President Donald Trump's proposed tariffs on steel and aluminum imports are "straight up stupid," a top economist told CNBC on Friday. Trump's tariffs announcement Thursday night has been broadly condemned by both sides of the U.S. political spectrum and the international community at large. "This is just straight up stupid," Adam Posen, president of the Peterson Institution for International Economics, said Friday. "This is fundamentally incompetent, corrupt or misguided." Posen, who served at the Federal Reserve Bank of New York in the mid-1990s, currently sits on the panel of economic advisors to the U.S. Congressional Budget Office. The president's protectionist move, which would impose a 25 percent levy on all steel imports and 10 percent on aluminum, has garnered widespread criticism, with trade partners of the U.S. already threatening to retaliate. Downstream producers of steel and aluminum products — including car, plane and consumer goods manufacturers — say it will jack up their costs, and experts fear a trade war that could harm global growth. The Dow promptly tanked more than 400 points on Thursday trading. U.S. automaker and plane manufacturer stocks closed sharply lower, and metals producers in Europe and Asia saw losses on Friday morning's market open.

Rescuing US steel

"Steel is just a tiny input in the U.S. gross domestic product (GDP) — which is why it's so crazy. You mess up your entire trading system for an industry that has a total of 80,000 jobs," Posen argued. 2015 census data showed roughly 140,000 Americans employed in steel mills, contributing $36 billion to the national economy. By comparison, steel-consuming industries, which experts believe will be hardest hit by the tariffs, employ 6.5 million Americans and add about $1 trillion to U.S. GDP, according to the census. Trump, who campaigned on the promise of reviving long-suffering U.S. industries, argued the move will protect American jobs. The U.S. imports 30 percent of its steel. Once a bastion of steel production, shifts in global trade and automation saw hundreds of thousands of steel jobs eviscerated from the 1970s onward. America's steel industry has long called for a clampdown on imports and global steelmaking overcapacity, and is now lauding Trump's decision. U.S. steel and aluminum shares jumped sharply before markets closed Thursday, with some companies seeing share price increases of up to 9 percent.

Overturning the TPP will not help the U.S. steel industry much, says Wood Mackenzie Thomas Barwick | Stone | Getty Images

Critics of the policy, which reportedly include Trump's own chief economic advisor Gary Cohn and Council of Economic Advisers chair Kevin Hassett, as well as multiple right-leaning free trade groups, warn that the protectionist measures will only hurt the U.S. in the long run. But the move appears to be in line with what many people see as Trump's prioritization of pleasing his "base." "It's time we stop pretending there was any sense at all," Posen said. "It's all about giving a handout to particular U.S. steel and aluminum producers. In national security terms, it makes no sense because it's hitting our military allies like western Europe, South Korea and Japan, and it doesn't directly hit China. It's a tax on consumers." Posen noted that China isn't even among the top five steel exporters to the U.S. — these are Canada, Brazil, South Korea, Mexico and Russia. The premise for the decision, known as Section 232, was on national security grounds. The White House claimed that relying on foreign steel could threaten the U.S. defense industry. But even Secretary of Defense James Mattis said that the Pentagon "continues to be concerned about the negative impact on our key allies regarding the recommended options within the [Section 232] reports." "The biggest thing is they think this is going to scare the Chinese," Posen said. "This isn't going to scare anybody. This is going to scare markets, but it's not going to scare the Chinese government, it's going to lead to retaliation possibly. And that's where things get bad." A spokesperson for the White House was not immediately available for comment when contacted by CNBC.

A threat to global growth?