The annual Jackson Hole summer retreat hosted by the Kansas City Fed has become a highlight of the global central banking calendar.

Expectations this year have not soared to the heights of 2010 or 2011, but equity and commodity prices have been rising and could react badly if the U.S. central bank chief pulls his punches.

After all, "many" Fed policymakers judged at their Aug. 1 meeting that extra easing would probably be warranted fairly soon in the absence of signs of a "substantial and sustainable" economic recovery, according to the minutes of their deliberations.

Yet some economists reckon not even a third round of Fed asset purchases would do much to perk up world growth.

For a start, bond yields are already very low and the Fed has already said it expects short-term rates to remain close to zero at least until late 2014. The law of diminishing returns is at work.

Above all, buying more U.S. Treasuries or mortgage-backed securities would do nothing to address the euro zone's underlying problems or wean China off its dependence on an investment boom that looks unsustainable, said Julian Jessop with Capital Economics, a London consultancy.

"These, and not the health of the U.S. financial system, are now the biggest challenges facing the global economy," he said.

So whereas China's July exports to the United States were flat from a year earlier and shipments to Southeast Asia rose briskly, those to the European Union plunged 16 percent. As a result, total exports edged up just 1 percent.

"Most of the deceleration in export growth is explained by the European crisis," said Joy Yang, an economist for Mirae Asset Management in Hong Kong.

That's why markets will be glued not only to Bernanke's speech on Friday but also to policy signals from the euro zone ahead of a Sept. 6 meeting of the European Central Bank.

WAITING FOR DETAILS

Investors want the ECB to flesh out President Mario Draghi's promise to "do what it takes" to preserve the single currency despite a collapse of confidence in the capacity of the likes of Spain and Italy to tame their debts.

That evaporation of trust is weighing on the eurozone economy, which is set to shrink this quarter, polls of purchasing executives suggest. Germany's closely watched IFO survey, due on Monday, is likely to confirm the deteriorating trend.

By contrast, economists polled by Reuters expect revised U.S. gross domestic product data on Wednesday to show the economy expanded at a 1.7 percent annualised pace in the second quarter, up from an initial estimate of 1.5 percent.

The Fed has made plain its dissatisfaction with such tepid growth; the U.S. unemployment rate has fallen to just 8.3 percent from 9.1 percent in July 2011.

But markets are split on what policy steps Bernanke will flag. Many economists expect the Fed will promise to hold short-term rates extremely low for even longer while keeping the option of further asset purchases in reserve for now.

It's hard to see how that would be a game-changer for the rest of the world.

Indeed, bond strategists at Deutsche Bank said developments in the U.S. housing market and in credit conditions were already consistent with slightly above-trend growth; they said their key source of concern was a slowdown in Asia.

On that score, Yang at Mirae said China, Asia's powerhouse, might feel more comfortable in relaxing its own monetary policy if the Fed moved first.

But she said the key to a short-term recovery in growth was faster domestic investment. Yet project approvals were being held up by the approaching turnover in China's central leadership and the accompanying churn of provincial officials.

"Even if the U.S. started to ease more aggressively, the binding constraint on the fiscal side is the new leadership," she said. "Everything is waiting for the political transition."