For over a decade, borrowing yen at near-zero rates to invest it elsewhere was one of the most discreet and powerful sources of funding in global finance. That tap of cheap money is now closing, and it is doing so through a channel almost no one watches: Japan's bond market.
1 The mechanism: borrow low, invest high
Currency carry, the carry trade, is an old recipe gone colossal.
The tap of cheap money
Borrowing a zero-rate currency to invest elsewhere
Carry consists of borrowing in a very low-rate currency, the yen, to invest in better-paying assets around the world: US bonds, equities, emerging markets, sometimes crypto-assets. As long as the yen stays stable and Japanese rates low, the yield gap is pocketed with little apparent risk. For years, this funding fed markets across the planet.
Why it grew so large
①
Near-zero cost. Japanese rates stayed close to zero for nearly three decades, making the yen the cheapest borrowing currency in the world.
②
Leverage. Borrowing to invest multiplies gains, but also losses: that is the heart of the fragility.
③
Discretion. The funding is in yen, but the investments are scattered everywhere: no institution sees the whole, and no one truly measures the size of the thread.
The Achilles' heel
The bet rests on two conditions: a stable yen and low Japanese rates. If the yen suddenly appreciates, or if Japanese rates rise, the cost of funding swells and gains flip into losses. The unwind then becomes a rush for the exit.
2 Japan's fault line
A long-sleeping bond market woke up all at once.
One stimulus too many?
When a fiscal promise breaks the rate market
In January 2026, the new Prime Minister Sanae Takaichi called a snap election, pledging aggressive fiscal expansion, tax cuts and higher spending. In a country whose public debt reaches 236.7% of gross domestic product, one of the highest in the world, the announcement was enough to break the bond market in a single session.
Japan's 40-year yield
> 4 %
For the first time since this maturity was created in 2007 (January 2026).
Public debt
236,7 %
of GDP, leaving markets highly sensitive to any unfunded stimulus.
The Bank of Japan, which has begun a hiking cycle, from near zero to 0.25% in 2024 and toward 0.75%, is walking a tightrope: hiking too fast would trigger the unwind, too slowly would let the yen slide. By June 2026, the 40-year had eased back to around 3.76%, without dispelling the underlying tension.
3 The unwind and the contagion
When the funding withdraws, it never takes only the yen.
Echoes of August 2024
A still-fresh precedent
In late July 2024, a single Bank of Japan rate hike was enough to reverse the yen's path. Within a few sessions, the Nikkei lost nearly 20%, its worst fall since 1987, world markets wobbled, and bitcoin plunged from 64,000 to 49,000 dollars in 48 hours. All from a currency move.
Why it never stays in Japan
The unwind never stays confined to the yen, because the funding layer sits beneath a multitude of different positions across different markets. When it withdraws, it forces simultaneous selling almost everywhere: US Treasuries, European bonds, emerging markets like India. A thread pulled in Tokyo, and the whole global fabric tightens.
Positions still open
≈ $500B
of yen-funded carry, despite the partial unwind of 2024 (Morgan Stanley estimate).
August 2024
−20 %
The Nikkei over five sessions, its worst fall since the 1987 crash.
4 What it changes
Invisible plumbing, and the danger isn't where you'd expect.
The right diagnosis
The risk is not the level, it's the speed
What worries is not so much the level of Japanese rates as the speed of any reversal. An orderly, gradual unwind can be absorbed by markets. A brutal unwind, amplified by leverage, feeds on itself: selling pushes the yen up, the stronger yen worsens losses, which force more selling. It is this spiral mechanism to watch, more than any particular rate threshold.
Two takeaways
①
Markets are more linked than they look. Yen funding quietly connects Tokyo to Wall Street, Frankfurt and Mumbai. Apparent diversification hides a common dependency.
②
The danger hides in boredom. A market deemed dull, Japanese government bonds, can become the trigger of a global shock. The most serious risk is often the one no one watches anymore.
The cheapest funding in the world carries a hidden price: it makes markets across the entire planet dependent on a single rate, set in Tokyo.