Let’s agree that global warming is happening: the earth’s atmosphere and oceans have been warming up since the beginning of the last century and continue to do so. Let’s also agree that human activity, especially the emission of greenhouse gases, makes a significant contribution to this process of global warming. On the basis of my admittedly amateurish reading of some of the relevant literature, including the Stern Report, I conclude that while there may be some local benefits from global warming, the overall global impact is negative. That still leaves unanswered the following question: what is the optimal global temperature (or perhaps: is there an optimal global temperature)? For the moment, define optimal as optimal for humanity.
The answer to this question matters, because with the many proposals and plans for mitigation and adaptation we are approaching a situation where the first stage of ‘terraforming’, the collective management and regulation of the average temperature of the globe, could become an option. Strangely enough, none of the literature I have seen addresses this question. Indeed most of the literature either assumes or attempts to demonstrate that any and all change in average level of the global temperature is bad. What the current level is does not seem to matter much, it’s the change that is costly.
Up to point, a rule for guiding global temperature policy that can be summarised as "whatever the current temperature is, don’t plan or expect it to change", (or, more pedantically, the optimum average temperature follows a Martingale) makes sense. With the speed of adaptation of biological species other than humans to climate change measured, if not quite in aeons, then certainly not in years either, but rather in centuries, millennia or even longer time-spans, more rapid climate change is likely to be more costly than less rapid climate change. But would it really be best simply to stabilise the average global temperature at its current level? Is it obvious we should not instead let is rise a few degrees, say 5 or 10, over some period of time, and then stabilise it at this higher level? Or should we aim to lower it a few degrees, again, say 5 or 10 degrees over a period of years, decades or centuries, and then stabilise it?
The historical record either shows that the present is unusually warm (if you have limited recall) or that it is unusually cold (if you take the long view).
Wikipedia (yes, I know) informs me that "The last 3 million years have been characterised by cycles of glacials and interglacials within a gradually deepening ice age". Instrumental measurement-based records only exist for the last 150 years or so, and show the now well-documented increase in temperature since about 1910, shown in Chart 1, taken from Wikipedia, below:
This picture of recent global warming to unprecedented levels is broadly confirmed if we take the 1000-year view of Chart 2, below (also from Wikipedia).
However, taking the longer view, as is done in Charts 3, 4 and 5 (all from Wikipedia) for ever-lengthening horizons, it is clear that we are in a spectacularly cold period from a long-run perspective.
When you consider the near-infinite variety of species that must have been wiped out the onset of our current cold spell (nothing like it has been seen for 450 million years), it may seem only fair that temperatures appear to be rising from what is obviously a very low level. Perhaps we should give evolution a chance to come up with life forms that are better suited to a warmer climate. A longer perspective like the one in Chart 5 can make one question the human-centric approach to climate change that virtually everyone in the climate change debate seems to adopt. At most a concern is expressed for a few ‘higher’ life forms, or for ‘biodiversity’ as a resource that may be useful for longer-term human survival and well-being. Let’s hear it for the cockroach instead, and for the one-celled organisms that can survive extreme temperatures! Or even, let’s hear it for the inorganic forms of creation, for whom all manifestations of life must be essentially parasitic.
From the usual crass anthropocentric perspective, it is obvious that, if we value the continued existence of humanity at all, a climate policy summarised as "never mind what level average global temperature is at, just keep it at its current level" cannot make sense for all temperature levels. At an average global temperature of 100 degrees Celsius many currently existing life forms, including humanity, would perish, and the same would hold for a sufficiently low average temperature.
Perhaps things are so dire for humanity, that the question as to what the optimum average global temperature is, need not be asked for the next 200 years. Perhaps the current rate of increase in the average global temperature threatens to outstrip our ability to adapt and cope by such a wide margin, that the need to slow down this rate of increase, and preferably to halt the increase in temperature altogether, should be our dominant or even our only concern. Still, I would like to know an answer to that question. Until I get a better sense of the long-run desired level of the average global temperature, I will not be fully convinced convinced that the argument for stopping it from rising further is well thought out.