Even though the underlying research is paywalled, the methodology is openly available and very interesting. It lists all the organisations in the study and their funding sources, plus the structure used for categorising their involvement. Look under Supplementary Material here: http://link.springer.com/article/10.1007%2Fs10584-013-1018-7
I find the methodology questionable. First, build a list of denier organizations based on anyone who has ever been remotely associated with known climate-deniers. This includes being labeled a conservative organization. Second, classify the organizations on a scale that categorizes anyone who neither remains totally silent nor actively encourages immediate government action as some form of denier. Third, pull all the publicly available funding data you can find on those organizations since 2003 and attribute it all to climate change denial.
Lots of these organizations are almost totally focused on non-climate change issues. There is no attempt made to figure out what fraction of their revenue might actually support climate change work, if any. (It would be difficult, given how little data 990s provide, but there are proxy measurements: number of staff working on climate change relative to total staff, number of climate change studies relative to total studies, etc.) Instead, if an organization so much as expresses some small doubt that immediate government action is the best solution to climate change problems, all of their revenues for years are considered to deny climate change.
This study is like the Glenn Beck chalkboard of climate change denial conspiracies.
Without seeing the actual paper I don't think it's fair to assume that no adjustment is made to an organisations funding to account for their level of involvement with climate change debate. The methodology paper appears to categorise them into 4 levels of involvement which could be used in the paper to weight the funding amounts.
