2021 Was Earth’s Fifth-Hottest Year, Scientists Say

Last year was Earth’s fifth hottest on record, European scientists announced on Monday. But the fact that the worldwide average temperature didn’t beat the record is hardly reason to stop worrying about global warming’s grip on the planet, they said.

Not when both the United States and Europe had their warmest summers on the books. Not when higher temperatures around the Arctic caused it to rain for the first time at the Greenland ice sheet’s normally frigid summit.

And certainly not when the seven hottest years ever recorded were, by a clear margin, the past seven.

The events of 2021 “are a stark reminder of the need to change our ways, take decisive and effective steps toward a sustainable society and work toward reducing net carbon emissions,” said Carlo Buontempo, director of the Copernicus Climate Change Service, the European Union program that conducted the analysis made public on Monday.

The mean temperature globally last year was 1.1 to 1.2 degrees Celsius (2 to 2.2 degrees Fahrenheit) higher than they were before industrialization led humans to begin pumping large quantities of carbon dioxide into the air.

The year was fifth warmest by a slight margin over 2015 and 2018, by Copernicus’s ranking. The hottest years on record are 2016 and 2020, in a virtual tie.

The steady warming corresponds with the scientific consensus that increasing levels of greenhouse gases in the atmosphere are causing long-lasting changes in the global climate. Copernicus said its preliminary analysis of satellite measurements had found that concentrations of heat-trapping gases continued to rise last year, helped by 1,850 megatons of carbon emissions from wildfires worldwide.

One big reason for 2021’s lower mean temperature was the presence during the early part of the year of La Niña conditions, a recurring climate pattern characterized by lower surface temperatures in the Pacific Ocean. (La Niña has returned in recent months, which could presage a drier winter in the Southern United States but wetter conditions in the Pacific Northwest.)

Those effects were offset in the 2021 average, however, by higher temperatures in many parts of the world between June and October, Copernicus said.

“When we think about climate change, it’s not just a single progression, year after year after year being the warmest,” said Robert Rohde, the lead scientist at Berkeley Earth, an independent environmental research group.

“The preponderance of evidence — which comes from looking at ocean temperatures, land temperatures, upper atmospheric temperatures, glaciers melting, sea ice changes — are telling us a coherent story about changes in the earth system which points to warming overall,” Dr. Rohde said. “Slight variations up or down, a year or two at a time, don’t change that picture.”

Berkeley Earth is expected to issue its own analysis of 2021 temperatures this month, as are two U.S. government agencies: NASA and the National Oceanic and Atmospheric Administration.

Unlike those groups, Copernicus uses a method called re-analysis, which produces a portrait of global weather conditions using a computer model that fills in the gaps between temperature measurements. Even so, the different groups’ conclusions usually line up quite closely.

As ever, higher average temperatures were not observed uniformly across the planet last year. Most of Australia and parts of Antarctica experienced below-normal temperatures in 2021, as did areas in western Siberia.

Europe’s summer last year was the warmest on record, though 2010 and 2018 were not far behind, according to Copernicus. Severe rainfall and flooding caused destruction and death in Belgium, Germany, Luxembourg and the Netherlands. Heat and dryness set the stage for wildfires that ravaged Greece and other places around the Mediterranean.

The western side of North America experienced off-the-charts heat, drought and wildfires last summer. Canada’s maximum temperature record was broken in June when the mercury in a small town in British Columbia hit 121.3 degrees Fahrenheit, or 49.6 Celsius.

Scientists have concluded that the Pacific Coast heat wave would have been practically impossible in a world without human-induced warming. The question is whether the event fits into the present meteorological understanding, even if it is without precedent, or is a sign that the climate is changing in ways that scientists do not fully grasp.

“From where I sit right now, I would tend to think that this was probably still a very rare event, even in the modern climate,” Dr. Rohde said. “But there’s a degree of ‘wait and see’ involved.”

If the planet does not experience heat events of similar intensity in the coming decades, scientists are likely to look back and regard 2021 as an extreme fluke, he said. “If we do, it’s telling us that something is changed in a more fundamental way.”