by Roberta B. Bowman In 1999, fewer than 500 companies issued sustainability reports. That number is now over 3,500. Once only a concern for a few niche companies with sustainability as a core value and brand differentiator, corporate sustainability has moved categorically into the mainstream. Today, there is a growing demand for companies to demonstrate a responsible, sustainable, long-term approach to business,  to produce a statement of their sustainability activities, and to clearly show how this relates to their core business strategy. Corporate sustainability  reports now contain detailed performance metrics…