Where have all the billionaires gone?

The number of billionaires around the world is shrinking — and so are their bank accounts.

According to a new report from the research firm Wealth-X, the global population of billionaires declined 5.4 percent to 2,604 in 2018. Their total net worth dropped 7 percent to $8.6 trillion, amid market volatility and uncertainty over global trade.

In fact, billionaires in Asia — whose appetite for luxury properties has fueled the real estate market in New York and other cities — suffered the biggest blow in 2018, Wealth-X found. Their ranks dropped by more than 13 percent to 677. (In China, there were 285 billionaires in 2018, a 15.7 year-over-year drop.) Collectively, Asian billionaires’ wealth slid more than 8 percent to $2.2 trillion.

“Billionaires are really exposed to the stock markets,” said Maeen Shaban, Wealth-X’s director of research and data analytics who co-authored the report. “In China, you were talking about a more than 21 percent drop in the Shanghai stock market and more than 18 percent in Beijing. It was really terrible.”

Anecdotally, brokers in New York say Chinese buyers retreated from the market more than a year ago as it became increasingly difficult to move money outside of the country. More recently, “the Chinese buyer is basically on hold now until a U.S.-China trade deal is struck,” said Compass’ Leonard Steinberg.

But while Chinese billionaires are in retreat, North America’s population of billionaires grew, according to Wealth-X, thanks to a strong U.S. dollar, interest-rate hikes and tax reform that boosted corporate earnings.

In the U.S., the number of billionaires grew 3.7 percent to 705. The report also found that 15 cities were home to 30 percent of the world’s billionaire population in 2018. New York led the pack with 105 billionaires, followed by Hong Kong with 87 and San Francisco with 75. Los Angeles had 39. San Francisco also had the highest number of billionaires per capita — one for every 11,600 residents.

Overall, the decline of billionaires doesn’t bode well for real estate — an asset class that 21 percent of billionaires consider to be among their “top interests,” Wealth-X found.

Luxury real estate, in particular, is already weighed down by excess inventory, nevermind the fact that the number of buyers is ostensibly shrinking.

“One of the byproducts of this development boom was the notion that we were awash with billionaires, and billionaires would be buying all these units,” said Jonathan Miller, founder of appraisal firm Miller Samuel. “The problem with that math is that everyone got the same idea at the same time.”

Miller said 55 percent of the units on Manhattan’s Billionaire’s Row remain unsold. Miller’s analysis looked at eight buildings, where 496 units out of 907 were not sold.

Prices in New York’s ultra-luxury market — defined as the top 20 percent of sales — are down 4.7 percent from a year ago, according to Streeteasy.

“We still continue to see inventory building up,” said Grant Long, StreetEasy’s top economist. “The luxury market has been headed downward since 2014-2015, but that’s accelerated in the last two years.”

But billionaires can’t single-handedly tank the market. They actually represent a small portion of luxury buyers overall, said Ed Mermelstein, a real estate attorney. “We rely more on the next several layers below; those who are are worth between $100 million and $500 million,” he said. “That’s the demographic that’s going to be buying apartments for $15, $20 and $30 million.”

And on the ground in New York, Steinberg said he’s seen a “notable uptick” in the luxury market starting in February. “The wealthiest audience is out there shopping,” he said. Many have been waiting on the sidelines for the bottom of the market. “They’re exhausted from waiting.”