Physical signs of climate change are accelerating as temperatures drive towards increasingly dangerous levels.
"We have seen a growing amount of disasters because of climate change," said World Meteorological Organization Secretary-General Petteri Taalas. He said since 1998, about 4.5 billion people around the world have been hurt by extreme weather.
The WMO's annual state of global climate report says Earth is nearly 1 degree Celsius warmer than when the industrial age started.
The planet may see temperatures increase 3-5C by the end of the century, said WMO's Taalas.
UN chief Antonio Guterres warned countries to come with concrete plans at an upcoming climate summit as he released the flagship report on Thursday.
The destruction wrought by Cyclone Idai in Africa earlier this month was a stark reminder of a planet out of whack due to greenhouse gas emissions blamed for climate change, he said, describing how extreme weather last year disrupted a growing number of lives.
The WMO report comes as Guterres drums up support for a September climate summit to ramp up the political will to tackle the climate crisis.
"I am telling leaders don't come with a speech, come with a plan," he said during a press conference. "If not, it will be irreversible."
The 2015 Paris Agreement accepted by nearly 200 nations aims to limit a rise in average world temperatures to "well below" 2C above pre-industrial times, including by reducing the use of polluting fossil fuels.
Scientists have linked global warming to accelerating sea-level rise, shrinking sea ice, glacier retreat and extreme events like heatwaves, the UN agency noted in its report.
A dramatic increase in the average number of people exposed to potentially lethal heat waves - some 125 million since the beginning of the century - exemplified climate change's fast-expanding cost to public health, said Guterres.
The world's warming trend is expected to continue, with the past four years the hottest on record, said the report. Several countries had already signalled they would bring new plans to the New York climate summit, said Taalas, who is among the event's organisers.
Cyclone Idai, which caused massive flooding nearly 1,000 casualties in its wake after slamming into Mozambique and rippling through Zimbabwe and Malawi bore the fingerprints of climate change, said Taalas.
The "clear link" between climate change and social instability should also alarm nations worldwide, added Guterres, who reminded historical instances of weather crises preceding upheavals such as the French Revolution and the Arab Spring.
Worldwide, disasters affected nearly 62 million people in 2018, mostly in floods, the WMO report said.