Susan-Marie,

the tobacco example is true. But it is easy to compile an impressive list of past warnings that have turned out to be not true. Looking who is it (and what his affiliation is) who says something is true or can still be doubted can help to determine how much one personally trusts a source, but it is not much more than an indication.

A scientist can have a personal reason (like an industry link) to adopt one particular position and can be right, another can have an impeccable record of working for the benefit of mankind and be wrong. Those warning about the global warming have at least one personal interest: inflating the potential danger increases the chances for grant monies. The personal interests of the other camp have been mentioned by Brucie in one old post.

One item from the Scientific American News section in June this year:
Rising Sun
Humans may be shouldering too much of the blame for global warming, according to a new look at data from six sun-gazing satellites. They suggest that Planet Earth has been drenched in a bath of solar radiation that has been intensifying over the past 24 years--an increase of about 0.05 percent each decade. If that trend began early last century, it could account for a significant component of the climatic warm-up that is typically attributed to human-made greenhouse gases, says Richard C. Willson of Columbia University's Center for Climate Systems Research in Coronado, Calif. Willson concedes that the climate's sensitivity to such subtle solar changes is still poorly understood, but the evidence merits keeping a close eye on both the sun and humans to better gauge their relative influences on global climate. "In 100 years I think we'll find the sun is in control," he says. His team's report appears in the March 4 Geophysical Research Letters. (I wouldn't see yet that humans can be held responsible for that)

A policy statement of the American Association of State Climatologists (I can agree with much of what they write)

Wolfgang