STATE OF THE CLIMATE IN 2019

In 2019, the dominant greenhouse gases released into Earth’s atmosphere continued to increase. The annual global average carbon dioxide concentration at Earth’s surface was 409.8 ± 0.1 ppm, an increase of 2.5 ± 0.1 ppm over 2018, and the highest in the modern instrumental record and in ice core records dating back 800 000 years. Combined, greenhouse gases and several halogenated gases contributed 3.14 W m−2 to radiative forcing, representing a 45% increase since 1990. Carbon dioxide is responsible for about 65% of this radiative forcing. The annual net global uptake of ∼2.4 billion metric tons of carbon dioxide by oceans was the highest in the record dating to 1982 and 33% higher than the 1997–2017 average. A weak El Niño at the beginning of 2019 transitioned to ENSO-neutral conditions by mid-year. Even so, the annual global surface temperature across land and oceans was still among the three highest in records dating to the mid- to late 1800s. July 2019 was Earth’s hottest month on record. Well over a dozen countries across Africa, Europe, Asia, Australia, and the Caribbean reported record high annual temperatures. In North America, Alaska experienced its warmest year on record, while the high northern latitudes that encompass the Arctic were second warmest, behind only 2016. Stations in several countries, including Vietnam, the Netherlands, Belgium, Luxembourg, France, and the United Kingdom, set new all-time daily high temperature records for their nations. Australia set a new nationally averaged daily maximum temperature record of 41.9°C on 18 December, breaking the previous record set in 2013 by 1.6°C. Daily temperatures surpassed 40°C for the first time in Belgium and the Netherlands. Lake temperatures increased on average across the globe in 2019; observed lakes in the Northern Hemisphere were covered in ice seven days fewer than the 1981–2010 average, according to phenological indicators. Over land, the growing season was an average of eight days longer than the 2000–10 average in the NH. Above Earth’s surface, the annual lower troposphere temperature was third highest to record high, and the lower stratosphere temperature was third lowest to record low, depending on the dataset analyzed. Middle- and upper-stratospheric temperatures were lowest on record since satellite records began in 1979. In September, Antarctica experienced a dramatic upper-atmosphere warming event that led to the smallest ozone hole since the early 1980s. Below-average Antarc