The last four years have been the world's hottest since record-keeping began, with 2018 the fourth warmest on record, according to data published by US research group Berkeley Earth.
Temperatures in 2018 were around 1.16 degrees Celsius (2.09 degrees Fahrenheit) above the average temperature of the second half of the 19th century, from 1850-1900, often used as a pre-industrial baseline for global temperature targets.
"Global mean temperature in 2018 was colder than 2015, 2016, and 2017, but warmer than every previously observed year prior to 2015," the report said.
"Consequently, 2016 remains the warmest year in the period of historical observations. The slight decline in 2018 is likely to reflect short-term natural variability, but the overall pattern remains consistent with a long-term trend towards global warming."
The US government normally conducts a similar analysis, but the partial shutdown of operations over a budgetary impasse has put a hold on it for now.
The European Union's Copernicus Climate Change Service published its own data on 7 January that also found 2015-2018 to be the hottest years on record.