Ark type scenario. The future survival of the life that has arisen on the Earth clearly depends on human technology.
The considerations in the previous paragraph apply to the distant future. Let us now consider the more immediate survival prospects for the life that has arisen on the Earth. This is a complex issue with many aspects; I cannot go into all of these aspects here (they will appear at some stage in future blogs, and I have written 3 books in which I try to cover all of these aspects). There is one thing that is crystal clear. If there is no geoengineering then the Earth will become devoid of life much sooner (and seemingly very very much sooner) than if such geoengineering occurs. The non-technological ability of the ageing Earth to keep the conditions of the Earth favourable for complex life-forms is already weakening due to the ageing process. However, just as with an individual human, technology can extend life. If humans can successfully geoengineer the Earth (in particular the temperature of the atmosphere) then the life-forms of the Earth can look forward to a long and rosy future. If not, there is just a downward spiral of decay and death.
So, there is no doubt about the need for geoengineering. The only question is when such geoengineering is required. If the human perturbations to the cycles of the Earth hadn't occurred there would probably be quite a few thousand years before complex life on Earth died. The human movement of fossil fuels from their underground storage areas to the temporary storage areas of the slow moving deep ocean currents (the thermohaline circulation) leads me to believe that we have, at most, until the year 3000 to be successfully geoengineering the temperature of the Earth's atmosphere.
There is no certainty about the date; there is just certainty about the need.