US Secretary of State John Kerry has warned that failing to act immediately and decisively on climate change will have "catastrophic" and wide-ranging consequences.
The top US diplomat was reacting to a UN expert panel report that said soaring carbon emissions will amplify the risk of conflict, hunger, floods and migration this century.
"Unless we act dramatically and quickly, science tells us our climate and our way of life are literally in jeopardy," Kerry, in Paris on Sunday for crunch talks with Russia over Ukraine, said in a statement.
"Denial of the science is malpractice."
"There are those who say we can't afford to act. But waiting is truly unaffordable. The costs of inaction are catastrophic," he added.
News that makes sense
Your trusted source for staying up-to-date with the world around you. Get free daily news updates and analysis, straight to your inbox.
The United Nations report said that, left unchecked, greenhouse gas emissions may cost trillions of dollars in damage to property and ecosystems, and in bills for shoring up climate defences.
The United States and China are among the world's biggest polluters but Kerry said that "no single country causes climate change, and no one country can stop it".
Kerry cautioned that water scarcity and flooding were security risks.
"The clock is ticking. The more we delay, the greater the threat. Let's make our political system wake up and let's make the world respond."
