Strains on the U.S. economy are so profound the Federal Reserve will be shackled to its zero interest rate policy for years to come, a San Francisco Fed economist argues in new research.

Writing for the bank’s Economic Letter, Glenn Rudebusch says the Fed’s history and its current economic expectations indicate “the funds rate should be near its zero lower bound not just for the next six or nine months, but for several years.”

The economist said the Fed will need to maintain this stance in part because its current interest rate policy is not easy enough, having been constrained by an inability to go below zero.

“In order to deliver a degree of future monetary stimulus that is consistent with its past behavior, the FOMC would have to reduce the funds rate to -5% by the end of this year — well below its lower bound of zero,” he wrote. That’s not possible, and it explains why the Fed will have to keep rates where they are for such a long time.

The policy Rudebusch is referring to is the one the Fed put in place at the end of last year, as it struggled to stimulate a rapidly faltering economy. The Fed then took a step unprecedented in the era of modern monetary policy-making and pegged its overnight target rate in a band between 0% and 0.25%. That was down from the overnight target of 5.25% that was in place at the start of the crisis in late summer 2007.

Pegging overnight lending rates so low allows monetary policy to add the most possible support to the broader economy. But economic and banking troubles have been such that policy makers have been forced to adopt a radical agenda of emergency lending and direct market interventions, to create an environment where those rock-bottom borrowing rates can be more effective.

It’s these more interventionist policies that have become the focus of market participants and economists. Indeed, FOMC meetings are now more watched for what the Fed says about its intentions regarding purchases of Treasury or mortgage-related debt. Most already expect, as Rudebusch predicts, monetary policy isn’t going anywhere anytime soon.

But even if rate policy decisions aren’t the star right now, they still matter. The stance of policy has a lot to say about how weak the economy has become, and how much ground has been lost since the recession began.

Rudebusch wrote, “Given the severe depth of the current recession, it will require several years of strong economic growth before most of the slack in the economy is eliminated and the recommended funds rate turns positive.”

He notes that “like many private forecasters, FOMC participants foresee persistently high unemployment and low inflation as the most likely outcome over the next few years.”