Pedestrians walk past the New York Stock Exchange before the closing bell in New York.

A good part of 2019 earnings expectations could depend on a tariff deal between President Donald Trump and China's President Xi Jinping.

Now that the highly anticipated Jerome Powell speech is out of the way and the G-20 summit is gathering, the market can begin to try to decipher 2019 earnings estimates.

The Powell speech on Wednesday removed some uncertainty about rate hikes, but there's still plenty of murkiness around three other issues that will affect 2019 earnings: tariffs, higher business costs and global growth.

Predicting earnings in the following year has always been difficult, but it's been almost impossible for 2019, and that has been why the markets have been so volatile. "Uncertainty around the Fed's rate-hike plans and tariffs has made it really tough to figure out 2019 earnings," David Aurelio, who tracks corporate earnings for Refinitiv, told CNBC.

Mining company Freeport McMoRan's CEO, Richard Adkerson, said in a recent conference call: "This uncertainty is causing us certainly to slow down our long-term plans to develop our resources. I mean, we're going to defer those investments decisions until there is clarity in the marketplace."

According to Refinitiv, S&P earnings are expected to increase 8.3 percent next year compared with this year's 23 percent gain, about half of which was due to the effect of tax cuts.

Morgan Stanley's Mike Wilson, in a recent note, said the Street is overshooting that estimate and that actual corporate profits will only be up 4.3 percent in 2019.

Still, the market has rallied this week because there's one less worry for the markets: a more aggressive Fed. "[Fed Chair] Powell did the right thing [by changing rate hike expectations], because rising rates were playing into profit expectations in 2019," Earnings Scout's Nick Raich told CNBC. "If growth expectations are starting to deteriorate, and inflation is not a problem, why would you keep tightening?"

OK, that's one less worry, but several others are still unresolved.

1. Tariffs. If Trump and Xi agree that there will be no more increases in tariffs while they negotiate a trade deal, that would be a big boost for earnings in 2019 and help the consensus around higher estimates. If not, "the uncertainty will bleed into the global economy, potentially leading to a broader global slowdown," CFRA's Lindsey Bell told clients in a recent note.

2. Higher input costs. These could erode profit margins and cause another leg down in earnings estimates.

3. Global slowdown. Tariffs are part of the global slowdown story, but only part. About 40 percent of profits come from overseas. Bell noted that slower growth is typically a negative for markets. "Slower growth typically comes with lower price-to-earnings (P/E) multiples."

She notes that year to date, the forward P/E for the S&P 500 has averaged 17.1, but with earnings growth of only roughly 8 percent in 2019, that will likely decline. In 2014, the last year to show similar EPS growth, the forward P/E averaged 15.9, which is in line with the current forward multiple. If earnings growth is lower, say, 4 percent, the multiple will likely decline to a low-15 level. That could imply a flat or even lower S&P next year.

That's the prediction of Morgan Stanley's Wilson: a flat S&P 500 for 2019.

It's going to be hairy going into the end of the year, but hopes for a Santa Claus rally could very much depend on tariffs. "Everyone is betting the economic upcycle will be over in 2019," Raich said. "The way to keep it going is for the Fed to pause, and Trump to make a deal with China."

One down, one to go.