Outgoing BP chief Bob Dudley on Wednesday criticized sweeping climate proposals from Sen. Bernie Sanders, a top-tier 2020 Democratic presidential candidate, and Rep. Alexandria Ocasio-Cortez, a champion of the far left.

"They have a completely unrealistic idea of the complexity of the global energy system," Dudley told CNBC's "Squawk Box" from the World Economic Forum in Davos, Switzerland. "It's very complex."

Nearly a year ago, Ocasio-Cortez, with Sen. Ed Markey, released the Green New Deal, an ambitious plan to remake the U.S. economy and drastically reduce the nation's greenhouse gas emissions. Sanders, who endorsed the proposal, later released his own detailed plan, building on the Green New Deal and calling for the U.S. to move to renewable energy across the economy by 2050.

Sanders later Wednesday responded on Twitter to Dudley's comments.

"What's 'unrealistic' is thinking we will let fossil fuel companies continue to destroy the planet. The outrageous greed of companies like BP got us into this crisis. We are going to solve the climate crisis, end their greed and hold them accountable for the damage they caused," Sanders tweeted.

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The office of Ocasio-Cortez did not immediately respond to CNBC's requests for comment.

Dudley on Wednesday did not entirely shoot down the need for energy transition. He stressed that "we must reduce carbon emissions" and that "renewables need to grow."

But he said there's other factors to be considered. "The world needs all forms of energy," he said. "Less emissions, but all forms of energy."

Industry executives have been focused on sustainability issues this week in Davos and around the global. The theme at this year's annual WEF in the Swiss Alps is "stakeholders for a cohesive and sustainable world."

Days before the forum opened, BlackRock chief Larry Fink called on industry leaders to prepare for a long-term climate crisis that will reshape finance.

Fink, whose BlackRock surpassed $7 trillion in assets under management, used his annual letter to the world's biggest companies to sound the alarm. "Climate change has become a defining factor in companies' long-term prospects. … But awareness is rapidly changing, and I believe we are on the edge of a fundamental reshaping of finance."

BlackRock will put "sustainability at the center of our investment approach," he wrote — from portfolio construction to launching new investment products that screen fossil fuels.

Dudley said he spoke with Fink on Tuesday. Fink explained, according to Dudley, that the letter was not a call to "get out of hydrocarbons." Dudley said Fink "made that clear."

In November, Dudley told CNBC that there was a lack of realism from environmentalists and lawmakers who want the energy industry to immediately stop carbon-emitting activity.

"There's just a lot of people, very well-meaning people, who want to believe that there is a simple solution," Dudley said at the Abu Dhabi International Petroleum Exhibition & Conference.

He said that natural gas, which emits roughly half of the carbon as coal to produce the same level of energy, needed to be a big part of the energy transition story. He said fast-developing technology to monitor gas leakage, such as satellite imagery and drone inspection, would soon help to reduce waste further.

Dudley will be stepping down on Feb. 4 as CEO and from the board. He's the company's longest-serving chief executive, with nearly a decade at the helm. He will retire at the end of March.

— CNBC's David Reid contributed to this report.