Global Climate

Abstract

For reasons other than the climate, 2020 was an extraordinary year. The COVID-19 pandemic has affected almost all of us, changing the lives of many people around the globe. While the economic disruption associated with COVID-19 led to modest estimated reductions of 6-7% (e.g., le Quere et al. 2020;Friedlingstein et al. 2020; BP Statistical Review of the World Energy 2021) in global anthropogenic carbon dioxide (CO 2 ) emissions, atmospheric CO 2 levels continued to grow rapidly-a reminder of its very long residence time in the atmosphere and the challenge of reducing atmospheric CO 2 . As we show in this chapter, the climate has continued to respond to the resulting warming from these increases in CO 2 and other greenhouse gases such as methane and nitrous oxide, which also experienced record increases in 2020.The year 2020 was one of the three warmest since records began in the mid-to-late 1800s, with global surface temperatures around 0.6°C above the 1981-2010 average, despite the El Niño-Southern Oscillation progressing from neutral to La Niña conditions by August (see section 4b). Lower tropospheric temperatures matched those from 2016, the previous warmest year. Meanwhile, stratospheric temperatures continued to cool as a result of anthropogenic CO 2 increases. Along with the above-average surface temperatures, an unprecedented (since instrumental records began) geographic spread of heat waves and warm spells occurred. Antarctica observed its highest temperature on record (18.3°C) at Esperanza in February. In August, Death Valley, California, reported the highest temperature observed anywhere on Earth since 1931 (preliminary value of 54.4°C).Consequently, many permafrost measurement sites experienced their highest temperatures on record; Northern Hemisphere (NH) snow cover was below the 51-year average and the fourthleast extensive on record. Glaciers in alpine regions experienced their 33rd consecutive year of negative mass balance and 12th year of average losses of more than 500 mm depth. On average, NH lakes froze over 3 days later and thawed 5.5 days earlier than the 1981-2010 average during the 2019/20 winter, which was the third-shortest ice cover season since 1979/80.The atmosphere responded to higher temperatures accordingly by holding more water. Total column water vapor was high relative to the 1981-2010 average, ranging from 0.75 to 1.06 mm over ocean and 0.58 to 0.94 mm over land, but did not reach the record values of 2016. At the surface, specific humidity over oceans was at record high levels (0.23 to 0.41 g kg −1 ) and was well above average over land (0.14 to 0.36 g kg −1 ). Conversely, relative humidity was well below average over land (-1.28 to -0.68 %rh), continuing the long-term declining trend. Precipitation increased compared to 2019, driven largely by land values, but there were few exceptional extreme precipitation events, coupled with below-average cloudiness over most of the land. More lakes showed positive water level anomalies than 2019, and in East Africa, Lake Victoria's level ...

State of the UK Climate 2021

“…The 2020 BAMS state of the global climate report (Dunn et al, 2021) show that on a continental and global scale prior to around 2010 there was a widespread and general slowdown of near-surface winds termed "global stilling." More recently there are some indications of a reversal of this trend.…”

“…More recently there are some indications of a reversal of this trend. Dunn et al (2021) and references within conclude that this is likely primarily a combination of internal variability and anthropogenic greenhouse gas forcing, with secondary impacts from land use change, urbanization and instrumentation issues. The UK mean speed series shown in Figure 43 broadly reflects the behaviours documented globally, although in the case of the UK the decline has stopped rather than reversed as reported for the global series.…”

Global total ozone recovery trends derived from five merged ozone datasets

“…The latest version of the GOME-type Total Ozone Essential Climate Variable (GTO-ECV) data record (Coldewey-Egbers et al, 2015, 2021Garane et al, 2018) has been generated as part of the European Space Agency's Climate Change Initiative+ ozone (ESA_CCI+ ozone) project. Total columns from six sensors (GOME, SCIAMACHY, OMI, GOME-2A, GOME-2B, and TROPOMI), retrieved with the GOME Direct Fitting (GODFIT) version 4 algorithm (Lerot et al, 2014;Garane et al, 2018),…”

Using machine learning to identify novel hydroclimate states

“…For example, if we set the contamination parameter ν to be 0.01, this sets the threshold for the decision function such that 1% of years over the training period are considered to be outliers. To extend drought atlas data to 2020, we use instrumental PDSI values from the CRU TS dataset [37,38]. While drought atlases are calibrated using recent instrumental data, the two datasets are not identical, and there are some differences in the decision functions between the instrumental record and the reconstructions.…”