Natural variation and the influence on global temperature

2021 was another warm year globally – consistent with ongoing warming from greenhouse gases.

The global average surface temperature in 2021 was 0.76 ± 0.04 °C above the 1961-1990 average according to the Met Office and University of East Anglia’s HadCRUT5 data set.

Regional differences in average temperature in 2021 are particularly pronounced with above average warming clearly evident in the Arctic.

This places 2021 as joint sixth warmest year since records began 172 years ago. Other climate monitoring centres rank 2021 from the fifth to the seventh warmest year in their records.

In the seven years from 2015, the planet’s annual average surface temperature – as measured by the global network of weather stations, ships and ocean buoys – has been higher than in any year prior to 2015 in the recorded series beginning in 1850.

As the concentration of greenhouse gases in the atmosphere increases each year, why doesn’t the annual global temperature increase every year too? The simple answer is that the planet’s temperature is subject to a certain element of natural variation, especially in the exchange of heat between the ocean and the atmosphere, especially in the tropical Pacific.

A cycle of variability known as the El Niño Southern Oscillation (ENSO) allows temperature to fluctuate between warm ‘El Niño’ years and cooler ‘La Niña’ years. When La Niña conditions prevail parts of the tropical Pacific are cooler than normal and this cooling has an impact on the overall average temperature of the earth – the opposite effect happens in an El Niño year.

Colin Morice is a Met Office scientist. Dr Morice said: “The variation in temperature in the tropical Pacific arising from ENSO events broadly coincides with the northern hemisphere winter. During 2021 the year was ‘book-ended’ by successive La Niña events which has had a modest effect on the average temperature, making 2021 cooler than recent extreme years.

The past seven years, which are the seven warmest years on record, encompassed an exceptionally strong El Niño event in late 2015/early 2016 and a “double dip” La Niña in 2020/21 and 2021/22. Dr Morice continued: “Variability in global temperature – particularly that associated with El Niño and La Niña events – mean we don’t expect each year in the global temperature record to be warmer than the last. However, over the full record, all 172 years, the long-term warming is clear.”

Taking measurements from weather stations and the temperature of the sea’s surface from ships has been a key part of meteorology for over 170 years. A fuller picture of the changing climate can be provided by considering other climate indicators alongside surface temperature.

Ocean heat content measures change in the energy in the global ocean. It has been estimated that over 90% of the excess heat trapped in the earth system by human-emitted greenhouse gases goes into the ocean. Dr Morice explained: “Measurements from different layers of the ocean are allowing scientists to record ocean heat content, and it is clear to see that a lot of the heat from global warming is being taken down into the deeper layers of the ocean. In contrast to surface temperature, ocean heat content rises more steadily as excess energy continues to accumulate in the climate system.” Recent results show that ocean heat content reached a new record high in 2021, as it did in 2020.

The effects of accumulating greenhouse gases and this excess of energy are seen across the earth system. Key climate indicators can be used to track some of these effects at the broadest scales.

Learn more about key climate indicators with our climate dashboards.


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