Ray Dalio thinks the ability of central banks to reverse an economic downturn is coming to an end as the global economy enters what he says are the late stages of the long-term debt cycle.

Dalio also is warning that current conditions remind him of the Depression-era environment.

He cited three factors: Ineffective central banks, high levels of debt and the U.S.-China conflict.

“Interest rates get so low that lowering them enough to stimulate growth doesn’t work well,” the billionaire founder of investment management firm Bridgewater Associates wrote in an essay published on LinkedIn this week..

“If/when there is an economic downturn, that will produce serious problems in ways that are analogous to the ways that the confluence of those three influences produced serious problems in the late 1930s,” Dalio wrote.

Dalio says the last time similar forces were creating a similar dynamic was from 1935 to 1945.

“There is a lot to be learned by understanding the mechanics of what happened then,” he wrote.

In the latest essay, he spoke of how central banks are being forced to keep interest rates low and “print money to buy financial assets” in order to prop up markets and make huge fiscal deficits affordable. He said there are “strong deflationary forces at work” as capacity has surged.

Money printing and buying financial assets won’t work either, Dalio said, as it doesn’t produce adequate credit in the real economy and creates the need for large budget deficits and then their monetization.

“These forces are creating the need for extremely loose monetary policies that are forcing central banks to drive interest rates to such low levels and will lead to enormous deficits that are monetized, which is creating the blow-off in bonds that is the reciprocal of the 1980-82 blow-off in gold,” he said.

Dalio directed readers to study the economic and investing conditions of 1935-45 as “there is a lot to be learned by understanding the mechanics of what happened then (and in other analogous times before then) in order to understand the mechanics of what is happening now. It is also worth understanding how paradigm shifts work and how to diversify well to protect oneself against them.”

U.S. President Donald Trump on Wednesday renewed criticism of the Federal Reserve, saying in a tweet that it had “no clue.” Earlier this month, he said the bank needed to cut rates by at least 100 basis points.