For years, the chief executive officer was seen as the key individual responsible for viewing a company through the lens of sustainability. That made sense: If you can’t get the person at the top to pay attention, why should anyone else?

That’s changing. Today, it’s the chief financial officer who reigns supreme in sustainability in some companies. And that also makes sense: He or she who manages risk, compensation, disclosure, and financial performance will more or less set the stage for how a company addresses its environmental and social impacts.

But getting CFOs on board has been a lot harder than engaging CEOs. The latter has higher visibility — and, thus, higher exposure. He or she is more likely to speak publicly to customers, analysts, and thought leaders; is more likely to have those utterances amplified by the media or blogosphere; and is generally seen as the company’s figurehead. What the CEO says about the company’s sustainability commitments, internally and externally, sets the tone. The CFO does few of these things.

That’s not the only challenge. CFOs, by definition, are primarily focused on the company's financial well being. But not everything related to sustainability can be fit onto a conventional balance sheet or profit-and-loss statement.

A recent report from the global accounting firm Ernst & Young aims to help companies connect CFOs with their company’s sustainability efforts. (Download here as PDF.) “Sustainability issues and financial performance have begun to intertwine,” it begins. “CFOs are getting involved in the management, measurement and reporting of the companies’ sustainability activities. This involvement has expanded the CFO’s role in ways that would have been hard to imagine even a few years ago.”

There’s good reason for the shift. Institutional investors like pension funds, insurance companies, and university endowments increasingly are viewing sustainability issues — climate change, toxic ingredients, natural resource constraints, labor issues — as risk factors bearing on a company’s reputation and financial performance. Equity analysts — individuals who study companies and markets in order to assess a company’s future financial and stock performance — are beginning to incorporate sustainability metrics. Today, more than 300,000 Bloomberg terminals provide corporate sustainability data — on emissions, energy consumption, sustainability and governance policies, and more — to analysts and traders worldwide.

That is to say: The CFO’s principal stakeholders, both internally and externally, are tuning in to sustainability, as my colleague Paul Baier noted recently.

According to Ernst & Young, there are three key areas where CFOs play a role in sustainability:

1. Investor relations. E&Y describes this as “the art of storytelling,” which surprised me, given investors’ interest in numbers, not stories. But sustainability, says the report, “can be viewed as a new character introduced into a familiar plotline. The story is still about financial promise, but with a new twist: increasingly, a company’s sustainability story is being heard and read by the same people who read its annual financial reports.”

As sustainability issues intertwine with business strategy, institutional investors are starting to view financial and non-financial performance as two sides of the same coin. The report urges CFOs to “Work with your sustainability team to develop a sustainability story for your organization. If current trends continue, the CFO could be the one telling it.”

2. External reporting and assurance. Transparent reporting of sustainability performance is important, and not just to investors and ratings agencies. Business customers are requesting information about a company’s environmental footprint — witness the growing number of supply-chain initiatives centered around a company’s sustainability commitments and performance.

E&Y also points to the growth of corporate sustainability reports, but especially to the growing wave of integrated reports that combine sustainability metrics and conventional financial reporting. “It may be a voluntary trend that gains momentum, or a development driven by government regulation,” writes E&Y. “Either way, the potential shift in the direction of integrated reporting adds to the importance of involving the corporate finance team in the sustainability reporting process today.”

3. Operational controllership and financial risk management. In 2010, the Securities & Exchange Commission issued guidance to companies regarding their responsibility to disclose material risks related to climate change. The guidance notes that a company’s CEO and CFO must certify that the company has installed “controls and procedures” enabling it to discharge its climate change disclosure responsibilities. “In other words,” says E&Y, “sustainability has found its way into the realm of controllership and financial risk management.”

To quantify inputs and outputs related to climate change, CFOs will need accounting systems that track sustainability-related events that are significant from a financial reporting perspective. This needn’t be a major leap, since purchases of raw materials, transportation, energy, etc., already are tracked in traditional financial reporting systems. E&Y advises: “Start analyzing data such as water and energy use, emissions output, employee transportation, telecommuting, virtual conferencing, copy-paper purchases, supply and distribution chain policies and practices — anything that contributes to your company’s environmental impact.”

So far, so good. But it’s unclear how many CFOs actually “get" sustainability. Relatively few companies have yet integrated sustainability and financial reporting, and CFOs rarely speak out on sustainability topics. One of the few who has is Kurt Kuehn, CFO of UPS, who last year offered his perspective on the topic. But such voices remain few and far between. To the extent things truly are shifting, it’s — well, a quiet revolution.

I remain a tad skeptical of the shift, given what I hear from companies. I am not encouraged by the few pieces of survey data, such as this 2009 research by Robert Half Management Resources, which found two-thirds of CFOs saying they expected no increase in their companies’ emphasis on green initiatives. More recently, UK consultant Christopher Gleadle noted, “Research shows CFOs who do not feel the pressure for a sustainability strategy from the CEO are left feeling underwhelmed by the sustainability agenda – which exposes a paradox.”

And I’m reminded how long the conversation about CFOs and sustainability has been around — witness my colleague Gil Friend, who penned this perspective on CFOs, risk, and sustainability ... in 2004.

Ever the optimist, I believe that Ernst & Young is on to something here, and I suspect that a venerable company with such global perspective isn’t just whistling in the dark. So, I’ll leave you with one of E&Y’s “five actions CFOs can take now to enhance corporate value through sustainability.” Action No. 2 states:

Ensure that those responsible for sustainability matters do not operate in isolation from the rest of the enterprise — especially the finance function. The financial organization, through its accounting system, must provide the sustainability function with the information needed to do its job. That information should be timely, accurate and complete — the very same attributes that financial accounting information should possess. No matter how the company structures these responsibilities, the CFO is responsible for providing the sustainability function with the necessary information.

It’s part of a sound agenda for any executive who wants to ensure that sustainability becomes woven into the corporate fabric.