One of the common elementary errors in scientific analysis is to confuse correlation with causation. If two phenomena, A and B, are strongly correlated, it is very likely that there is a causal link. But it doesn’t prove that A caused B. Perhaps B caused A. Or perhaps some other phenomenon, C, caused both A and B. (Timing can be a factor here: if A precedes B in terms of time, we can be pretty sure that B did not cause A).
This particular confusion bedevils a great deal of climate science. In Al Gore’s famous movie “An Inconvenient Truth”, he demonstrates a close correlation over hundreds of thousands of year between atmospheric CO2 levels and mean global temperatures, and BINGO! He’s proved that it was CO2 wot dun it. But he is hung out to dry on the timing issue, because on more detailed analysis the temperature peaks precede the CO2 peaks by around 800 to 1000 years. If anything, it looks as if temperature drives CO2, not vice versa.
On the face of it, the case for anthropogenic global warming (AGW) is pretty clear. The level of atmospheric CO2has been increasing steadily as long as we’ve had good records. This has all taken place during the period since the Industrial Revolution started, with a steady increase in fossil fuel use. (Leave aside, for the moment, the fact that the current slight warming started well before the Industrial Revolution got into its stride).
Everyone (well nearly everyone) accepts that CO2 is a greenhouse gas, and therefore might have some climate impact. Challenges to the orthodoxy usually focus first of all on the “Sensitivity” of the climate system to CO2. Because the theoretical warming effect of CO2 is negative logarithmic, CO2 has to increase geometrically to achieve a linear increase in temperature. So sensitivity is expressed as the temperature increase resulting from a doubling of CO2 – which (other things being equal) will always be the same. A doubling from 100 ppm to 200 ppm would cause the same temperature rise as a doubling from 400 ppm to 800 ppm – although the latter involves a four times larger increase.
The climate is so chaotic and the weather so variable that it is quite difficult to decide what the sensitivity might be. The IPCC likes to work on 3 to 4oC per doubling. Some scientists believe the figure is much lower.
The other main ground of debate is “feed-back mechanisms”. For example: warming could increase the level of water vapour, a more powerful greenhouse gas than CO2, so augmenting warming – a positive feedback. But water vapour results in more cloud cover, higher albedo, less sunlight reaching the earth’s surface, so less warming – a negative feedback. The IPCC tends to assume that the aggregate of all feedbacks is positive. Some scientists believe they may be negative.
The failure of global temperatures to increase for the last 18 years, despite continuing increases in CO2, argues for low sensitivity and possibly negative feedbacks.
But although I have been involved in this debate for a decade, it had never occurred to me to question the first link in the chain – that rising CO2 reflects anthropogenic emissions. I mean, it’s obvious, isn’t it? Obvious, but maybe not true.
I have often remarked on the 1000 year cyclical pattern of temperature which we have observed for at least ten thousand years. On that basis, we should have expected a recovery from the Little Ice Age in the 17th century – and that is exactly what we have observed. It is entirely reasonable to suppose that the observed warming since then would have taken place even without human activity.
The key insight is this: that natural cyclical changes in temperature can themselves drive levels of atmospheric CO2. The rise we have seen may be partly, or largely, natural and not anthropogenic. If so, then the vastly expensive measures we are taking to “combat climate change” and to reduce emissions are even more futile than they appear already. It is worth recalling that man-made emissions are dwarfed by the scale of the natural carbon cycle. They represent only around 3% of the total.
The mechanism is very simple. There is an awful lot of CO2 in the atmosphere. On one estimate, 3000 gigatonnes (3 x 1012). But there is a great deal more in the oceans – again, on some estimates, fifty times as much. And the relationship between the two is dynamic, not static.
Those who challenge my position on these issues often accuse me (wrongly) of ignoring basic science. Well here’s some basic science they’ve ignored: cool water can hold more CO2 in solution than warm water. If the ocean cools, it will dissolve more CO2 out of the atmosphere, and atmospheric CO2 levels will drop. Vice versa, if the water gets warmer, CO2 will be released out of solution and into the atmosphere – raising levels of atmospheric CO2.
It is conceivable that much, or most, of the increase in atmospheric CO2 which we have observed is entirely due to natural causes, in which case our efforts to curb emissions, which already look totally ineffectual, will be even more futile.
There are two objections to this idea, which I expect to be raised by the alarmist brigade. First, the balance of 12C vs 13C (isotopes of carbon) is claimed to support an anthropogenic/fossil fuel source for the increased CO2. Second, the current level of CO2 at 400 ppm is higher than that recorded for previous interglacials (by about 100 ppm). There are satisfactory explanations of both of these points – though they rather go beyond the reasonable bounds of a political blog. However they are dealt with by Professor Murry Salby of Macquarrie University and the University of Colorado
Professor Salby’s thinking is covered in more detail in a booklet written by Mike Haseler, Chairman of the Scottish Climate and Energy Forum, for The Bruges Group, “Climate: What we know, and what we don’t”. I recommend it. Professor Salby also presents intriguing evidence that CO2 emissions from land surfaces are greatly enhanced by increases in surface temperature and soil humidity – more evidence that temperature drive CO2, rather than the orthodox view, which is the opposite.