This is the second blog post in a multi-part series on the Bingaman Clean Energy Standard. Read part 1.

One question on the eve of the debut of Sen. Jeff Bingaman’s (D-NM) Clean Energy Standard bill:  What is “clean”?

Broadly speaking, a clean energy standard requires an electric utility to generate or purchase a certain percentage of its supply from “clean” sources. Judging by previous Congressional proposals for promoting renewable or other low-emitting energy sources – not to mention the electricity portfolio standards in place in 31 states and the District of Columbia – cleanliness is in the eye of the beholder.

When President Obama proposed a clean energy standard in his 2011 State of the Union speech, he drew from earlier Republican proposals to include not only renewable energy, but nuclear power, hydropower, coal with carbon capture and storage, and, getting partial credit, natural gas. But is nuclear power clean?  Is hydropower clean? Is clean coal clean?  Even some renewable sources have their environmental challenges – just ask the desert tortoise or residents of Cape Cod.  How do we define “clean”? It depends on the objective of the bill.

Like the Obama proposal, the Bingaman CES is expected to use carbon intensity as its central measure. This may be viewed as a statement that carbon dioxide is a reasonable surrogate for the other environmental objectives.

One proposal in the last Congress suggested that the objective shouldn’t be “clean” energy, but the maintenance of an affordable and diversified energy mix. The recent advances in the production of natural gas can be a great boon to our country, but are we in danger of retarding progress in the development of renewable energy, nuclear power, and carbon capture and storage?  Should we bet the farm on natural gas being cheap for decades to come? Or should we hedge our bets and maintain a diverse portfolio of energy sources, just in case?

Another objective, suggested by some state laws, is to advance technologies we anticipate will be major factors in the global race to develop and sell clean energy technology. China and Europe have been making massive investments in low-carbon energy technologies, anticipating a global market that will be one of the major economic engines of the 21st century. Shouldn’t the United States be in the clean energy race as well?

Fighting climate change, protecting the environment generally, keeping electricity affordable by maintaining a diverse energy mix, or keeping the United States competitive in the global economy – what’s the right objective? A clean energy standard can help us achieve them all. The trick is in striking the right balance.

For more on how a CES can be designed to balance the many important objectives, see our Illustrative Framework for a Clean Energy Standard for the Power Sector. For more on the basics of a CES, listen to our Clean Energy Standard 101 podcast.

Manik Roy is Vice President for Strategic Outreach at C2ES.