*Yes, I'm a nerd and typed that in Microsoft Equation editor.
For this idea to truly bring about change, you'd also need companies to include the emissions due to their administrative usages (ie. carbon emissions due to office lighting, etc.). It's of utmost importance that the sum of the products adds up to the total for the company.
Two questions that pop out at me:
-What if you just buy the products from a warehouse, in better terms, how far down the supply chain do you take this figure? The carbon emissions rating (CER) should be inclusive of everything from the beginning up. Theoretically, if the CER were international, the sum of all the CERs would equal the global carbon emission due to non-natural sources.
-How do you include transportation of products? Tough question, in a utopia, that information would be avaliable to the customer at each possible purchasing location (or based on delivery for remote purchasing). In reality, that's a very difficult task, and I'd never vote to require business to do that. Transportation does need to be included.
If it was shown that a CER would actually affect any significant consumers, businesses everywhere would be scrambling to install motion sensors that turn of their newly installed compact fluorescents. They'd be pressuring those down their supply chain to do the same. If consumers actually cared, even just a little bit, the corporate fighting of global warming would be much more profitable (For the record, I work as an auditor for the Industrial Assessment Center, and I can tell you for a fact that a lot of energy saving policies are extremely financially justified, it's just a matter of management realizing it).
Businesses that sell products without packaging would be required to have the information avaliable (like the little fliers in fast-food restaurants). You can bet if there was any response in consumer trials you'd see great "who can be the greenest" corporate competition, combined with smear campaigns agaisnt the heavy carbon hitters (fast food would have to change a lot, except for In-and-Out burger, who buys materials locally).
Would it actually work? You speculation is as good as mine, but I think a big part of the answer is how the information is displayed (is it on the front in red, or tucked in between Thiamine and riboflavin?
Why stop with carbon? That would give an unfair advantage to companies who happen to get their power from a nuclear energy plant right now. Perhaps, it'd be better to have a nutritional informationesque rating for the most popular non-sustainable sources... Wind solar, and hydro power would bear no penalty; would burning trash (even though highly inefficient and rarely used) be counted as a plus or a negative?
Would you pay twenty cents more for a tube of toothpaste that was twice as green as it's competitor?
Should the CER be based on a quantity? That way a 4 lb jar of peanut butter's CER wouldn't look menacing compared to much less environmentally friendly single serving packages?
And NO, companies would not be allowed to buy carbon credits from goat farmers in west Niger. That's a really stupid idea, and I can't believe any governments are acutally supporting it.
I think this idea has about 0.00001% chance of being realized. When I started this post I didn't think it was a great idea, but I'm started to convince myself.