The financial crisis, which began 10 years ago next month when Lehman Brothers collapsed, led the Federal Reserve to take desperate measures.

It cut the federal funds rate to its lowest level ever, effectively zero percent. But as the Great Recession deepened, Fed Chairman Ben S. Bernanke embarked on a novel policy: The central bank bought trillions of dollars of debt — mostly Treasuries and mortgage-backed securities — to keep long-term rates low and stimulate economic activity.

Ahead of tomorrow’s Fed announcement:Debate over Fed meeting centers on just two words

That massive bond-buying program, dubbed “quantitative easing” in stilted Fedspeak, quintupled the size of the Fed’s balance sheet to $4.4 trillion. At the Fed’s annual economic symposium in Jackson Hole, Wyo., in 2012, Bernanke proclaimed it a big success: “There is substantial evidence that the Federal Reserve’s asset purchases have lowered longer-term yields and eased broader financial conditions,” he said.

The first two rounds of QE “may have raised the level of output by almost 3% and increased private payroll employment by more than 2 million jobs, relative to what otherwise would have occurred,” Bernanke said.

Some academics also estimated these “large-scale asset purchases” altogether cut 10-year Treasury yields TMUBMUSD10Y, 0.656% by 100 basis points, or one percentage point, which economists estimate would have boosted total gross domestic product growth by 0.5% to 1% percentage point.

But in a recent study, other researchers found that QE had much less direct impact on the bond market.

Two prominent academic economists, James Hamilton of University of California San Diego and Kenneth West of the University of Wisconsin, and two leading Wall Street economists, David Greenlaw of Morgan Stanley and Ethan Harris of Bank of America Merrill Lynch, closely monitored market reactions to Fed announcements and other news from QE’s introduction in November 2008 through 2014, when the Fed stopped buying securities.

In a paper presented at the annual U.S. Monetary Forum earlier this year, the researchers found that “Fed actions and announcements were not a dominant determinant of 10-year yields and that whatever [their] initial impact…, the effects tended not to persist.”

They called the impact of QE “likely more modest than generally claimed” — reducing 10-year yields by only ½ to one percentage point — and determined that the Fed’s balance sheet “should not be viewed as a primary tool of monetary policy going forward.” This suggests the Fed may have even less maneuvering room during the next recession than it did in the last one.

“It’s a modest stimulus. It’s not like this is a game changer,” Harris told me in a phone interview. He called Bernanke’s two-million jobs claim “on the optimistic end of the spectrum.”

“Their ability to lower bond yields was less than what people think. Even the earlier rounds weren’t quite as effective as they seem…because there was some give-back in the markets after the fact.”

In fact, the study pointed out, “European news — including political events, economic data and monetary policy developments — was a major persistent driver of the U.S. bond market during this period,” lowering 10-year Treasury yields by some 150 basis points during the sample period.

Events like the Greek debt crisis, the European Central Bank’s own QE program, and fears the European Union might collapse were “a bigger driver of the U.S. bond market than what the Fed was doing,” Harris said.

Actually, Harris supports the Fed’s actions, because of what he called its “confidence effect.”

“If the Fed had come out and announced we’re not going to do anything beyond rate cuts, we’re out of ammunition,” he told me, “I think the outcome would have been much worse.”

Still, he added, “we’d rather the Fed have room to cut interest rates and rely on policy conventionally than rely on a tool that is probably weaker and certainly less certain in its impact.”

Federal Reserve Board Chairman Jerome Powell. Getty Images

And that’s the problem now: Even after several rate hikes, the federal funds rate sits at around 2%. A 1/4-point rate hike every quarter would put it at only 3.5% by the end of 2019 — far short of the 5.25% at its peak during the last cycle, 2006-2007. The Fed, the study said, would thus “have significantly less ability to fight a recession by lowering nominal rates than it generally ended up needing in recent historical downturns.”

“This is the number one question facing the Fed in the next several years is what do you do about your low ammunition problem?” he said. “Both the European Central Bank and the Bank of Japan are at the zero lower bound still on interest rates. They can’t cut.”

“You have lack of ammunition at the Fed, lack of ammunition at other central banks, and potential constraints on fiscal stimulus,” Harris said.

Harris doesn’t see a recession coming in the next couple of years, and by the time the next one comes, the Fed may have raised rates even more. But with trillions of dollars still on its balance sheet, could it rely on what’s proved to be a very weak tool to take up the slack?