May 24, 2017 0926 GMT
"We need to pay attention. All eyes are on US policymakers and how they will formulate their policies," Ayhan Kose, main author of the World Bank's global forecast, told AFP in an interview.
"What happens in the United States does not stay in the United States."
In the semi-annual Global Economic Prospects report, the World Bank lowered the estimate for global growth this year a tenth from the June forecast, to 2.7 percent compared to the 2.3 percent growth seen last year.
The World Bank maintained its US forecast at 2.2 percent following a sluggish 1.6 percent in 2016.
But those estimates could go much higher.
Even the table in the report displaying the forecasts for the major regions and economies comes with a footnote warning about the uncertainty: "The US forecasts do not incorporate the effect of policy proposals by the new US administration, as their overall scope and ultimate form are still uncertain."
Kose said it is simply too soon to draw any conclusions, however a simulation showed the individual and corporate tax cuts that Trump discussed during the campaign could on their own add three-tenths to the US growth rate this year, and more than double that in 2018.
And infrastructure spending could have an even larger impact.
"These are significant increases," he said.
"A healthy US economy is good for the rest of the world," since an increase in US growth of one full percentage point could boost advanced economies by eight-tenths after a year, and emerging markets by six-tenths.
So global growth potentially could rise another tenth this year to 2.8 percent, and get a three-tenths boost in 2018, he said.
Waiting for specifics
However, he was quick to stress the caveats to this estimated impact: It will depend on the timing of the tax cuts, how they impact inflation, and how the Federal Reserve reacts.
The same is true for other proposals like big spending on infrastructure, which would have a more direct impact on growth, but could exacerbate an already tight labor market, which also could fuel inflation.
The US central bank has cautioned that spending that fuels inflation could require more and faster increases in the benchmark lending rate.
Higher interest rates in turn would tend to slow economic growth.
Kose said the rising threat of protectionism in advanced economies also remains a concern, and that uncertainty "is a challenge to businesses in the US and abroad."
But again he said, it is too early to tell what the impact on global growth will be until the specifics are known — for example, whether US tariff increases Trump has threatened are limited to one product, like automobiles, or extend to an entire country.
"One thing we know well, open trade policies help promote economic growth," Kose said, in the closest he came in the interview to a word of caution to the new administration.