Nearly three-quarters of economists surveyed by the National Association for Business Economics said the U.S. will have entered a recession by 2021, with 38% of respondents stating that will happen by 2020 and 34% predicting 2021.

The percentage of economists stating a U.S. recession will start in 2020 dropped from 42% in February to 38% in August, the association's report showed, while the percentage of respondents expecting a recession in 2021 increased from 25% in February to 34% in 2021.

In a report published before the survey, S&P Global Ratings economists put the odds of a recession in the next 12 months at 30% to 35%, rising from 25% to 30% in the previous quarter.

"Our calculated odds based on financial market spreads — at 34.9% — is near the top of this range, reflecting the possibility that tightening financial conditions could lead to a sharper slowdown," S&P Global Ratings said in its Aug. 15 report. The report said the inversion of the yield curve between 10-year and 3-month Treasury rates "suggests that recession risk is rising."

"Having successfully 'predicted' all seven recessions dating back to 1970s, the inversion of the yield curve is undoubtedly raising a red flag for a potential downturn," the Ratings report said. "Still, we understand that correlation does not imply causation, and explanations for such correlation range from pure investors' risk sentiment changes to banks' unwillingness to lend."

There are less-followed indicators that also can imply recession risks, and according to Mark Yusko, CEO of Morgan Creek Capital Management, holds that Brent crude prices, London Metal Exchange copper prices and the Korea Composite Stock Price Index, along with 10-year Treasury yields, make up the "four horsemen of the growthpocalypse."

"Those four horsemen are all in full gallop downwards, which is usually a bad sign," he told S&P Global Market Intelligence before the release of the report by the National Association for Business Economics, or NABE. "A lot of people are calling for a 2020 recession. [I] think that's a pretty good call."

On a global scale, the largest percentage of respondents in the NABE survey said they believe global GDP growth will average between 3% and 3.49% in 2019, with 32% of respondents stating that growth will be between 3% and 3.24%, and 24% predicting 3.25% to 3.49%.