If we do not take urgent climate action now, then we are heading for a temperature increase of more than 3C by the end of the century.
This year has been one of the hottest on record, as the world comes to the end of a decade of "exceptional" heat, the World Meteorological Organisation has said.
The past decade, from 2010 to 2019, has almost certainly been the warmest in records dating back to the 19th century, and the past five years from 2015 have also been the hottest on record, the UN body said.
This year temperatures have been 1.1C above pre-industrial levels, putting 2019 on course to be the second or third hottest year on record, data from January to October shows.
The WMO provisional statement on the state of the global climate also warns that sea levels are rising ever faster, ice is melting and "once in a century" heatwaves and floods are now becoming more regular occurrences.
Millions of people were forced from their homes as a result of extreme events such as cyclones, hurricanes and flooding.
The past year has also seen droughts in many parts of the world and two major heatwaves in Europe in late June and late July - with a new temperature record of 38.7C set in the UK.
WMO Secretary-General Petteri Taalas: "If we do not take urgent climate action now, then we are heading for a temperature increase of more than 3C by the end of the century, with ever more harmful impacts on human well-being.
"On a day-to-day basis, the impacts of climate change play out through extreme and "abnormal" weather.
And he warned: "One of the main impacts of climate change is more erratic rainfall patterns.
"This poses a threat to crop yields and, combined with population increase, will mean considerable food security challenges for vulnerable countries in the future."
The report is released as countries meet in Madrid for the latest round of UN climate negotiations, known as "Cop25", amid pressure to increase their ambitions to cut the greenhouse gas emissions driving climate change.
The assessment warns concentrations of greenhouse gases in the atmosphere have reached new record levels in 2019.
And the rate at which sea levels are rising has increased as a result of melting ice sheets in Greenland and Antarctica, reaching new highs this year.
The world's oceans take in much of the extra heat and carbon dioxide caused by human activity, but it has serious impacts for the marine environment.
Ocean heat is at record levels, there have been widespread marine heatwaves and sea water is increasingly acidic, damaging wildlife.
At the poles, Arctic sea ice is in long term decline and has been a relatively low levels in Antarctica since a sudden drop in late 2016.
The year 2016, which began with an exceptional "El Nino" weather phenomenon in the Pacific that pushes up global temperatures, remains the hottest year on record.
The report draws on information from other UN bodies and three major global temperature datasets, including from the Met Office Hadley Centre and the University of East Anglia's Climatic Research Unit
Colin Morice, of the Met Office Hadley Centre, said: "Our global temperature figures are in agreement with other centres around the world that 2019 is set to join each of the years from 2015 as the five warmest years on record.
"Each decade from the 1980s has been warmer than the previous decade. 2019 will conclude the warmest decade in records that stretch back to the mid-19th century."
Prof Tim Osborn, from UEA's Climatic Research Unit, said: "The five warmest years for average global surface temperature since records began in 1850 have all occurred in the last five years; by contrast the five coldest years all occurred before 1912.
"This is climate change and not a coincidence."
Emily Beament is the PA environment correspondent.