Remember the commodities supercycle, that seemingly endless 2000s commodities boom? It drove oil, gold, copper and other commodities to record levels.

The supercycle was driven by exploding demand from China and other emerging countries, supply bottlenecks caused by years of not developing wells and mines, and rock-bottom interest rates that inflated demand for hard assets all around the world.

But now gold, oil and other commodities are well off their peaks, so far off, in fact, and for so long that they can only be described as in a supercycle in reverse, or a secular bear market.

If that’s true — and I’m pretty sure it is — investors who piled in to commodities are in for a bruising decade ahead unless they take profits or cut their losses.

“ ‘By 2025, we’re going to look back and say, wow, what a great period in U.S. equities.’ ” — Shawn Driscoll, T. Rowe Price New Era fund

Meanwhile, stocks, which run counter to commodities, may well go much higher, along with the U.S. dollar.

“We believe that we are in the initial years of a secular down cycle in commodities,” wrote Shawn Driscoll, manager of the natural resource-focused T. Rowe Price New Era Fund PRNEX, -1.12% , in the fund’s most recent semiannual report.

“Commodity cycles are very long on the way up and the way down,” he told me in a phone interview. They last around 13 to 15 years, because it takes that long for fundamentals of supply and demand to go to extremes.

When the most recent supercycle began in 1998, Driscoll said, commodities prices had plummeted, so producers shuttered old mines and wells and hadn’t opened new ones in a while.

But when demand revived, it took years for producers to catch up. Ultimately, companies built too much capacity just in time for the next peak.

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That’s where we are now, he said. The previous commodities bull market started in 1968, just when the 1960s go-go bull market in stocks faltered, and ended in 1981 when gold surged above $800 an ounce.

The current commodities supercycle ran from 1998 through 2011, said Driscoll — the same length as the last one. “We think of the peak in the commodities supercycle as roughly early April 2011,” he told me.

That’s when copper topped $4.40 a pound; it’s barely above $3 now, a drop of more than 30%. Gold, which peaked around $1,900 an ounce that year, changed hands near $1,150 on Monday, almost 40% off its high.

And Brent crude oil, trading just above $80 a barrel, is more than 40% below its July 2008 record high of $146.

Can anyone seriously believe this isn’t a long-term bear?

The supercycle coincided with China’s hypergrowth before and after the 2008 Beijing Olympics, when it scoured the globe for materials to build a gleaming modern infrastructure. China’s iron ore consumption rose by 14% a year throughout the 2000s.

China’s boom also lifted commodity-producing countries and their currencies. The Federal Reserve’s ultra-low interest rates fueled the blow-off phase of the U.S. housing bubble and led investors to scour the globe for higher returns.

The financial crisis and Great Recession ended that. As governments bailed out the financial system, hard-money conservatives warned that the Fed and other central banks would eventually have to monetize the accumulated debt, leading to hyperinflation. So gold became an alternative to unstable “fiat” currencies.

Yet none of that has happened. Europe and Japan are desperate to avoid deflation and the U.S. has had a mild recovery with none of the wage growth of previous rebounds. Commodities, said Driscoll, outperform during periods of accelerating growth and heightened inflation.

“However, global growth is currently modest, supply is accelerating, and inflation is tame,” he wrote. China’s GDP growth has dropped sharply, and it’s switching from an infrastructure-based to a consumption-driven economy.

But commodities producers built up massive capacity during the boom. Exhibit A: the U.S., whose shale-oil production has added 3 million barrels a day to the world’s output.

How low can prices go? Driscoll says oil may bottom out around $50 a barrel over the next 10 years, and gold at about $800. That’s more than three times the yellow metal’s lowest price from 1999 and around its peak in the previous supercycle.

There may be some tradable rallies, but they’re not for the faint of heart. “We think we’re in year four” of the secular bear, said Driscoll. He expects it to last another decade.

U.S. investors have responded by pulling more than $30 billion out of the SPDR Gold Shares GLD, -2.83% and iShares Gold Trust IAU, -2.84% ETFs since January 2013, according to ETF.com.

Meanwhile, Mr. Commodities himself, Jim Rogers, wrote: “Historically, there has been a negative correlation between the price movement of stocks and commodities.” (Rogers is still calling this “a normal correction.” Really, Jim? After 30%-plus losses and 3 ½ years?)

Driscoll, however, thinks commodities’ eclipse is good news for U.S. stocks, as it was in 1982, when the great bull market began. “By 2025, we’re going to look back and say, wow, what a great period in U.S. equities,” he told me.

If you’ve lost money in commodities for the past 3 ½ years, you could do a Rip Van Winkle and tune out for a decade or so, or you can pretend you’ve been right all along.

I have small ETF positions in gold, energy and the Australian dollar. I may sell some or all of them in the days ahead. You should think about it too.

Howard R. Gold is a MarketWatch columnist and founder and editor of GoldenEgg Investing, which offers free market commentary and simple, low-cost, low-risk retirement investing plans. Follow him on Twitter @howardrgold.