When GDP Growth Has to Outrun Debt Costs (And What Happens If It Doesn't)

The U.S. economy is growing at about 6% in nominal terms right now, and the 10-year Treasury yield just hit 5.16%. That gap matters more than most people realize.

Here's why: the federal government owes $40 trillion. Every year, it has to pay interest on that debt. If the economy grows faster than the interest rate, the debt becomes more manageable over time as a percentage of GDP. But if the interest rate catches up to or exceeds the growth rate, you get what budget analysts call a debt spiral. The debt grows faster than the economy's ability to service it.

Right now, nominal GDP growth is still ahead. The economy's running hot, boosted by AI infrastructure spending that's approaching railroad-mania levels and a $2 trillion federal deficit that's pumping money into the system. But Treasury yields have jumped more than a full percentage point since the Iran conflict started, and that gap between growth and borrowing costs is narrowing.

The Federal Reserve raised rates earlier this month because the economy isn't just resilient, it's thriving. That's creating a weird situation where strength becomes its own risk. More growth means more inflation pressure, which means higher yields, which means the hamster wheel spins faster.

The AI build-out is the biggest factor. Alphabet, Amazon, Microsoft, Meta, Oracle, and SpaceX are projected to spend $870 billion on capital expenditures this year, up from $470 billion in 2025. S&P Global estimates that number hits $1.3 trillion in 2027.

That's not just tech sector money anymore. Industrial companies like Caterpillar and GE are seeing massive orders tied to data center construction. UBS economist Jonathan Pingle wrote this week that "the breadth and magnitude of the AI investment impulse spilling over to other sectors is as surprising as it is extensive."

Economist Stijn van Nieuwerburgh says the AI spending wave is on track to be the biggest infrastructure boom in U.S. history, bigger than the railroads.

The federal deficit is the other engine. Most of that $2 trillion goes into consumers' pockets through entitlement payments, which eventually cycles into corporate profits and stock valuations. Research Affiliates pointed out earlier this year that deficit spending is effectively stimulus, even if nobody wants to call it that.

A recent gauge of U.S. business activity for September hit a five-year high, and third-quarter GDP could show even more acceleration. Gas prices and consumer sentiment about cost of living have been negative, but the actual economic data has been strong.

The problem isn't today. It's what happens when something changes.

If AI spending slows down, the economy's hottest growth engine cools off. Some Wall Street analysts think the AI bubble is close to popping. Instances of AI agents going rogue and fears about the technology's risks have led to calls for slower development, which could mean less investment.

Higher borrowing costs could also reduce AI spending. Rockefeller International Chairman Ruchir Sharma said in a Financial Times op-ed that the bubble could pop when the 10-year yield decisively exceeds 5%, signaling "a new era of tighter money, in which AI mega projects will be harder to fund."

And here's the key part: yields topping 5% would start to approach nominal GDP growth. If borrowing costs match or exceed the economy's growth rate, the national debt math breaks.

The Committee for a Responsible Federal Budget has been warning about this for years. They said Wednesday that "with interest rates on new Treasury bonds and notes at around 5% and medium-term nominal economic growth expected to be closer to 4%, the U.S. is entering a debt spiral."

That could lead to a fiscal crisis, which could show up as exploding unemployment, crashing asset values, surging inflation, falling incomes, sharp tax increases, cuts in government support, or some combination.

You'd think slower economic growth would bring down bond yields, but it doesn't necessarily work that way.

Other heavily indebted countries and AI companies are competing for bond investors' capital. Treasury auctions need attractive yields to draw enough demand, regardless of what the economy's doing.

Then there's the geopolitical environment. The Iran conflict, trade friction, and frequent shocks have created a less stable world. That risk gets priced into yields. The market's not treating these as one-off events anymore.

The Fed's willingness to keep fighting inflation is also uncertain. Chairman Kevin Warsh earned credibility with his hawkish stance, but the market could turn on him if inflation stays high and the Fed backs off.

Wall Street veteran Ed Yardeni wrote Wednesday that "in the past, especially during the 1980s, the Bond Vigilantes pushed the bond yield above nominal GDP to slow the economy. They haven't done that so far. The risk is that they will do that if the Fed fails to subdue inflation."

Bond vigilantes are investors who sell Treasuries (pushing yields higher) to punish the government for fiscal or monetary policy they think is reckless. If they decide current policy is unsustainable, they don't wait for permission to act.

The spread between nominal GDP growth and the 10-year yield is the key number. As long as growth stays above borrowing costs, the debt load stays manageable. If that flips, the math gets ugly fast.

Right now, we're at roughly 6% nominal GDP growth versus 5.16% on the 10-year. That's still positive, but it's tightening.

AI capital expenditure reports from the hyperscalers will show whether the infrastructure boom is sustaining or slowing. If those numbers start coming down, the economy's biggest growth driver weakens.

Fed commentary about inflation expectations will signal whether rates are likely to go higher or hold. If the Fed signals it's done raising, yields could stabilize. If it suggests more hikes are coming, the hamster wheel speeds up.

Treasury auction results will show how much demand there is for U.S. debt at current yields. Weak demand means higher yields to attract buyers. Strong demand means the market's still comfortable financing the deficit.

And geopolitical developments matter because they directly affect yields through risk pricing. Market structure responds to uncertainty by demanding higher compensation for holding longer-dated assets.

The U.S. economy is strong right now, but it's also stuck in a situation where it has to keep running faster just to stay ahead of the debt it's accumulating. That works until it doesn't.