Governments must speed up talks on a joint response to global warming, UN Secretary-General Ban Ki-moon has warned.
Mr Ban addressed the opening of the high-level segment of annual UN climate talks in Qatar, involving environment ministers and climate officials from nearly 200 countries.
Pointing to the destruction caused by Hurricane Sandy in North America and the Caribbean in late October, along with other weather disasters this year, Mr Ban said "abnormal" has become the new normal as the world warms, presenting a "crisis, a threat to us all, our economies, our security and the well-being of our children."
Climate scientists say it is difficult to link a single weather event to global warming, but some say the damage caused by Sandy was made worse by the rise of sea level.
"No one is immune to climate change, rich or poor," Mr Ban said.
"It is an existential challenge for the whole human race."
He warned "the pace and scale of action are still not enough."
Mr Ban said countries are "in a race against time" to reach their goal of keeping the temperature rise below a threshold of 2 degrees C, compared to pre-industrial times.
Climate scientists have observed changes including melting Arctic ice and permafrost, rising sea levels and acid content of oceans, shifting rainfall patterns with impacts on floods and droughts.
They say low-lying Pacific island states, in particular, are losing shoreline to rising seas, expanding from heat and the runoff of melting ice.
Mr Ban noted that time is running out for governments to act, citing recent reports showing rising emissions of greenhouse gases, which most scientists say are causing the warming trend.
A small minority of climate scientists still reject that.
"Let us avoid all the scepticism. Let us prove wrong all these doubts on climate change," Mr Ban said.
Governments represented at the Doha conference have started talks on crafting a new global climate treaty that would take effect in 2020.
They are also discussing how to rein in greenhouse gas emissions before then, partly by extending the Kyoto Protocol, a treaty limiting the emissions of most industrialised countries that expires this year.
One of the most hotly debated issues in the talks that started last week has been the pledges by rich countries three years ago to deliver financing to help poor countries to switch to cleaner energy sources and adapt to climate change.
Developing countries complained of the lack of firm commitments on financing in Doha.