The Japanese yen fell overnight to its lowest level against the dollar since April 2009 as investors continued to respond to policymakers’ aggressive efforts to stimulate the Asian nation's moribund economy.

The yen has been weakening since late 2012 as Prime Minister Shinzo Abe campaigned on a platform to boost economic growth via massive fiscal and monetary stimulus. The trend continued after his election victory and accelerated in recent weeks after newly appointed Bank of Japan Governor Haruhiko Kuroda announced plans to double the BOJ’s monetary base by the end of 2014.

Last week, Kuroda announced an aggressive program of bond buying and set an inflation target of 2% for Japan, which has been struggling with deflation and lackluster economic growth since the early 1990s.

“The new government under Abe and Kuroda is trying to print money and really go for the fences,” says Washington Post columnist Neil Irwin, author of The Alchemists: Three Central Bankers and a World on Fire.

While Fed Vice Chairman Janet Yellen (among others) called the BOJ’s actions “very appropriate,” investing legends such as Bill Gross and George Soros (among others) have warned it could lead to ruin.

Related: GARY SHILLING: The Japan Train Wreck Is Accelerating

Irwin is non-committal on how this plays out but is certainly aware of the risks Japan is taking.

“It’s a giant bet,” he says of what’s being called Abe-nomics. “Is there a tipping point where their debts can become the mother of all debt crisis?”

At issue is Japan’s debt-to-GDP, which is already the largest in the developed world at 200%, a problem complicated by the nation's lackluster growth and aging population.

“It’s a narrow needle to thread,” Irwin says. “ You want enough inflation and growth to get the economy back on track [which] will bring down deficits, but not so much your bond yields rise and then you’re in fiscal trouble.”

Related: “Obscene” Stimulus Will Trigger ‘Made in Japan’ Crisis in 2013: Mauldin

While many Americans rightly worry about what the Fed is doing, most experts agree Japan’s stimulus efforts pose a more immediate threat to a fragile global economy even as financial markets continue to run fast and hot on easy money.

I agree with Irwin’s analysis that events in Japan are “the most interesting story in global economics” right now and will likely prove to be the biggest story of 2013, possibly beyond.

Related: Bernanke Is "One of the Most Consequential Central Bankers of All Time": Neil Irwin

Aaron Task is the host of The Daily Ticker and Editor-in-Chief of Yahoo! Finance. You can follow him on Twitter at @aarontask or email him at [email protected]