2021 In Climate Change

This article documents notable events, research findings, effects, and responses related to global warming and climate change during the year 2021.

Summaries

* 26 February: The United Nations Synthesis Report on ''Nationally Determined Contributions under the Paris Agreement'' stated that "estimated reductions referred to in paragraphs (on greenhouse gas emissions) fall far short of what is required, demonstrating the need for Parties to further strengthen their mitigation commitments under the Paris Agreement".
* 21 June: the World Meteorological Organization wrote that "2021 is a make-or-break year for climate action, with the window to prevent the worst impacts of climate change—which include ever more frequent more intense droughts, floods and storms—closing rapidly."
* 28 July: a follow-on to the 2019 ''World Scientists' Warning of a Climate Emergency'' noted "an unprecedented surge in climate-related disasters since 2019" and stated there is "mounting evidence that we are nearing or have already crossed tipping points associated with critical parts of the Earth system".
* 6 September: editors from over 200 health journals published a joint editorial stating "The science is unequivocal; a global increase of 1.5 °C above the pre-industrial average and the continued loss of biodiversity risk catastrophic harm to health that will be impossible to reverse.... The greatest threat to global public health is the continued failure of world leaders to keep the global temperature rise below 1.5 °C and to restore nature."
* 30 September: UN Secretary-General António Guterres stated that "time is running out. Irreversible climate tipping points lie alarmingly close." He called for more ambition as current NDCs will lead to a rise of 2.7 °C, saying that "all leaders must recognize that we are in the middle of a climate emergency".

Measurements and statistics

* 25 January: a review article published in '' The Cryosphere'' reported that Earth lost 28 trillion tonnes of ice between 1994 and 2017, 68% being from atmospheric melting and 32% by oceanic melting. The ''rate'' of ice loss rose 57% since the 1990s–from 0.8 to 1.2 trillion tonnes per year–raising global sea level 34.6 ±3.1 mm in that time period.
* 9 February: a study published in ''Environmental Research'' concluded that airborne fine particulate matter (PM_2.5) caused by burning fossil fuels causes 8.7 million premature deaths annually, including China (2.4 million), India (2.5 million) and parts of eastern US, Europe and Southeast Asia.
* 16 February: study results published in the ''PNAS'' (study's time period: 1990–2018) reported widespread advances and lengthening of pollen seasons (up to 20 days) and increases in pollen concentrations (up to 21%) across North America, with human forcing of the climate system contributing about 50% of the trend in pollen seasons and about 8% of the trend in pollen concentrations.
* Atlantic meridional overturning circulation (AMOC):
::* 25 February: a ''Nature Geoscience'' article reported an "unprecedented" (since AD 400) decline in the twentieth century of the Atlantic meridional overturning circulation (AMOC), which is now in its weakest state in more than 1,000 years. The AMOC redistributes heat on the planet and has a major impact on climate. In particular, weakness in the AMOC, which includes the Florida Current and the Gulf Stream, counteracts its moderating effect on the climate in Europe.
::* 5 August: a study published in ''Nature Climate Change'' presented "spatially consistent empirical evidence that, in the course of the last century, the AMOC may have evolved from relatively stable conditions to a point close to a critical transition".
* February: measurements from Mauna Loa Observatory showed that, for the first time, atmospheric levels reached 417 parts per million (ppm), a concentration 50% higher than the 278ppm pre-industrial level.
* 8 March: a study published in ''Nature Climate Change''—studying the combined effects of average global sea level rise and natural and human-induced subsidence—estimated that subsiding coastal locations may locally experience up to four times more ''relative'' sea level rise than could be attributed to global sea level rise alone.
* 17 March: a study by the International Federation of Red Cross and Red Crescent Societies estimated that, globally between September 2020 and February 2021, 12.5 million people were displaced by adverse impacts of climate change, the annual average exceeding 20 million.
* 17 March: a study published in ''Nature'' estimated that trawling's disturbance of carbon stored in sea beds can re-mineralize sedimentary carbon into amounts equivalent to 15–20% of the atmospheric absorbed by the ocean each year, and comparable to that of terrestrial farming.
* 18 March: a study accepted for publication in '' Environmental Research Letters'' estimated that the severity of heatwave and drought impacts on crop production in Europe roughly tripled over the preceding 50 years, from –2.2 (1964–1990) to -7.3% (1991–2015).
* 1 April: a study published in ''Nature Climate Change'' estimated that anthropogenic climate change has reduced global agricultural total factor productivity by about 21% since 1961, and 26–34% in warmer regions such as Africa, Latin America and the Caribbean.
* 7 April: NOAA reported carbon dioxide levels were higher than at anytime in the past 3.6 million years, in the Mid-Pliocene Warm Period when sea level was about 24 mm (78 ft) higher than today and the average temperature was about 4 °C (7 °F) higher than in pre-industrial times.
* ''NOAA's redefinition of "average" and "normal":''
::* April: NOAA's Climate Prediction Center (CPC) states it will use 1991–2020 as the new 30-year period of record, with "average" numbers of named Atlantic storms rising from 12 to 14, hurricanes from 6 to 7, and major hurricanes remaining at 3; Eastern Pacific and Central Pacific numbers remain unchanged over 1981–2010.
::* May: NOAA's NCEI supersedes weather and climate data from 1981–2010 with data from 1991–2020 to change its designation of "Climate Normal

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