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I would agree that the probable motivation behind the Grimes PLOS One paper, "On the Viability of Conspiratorial Beliefs", is to discredit AGW sceptics.
Stephan Lewandowsky, now a professor at Bristol University, is the leading advocate of the idea that AGW sceptics are just conspiracy theorists and are liable to believe in a range of other conspiracy theories. (The origin of this particular idea probably goes back to Al Gore in his "Inconvenient Truth" film). The Grimes paper references four Lewandowsky papers, thanks Lewandowsky in the 'Acknowledgements' section, and the first two words in the abstract of the paper are "conspiratorial ideation", very similar to the obscure phrase "conspiracist ideation" which appears in the title of two of Lewandowsky's papers. It is a reasonable assumption that Grimes shares Lewandowsky's views.
In fairness to grant agencies, I don't think they actually funded Grimes for this paper. In the paper it states "Funding: The author has no support or funding to report". Also to publish in PLOS One, an author is charged a publication fee of $1495, which presumably Grimes has paid himself (PLOS One is one of a small number of scientific journals that cover their costs by getting the author of a paper to pay rather than the reader or a library).
It depends on how you define 'conspiracy' as to whether AGW can be regarded as a conspiracy. If we adopt the definition used in Grimes paper:
"Conspiratorial ideation is the tendency of individuals to believe that events and power relations are secretly manipulated by certain clandestine groups and organisations", that secrecy element isn't really present for climate science. Climate science is actually pretty transparent - for example they make predictions of future global temperatures that are always well above what actually happens, suggesting their computer models are not valid to anybody who cares to notice. I would regard AGW as being more of a charade than a conspiracy.
AGW reminds me a quite a bit of the situation with Jimmy Savile in the UK. There was no conspiracy in what Savile was doing decades ago - he was hiding in plain sight. Savile relied on the fact that people in management have surprisingly little observational skill and are often unwilling to go against what they think is the prevailing opinion. In this analogy, AGW sceptics would be the equivalent of the numerous people not in management who pointed out Savile was up to no good, and the British political class would be the equivalent of the BBC managers and hospital managers who didn't appear to notice anything or chose to overlook it.
Besides, Grimes assumptions are fatuous in the first place. So many conspiracies when later examined upon revelation, prove to have had 'whistle blowers' who could not get anyone on the outside to take any notice.