Energy Secretary Steven Chu, a Nobel Prize-winning physicist frequently the target of Republican criticism, announced Friday that he was stepping down in the latest shake-up of President Barack Obama's Cabinet. Chu, who disclosed his decision in a letter to Energy Department staff, frequently clashed with GOP lawmakers over gas prices as well as government backing for green-energy companies like the failed firm Solyndra.

In his letter, Chu took aim directly at his critics, saying the clean-energy efforts were a success—and blasted climate-change skeptics as trapped in "the Stone Age."

"While critics try hard to discredit the program, the truth is that only one percent of the companies we funded went bankrupt," he wrote. "That one percent has gotten more attention than the 99 percent that have not."

Chu added: "The test for America’s policy makers will be whether they are willing to accept a few failures in exchange for many successes. America’s entrepreneurs and innovators who are leaders in the global clean energy race understand that not every risk can—or should—be avoided. Michelangelo said, 'The greater danger for most of us lies not in setting our aim too high and falling short; but in setting our aim too low, and achieving our mark.'"

He also scolded climate-change skeptics and urged a shift from fossil-fuels to other sources of energy.

"The overwhelming scientific consensus is that human activity has had a significant and likely dominant role in climate change," Chu warned in his letter. "There is also increasingly compelling evidence that the weather changes we have witnessed during this thirty year time period are due to climate change."

Chu underlined that China was pushing ahead with clean-energy investments—outpacing U.S. efforts.

"While we cannot accurately predict the course of climate change in the coming decades, the risks we run if we don’t change our course are enormous. Prudent risk management does not equate uncertainty with inaction," he said. "The Stone Age did not end because we ran out of stones; we transitioned to better solutions."

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In a written statement, Obama praised Chu for giving the Energy Department "a unique understanding of both the urgent challenge presented by climate change and the tremendous opportunity that clean energy represents for our economy."

Obama added, "And during his time as Secretary, Steve helped my Administration move America towards real energy independence. Over the past four years, we have doubled the use of renewable energy, dramatically reduced our dependence on foreign oil, and put our country on a path to win the global race for clean energy jobs. I wish him all the best."

Chu will likely stay in his post through the end of February, though it will depend on the confirmation of his successor.