Greenhouse gases grew to their highest levels on record last year, a report by the National Centers for Environmental Information found.
Oceans also saw striking extremes with record warming and sea level rises, according to the State of the Climate 2021 report published on Thursday.
The peer-reviewed report is an international annual summary of changes and significant events in the global climate. It is based on contributions from more than 530 scientists from 67 countries around the world and reflects tens of thousands of measurements from multiple independent datasets.
Greenhouse gases were the highest on record
The report found the most dominant greenhouse gases - methane, nitrous oxide, and carbon dioxide (CO2) - reached new record highs in 2021. The gases are the three largest contributors to Earth's warming.
The annual global average CO2 concentration grew at its fifth-highest rate since 1958, the start of the instrumental record. CO2 also reached the highest concentration in the modern record and in records dating back 800,000 years via data derived from ice cores.
The annual average atmospheric methane concentration was also the highest on record, and the annual increase of 18 parts per billion (ppb) was the highest since measurements began.
The report also showed the annual increase in methane has significantly accelerated since 2014 and the annual increase of 1.3 ppb for nitrous oxide was the third highest since 2001, contributing to a global annual average atmospheric concentration of 334.3 ppb.
Earth's warming trend continued
The report also found a range of scientific analyses shows global surface temperatures were 0.21-0.28 of a degree C above the 1991-2020 average.
This places 2021 among the six warmest years since records began in the mid to late 1800s.
It also found the last seven years (2015–2021) were the seven warmest years on record, and the average global surface temperature has increased at an average rate of 0.08-0.09 of a degree C per decade since the start of record-keeping and at a rate more than twice as high since 1981.
Ocean heat and global sea level were the highest on record
Oceans are continuing the heat up and reached record highs in 2021, the report found. It was the tenth consecutive year in a row where global average sea level rose to a new record high and was about 97.0 mm higher than the 1993 average.
La Niña conditions lowered sea surface temperatures
The report found La Niña conditions, which began in mid-2020, continued for most of 2021. As a result the global surface temperature was lower than 2019 and 2020 but was still 0.29 of a degree C higher than the 1991–2020 average.
It also found approximately 57 percent of the ocean surface experienced at least one marine heatwave last year.
Temperatures were mixed in the Southern Hemisphere
La Niña contributed to the warmest year on record for New Zealand, but also to the coolest year since 2012 for Australia.
Tropical cyclone activity was well above average
There were 97 named tropical storms during the Northern and Southern Hemisphere storm seasons last year, well above the 1991–2020 average of 87. Seven tropical cyclones reached Category 5 intensity on the Saffir–Simpson Hurricane Wind Scale.
Reaction to the report
Te Herenga Waka—Victoria University of Wellington climate science lecturer Dr Kyle Clem is the lead editor of the report's chapter on Antarctica and the Southern Ocean, which shows 2021 was another year of extremes in the region.
Dr Clem said the region saw huge and rapid swings in sea-ice coverage last year with new record lows of daily coverage in October and December.
"Sea-ice extent began well below average during January and February 2021. It then grew rapidly in March and remained above average through much of autumn and winter, with near record-high daily sea ice in August. Just as quickly as it grew, it rapidly diminished in September and October, plummeting to new daily record-lows," he said.
He said the swings are alarming because Antarctic sea ice is a crucial component of the Earth's climate system.
Antarctica has also been losing billions of tons of sheet ice mass since 2003, a trend that continued last year.
"In 2021, this trend continued, with large mass losses and thinning of glaciers in the West Antarctic region. A net continent-wide mass loss of 50 gigatons was observed, adding about 0.14 mm of global sea level rise."
But Dr Clem said total ice mass loss in 2021 was lower than the long-term trend because there were significant surface mass gains from heavy snowfall in other parts of Antarctica, linked to an unusually high number of extreme weather events which brought large dumps of snow to the continent at the end of the year.
"These heavy precipitation events, which occur sporadically when atmospheric rivers in middle latitudes are guided to Antarctica, are likely to become more intense in a warming atmosphere that can hold more moisture.
"2021 showed the interaction between extreme precipitation events and melting—both governed by warming—is an important factor affecting Antarctica's year-to-year contribution to sea level rise."
In other extremes, the entire Antarctic Peninsula had a very warm year, along with huge losses in sea-ice coverage along the coast, and the Esperanza station tied for its warmest year on record.
Meanwhile, the South Pole experienced its coldest winter on record but it still continues to warm in the long term.
Dr Clem said the record-cold winter did not significantly affect long-term warming because the region continues to warm more than twice as fast as the global average.
Last year also saw the second longest-lived ozone hole on record which has a particular impact on New Zealand.
"The Antarctic ozone hole, which occurs between late August to November, is still a significant feature of the southern polar region," he said.
"It remains a key driver of Southern Hemisphere weather and climate during summer, including across middle latitude regions such as New Zealand where it influences the north-south location of summer storms."
But Dr Clem said the ozone hole showed signs it was "healing", due to decreasing amounts of ozone-depleting substances in the atmosphere.
"The 2021 ozone hole, although long-lived, had a slow growth rate and both its size and minimum ozone concentration values were less severe than those seen in the early 2000s when the ozone hole was at its strongest."