Bernie Sanders hit back at Joe Biden after the former vice president suggested the Vermont senator's plan to address climate change was unreasonable.

"The climate crisis is an existential threat, putting at risk our very existence. I have proposed the most aggressive, comprehensive plan to address this crisis. Why? Because we have no other choice," Sanders, 78, said in a tweet on Friday.

Earlier on Friday, Biden, 77, took a swipe at Sanders's climate plan.

"Not a single, solitary scientist thinks it'd work," Biden told a voter in Claremont, New Hampshire, who asked him about his plan to address climate change. "You cannot get to zero emissions by 2030. It's impossible."

The Sanders campaign's Green New Deal plan calls to "reduce domestic emissions by at least 71% by 2030 and reduce emissions among less industrialized nations by 36% by 2030," as well as "achieve 100% sustainable energy for electricity and transportation by no later than 2030 and to fully decarbonize the economy by 2050 at the latest."

Biden's climate plan calls to "establish an enforcement mechanism to achieve net-zero emissions no later than 2050." Protesters at Biden's event on Friday chanted, "2050 is too late," after the former vice president's speech.

Sanders has gone on the attack against Biden in the weeks leading up to the Feb. 3 Iowa caucuses and Feb. 11 New Hampshire primary, attacking him for his 2002 vote in favor of the Iraq War and his record on Social Security.

