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Despite La Niña, 2021 still ranks among the top 10 hottest years on record

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Despite La Niña, 2021 still ranks among the top 10 hottest years on record
Despite La Niña, 2021 still ranks among the top 10 hottest years on record

With their assessments of 2021 complete, NASA and NOAA agree: despite a slight cooling due to La Niña, last year still ranked as the 6th hottest year in the record books.

After poring through all of the temperatures gathered around the globe during 2021, NASA and NOAA have now weighed in where the year ranked among the hottest on record. According to NOAA, the global average temperature was 0.84°C above the 20th century average in 2021, which ranks the year as the sixth warmest on record. In NASA's records, the global average was 0.85°C above the 1951-1980 average (or 0.87°C above the 20th century average). The space agency found that 2021 was in a statistical tie with 2018 for 6th warmest on record.

Global-land-ocean-anomalies-2021-NOAA
Global-land-ocean-anomalies-2021-NOAA

NOAA's assessments of global temperatures from 1880 to 2021, with 2021 coming in as the 6th warmest year. Credit: NOAA NCEI

Global-Annual-Temperature-graph-NASA-GISS
Global-Annual-Temperature-graph-NASA-GISS

Although produced using different data and methods of analysis, NASA's annual temperatures from 1880-2021 track closely with those from NOAA, with 2021 ranking as tied with 2018 for 6th warmest. Credit: NASA GISS

According to both agencies, even with their independent data and analyses, the top 10 hottest years, since record-keeping began 142 years ago, have all occurred since 2005. The last 8 years, from 2014-2021, all occupy the top 8 spots on that list.

Top-10-Warmest-Years-as-of-2021-NOAA
Top-10-Warmest-Years-as-of-2021-NOAA

The top 11 hottest years (counting both 2013 and 2005 as tied for 10th place), with 2016 still ranked as the hottest, partly due to the extreme El Niño event that year. Credit: NOAA

While 2021 was sixth warmest on record for the globe, NOAA's records show that over land in the Northern Hemisphere, which is where over 85 per cent of the human population calls home, it was the third warmest year on record.

In their own analyses, the Japanese Meteorological Agency (JMA) also ranked 2021 as 6th warmest, while Europe's Copernicus Climate Change Service state it was the 5th warmest, "but only marginally warmer than the years 2015 and 2018."

These different agencies each work with different sets of temperature data, and they each have their preferred ways to analyze the data. Thus, there are often slight differences in results. Regardless of this, they all show the same overall trends, and they both reveal the magnitude of global warming.

"The most important thing for people to understand is that the long term trends are very clear and are now large enough to be having significant impacts on local weather and extremes," Gavin Schmidt, the director of NASA's Goddard Institute for Space Studies, told The Weather Network. "Global warming is no longer of purely academic interest."

NO SHORTAGE OF EXTREMES

In a press conference on Thursday, Russell Vose, the chief of NOAA's Analysis and Synthesis Branch at the National Centers for Environmental Information (NCEI), told reporters that "there was no shortage of extremes" experienced around the world in 2021.

Climate Extremes Annual 2021 NOAA
Climate Extremes Annual 2021 NOAA

A map of significant climate anomalies and events around the world in 2021, such as the extreme heatwave experienced in the US northwest and western Canada. Credit: NOAA

Only a few of these events rank as record-breaking.

Two examples of note are the extreme heat wave in western North America, which produced Canada's new all-time hottest temperature, and intense rainfall produced by Hurricane Ida.

"Both of these were 'juiced' by global warming and by our impacts on climate," Schmidt stated. "The point being that you don't need to have a warmest year record globally to be seeing the impacts of the long term trends locally."

SLIGHT COOLING FROM LA NIÑA

The carbon dioxide concentration of the atmosphere rose again in 2021, which contributed to the warming of the planet, but last year was significantly cooler than 2020. One factor that contributed to this was La Niña.

2021-Annual-Global-Temperature-Anomalies-NASA-GISS
2021-Annual-Global-Temperature-Anomalies-NASA-GISS

This map plots global temperature anomalies for 2021, compared to the past. Much of the map indicates warmer than average temperatures, with the hottest regions across northeastern Canada and the Arctic. A significant cool region across the equatorial Pacific Ocean indicates the presence of La Niña, the cool phase of the El Niño Southern Oscillation (ENSO). Credit: NASA GISS

The typical pattern of winds and ocean currents across the equatorial Pacific Ocean sees most of the warmth pushed off towards the west, where it 'piles up' around Indonesia, while there is an upwelling of cooler waters off the west coast of South America. Periodically, this pattern breaks down and the warmer waters shift back towards the east, causing what we call El Niño. As an El Niño event tends to release significant amounts of heat from the oceans, we tend to see extra warming during these times. Thus, 2016, the year we experienced one of the strongest El Niño patterns in the record books, is currently the warmest year since the start of the industrial revolution.

La Niña is the opposite of El Niño. Instead of warmer water sloshing back towards the east, the westward push grows stronger, causing the cooler waters along the coast of South America to spread out across the ocean basin. As a result of this spread of cooler ocean surface temperatures, La Niña years tend to be slightly cooler on the global scale.

2021 began with a weak La Nina pattern already in place. When that pattern dissipated during the summer, another moderate La Nina developed towards the end of the year. Based on NASA's analysis, this reduced the year's global temperature average by about 0.03°C.

ENSO-corrected-global-temps
ENSO-corrected-global-temps

NASA's official GISTEMP record of global temperatures (black line) is presented here along with an analysis that removes the impacts of ENSO (red line). Without El Niño and La Niña, it appears that 2020 would have been the warmest year on record, and 2021 would be fourth warmest, behind 2017 and 2019. Credit: NASA GISS

Even without the influences of El Niño and La Niña (shown by the red line in the graph above), 2021 was still significantly cooler than 2020.

Although many factors can weigh into these differences, in Thursday's press conference Schmidt discussed one in particular — aerosols.

Aerosols are tiny particles floating in the air, which act to block sunlight from reaching the ground. Thus more aerosols in the air tends to produce a slight cooling effect, while fewer aerosols results in slightly more warming.

As Schmidt told reporters, a reduction in the amount of aerosols in the air through 2020, due to the pandemic's impact on global transportation activity, was linked to why 2020 ranked so high in the list of warmest years. It was a paper published in March of 2021 where researchers reported that the reduced aerosol content of the atmosphere had resulted in warming of a few hundredths of a degree Celsius over the year.

With transportation activity increasing again through 2021, this would cause a corresponding increase in aerosols. Schmidt stated that they would expect it to cause a similar decrease in temperature. However, more research is needed to know the precise impact.

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