The nine years spanning 2013-2021 all rank among the 10 hottest on record, according to an annual report by the US National Oceanic and Atmospheric Administration (NOAA) published Thursday, the latest data underscoring the global climate crisis.
For 2021, the average temperature across global surfaces was 1.51 degrees Fahrenheit (0.84 degrees Celsius) above the 20th-century average, making the year the sixth-hottest in the overall record, which goes back to 1880.
NOAA also uses the 21-year span from 1880 to 1900 as a surrogate to assess pre-industrial conditions, and found the 2021 global land and ocean temperature was 1.87F (1.04C) above the average.
A separate analysis of global temperature released by NASA had 2021 tying with 2018 as the sixth-warmest on record.
Both data sets vary slightly from the European Union's Copernicus Climate Change Service in their assessment, which had 2021 as the fifth warmest in records tracking back to the mid-19th century.
Increases in abundance of atmospheric greenhouse gasses since the industrial revolution -- such as carbon dioxide, methane, nitrous oxide, and halogenated compounds -- are mainly the result of human activity and are largely responsible for the observed increases.
Climate scientists say it is crucial to hold end-of-century warming to within a 1.5C (2.7F) rise to avert the worst impacts -- from mega-storms to mass die-offs in coral reefs and the decimation of coastal communities.
The impacts have been increasingly felt in recent years -- including record-shattering wildfires across Australia and Siberia, a once-in-1,000-years heatwave in North America and extreme rainfall that caused massive flooding in Asia, Africa, the US and Europe.
The heat records observed in 2021 came despite the year beginning in a cold phase thanks to an El Nino Southern Oscillation (ENSO) episode across the central and eastern tropical Pacific Ocean.
Notably, the Northern Hemisphere land surface temperature was the third highest on record. The 2021 Southern Hemisphere surface temperature was the ninth highest on record.
Land heat records were broken in parts of northern Africa, southern Asia, and southern South America in 2021, while record-high sea surface temperatures were observed across parts of the Atlantic and Pacific oceans.
There were no cold records broken for land or ocean areas.
Average annual Northern Hemisphere snow cover was 9.3 million square miles (24.3 million square kilometers), the seventh-smallest annual snow cover extent in the 1967-2021 record.
Meanwhile, with the exception of September and December, each month of 2021 had Arctic sea ice levels in the top-10 lowest levels for those respective months.
On the other hand, the year 2021 had an above-average global tropical cyclone activity with a total of 94 named storms, tying 1994 as the tenth highest in a year.
Climate change increases sea surface temperatures, a key factor that influences cyclone formation and behavior.