Has sustainable and responsible investing (SRI) in the United States become bigger than the Rocky Mountains?

That might sound like an odd question, but it's one worth pondering. It certainly means something when the long-running SRI in the Rockies conference for sustainable and responsible investors is now such a big deal that it is moving to the decidedly non-Rockies location of New Orleans.

The truth is that the evolution of SRI in the Rockies (October 2-5 this year) -- from a regional event to a national conference that was held in the same region every year, to what is now a traveling national conference -- mirrors what's happening in the field in general.

Far from being a faddish niche, SRI is now very much part of the investing world, with more than $3 trillion in assets under professional management in the U.S. alone, according to the 2010 Report on Socially Responsible Investing Trends in the United States from U.S. SIF, the Forum for Sustainable and Responsible Investing. SRI hinges on use of ESG (environmental, social, governance) analysis, shareholder advocacy, and "community investment" strategies.

That $3 trillion in publicly traded securities in the U.S. represents a more than 13 percent increase in assets under management between 2007 and 2010. Over the same period, the broader universe of professionally managed assets grew by less than 1 percent.

So, here is the $3-trillion question: Why has the SRI space enjoyed such robust growth, during a period of global economic slowdown? There are several possible answers:

First, socially conscious investors have benefited from an expansion of quantity and improvement in quality of investment products and services designed to make money and make a difference. The global growth of the SRI industry has tracked the increasing public interest in sustainability and all things "green."

From slow food, to green building, to climate change, people are paying more attention to their impacts on the world; they are asking more questions about how their actions impact the commons known as planet earth.

Second, although many tend to focus on the "E" (for Environment) in ESG, interest in the role of corporate governance (the "G") has risen tremendously over the last few years.