U.N. Secretary General Ban Ki-moon urged world leaders on Monday as the COP21 climate summit kicked off in Paris to take more urgent action on climate change in the face of a risky potential rise in temperatures that would pose a major threat to the planet.

“Paris must mark a decisive turning point,” said Ban. “We need to go much faster, much further if we are to limit the global temperature rise to below 2 degrees Celsius.”

The science is clear, even a 2 degree rise will have consequences for our planet - UN SG Ban Ki Moon #COP21 pic.twitter.com/m0KEcG15yl

— UNDP Live tweeting (@UNDPLive) November 30, 2015

Experts emphasize the importance of keeping global warming within this range to avoid irreversible and catastrophic damage. Scientists say 2 C warming could be the breaking point that could end human life on earth as we know it.

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Ban said that while national emissions reductions pledges are a positive step, curbing global warming to avoid the worst impacts of climate change will take much more urgent commitments on an international level.

Ahead of the start of the two-week long COP21, the U.N. Framework Convention on Climate Change had received voluntary climate actions plans from some 180 countries.

"Climate change does not respect national borders", says @UN chief Ban Ki-moon one day ahead of #COP21 https://t.co/GRrrLtnMKV

— UN Climate Action (@UNFCCC) November 29, 2015

But Ban stressed that climate change supercedes national borders and that the “time for action is now” to rapidly ramp up “political momentum” to tackle climate change globally. He said a strong climate deal must be durable, dynamic, credible, and embody solidarity.

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Despite the warnings of climate change commitments moving too slowly for the severity of the issue, Ban said he is “reasonably optimistic” that world leaders will agree to an “ambitious universal climate change agreement” during COP21.

Global leaders are set to sign a climate deal at COP21, which kicked off Monday in Paris and will continue until Dec. 11.