Climate change: even La Niña's cooling effect could not temper 2021, one of the seven warmest years on record

La Niña episodes between 2020 and 2022 led to a temporary reduction in global average temperatures, but 2021 still became one of the seven warmest years on record, according to six major international datasets consolidated by the World Meteorological Organization (WMO). 

La Niña is a phenomenon that produces large-scale cooling of ocean surface temperatures in the central and eastern parts of the equatorial Pacific, in addition to other changes in tropical atmospheric circulation. Its effects on weather and climate are usually the opposite of those of El Niño. La Niña exerts a transient cooling effect on a global scale, which is usually strongest in the second year of the episode. 

Last year, the global average temperature was about 1.11 (±0.13) °C above pre-industrial levels (1850-1900). Thus, 2021 is the seventh consecutive year (2015-2021) in which the global temperature has exceeded pre-industrial levels by more than 1 °C. 

With the new record in 2021, global warming, as well as other long-term climate change trends, looks set to continue as a result of unprecedented levels of heat-trapping greenhouse gases in the atmosphere. 

There is definite evidence that global warming is approaching the lower limit of the temperature increase envisaged in the Paris Agreement, which is to try to limit that increase to 1.5°C above pre-industrial levels. 

The seven warmest years have all occurred since 2015, with the top three in the rankings being 2016, 2019 and 2020. The exceptionally strong El Niño event in 2016 contributed to unprecedented global average warming. 

After learning of the new data, the secretary-general of the World Meteorological Organisation, Professor Petteri Taalas, said that "long-term global warming from increased greenhouse gas concentrations is now much larger than the inter-annual variability of global average temperatures caused by naturally occurring climate forcing". 

"The year 2021 will be remembered for record temperatures of nearly 50°C in Canada (comparable to those seen in the hot Saharan desert of Algeria), exceptional rainfall and deadly floods in Asia and Europe, as well as drought in parts of Africa and South America. The impacts of climate change and climate-related hazards had devastating effects that disrupted the lives of communities on all continents," he added. 

According to scientific criteria, the place each particular year occupies in the global ranking must be interpreted from a long-term perspective, especially as the differences between specific years are sometimes minimal. Since the 1980s, each new decade has been warmer than the previous one, and this trend is expected to continue. 

Temperature is only one indicator of climate change, to which can be added greenhouse gas concentrations, ocean heat content, ocean pH, global mean sea level, glacier mass and sea ice extent. 

To make the temperature assessment as comprehensive and reliable as possible, the UN agency uses six international data sets. The same data are used in the annual State of the Climate reports that the Organisation produces to provide the international community with global climate indicators. 

WMO uses data sets based on monthly climate data from observing sites and from ships and buoys that are part of global marine networks. 

This information is developed and maintained by the United States National Oceanic and Atmospheric Administration (NOAA), the National Aeronautics and Space Administration (NASA) Goddard Institute for Space Studies, the UK Met Office Hadley Centre, the UK University of East Anglia Climate Research Unit (HadCRUT) and the Berkeley Earth Group. 

WMO also uses reanalysis data sets from the European Centre for Medium-Range Weather Forecasts (ECMWF), the Copernicus Climate Change Service and the Japan Meteorological Service (JMA). The reanalyses combine millions of meteorological and marine observations - including satellite observations - with additional model values to produce comprehensive atmospheric reanalyses. The combination of observations and model results makes it possible to estimate temperatures occurring at any time and place on the planet, even in areas for which little data is available, such as the polar regions. 

The Copernicus climate change service estimates that 2021 was the fifth warmest year on record, although it was only slightly warmer than 2015 and 2018. NOAA and Berkeley Earth consider 2021 to be the sixth warmest year in nominal terms. According to NASA's GISTEMP dataset and HadCRUT, 2021 was, along with 2018, the sixth warmest year on record. Finally, the JMA reanalysis data place 2021 as the seventh warmest year in nominal terms. The small differences between these datasets indicate the margin of error that is used to calculate the global mean temperature. 

The temperature values will be incorporated into the final version of the State of the Climate in 2021 report, which will be released in April 2022. This publication, which is the updated version of the interim report published in October 2021 to coincide with the 26th session of the Conference of the Parties (COP26) to the United Nations Framework Convention on Climate Change (UNFCCC), includes information on the set of key climate indicators and selected climate-attributed effects.