“Alarming new records” for sea levels, ocean heat and concentrations of greenhouse gases in the atmosphere were set in 2021, experts said.
The annual “state of the global climate report” from the UN’s World Meteorological Organisation (WMO) also said the oceans were becoming ever more acidic as carbon dioxide dissolved into sea water, threatening wildlife.
The report also warned that extreme weather created food and water shocks, displaced millions of people and caused billions of dollars in losses in 2021, as global temperatures reached 1.1C above what they were in the 19th century.
Last year was one of the seven warmest years on record, lower than other recent years due to the cooling effects of the natural La Nina weather pattern, but the years since 2015 have been the hottest seven years on record.
And it is “just a matter of time” before the world sees another warmest year on record, outstripping 2016, WMO secretary-general Petteri Taalas warned.
UN Secretary-General Antonio Guterres described the new report as a “dismal litany of humanity’s failure to tackle climate disruption”.
He warned fossil fuels were a dead end, as he called for action to accelerate the shift to renewable energy, including sharing technology, driving forward battery storage for clean power, scaling up supply chains and investment, and halting subsidies for polluting oil, gas and coal.
Mr Guterres said: “The global energy system is broken and bringing us ever closer to climate catastrophe. Fossil fuels are a dead end — environmentally and economically.
“The war in Ukraine and its immediate effects on energy prices is yet another wake-up call. The only sustainable future is a renewable one.”
The annual report from the WMO warns that four key climate indicators set new records in 2021.
Greenhouse gas concentrations reached a new global high in 2020, with carbon dioxide levels in the atmosphere climbing 49% above what they were in pre-industrial times, and then continuing to rise in 2021 and into 2022.
Ocean heat also reached record highs in 2021, with warming rates in the top 2,000 metres of the seas increasing over the past two decades and warmth reaching to ever deeper levels.
Much of the ocean experienced at least one “strong” marine heatwave in 2021.
Global average sea levels rose to a new record high in 2021, the report said.
Seas were rising at an average 4.5mm a year between 2013-2021, more than double the rate they were rising between 1993 and 2002, as ice loss from the ice sheets accelerated, raising risks for hundreds of millions living on the coasts.
The report also warned that the ocean absorbs nearly a quarter of the annual carbon dioxide emissions from human activity, where it reacts with water to make the seas more acidic, threatening wildlife and habitats such as coral and hitting food security, tourism and coastal protection provided by reefs.
The assessment, which draws on weather and science bodies as well as UN agencies, also highlighted the exceptional heatwaves that hit last year, with the mercury setting a Canadian record of 49.6C in British Columbia in June and a provisional European record of 48.8C in Sicily in August.
Flooding caused heavy loss of life and huge economic losses in places including China and Western Europe, while drought affected many parts of the world, with potentially devastating impacts in Eastern Africa.
Extreme weather events in 2021 worsened food insecurity and the risk of famine, and displaced millions of people from their homes, the report said.
And rain fell for the first time at Summit Station, the highest point on the Greenland ice sheet, at 3,216m.
Prof Taalas said: “Our climate is changing before our eyes. The heat trapped by human-induced greenhouse gases will warm the planet for many generations to come.
“Sea level rise, ocean heat and acidification will continue for hundreds of years unless means to remove carbon from the atmosphere are invented.
“Some glaciers have reached the point of no return and this will have long-term repercussions in a world in which more than two billion people already experience water stress.”
Commenting on the report, Dr Shaun Fitzgerald, director of the Centre for Climate Repair at Cambridge, said: “We will see many more millions of climate refugees as severe weather events increase in frequency and severity, and with sea level rising every year we will see more and more coastal regions being overcome.
“The need for climate repair has never been greater, and unfortunately that need isn’t going away.”
Reducing emissions was absolutely critical, he said, but warned it would only curb the rate at which the problem was getting worse, so there was a need to go “beyond zero emissions” and develop schemes to remove greenhouse gases from the atmosphere.