In 2021, the dominant greenhouse gases released into Earth’s atmosphere continued to increase. The annual global average carbon dioxide (CO2) concentration was 414.7 ± 0.1 ppm, an increase of 2.6 ± 0.1 ppm over 2020, the fifth-highest growth rate since the start of the instrumental record in 1958. This brings the concentration of CO2 to, once again, the highest in the modern record and ice core records dating back 800,000 years. The growth rate for methane (CH4) was the highest on record and the third highest for nitrous oxide (N2O), contributing to new record high atmospheric concentration levels for both gases. Weak-to-moderate La Niña conditions were present in the eastern equatorial Pacific during most of 2021, a continuation from 2020. La Niña tends to dampen temperatures at the global scale; even so, the annual global surface temperature across land and oceans was still among the six highest with records dating as far back as the mid-1800s. While La Niña conditions contributed to Australia’s coldest year since 2012, New Zealand and China each reported their warmest year on record. Europe reported its second-hottest summer on record, after 2010. A provisional new European maximum temperature record of 48.8°C was set in Sicily (Italy) on 11 August. In North America, exceptional heat waves struck the Pacific Northwest, leading to a new Canadian maximum temperature record of 49.6°C, set at Lytton, British Columbia, on 29 June, breaking the previous national record by over 4°C. In the United States, Furnace Creek in Death Valley, California, reached 54.4°C on 9 July—equaling the temperature measured at that location in 2020, which was the hottest temperature measured on Earth since 1931. The effects of warming temperatures were apparent across the Northern Hemisphere, where lakes were frozen 7.3 fewer days on average. Lake Erken, in Sweden, lost the most ice cover during the 2021 winter, with 61 days less ice cover compared to the 1991–2020 normal in response to an anomalously warm winter. The average growing season was six days longer than the 2000–20 base period. In Kyoto, Japan, full bloom dates for a native cherry tree species, Prunus jamasakura, were the earliest in the entire record, which began in AD 801, breaking the previous earliest date set in the year 1409. While fewer in number and locations than record high temperatures, record cold was also observed in various locales during the year. In Spain, a new all-time national minimum temperature record of −34.1°C was set on 6 January at Clot del Tuc de la Llança in the Pyrenees. Slovenia reported a national low temperature record of −20.6°C for the month of April, set at station Nova vas Bloka. Over Antarctica, a persistently strong and stable polar vortex helped maintain the second longest-lived ozone hole on record (shorter only than 2020), which did not close until 23 December, and contributed to the coldest extended winter on record at the South Pole. But on the northeastern Antarctic Peninsula, Esperanza and Marambio stations received persistent warm northerly winds, contributing to their warmest (tied) and second-warmest years on record, respectively. Across the global cryosphere, glaciers lost mass for the 34th consecutive year, and permafrost temperatures continued to reach record highs at many high latitude and mountain locations. In the high northern latitudes, the Arctic as a whole (poleward of 60°N), observed its coolest year since 2013, but 2021 was still the 13th-warmest year in the 122-year record. Extreme heat events occurred during the summer. Related to the western North American heat waves, a temperature of 39.9°C was recorded in Fort Smith, Northwest Territories, Canada, on 30 June, the highest temperature ever recorded north of 60°N. A widespread melt event on the Greenland Ice Sheet on 14 August—the latest on record—coincided with the first observed rainfall in the 33-year record at the Summit Station (3216 m a.s.l.). The seasonal Arctic minimum sea ice extent, typically reached in September, was the 12th-smallest extent in the 43- year record; however, the amount of multiyear ice—ice that survives at least one summer melt season—remaining in the Arctic at this time was the second lowest on record, indicating the Arctic’s sustained transition to a younger, thinner ice cover. The oldest ice, more than four years old, has declined by 94% since the start of the record. While the rate of decline in minimum sea ice extent over the 2010–21 period has slowed compared to previous decades, Arctic sea ice volume continues to rapidly shrink. Across the world’s oceans, global mean sea level was record high for the 10th consecutive year, reaching 97.0 mm above the 1993 average when satellite measurements began, an increase of 4.9 mm over 2020. Globally-averaged ocean heat content was also record high in 2021, while the global sea surface temperature cooled compared to 2019 and 2020 due to the ongoing La Niña conditions in the tropical Pacific. Still, approximately 57% of the ocean surface experienced at least one marine heatwave in 2021. A total of 97 named tropical storms were observed during the Northern and Southern Hemisphere storm seasons, well above the 1991–2020 average of 87, but well below the record 104 named storms of 1992. In the North Atlantic, 21 tropical cyclones formed, the third most for the basin, behind the record 30 cyclones in 2020 and 28 in 2005. There were seven Category 5 tropical cyclones across the globe—four in the western North Pacific and one each in the South Indian Ocean, Australian region, and the Southwest Pacific. Super Typhoon Rai was the third costliest typhoon in the history of the Philippines, causing about $1 billion (U.S. dollars) in damage and more than 400 deaths. While not reaching Category 5 status, Hurricane Ida was the most impactful storm in the Atlantic. At $75 billion (U.S. dollars) in damage, Ida was the costliest U.S. disaster of 2021 and the fifth most expensive hurricane on record (since 1980). As is typical, some areas around the world were notably dry in 2021 and some were notably wet. In August, 32% of global land areas were experiencing some level of drought, a new record high. A “mega-drought” continued in central Chile for the 12th consecutive year, becoming the longest drought in the historical record in the region. Drought intensified and expanded through most of the western United States and elsewhere along a large stretch of northeastern Siberia and the Far East region of Russia, which led to unprecedented wildfires. Most of the Middle East, from Türkiye to Pakistan, also saw an intensification of drought conditions. In parts of equatorial East Africa, the annual total rainfall was the lowest on record, leading to three consecutive failed rainy seasons that resulted in one of the worst threats to food security in 35 years for more than 20 million people in the region.. Conversely, on 20 July, a 1-hour precipitation total of 201.9 mm was recorded in Zhengzhou—capital of Henan province in central China and home to more than 10 million people—the highest hourly precipitation on record for mainland China. On 4 October, a new European 12-hour rainfall record was set in Rossiglione (northwest Italy), with a total of 740.6 mm, which was more than 50% of its annual average of 1270 mm. Following months of above-average rain, the Rio Negro River at Manaus (central Brazilian Amazon) rose and remained above its emergency threshold for a total of 91 days, reaching a record high level of 30.02 m on 16 June. The overflow of the river caused damaging floods that surpassed the “once-in-a-century” Amazon flood in 2012.