It's not too late for people to recalibrate their methods of consuming energy, but according to environmentalist and 350.org co-founder Bill McKibben, that action should've gone into effect years ago.

"If the question's when do we need to make change, the correct answer is 25 years ago," he told HuffPost Live's Alyona Minkovski in a Thursday appearance to promote the People's Climate March, happening in New York City on Sunday.

Proof of environmental damage includes a rise in average temperatures, which has the potential to impact human life on an extreme scale.

"We've raised temperature one degree. We're headed for two degrees even if we do everything right. The trouble is, we're not doing everything right, so at the moment we're headed to an increase of four or five degree celsius, eight or nine degrees Fahrenheit this century. If we let this happen, we can't have civilizations like the ones we're used to," he explained. "We just won't be able to grow the grain to support them."

While environmental reform is still a viable option, McKibben feels that the fight for climate change has become "desperate," and that we're "cutting it very very close" when it comes to progress.

"The damage that we've done already really can't be reversed on any human timescale," he said. But if people start relying on sun and wind -- "the only things we're not going to run out of" -- it could prove to have lasting benefits.

"If we stop burning coal and gas and oil, the planet's forests and oceans still retain some ability to cycle carbon out of the atmosphere," he explained. "If we did everything right, by the end of this century, we'd be back down to some place around 350 parts per million CO2."

"A lot of damage would've happened in the meantime, but we probably would've staved off real disaster," he concluded. "That's the real hope."

Watch the rest of Bill McKibben's conversation with HuffPost Live here.

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