New York Times: AT an annual gathering of the advertising industry a year ago in Cannes, the environment was the topic du jour. “Be seen, be green,” one agency urged on the invitation to its party at a hillside villa. Al Gore, invited by another agency, delivered a message linked to “An Inconvenient Truth,” his book and film about climate change: That the ad industry could play an influential role in encouraging businesses and consumers to change their ways and slow global warming. The sun …