The dismissive response on Real Climate and on Grist to this letter do not provide the objective scientific rebuttal to these science claims. This is unfortunate and is misleading policymakers, but, as we have learned and reported many times on at Climate Science and elsewhere (e.g. see and see), this is the way the IPCC and CCSP community deals with solid science that disagrees with their perspective.
Open letter by the cato institute on climate science
UPDATE: Also see what that nice man MONBLOT has to say about this from the Guardian
Please download PDF file to read FULL report