The number flashing across the Treasury market has become difficult for investors to ignore.

The yield on the benchmark 10-year U.S. Treasury surged above 5.1% this week, reaching levels not seen since 2007 and delivering a fresh shock to investors who had spent years becoming accustomed to dramatically lower interest rates.

The move matters because the 10-year Treasury is not merely another bond.

It is one of the most important reference points in global finance.

Its yield influences mortgage rates, corporate borrowing costs, valuations for technology companies and the return investors demand across a wide range of assets.

When it moves sharply higher, the impact can spread through markets almost immediately.

On Wednesday, September 23, the 10-year Treasury yield climbed about 14 to 15 basis points to around 5.11%, while an intraday move took it as high as roughly 5.14%. The level was the highest since July 2007. The 30-year Treasury yield also moved above 5.4%, while the five-year yield pushed above 5% for the first time since 2007.

That combination is what has investors paying especially close attention.

This is not a small move concentrated in one part of the Treasury curve.

Shorter- and longer-dated yields are rising together, suggesting markets are repricing both the expected path of Federal Reserve policy and the longer-term cost of money.

The immediate catalyst was surprisingly strong economic data.

S&P Global’s September flash purchasing managers’ survey showed U.S. business activity accelerating at its fastest pace since July 2021. The composite index rose to 58.4, with services and manufacturing both showing stronger activity. Manufacturing alone climbed to 57, while services reached 58.7.

Normally, strong economic growth would be welcomed by investors.

But the bond market interpreted the report differently.

A stronger economy gives the Federal Reserve more room to keep interest rates elevated if inflation remains too high.

And inflation is exactly what investors are worried about.

The same data indicated that input prices were rising, with energy, transportation and wage costs contributing to renewed price pressure. Oil prices have also climbed sharply as geopolitical tensions surrounding Iran and the Middle East threaten to keep energy markets volatile.

That is a difficult combination for the Fed.

If economic activity is weakening and inflation is cooling, the case for lower interest rates becomes easier to make.

But when growth is strong while prices remain under pressure, policymakers have less reason to ease and more reason to consider tightening.

Federal Reserve Governor Michael Barr added to those concerns on Wednesday, saying further rate increases were likely to be required to bring inflation back toward the central bank’s target. His comments arrived just as the latest economic data were reinforcing the argument for keeping policy restrictive.

Markets responded quickly.

Expectations for another Fed rate increase in October moved sharply higher, with estimates from market-based measures varying as trading conditions changed. The key point was that traders were no longer treating additional tightening as a remote scenario.

That repricing hit stocks almost immediately.

The Nasdaq Composite fell 1.13% on September 23, the S&P 500 declined 0.75% and the Dow Jones Industrial Average lost 0.68%. Smaller companies were hit even harder, with the Russell 2000 dropping about 1.8%.

The reason is rooted in valuation.

Stocks are priced based partly on expectations for future earnings.

When long-term interest rates rise, the value investors assign to those future earnings can fall because the discount rate becomes higher.

That effect can be especially pronounced for growth companies whose expected profits lie far in the future.

Technology stocks therefore tend to be particularly sensitive to large moves in Treasury yields.

The irony is that many of those same technology companies are currently spending enormous amounts of money on artificial intelligence.

AI investment is pushing companies to raise capital for data centers, chips, electricity infrastructure and networking equipment.

Higher Treasury yields increase the baseline cost of borrowing.

That can make the AI infrastructure race more expensive at precisely the moment when companies are committing unprecedented amounts of capital.

The bond selloff also carries a psychological dimension.

Investors remember what happened when rates rose rapidly earlier this decade.

The 2022 market shock was particularly painful because stocks and bonds both suffered as central banks aggressively tightened policy to fight inflation. The traditional assumption that bonds would provide protection when stocks fell proved unreliable because inflation was driving interest rates higher at the same time as equity valuations were contracting. The episode left many investors acutely aware of what sustained rate shocks can do to diversified portfolios.

Today’s situation is not identical.

Inflation, economic growth and monetary policy are operating in a different environment.

But the memory remains.

That is partly why a 10-year yield above 5% feels so psychologically significant.

It changes the relative attractiveness of assets.

When Treasury yields were extremely low, investors often had little choice but to move into riskier assets to seek meaningful returns.

At 5%, government bonds can provide a much more substantial income stream.

That can alter the competition for capital.

Investors who previously accepted high equity valuations because safe assets offered little yield now have a different alternative.

The financial mathematics are changing too.

Suppose an investor expects a company to generate a certain amount of earnings ten years from now.

At a low discount rate, those earnings have a relatively high present value.

At a higher discount rate, their present value declines.

That does not mean stocks automatically collapse whenever yields rise.

But it does mean investors have to reassess how much they are willing to pay for growth.

The bond market itself has another problem to digest: supply.

The U.S. government needs to finance a very large amount of debt, and the Treasury market must absorb ongoing issuance.

At the same time, corporations are increasingly issuing debt for major capital projects, particularly around AI infrastructure.

That means government and corporate borrowers are competing for investor capital.

When supply is abundant, investors may demand a higher yield before purchasing the debt.

This can become a self-reinforcing cycle.

Higher yields raise borrowing costs.

Higher borrowing costs can increase interest expenses.

Greater interest expenses can increase financing needs.

The market then demands more compensation to absorb additional debt.

That is one reason investors are watching long-term yields so closely.

Another warning came from Wednesday’s five-year Treasury auction.

The government sold $70 billion of five-year notes at a yield of about 5.033%, while demand metrics were weaker than recent averages. A less enthusiastic auction can amplify concerns that investors are becoming more demanding about the price they receive for holding U.S. debt.

Still, the bond market does not have only one possible path.

If economic activity eventually slows, inflation cools and the Fed becomes less inclined to raise rates, Treasury yields could retreat.

The market’s current pricing therefore remains highly dependent on data.

That is why every new inflation report, employment number, business survey and energy-price move has become potentially important.

For households, the consequences of the yield surge are tangible.

Mortgage rates tend to move with long-term Treasury yields.

Corporate loans become more expensive.

Auto financing can face upward pressure.

Credit conditions can tighten.

For investors, the changes are broader.

A higher risk-free rate can pressure expensive growth stocks, reshape bond allocations and change the relative appeal of different asset classes.

It can also strengthen the U.S. dollar if foreign investors expect higher American interest rates.

That, in turn, can affect commodities and emerging-market assets.

The 10-year Treasury is therefore doing much more than signaling what the bond market thinks about rates.

It is transmitting a message through the entire global financial system.

And the message today is unusually loud.

The U.S. economy is proving resilient.

Inflation risks have not disappeared.

Oil remains a potential source of additional price pressure.

The Fed is facing renewed tightening expectations.

And investors are demanding higher yields to lend money for the long term.

At roughly 5.1%, the 10-year Treasury is back in territory that markets have not seen in almost two decades.

That does not guarantee another crisis.

But it does guarantee that investors are once again living in a world where the cost of money matters.

After years of near-zero interest rates, that may be the biggest adjustment of all.