New York (CNN Business) Wall Street has swiftly marked down its hopes for corporate profits because of decelerating economic growth, nagging trade tensions and turbulent oil prices.

Since mid-October, analysts have slashed their estimates of S&P 500 earnings for the first half of 2019 by 4.5%, according to FactSet. That's the sharpest pace of downgrades in four years and nearly twice the 15-year average, FactSet said.

"The pessimism on corporate earnings is palpable," Richard Turnill, chief investment strategist at BlackRock, wrote in a Tuesday report.

Turnill said that profits are becoming "harder to forecast as the economic cycle moves into its late stage."

In other words, the threat of an eventual recession is clouding the view of companies and analysts alike.

Even though the US stock market has recovered about half the gains lost during the recent downturn, investors have been repeatedly caught off guard by alarming profit warnings from major companies in recent weeks.

Stanley Black & Decker SWK plunged 15% on Tuesday after telling shareholders that next year's bottom line will be much smaller than expected.

"As we enter 2019, the external challenges don't magically disappear," Stanley Black & Decker CEO James Loree told analysts during a conference call.

Loree said that while Black & Decker is "well-prepared," there are signs that "the US economy may soon be coming to the end of one of the most enduring recoveries in US history."

FedEx FDX Last month,, long a bellwether for the global economy, dramatically slashed its profit outlook for the fiscal year that ends on May 30. Tellingly, FedEx had just raised that forecast three months earlier.

"China's economy has weakened due in part to trade disputes," FedEx CEO Fred Smith told analysts.

Tesla TSLA shares fell sharply last week after the electric car marker announced plans to slash its workforce by 7% . CEO Elon Musk told employees that with "great difficulty, effort and some luck," Tesla would target a "tiny profit" for the first quarter. Wall Street had been anticipating a bigger profit.

'Earnings valley ahead'

The darker view on earnings isn't just a problem for the United States.

BlackRock said the number of analyst upgrades versus downgrades of global companies has tumbled to the weakest level since mid-2016.

Even though this earnings revision ratio is nearing previous non-recessionary lows, BlackRock sees room for more downgrades because of trade tensions and pressure on corporate profit margins.

Barry Bannister, head of institutional equity strategy at Stifel, expects further negative earnings revisions, especially in the energy, materials and financial sectors.

"Investors foresee [an] EPS recovery, but they may find the earnings valley ahead is deeper than expected," Bannister wrote to clients Tuesday.

S&P 500 earnings growth is projected to slow to just 6.5% in 2019, compared with an estimated 19.9% for 2018, according to FactSet.

The deceleration is even more pronounced for the first quarter, when earnings growth is expected to slow to just 1.3%.

"US earnings are coming off a 'sugar high,' with 2018's fiscal stimulus and tax cuts setting a high bar to clear," Turnill wrote.

Oil crash takes its toll

The more muted profit outlook isn't just about slowing economic growth. It's also about the crash in oil prices . Despite a rebound late last year, US crude is down by 30% since early October.

Analysts have slashed their earnings estimates on energy companies by nearly 30% since mid-October, according to FactSet.

Hess HES Apache APA Anadarko Petroleum APC In fact, energy companies account for nine of the top 10 S&P 500 companies experiencing the sharpest decline in first-quarter earnings estimates, FactSet said. That includes deep cuts to the projected earnings of oil companiesand

Of course, a certain amount of this earnings slowdown was already priced in by Wall Street when stock prices plunged late last year.

It's possible that analysts are simply overreacting to negative headlines and the turmoil in the stock market.

Goldman Sachs GS IBM IBM Procter & Gamble PG United Technologies UTX Consider the upbeat results reported in recent days by blue-chip companies includingand

Corporate profits could surprise even more, further bolstering the stock market. In the meantime, the sharp selloff in stock prices has made companies far cheaper than they were just a few months ago.

The S&P 500 is now trading at 15.3 times projected earnings, which is below the five-year average of 16.4, according to FactSet.

Global stock valuations have tumbled to 13 times forward earnings, compared with 16 a year ago,

"It's not all doom and gloom," Turnill wrote.