Al Gore has spent the better part of this week flooding the media with the idea that oil and gas stocks are sub-prime investments. He has likened the "absurd overvaluation" of fossil fuel companies to the subprime mortgage catastrophe that hammered our economy. He's encouraging investors to dump their holdings in fossil fuel companies.

He's not the only one calling for divestment. A growing chorus of voices is urging investors to rethink carbon assets on corporate balance sheets. Indeed, a coalition of investors just released a guide on how to eliminate carbon from your portfolio, and reinvest that money in other strategies. Co-authors, Green Century Funds, and Trillium Asset Management are involved in a movement that is gaining traction.

The idea is that fossil fuels companies value their carbon-based assets on the assumption that they can all be burned, but that might be unrealistic. If the world gets serious about constraining climate change and keeping global warming below two degrees Celsius, then about two-thirds of carbon assets will have to be left in the ground, rendering them "unburnable reserves." That, in theory, would force a major writedown of assets for some of the world's biggest companies. ExxonMobil (NYSE:XOM), Arch Coal (NASDAQOTH:ACIIQ), and ConocoPhillips (NYSE:COP) are right at the top of that list.

This concept is known as the "carbon bubble," and its implications for corporate balance sheets are profound.

Other voices disagree with the divestment movement. Simon Billenness of CSR Strategy Group argues that shareholder activism is a more effective strategy. His point is that shares constitute influence, and he gives the example of important changes at ExxonMobil that came as a result of shareholder activism.

Either way, long-term investors need to rethink carbon. Watch the video below to explore the topic further.