A draft assessment from the Intergovernmental Panel on Climate Change leaked last week by blog Watts Up With That suggests solar activity, or so-called solar forcing, is contributing to global warming trends rather than human activity exclusively.
Peter Frumhoff, policy director at the Union of Concerned Scientists, cautioned that the leaked report is a draft that's subject to change.
"The release of early drafts and selective reporting of interim results for political purposes stifles their (IPCC's) work," he said in a statement. "The IPCC should use this as an opportunity to strengthen its commitment to transparency and openness."
Frumhoff helped write the IPCC's 2007 report, which was criticized for a series of errors.
Last week, the IPCC said leaking draft reports "may lead to confusion because the text will necessarily change in some respects once all the review comments have been addressed."
Pennsylvania State University climatologist Michael Mann told the Huffington Post last week that there's nothing new in the leaked IPCC report, however.
"Solar forcing cannot possibly explain the warming of the past half century," he stated. "In fact, solar forcing has been flat over the past 50 years during which we've seen the greatest amount of warming."