Sens. Bernie Sanders (I-Vt.), Jeff Merkley (D-Ore.), and Edward J. Markey (D-Mass.) on Thursday introduced legislation to build a 100 percent renewable energy economy by 2050—a bill that environmental group 350.org previously termed "the most ambitious piece of climate legislation Congress has ever seen," although others caution that it is not enough to meet the real challenges of climate change.

The "100 by 50" Act calls for 50 percent of U.S. electricity to be produced by renewable energy sources like wind or solar by 2030, and 100 percent by 2050. It would require zero carbon emission vehicle standards and prohibit federal approval of oil and gas pipelines, among other measures.

"With an anti-science Congress and President in power right now, some might doubt that this is the right time to push for a bold new strategy to tackle climate change and make a massive fundamental shift in the way we produce energy," Merkley wrote in a Facebook post earlier this month when he announced plans to introduce the bill. "But the fact is, we don't have four years to wait to begin this rapid transition. We must act now."

It was embraced by alternative energy groups, such as NY Renews, which said in a statement Thursday that the senators were "setting the bar for climate leadership in the wake of President [Donald] Trump's assault on our lungs, our communities, and our planet."

350.org co-founder Bill McKibben said, "100 is an important number. Instead of making changes around the margins, this bill would finally commit America to the wholesale energy transformation that technology has made possible and affordable, and that an eroding climate makes utterly essential."

"This bill won't pass Congress immediately—the fossil fuel industry will see to that—but it will change the debate in fundamental ways," McKibben said.

The group's executive director May Boeve added, "While fossil fuel billionaires supporting Trump's administration put profits before people, we now have a legislative roadmap to phase out this dirty industry once and for all. This bill deploys clean energy in communities that need it most and keeps fossil fuels in the ground."

Others were sharply critical of the bill, warning that it would not solve the climate crisis and uses a too-long timeline for a 100 percent withdrawal from fossil fuel use.