Arguably, only professional scientists working in the field of climate have had the time to look at enough of the available evidence to make a reasonable determination. I personally don't think that is true, but if someone were to examine the evidence regarding relevant scientific opinion non-selectively, I suppose that might be a reasonable means of determination of the causes. This could only result in one conclusion.

Other conclusions would only be reasonable in so far as they take account of available evidence in a reasonable manner. No-one has succeeded in reaching an opinion which is not trumped by scientific evidence on this basis, but I suppose one could still argue that isuch a determination may nonetheless be reasonable.

Since 'Mike' has at least set out the rudiments of a coherent argument here, which is contrary to the scientific view, I think it would be instructive to examine it in detail so that you can draw a reasonable conclusion about whether or not it and similar ratiocinations constitute 'reasonable determinations'.

To summarise what I have to say about it briefly, the person in question appears to believe that some decisive and demonstrable destabilisation of a 'natural order' is necessary, perhaps requiring positive feedbacks indicative of the acquiescence of the pre-existing natural system, before any human influence can be deemed to have/to have had a significant effect on climate.



He begins with the contention that "the default is natural variation", meaning (I suppose -please correct me by any proper means if I am wrong, Mike) that all changes in climate should be supposed to be caused by natural processes until there is sufficiently strong evidence to the contrary. Please reflect, however, as I have, how this squares with his acceptance that "doubling CO2 levels creates about 1.2C of warming." I think you will agree that this consideration sheds light on his own expansion of his assumption: "CO2 and other causes should be evaluated as to whether they can be ruled to be the definitive answer instead of natural variation". He appears to believe that natural causes and man-made causes are mutually exclusive. How reasonable is this? I decided to cancel a cycle ride earlier this week, partly because it was raining and partly because I felt tired. Why does it have to be one or the other? Can't two different factors have an influence on the same thing?

He then contends that the effect of CO2 alone is not large enough to tell whether its effects are actually taking place in the presence of natural processes. Here, he appears to lean heavily on his assumption that changes should be attributed to natural processes 'first'; so much so as to contend that we ought to suppose that they are negating man-made changes without anyone realising. This is taking his starting assumption, which is fair enough as it is explicitly stated, significantly further. It now has the form (again, please correct me, Mike, if I am wrong here) and I do not quote, 'all changes in climate should be supposed to be caused by natural processes and it should be supposed that natural processes cancel out all man-made influences until there is significantly strong evidence to the contrary.' How is this additional component of his assumptions not simply 'magical thinking'? Why should man-made influences be cancelled out, but not those processes which change the climate naturally, producing what he terms "natural variation"? He is suggesting, I suppose, that climate is decidedly self-determining, or rather that certain natural processes more generally determine it. He is not saying that it is impossible for humans to influence it, but that it is proper to suppose that the climate, in all its variability, is exclusively naturally-determining until it has been demonstrated otherwise.

I have put aside scientific reseach cantradicting Mike's contention that 'it is not possible to tell'. Scientists working in the field believe overwhelmingly that it is possible to tell, so that even those of them who somehow share his peculiar background assumption, would still reject his argument. My main concern has been to address the structure of the argument, which is the ostensible basis for this particular 'sceptical' opinion.

But in order to avoid making a 'straw man' of Mike's arument, I should qualify what I have said as follows: it rests on the supposition that the CO2 effect specifically, not human influence in general must have a demonstrated impact (and that no such demonstration has been made/is as yet possible), but he would have to clarify that point for himself. To make that clarification on his behalf, I suppose he would agree that evidence is not necessary to support the contention that a sufficiently large reflective surface interposed between the Earth and the sun would significantly affect the climate.

But here I come to the realisation that the magnitude of the human influence is the critical feature as far as he is concerned. If it exceeds (sufficiently) the magnitude of 'natural variation' then it can be supposed to exert an influence. Otherwise, it must be presumed to have no effect at all or at least no significant effect, being countered by natural forces; that is, until it has 'overpowered' natural influences. This refers back to his previous assertion that either one or the other must be active and not both.

On its own terms, this argument makes sense, but its assumptions, as I say, strike me as peculiar. How widely they are shared is not for me to say.

There is no way scientists can differentiate between man-made and natural causes, unless you consider the 17yr pause in warming with CO2 still rising, that would make all warming through natural causes.

the debate is only in politics and public, not in the scientific community.

start at skeoticalscience for the 'debate'

get the real science at NASA.

There is currently debate about whether the recent global warming trend is caused by natural or man-made causes. Using the knowledge you’ve gained about climate, the causes of climate change, and any additional research you might need, write an essay describing the differences between natural and man-made warming of Earth’s climate. How could scientists and ordinary citizens examine the evidence to make a reasonable determination of the causes of the recent global warming trend?