We’re wising up to green-washing, and we’re doing it wisely. Not every ad featuring glossy images of the verdant globe is greeted with full-on skepticism.  Put another way, we may be dumb, but we’re not so dumb that we buy GM’s “gas-friendly to gas-free” ad campaign.

Consumers are skeptical about a wave of green advertising even as the overall value of that market is exploding, according to results of market research issued today by Havas Media, a unit of Havas, a global marketing company based in Paris.

The study, Sustainable Futures ‘09, finds that many companies are unable to get their messages through to consumers because some marketing campaigns are seen as little more than opportunism on the part of big business.

“This lack of connection between what companies are doing and how they are perceived, threatens to weaken relationships between brands and consumers,” Havas warned.

The firm found that 64 percent of respondents viewed sustainability campaigns as a little more than a “marketing tool.” That result, said Havas, was particularly problematic because consumers continued to show strong interest in buying goods and services from responsible sources.

“It is not a case of consumers being fickle, but rather a case of businesses being perceived as unauthentic,” said Havas, which noted that companies in sectors like food, retail and telecommunications had been more successful so far than companies in the oil and automotive sectors at using messages pegged to sustainability.