WASHINGTON — The global economy has begun to shudder.

On Wednesday, the U.S. stock market tumbled after a reliable predictor of looming recessions flashed for the first time since the 2008 financial crisis. The Dow Jones industrial average fell 800.49 points, or 3%, and has lost close to 7% in the past three weeks.

Two of the world’s largest economies, Germany and the United Kingdom, appear to be contracting. Argentina’s stock market fell nearly 50% in recent days, and growth in China has slowed.

Whether the events presage an economic calamity or just an alarming spasm are unclear.

But unlike during the Great Recession, global leaders are not working in unison to confront mounting problems and arrest the slowdown. Instead, they are increasingly at each other’s throats.

And President Donald Trump has responded by claiming the economy is still thriving while dramatically ramping up his attacks on Federal Reserve Chairman Jerome Powell, seeking to deflect blame.

Wednesday’s sharp sell-off was caused by an unusual development in the bond market, called an “inverted yield curve,” that often foreshadows a recession.

For the first time since the run-up to the Great Recession, the yields — or returns — on short-term U.S. bonds eclipsed those of long-term bonds.

Normally, the government needs to pay out higher rates to attract investors for its long-term bonds. But with so many losing confidence in the near-term prospects of the economy and rushing to buy longer-term bonds, the U.S. government now is paying more to attract buyers to its 2-year bond than its 10-year note.

This phenomenon, which suggests investor faith in the economy is faltering, has preceded every recession in the past 50 years.

“The stars are aligned across the curve that the economy is headed for a big fall,” said Chris Rupkey, chief financial economist at MUFG Union Bank. “The yield curves are all crying timber that a recession is almost a reality, and investors are tripping over themselves to get out of the way.”

It’s the latest in a string of worrisome news about the U.S. economy.

The government is expected to spend roughly $1 trillion more than it brings in through revenue this year, creating a ballooning deficit. Business investment has begun to contract — largely due to the uncertainty surrounding Trump’s trade war — and manufacturing jobs have begun to slide. The big hiring and investment announcements that piled up at the beginning of the Trump administration have ceased, as have the announcements of bonuses and pay increases that came after a tax cut law was passed in 2017.

Several White House officials have become concerned that the economy is weakening faster than expected, but they are not working on proactive plans to try and change its course. The Treasury Department has had an exodus of senior advisers in recent months, and the White House just announced a replacement for chairman of the Council of Economic Advisers.

Instead of rolling out new policies, Trump and other top aides have escalated their attacks on the Federal Reserve, trying to pin much of the U.S. problems on what Trump alleges is elevated interest rates that are strangling growth.

In a series of Twitter posts Wednesday, Trump appeared to try to calm investors while also unloading vicious language aimed at Powell, whom he nominated in late 2017.

“China is not our problem, though Hong Kong is not helping,” Trump wrote. “Our problem is with the Fed. Raised too much & too fast. Now too slow to cut ... Spread is way too much as other countries say THANK YOU to clueless Jay Powell and the Federal Reserve. Germany, and many others, are playing the game! CRAZY INVERTED YIELD CURVE! We should easily be reaping big Rewards & Gains, but the Fed is holding us back. We will Win!”

The Twitter posts reflected anxiety within the White House about problems in the economy, particularly because just a few hours earlier Trump had tried to spin the inverted yield curve as a positive, saying it has occurred because “Tremendous amounts of money pouring into the United States. People want safety!”

The U.S. economy has shown signs of weakening in recent months, but high levels of consumer spending in the United States have helped enormously.

Still, the escalating trade war between Trump and Chinese leaders has stopped many businesses from investing. And there are signs that the large tariffs he has placed on many Chinese imports is costing U.S. businesses and consumers billions of dollars.

In a rare admission of the economic consequences of his adversarial trade approach, Trump on Tuesday announced he was delaying many of the tariffs he had promised on cellphones and laptop computers until Dec. 15.

That announcement brought the stock market up sharply higher Tuesday, but all of those gains evaporated in minutes Wednesday amid fears about the yield curve.

Aside from the drop in the Dow, the Standard & Poor’s 500 stock index, a broader measure of stocks, was down 2.9%, and the tech-heavy Nasdaq composite index dropped 3%.