SHANGHAI — The global economy looks shaky. Markets for things as diverse as oil and European bank shares have plumbed new lows. The tried-and-true cures no longer seem to work.

The task for global leaders this week: Come up with Plan B.

Finance ministers and central bank governors from the world’s biggest industrialized and developing economies now arriving in Shanghai face their toughest task since the worst days of the global financial crisis. Europe is mired in low growth and Japan is teetering on the brink of recession, even though the Bank of Japan and a lengthening list of European central banks are pushing interest rates ever deeper into negative territory.

China, the world’s leading engine of economic growth in recent years, is struggling with heavy debts, a slowdown in manufacturing, stagnant exports and a flood of money leaving its borders. Countries that depend on selling oil and other resources are struggling under persistently low prices. The United States is facing a drag on growth as the strong dollar makes it cheaper for many consumers and companies to import goods instead of buying them from American businesses.

That leaves a difficult situation for the economic policy makers of the Group of 20 nations, who will be hard pressed to come up with solutions during their meetings in Shanghai.

“What to do about the increasing risks to the recovery is perhaps the single most important question before the finance chiefs in Shanghai,” the International Monetary Fund said this week. It added: “The G-20 must plan now and proactively identify policies that could be rolled out quickly, if downside risks materialize.”

During and right after the global financial crisis in 2008 and 2009, the Group of 20 produced broad agreements on many financial regulations, most notably a tightening of international capital standards. But in the years since, the disparate collection of economies from Argentina to the European Union has been known mostly for agreeing on generalities and making few changes, culminating in an unproductive series of meetings in Turkey last year.

“The Turkish presidency of the G-20 led to no progress on any front,” said Ken Courtis, a prominent international economist and fund manager.

China, which holds the Group of 20 presidency this year, is one major concern. In August, China devalued its currency, the renminbi, by 4 percent with no warning, setting off sharp sell-offs in global markets. China now faces pressure to weaken the currency further as money leaves the country because of slowing growth — a prospect that could further hurt the global economic outlook.

“What we’re concerned about is China. Could there be a break on China’s currency that could tip the global economy into recession?” said Charles Collyns, a former assistant secretary of the Treasury for international finance in the Obama administration who is now the chief economist of the Institute of International Finance.

Chinese officials appear to have gotten the message that silence is not always golden. At a financial conference here on Thursday, officials described how they would use government deficit spending to spur growth. A senior Chinese central banker pledged to preserve the value of the renminbi by firmly tying its value to a group of other currencies. Zhou Xiaochuan, the governor of the central bank, has agreed to give a public speech on Friday morning in Shanghai, then hold a rare news conference afterward.

China’s devaluations put pressure on rival exporters to do the same or risk losing their competitiveness. Many emerging markets, like Russia and Brazil, have allowed their currencies to fall steeply against the dollar or been forced into it, raising fears of devaluations of the sort seen in the 1930s, which led to a collapse of international trade. Countries with currencies that have stayed strong as safe havens, including the dollar and the Japanese yen, have seen growth suffer.

One of the biggest question marks over the group lies in how much it might do on currencies, beyond a general call for more stable exchange rates. But Chinese officials have worked to deflate expectations that the Shanghai meeting could be a reprise of the Plaza Accord, a deal between major economies in 1985 to fix volatile currency markets.

A senior United States Treasury official said that he expected countries to reiterate previous commitments to manage their currencies responsibly, rather than shift their value. “I see those commitments as being a strong indication from G-20 members that they will manage their currencies in ways that are globally consistent,” he said.

The official, who insisted on anonymity because of diplomatic considerations, said that the United States would push instead to call on countries with broadly measured surpluses on trade and royalties to stimulate domestic spending, particularly by consumers.

Yet China has already stepped up infrastructure spending so much in the last decade that it is becoming harder to find projects that justify the investment. In Europe, the longstanding caution of the German government about large-scale spending has been a deterrent. In the United States, Republicans in Congress have been leery of the extra borrowing that could be needed to finance additional roads, bridges and rail lines.

One question is whether China’s presidency this year can make the group more productive.

Except on climate change, for which it made a large commitment to slow down emissions before the Paris agreement in December, China has sought to be recognized as a leader of global diplomacy without taking responsibility for specific results. In Syria, in South Sudan, and at times even on its own border in North Korea, it has been leery of trying to dictate any resolution to longstanding international concerns.

In the Group of 20, China has to cope not only with its own reluctance to take strong stands but also with the lack of a strong international consensus among the disparate nations on what the world should do to strengthen economic growth. “It’s hard in the context of a G-20 communiqué to get really concrete initiatives,” Mr. Collyns said. “So what I would expect is some kind of framework.”