Perhaps we should all begin to worry. Have you seen the advertisements, the ones from the oil companies?

I originally ignored them, going all the way back to 2000 when British Petroleum changed its corporate symbol from a shield to a flower with a bright yellow sunburst and started telling the world that "BP" stood for "Beyond Petroleum."

I like clever advertising. I thought that was a great gimmick, a chutzpah-filled hoot to have a mega-billion-dollar oil company warning that the global oil barrel would not remain eternally full and suggesting that we need to consider other sources for powering the future.

My cynicism was stoked by myriad environmental groups, many of which lost no time pointing out that BP spent -- as it pretty much continues to spend -- 50 times more money pulling oil from the earth and seas than capturing energy from the wind or sun, or exploiting the possibilities of hydrogen.

The same year BP began its green-energy marketing campaign, members of the activist environmental group Greenpeace International occupied a BP barge on its way to an Arctic Ocean site where the company was preparing to drill for oil. It all made for a good laugh -- nothing to be taken seriously.

But then the Royal Dutch/Shell Group of Companies started mouthing BP-type warnings and started talking about alternative energy exploration -- topics that later seemed especially prescient for Shell given that company's recent overestimates of its exploitable oil reserves. Now, Exxon Mobil Corp. has joined the campaign, including a multimillion-dollar marketing strategy and a resurfacing of its once-ignored report, "The Outlook for Energy: A 2030 View."

The Exxon report, issued last year, forecasts a peak in oil production outside of the Middle East and other countries not belonging to the Organization of Petroleum Exporting Countries by 2010. By 2030, a decline in global oil production will be aggravated by a world demanding 50 percent more of the stuff than it uses today, the report said.

"Peak" in oil parlance refers to the top of a slippery slope. When we reach the top in terms of production, it's all downhill from there. That means countries such as the United States, Britain, Norway and Mexico, which now provide 60 percent of the world's oil, "are already in a production plateau," according to an industry analysis published in May by Alfred J. Cavallo, an energy consultant in Princeton, N.J.

It also means that countries in North America and Western Europe, the largest users of fossil fuels, increasingly will have to depend on OPEC countries to fuel American and European cars and trucks and American and European economies.

Most of us know this isn't a good thing and that something must be done about it. But, at least according to Exxon Mobil's numbers, we live in a world that consumes 80 billion barrels of oil and uses nearly 300 billion cubic feet of natural gas every day. We're gassed! And although some of us occasionally rumble into sobriety and scream warnings in anxiety, few of us are willing to give up the big car, big house, big trip or any of the other goodies afforded by big energy. We talk a good game but pass the check on to someone else, or, at least, we try to.

But it looks as if the jig is up, and those rich oil companies, the leaders of which have a distinct inability to elicit sympathy or faith from most us, appear to be trying to tell us as much. What bothers me is that their warnings stem from a credible source -- pure, unadulterated greed.

Pie-in-the-sky, holier-than-thou, I'm-cleaner-than-you environmental altruism is something I can easily ignore. It's like religion. You can believe it, accept it, or not. But greed is real. It's visceral. It seeks to sustain and preserve itself at any cost. It underlies the reality that oil companies are not so much about oil as they are about money. They have no allegiance to crude. Their crude loyalty is to cash.

Thus, if the oil companies think, as they apparently do, that declining oil reserves are threatening their future cash flow, maybe we all should start listening to them and pay closer attention to what once appeared to be slick public relations-only campaigns for developing alternative fuels.

The way I figure it, if the oil companies run dry without a plan, that's really bad for them. But in no way can that be good for us.

Energy companies have begun marketing alternative fuels, which may help the bottom line when oil runs low.