Editing IPCC:AR6/WGI/Chapter-Atlas (section)

=== Atlas.4.2 Assessment and Synthesis of Observations, Trends and Attribution ===

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Figure Atlas.11 shows observed trends in annual mean surface temperature and indicates it has been rising rapidly over Africa from 1961 to 2015 and with significant increases in all regions of 0.1°C–0.2°C per decade and higher over some northern, eastern and south-western regions ( ''high confidence'' ) (see also Interactive Atlas). This is confirmed by an independent analysis performed for a longer period (1961–2018) over areas where long-term homogeneous temperature time series are available ( [[#Engelbrecht--2015|Engelbrecht et al., 2015]] ). More specifically over the Horn of East Africa, the long-term mean annual temperature change between 1930 and 2014 showed two distinct but contrary trends: significant decreases between 1930 and 1969 and increases from 1970 to 2014 ( [[#Ghebrezgabher--2016|Ghebrezgabher et al., 2016]] ). North Africa has an overall warming in observed seasonal temperature ( [[#Barkhordarian--2012|Barkhordarian et al., 2012]] ; [[#Lelieveld--2016|Lelieveld et al., 2016]] ) with positive trends in annual minimum and maximum temperatures ( [[#Vizy--2012|Vizy and Cook, 2012]] ). Temperatures over West Africa have increased over the last 50 years ( [[#Mouhamed--2013|Mouhamed et al., 2013]] ; [[#Niang--2014|Niang et al., 2014]] ) with a spatially variable warming reaching 0.5°C per decade from 1983 to 2010 ( [[#Sylla--2016|Sylla et al., 2016]] ). West Africa has also experienced a decrease in the number of cool nights, as well as more frequent warm days and warm spells ( [[#Mouhamed--2013|Mouhamed et al., 2013]] ; [[#Ringard--2016|Ringard et al., 2016]] ). Similarly, East Africa has experienced a significant increase in temperature since the beginning of the early 1980s ( [[#Anyah--2012|Anyah and Qiu, 2012]] ) with an increase in seasonal mean temperature. Over South Africa, positive trends were found in the annual mean, maximum and minimum temperatures for 1960–2003 in all seasons, except for the central interior ( [[#Kruger--2004|Kruger and Shongwe, 2004]] ; [[#Zhou--2010|Zhou et al., 2010]] ; [[#Collins--2011|Collins, 2011]] ; [[#Kruger--2013|Kruger and Sekele, 2013]] ; [[#MacKellar--2014|MacKellar et al., 2014]] ), where minimum temperatures have decreased significantly ( [[#MacKellar--2014|MacKellar et al., 2014]] ). Within inland southern Africa, minimum temperatures have increased more rapidly than maximum temperatures ( [[#New--2006|New et al., 2006]] ).

Most areas lack enough observational data to draw conclusions about trends in annual precipitation over the past century. In addition, many regions of Africa have discrepancies between different observed precipitation datasets ( [[#Sylla--2013|Sylla et al., 2013]] ; [[#Panitz--2014|Panitz et al., 2014]] ). A statistically significant (95% confidence level) decrease in rainfall and the number of rainy days is reported in autumn over the eastern, central and north-eastern parts of South Africa in spring and summer during 1960–2010 ( [[#MacKellar--2014|MacKellar et al., 2014]] ; [[#Kruger--2017|Kruger and Nxumalo, 2017]] ). Central Africa has experienced a significant decrease in total precipitation, which is likely associated with a significant decrease of the length of the maximum number of consecutive wet days ( [[#Aguilar--2009|Aguilar et al., 2009]] ). Furthermore, rainfall decreased significantly in the Horn of Africa ( [[#Tierney--2015|Tierney et al., 2015]] ) with the largest reductions during the long rains season from March to May ( [[#Lyon--2012|Lyon and DeWitt, 2012]] ; [[#Viste--2013|Viste et al., 2013]] ; [[#Rowell--2015|Rowell et al., 2015]] ). Over mountainous areas significant increases are found in the number of rain days around the southern Drakensberg in spring and summer during the period 1960–2010 ( [[#MacKellar--2014|MacKellar et al., 2014]] ). Similarly, southern West Africa is observed to have had more intense rainfall from 1950 to 2014 during the second rainy season of September to November ( [[#Nkrumah--2019|Nkrumah et al., 2019]] ). The Sahel region also had more intense rainfall throughout the rainy season ( [[#Panthou--2014|Panthou et al., 2014]] , 2018a, b; [[#Sanogo--2015|Sanogo et al., 2015]] ; [[#Gaetani--2017|Gaetani et al., 2017]] ; [[#Taylor--2017|Taylor et al., 2017]] ; [[#Biasutti--2019|Biasutti, 2019]] ) during the period 1980–2010. Southern African rainfall shows a significant downtrend of –0.013 mm day <sup>–1</sup> year <sup>–1</sup> in recent decades and –0.003 mm day <sup>–1</sup> year <sup>–1</sup> for longer periods during 1900–2010 ( ''low confidence'' ) ( [[#Jury--2013|Jury, 2013]] ).

Temperature increases over Africa in the 20th century can be attributed to the strong evidence of a continent-wide anthropogenic signal in the warming (Figure 3.9; [[#Hoerling--2006|Hoerling et al., 2006]] ; [[#Min--2007|Min and Hense, 2007]] ; [[#Stott--2010|Stott et al., 2010]] , 2011; [[#Niang--2014|Niang et al., 2014]] ). More specifically over West Africa, the clear emergence of temperature change (Figure Atlas.11) is due to the relatively small natural climate variability in the region which generates narrow climate bounds that can be easily surpassed by relatively small climate changes ( [[#Niang--2014|Niang et al., 2014]] ). Warming over North Africa is largely due to anthropogenic climate forcing ( [[#Knippertz--2003|Knippertz et al., 2003]] ; [[#Barkhordarian--2012|Barkhordarian et al., 2012]] ; [[#Diffenbaugh--2017|Diffenbaugh et al., 2017]] ).

The drying observed over the Sahel in the 1960s to 1970s has been attributed to warming of the South Atlantic SST and southern African drying as a response to Indian Ocean warming ( [[#Hoerling--2006|Hoerling et al., 2006]] ; [[#Dai--2011|Dai, 2011]] ). Enhanced rainfall intensity since the mid-1980s over the Sahel ( [[#Maidment--2015|Maidment et al., 2015]] ; [[#Sanogo--2015|Sanogo et al., 2015]] ) is associated with increased greenhouse gases suggesting an anthropogenic influence ( ''medium confidence'' ) ( [[#Biasutti--2019|Biasutti, 2019]] ). In the last decade, the changes in the timing of onset and cessation of rainfall over Africa have been linked to changes in the progression of the tropical rainband and the Saharan heat low ( [[#Dunning--2018|Dunning et al., 2018]] ; [[#Wainwright--2019|Wainwright et al., 2019]] ). Moreover, later onset and earlier cessation of eastern Africa rainfall is associated with a delayed and then faster movement of the tropical rainband northwards during the boreal spring and northward shift of the Saharan heat low ( [[#Wainwright--2019|Wainwright et al., 2019]] ), driven by anthropogenic carbon emissions and changing aerosol forcings ( ''medium confidence'' ). Over East Africa, the drying trend is associated with an anthropogenic-forced relatively rapid warming of Indian Ocean SSTs ( [[#Williams--2011|Williams and Funk, 2011]] ; [[#Hoell--2017|Hoell et al., 2017]] ); a shift to warmer SSTs over the western tropical Pacific and cooler SSTs over the central and eastern tropical Pacific ( [[#Lyon--2012|Lyon and DeWitt, 2012]] ); multi-decadal variability of SSTs in the tropical Pacific, with cooling in the east and warming in the west ( [[#Lyon--2014|Lyon, 2014]] ); and the strengthening of the 200-mb easterlies ( [[#Liebmann--2017|Liebmann et al., 2017]] ). However, decadal natural variability from SST variations over the Pacific Ocean has also been associated with the drying trend of East Africa ( [[#Wang--2014|Wang et al., 2014]] ; [[#Hoell--2017|Hoell et al., 2017]] ) with an anthropogenic-forced rapid warming of Indian Ocean SSTs ( ''medium confidence'' ).

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