Here’s a curious observation. Some commentators who for years have been vocally decrying the IPCC consensus are lining up to support the ‘Ruddiman’ hypothesis. A respected paleoceanographer, Bill Ruddiman has recently argued that humans have been altering the level of important greenhouse gases since the dawn of agriculture (5 to 8000 years ago), and in so doing have prevented a new ice age from establishing itself. This intriguing idea is laid out in a couple of recent papers (Ruddiman, 2003; Ruddiman et al, 2005) and has received a fair degree of media attention (e.g. here, and here).
The basic idea mainly relies on paleoclimatic evidence that there would have been a new ice age by now under ‘normal’ circumstances, and since there isn’t one, anthropogenic influences are presumably to blame. Whether this is valid or not is now the topic of some debate at scientific conferences. One can argue about the details (and I have here for instance), but the relevant issues are quite technical and not really what I want to talk about. Instead, I will explore what acceptance of this idea implies.
The attraction to this idea for the contrarians is not hard to see. They have seen quickly that this is a controversial and non-mainstream idea (and even Bill Ruddiman will allow that!), and that Ruddiman appears to be ‘battling’ against the science establishment. So, following a sort of ‘the enemy of my enemy is my friend’ logic, they have been vocal in supporting Ruddiman’s ideas (I should hasten to add that this support does not reflect on the quality of Ruddiman as a scientist, nor on the validity of his thesis).
For instance, take this quote from TCS
Given Ruddiman’s findings the key question now is not “is industrial-age, human-caused global warming occurring?”, but rather “are we sure that the human effect on climate over the last 8,000 years has helped to prevent the occurrence of another glaciation?” Should the answer to that question be yes, then it prompts the further question: “do we wish to maintain the human warming effect, or instead to counteract it and allow Earth’s climatic cycle to drop back into its next (natural) glacial episode?”.
Or from the Idso’s site:
Hence, even if the IPCC is correct in their analysis of climate sensitivity and we are wrong in suggesting the sensitivity they calculate is way too large, the bottom line for the preservation of civilization and much of the biosphere is that governments ought not interfere with the normal progression of fossil fuel usage, for without more CO2 in the atmosphere, we could shortly resume the downward spiral to full-fledged ice-age conditions.
There are multiple strands of intellectual incoherence here. Firstly, people who have argued that there really hasn’t been a significant rise in CO2 since the 1800s, now accept the ice core results that show there was a much smaller rise from 8000 yrs ago. Secondly, those that argued that the 20th Century CO2 rise cannot be anthropogenic, appear to accept that the post-8000 BP change was. Thirdly, those who argue that the current increase in greenhouse gases has no significant climate effect, now appear to believe that the much smaller changes in the pre-industrial prevented an ice age.
This is like someone who believes the earth is flat buying a round-the-world ticket for their vacation.
Let me make it a little clearer. If one accepts Ruddiman’s hypothesis, one implicitly agrees that: i) CO2 and CH4 can be affected by human activity, ii) greenhouse gases have a significant forcing role, and iii) climate sensitivity is in the ballpark of mainstream estimates. (The opposite is not however true, one can agree on those three points (which is pretty much the IPCC consensus) without necessarily thinking that Ruddiman is correct). One must therefore conclude that those contrarians who have warmly welcomed Ruddiman’s thesis have now come around to mainstream opinion. Hmm…
But wait, they might say, we don’t believe any of those things, but we accept that we may be wrong, in which case, Ruddiman’s theory still implies that we should not do anything about increasing CO2 . No. At minimum it might indicate that reducing CO2 below pre-industrial levels might not be a good idea, but that is far from saying that any amount of further increases are beneficial. It certainly doesn’t follow that if a little bit of CO2 is good for the climate, then a lot more is better.
I think a more obvious explanation is that, for some critics, any argument will do – regardless of its coherence with the argument they had before, or the one they will pick next.