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@ARTICLE{Blunden:894657,
      author       = {Blunden, J. and Boyer, T.},
      collaboration = {Grooss, Jens-Uwe and Müller, Rolf},
      title        = {{S}tate of the {C}limate in 2020},
      journal      = {Bulletin of the American Meteorological Society},
      volume       = {102},
      number       = {8},
      issn         = {1520-0477},
      address      = {Boston, Mass.},
      publisher    = {ASM},
      reportid     = {FZJ-2021-03342},
      pages        = {S1 - S475},
      year         = {2021},
      abstract     = {In 2020, the dominant greenhouse gases stored in Earth’s
                      atmosphere continued to increase. The annual global average
                      carbon dioxide (CO2) concentration at Earth’s surface was
                      412.5 ± 0.1 ppm, an increase of 2.5 ± 0.1 ppm over 2019,
                      and the high-est in the modern instrumental record and in
                      ice core records dating back 800,000 years. While
                      anthropogenic CO2 emissions were estimated to decrease
                      around $6\%–7\%$ globally during the year due to reduced
                      human activities during the COVID-19 pan-demic, the
                      reduction did not materially affect atmospheric CO2
                      accumulation as it is a relatively small change, less even
                      than interannual variability driven by the terrestrial
                      biosphere. The net global uptake of ~3.0 petagrams of
                      anthropogenic carbon by oceans in 2020 was the highest in
                      the 39-year record and almost $30\%$ higher than the
                      1999–2019 average. Weak El Niño-like conditions in the
                      eastern equatorial Pacific Ocean in early 2020 cooled and
                      transitioned to a moderate La Niña later in the year. Even
                      so, the annual global surface tem-perature across land and
                      oceans was among the three highest in records dating to the
                      mid- to late 1800s. In Europe, 17 countries reported record
                      high annual mean temperatures, contributing to the warmest
                      year on record for the European continent. Elsewhere, Japan,
                      Mexico, and Seychelles also experienced re-cord high annual
                      mean temperatures. In the Caribbean, Aruba, Martinique, and
                      St. Lucia reported their all-time monthly maximum
                      temperatures. In the United States, Furnace Creek in Death
                      Valley, California, reached 54.4°C on 16 August—the
                      hottest temperature measured on Earth since 1931, pending
                      confirma-tion. North of 60°N, the annual mean temperature
                      over Arctic land areas was 2.1°C above the 1981–2010
                      average, the highest in the 121-year record. On 20 June, a
                      temperature of 38°C was observed at Verkhoyansk, Russia
                      (67.6°N), provisionally the highest temperature ever
                      measured within the Arctic Circle. Near the opposite pole,
                      an atmospheric river—a long, nar-row region in the
                      atmosphere that transports heat and moisture from
                      sub-tropical and midlatitudes—brought extreme warmth from
                      sub-tropical and midlatitudes to parts of Antarctica during
                      austral summer. On 6 February, Esperanza Station recorded a
                      temperature of 18.3°C, the highest temperature recorded on
                      the continent, surpassing the previous record set in 2015 by
                      1.1°C. The warmth also led to the largest late-summer
                      surface melt event in the 43-year record, affecting more
                      than $50\%$ of the Antarctic Peninsula. In August, daily sea
                      ice extent in the waters surrounding Antarctica shifted from
                      below to above average, marking the end of persistent
                      below-average sea ice extent since austral spring 2016.In
                      the Arctic, when sea ice reached its annual maximum extent
                      in March, thin, first-year ice comprised $~70\%$ of the ice;
                      the thickest ice, which is usually more than four years old,
                      had declined by more than $86\%$ since 1985 to make up just
                      $2\%$ of total ice in 2020. When the minimum sea ice extent
                      was reached in September, it was the second smallest except
                      for 2012 in the 42-year satellite record. The Northern Sea
                      Route along the Siberian coast was open for about 2.5
                      months, from late July through mid-October, compared to less
                      than a month typically.Glaciers across the global cryosphere
                      lost mass for the 33rd consecutive year, and permafrost
                      temperatures continued to reach record highs at many high
                      latitude and mountain locations. In the Northern Hemisphere,
                      lakes froze three days later and thawed 5.5 days earlier on
                      average. In Finland, the average duration of lake ice was 42
                      days shorter. Record high spring temperatures in central
                      Siberia drove rapid snow melt that contributed to the lowest
                      June snow cover extent across Eurasia in the 54-year
                      record.As is typical, some areas around the world were
                      notably dry in 2020 and some were notably wet. The Middle
                      East experi-enced an extreme drought during autumn, with
                      most places reporting no precipitation in October. In South
                      America, the Bolivian lowlands suffered one of its most
                      severe droughts on record during autumn. Drought also
                      spanned the Chaco and Pantanal in Bolivia, Paraguay, and
                      southern Brazil. The Paraguay River shrank to its lowest
                      levels in half a century. A decadal “mega drought” in
                      south-central Chile continued through its 11th year, with
                      extreme conditions in the most populated areas. Argentina
                      reported its driest year since 1995. In North America,
                      drought continued to prevail in the West. The lack of
                      moisture in drought-stricken regions often pro-vide ideal
                      conditions for fire. Total fire emissions in the western
                      United States in 2020 were almost three times higher than
                      the 2003–10 mean. The Arctic experienced its highest fire
                      year in terms of carbon emitted into the atmosphere,
                      surpassing the record set in 2019 by $34\%,$ with most of
                      the fires occurring in Arctic Asia. In the tropics, the
                      Amazon saw its highest fire activity since 2012, while fire
                      activity in tropical Asia—including Indonesia—was one of
                      the lowest on record, related to wet conditions as La Niña
                      evolved during the fire season. The 2020 Southwest Asian
                      Monsoon season (June–September) was the wettest since
                      1981, also coincident with the emergence of La Niña. The
                      Meiyu rainy season, which usually occurs between July and
                      August over the Yangtze and Huaihe River Valleys of China,
                      was extended by two months in 2020. The May–October total
                      rainfall averaged over the area was the most since the start
                      of the record in 1961. Associated severe flooding affected
                      about 45.5 million people.A widespread desert locust
                      infestation during 2019–20 impacted equatorial and
                      northern East Africa, as heavy rains and prevailing winds
                      were favorable for breeding and movement of swarms across
                      Kenya, Ethiopia, northeastern Somalia, Uganda, South Sudan,
                      and northern Tanzania. The massive infestation destroyed
                      thousands of square kilometers of cropland and pasture
                      lands, resulting in one million people in need of food aid
                      in Ethiopia alone. Extremely heavy rains in April also
                      trig-gered widespread flooding and landslides in Ethiopia,
                      Somalia, Rwanda, and Burundi. The Lake Victoria region was
                      the wettest in its 40-year record. Across the global oceans,
                      the average ocean heat content reached a record high in 2020
                      and the sea surface temperature was the third highest on
                      record, surpassed only by 2016 and 2019. Approximately
                      $84\%$ of the ocean surface experienced at least one marine
                      heatwave (MHW) in 2020. For the second time in the past
                      decade, a major MHW developed in the northeast Pacific,
                      covering an area roughly six times the size of Alaska in
                      September. Global mean sea level was record high for the
                      ninth consecutive year, reaching 91.3 mm above the 1993
                      average when satellite measurements began, an increase of
                      3.5 mm over 2019. Melting of the Greenland Ice Sheet
                      accounted for about 0.8 mm of the sea level rise, with an
                      overall loss of 293 ± 66 gigatons of ice.A total of 102
                      named tropical storms were observed during the Northern and
                      Southern Hemisphere storm seasons, well above the
                      1981–2010 average of 85. In the North Atlantic, a record
                      30 tropical cyclones formed, surpassing the previous record
                      of 28 in 2005. Major Hurricanes Eta and Iota made landfall
                      along the eastern coast of Nicaragua in nearly the same
                      location within a two-week period, impacting over seven
                      million people across Central America. In the western North
                      Pacific, Super Typhoon Goni was the strongest tropical
                      cyclone to make landfall in the historical record and led to
                      the evacuation of almost 1 million people in the
                      Philippines. Very Severe Cyclonic Storm Gati was the
                      strongest recorded cyclone to make landfall over Somalia.
                      Bosaso, in northeast Somalia, received 128 mm of rainfall in
                      a 24-hour period, exceeding the city’s average annual
                      total of 100 mm.Above Earth’s surface, the annual lower
                      troposphere temperature equaled 2016 as the highest on
                      record, while stratospheric temperatures continued to
                      decline. In 2020, the stratospheric winter polar vortices in
                      both hemispheres were unusually strong and stable. Between
                      December 2019 and March 2020, the Arctic polar vortex was
                      the strongest since the beginning of the satellite era,
                      contributing to record low stratospheric ozone levels in the
                      region that lasted into spring. The anomalously strong and
                      persistent Antarctic polar vortex was linked to the
                      longest-lived, and 12th-largest, ozone hole over the region,
                      which lasted to the end of December.},
      cin          = {IEK-7},
      ddc          = {550},
      cid          = {I:(DE-Juel1)IEK-7-20101013},
      pnm          = {2112 - Climate Feedbacks (POF4-211)},
      pid          = {G:(DE-HGF)POF4-2112},
      typ          = {PUB:(DE-HGF)16},
      doi          = {10.1175/2021BAMSStateoftheClimate.1},
      url          = {https://juser.fz-juelich.de/record/894657},
}