There's been a steady drum beat of wild-eyed clean-tech forecasts for more than a year now. What's one more trillion-dollar log on the fire?

But CERA is no pump-and-dump stock scam operating out of an abandoned warehouse in Jersey. It's a respected global advisory group made up of serious economists and energy analysts and headed up by Daniel Yergin, a recognized energy expert and Pulitzer Prize-winning author.

Unfortunately, the report doesn't appear to break any new ground with its forecast -- it mentions that alternative energy is fast approaching mainstream application, that continued advancements in technology could rapidly speed both deployment and acceptance of clean tech, and that nuclear energy will play a large role in low-emissions power.

But it does add a bookend -- albeit an incomprehensible one -- to the debate about the direction and pace of the alternative energy sector by attaching a dollar amount to the size of the potential investment market.

By and large, much of the financial reporting on the alternative energy market has followed a similar pattern, focusing on investments by venture capital firms in green-tech startups, the green investing revolution, the inevitable collapse of the alternative energy bubble, or the dubious benefits of ethanol.

Few folk have been brave or foolish enough to venture a guess on the future girth of the alternative energy market. It's hard enough just to get a reasonable estimate of current investments, let alone to project out 20-plus years down the road. If reports are to be relied upon, 2007 was the high-water mark for investments in the alternative energy market in the U.S. and Europe. Last year, investors poured $5.8 billion into the industry, according to one report by the Cleantech Group. New Energy Finance, a London-based research firm, estimated $117 billion was invested worldwide by companies and governments in 2007.

Normally, I'd describe the difference between $5.8 billion and $117 billion as a yawning gap, but compared to the spread between $117 billion and $7 trillion, it's a mere rounding error. Maybe the market and investors were wise to ignore the announcement.