New York — U.S. investors will have their first chance today to react to Standard & Poor’s decision to strip the U.S. government of its top credit rating. But the bigger issues facing Wall Street and stock markets worldwide remain debt-ridden countries in Europe and concerns that the global economy is weakening.

Friday’s first-ever downgrade of U.S. long-term debt from AAA to AA+ wasn’t unexpected and may have little impact on interest rates. But it’s the kind of news that stock markets don’t need when investors are already nervous.

Even before the downgrade, the Dow Jones industrial average last week fell nearly 700 points, or 6 percent. Investors were worried because economic signals in the U.S. and overseas were pointing toward trouble:

• On July 29, the government dramatically lowered its estimate of how much the economy grew during the first quarter. It had said the economy grew at an annual rate of 1.3 percent, but revised that number down to 0.4 percent. Second-quarter growth was also weak, a 1.3 percent rate.

• European officials are trying to help Italy — the world’s eighth-largest economy — avoid the kind of bailouts that Greece, Portugal and Spain were forced to accept to prevent them from defaulting on their debt. And those bailouts haven’t solved all the problems in those countries.

• The first reports on the economy during the third quarter have been mixed. Manufacturing, which helped pull the economy out of the recession, fell to its weakest level since July 2009 — the month after the recession officially ended. The Labor Department said 117,000 jobs were created last month. But that came after 99,000 jobs were created in May and June combined — and 250,000 new jobs are needed each month to reduce unemployment.

As a result, financial analysts interviewed Sunday said they expect markets to be volatile this week — and beyond.

“We are in unchartered territory and, therefore, should all brace for volatility over a number of days if not weeks,” said Mohamed El-Erian, CEO and co-chief investment officer of the bond mutual fund company PIMCO.

Mark Zandi, chief economist at Moody’s Analytics, said he expected the downgrade to cause a selloff today. “There’s a lot of fear and misunderstanding and confusion, and that all could come out in the stock and bond markets. I don’t think it takes much to unnerve investors given the current environment. I think anything could drive investors to sell given how fragile sentiment is,” he said.

Former Federal Reserve Chairman Alan Greenspan, who appeared on NBC’s “Meet the Press” Sunday, expects the selling to last for some time. “It is very unlikely that (this) isn’t going to take a while to bottom out,” he said.

The reason: “It depends on Europe, not the United States,” Greenspan said. “The United States was actually doing relatively well, sluggish but going forward until Italy ran into trouble.” He said that half of U.S. corporations operate in Europe, and that the region “has been a very important driving force in the overall earnings of U.S. corporations.”

The Dow fell 513 points Thursday alone after concerns about Italy’s were compounded by anxiety ahead of Friday’s jobs report from the Labor Department. That report came in better than expected; the economy got 117,000 new jobs in July. But it wasn’t enough to calm investors. The Dow has fallen nearly 10 percent in two weeks — a period that included the budget debate that averted a default on U.S. debt.

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