The last three years were the hottest on record, new data has shown.
It is bad news for polar bears and for the planet as a whole, where extreme weather events caused death and destruction in 2017.
The UN's World Meteorological Organization (WMO) said 2017 was indistinguishable from 2015 as the second or third warmest year behind 2016, and was "the warmest year without an El Niño" in records dating back to the late 19th century.
Average surface temperatures in 2017 were 1.1 degree Celsius (2.0 Fahrenheit) above pre-industrial times, creeping towards 1.5C (2.7F), the most ambitious limit for global warming set by almost 200 nations under the 2015 Paris climate agreement.
The USA had its most expensive year ever in terms of weather and climate disasters,
And the head of the WMO, Petteri Talaas, said in a statement that other countries saw their development slowed or reversed by tropical cyclones, floods and drought.
The WMO said 17 of the warmest 18 years since records began in the 19th century have now happened since 2000, confirming a warming trend driven by man-made greenhouse gases.
"We're in a long-term warming trend despite the ups and downs you get on an annual basis, even a decadal basis," said Gavin Schmidt, of NASA's Goddard Institute for Space Studies, whose data is used by the WMO.
Last year China ramped up its war on pollution - also a major problem in India. But celebrations as world leaders signed the Paris pact now seem lilke a distant memory - not least since President Donald Trump, who doubts climate change is caused by man-made emissions, is withdrawing the US from the deal.
Trump wants to promote US fossil fuel industries, at odds with the Paris accord's goals of phasing out emissions from coal, oil, and natural gas between 2050 and 2100.
By contrast, the UN Secretary-General’s Special Representative for Disaster Risk Reduction, Robert Glasser, described the pace of climate change as "an existential threat to the planet" that required a "drastic response".
Among extreme weather events last year, the Caribbean and the United States suffered a battering from hurricanes, the Arctic ended 2017 with the least sea ice for mid-winter and tropical coral reefs suffered from high water temperatures.