New York’s biannual billion-dollar “gigaweek” of sales, whose latest series starts on Monday, is the glittering pinnacle of the auction market for Impressionist, modern and contemporary art. But is instability in the wider world taking off some of that shine?

The week’s various auctions at Christie’s, Phillips and Sotheby’s (which in June returned to private ownership) are estimated to raise at least $1.2 billion. But the world’s wealthy will have to be in a free-spending mood to match the $2 billion raised in May at the equivalent spring sales, when Jeff Koons’s stainless steel “Rabbit” sculpture fetched $91.1 million, an auction high for any work by a living artist. This time around, there are few museum-quality works by the most famous artists to tempt billionaires — no painting or sculpture is estimated to sell for more than $45 million.

“First, there isn’t a major estate — this cycle doesn’t have a Rockefeller,” said Diana Wierbicki, global head of art law at Withers Bergman LLP, referring to Christie’s 2018 record-setting auction of the Peggy and David Rockefeller collection. Second, she pointed out, sellers and buyers are confronting the global economic uncertainty from a looming Presidential election in the United States, riots in Hong Kong and a Brexit-traumatized Britain.

And owners who could sell are thinking twice about offering trophy works at auction because President Trump in 2017 removed art from the assets eligible for 1031 exchanges — the strategy that allows investors to defer paying capital gains tax on property when it is sold, as long as a similar property is purchased for the same amount. “Now people are asking, ‘Does it make sense to sell if I’m facing a federal capital gains tax of 28 percent?’” Ms. Wierbicki said.