Characterizing the "real issue" as "one of democracy," he added, "At the very least, American voters deserve to know who is behind these efforts."
Advocates for a strong national response to global warming have hailed Brulle's work as exposing how big-money interests have manipulated the debate from the shadows.
Just as vigorously, climate-change skeptics have assailed his study as a flawed and misleading polemic. It was based on a "bizarre assumption that the science is settled, that the groups are illegitimate," said Richard Lindzen, a senior fellow at the Cato Institute, one of the conservative organizations Brulle examined.
The research is "less than worthless," said James Taylor, senior fellow for environment policy at the Heartland Institute, also a Brulle subject.
Taylor challenged the professor's selection of groups, many of which support other causes as well. In some cases, Taylor said, less than 10 percent of their funding is spent on climate-related efforts.
For his study, published in December in the journal Climatic Change, Brulle did not look at the income streams of environmental groups on the other side of the global-warming feud. He said he would do that next. When he does, Taylor predicted, Brulle will find not only more money, but more unsourced money from what are known as "donor-advised" or "donor-directed" funds.
The funds are "something new" and "are completely transforming the face of philanthropy," said Ray Madoff, a Boston College Law School professor who has urged tighter regulation of them. "We're talking about corporate entities spending money on issues, and now it's not traceable."
Instead of contributing directly to a cause, a donor gives money to a nonprofit fund, and specifies where it is to be sent. Such funds have to disclose where the money goes, but not its source.
For individuals, donor-advised funds can be a way to ward off incessant mailings from charities. But for backers of the global-warming opposition, Brulle contends, it's a detour past public scrutiny.
During the period he studied, Brulle found that Virginia-based Donors Trust Inc. and its associated Donors Capital Fund became increasingly prominent. "Launched" in 1999, according to its website, Donors Trust is "dedicated to the ideals of limited government, personal responsibility, and free enterprise."
By 2009, Donors Trust was responsible for 25 percent of all foundation contributions to the 91 groups Brulle researched; nearly half had gotten something. The largest amount was $7.7 million in 2010 to the Americans for Prosperity Foundation, nearly half of the group's $16.9 million grant income that year.
The rise of Donors Trust coincided with efforts by environmentalists to embarrass major patrons of conservative groups dismissive of climate change. Indeed, some cut back on direct contributions or declined or ceased.
The Heartland Institute used to disclose its funders, said senior fellow Taylor, but "environmental groups took the opportunity to harass our donors."
Between 2005 and 2010, Heartland received $13.5 million from Donors Trust, according to the study.
Brulle could not determine which contributors switched to the avenue of donor-advised funds. He found, though, that a foundation run by the billionaire Koch brothers started sending large grants to Donors Trust in 2008.The ExxonMobil Foundation, a former funder of climate change skeptic groups, cut funding completely, said spokesman Alan Jeffers: "We do not fund climate-change denial."
Donors Trust chief financial officer Jeff Zysik defended donor-advised funds as legitimate, with anonymity only one of many benefits. "This whole argument that [they] are shadowy ATM machines," he said, "is a straw man."
Last week, in a Senate floor speech, Rhode Island Democrat Sheldon Whitehouse called the process "an identity-laundering scheme . . . a deliberate, complex scheme of lies and propaganda."
Brulle's study followed the money, said Naomi Oreskes, a Harvard University science professor and author of Merchants of Doubt: How a Handful of Scientists Obscured the Truth on Issues from Tobacco Smoke to Global Warming.
"It is a public need to know who is behind this," she said, "and who is paying for it."