Projected Change in Temperature and Precipitation Over Africa from CMIP6

We analyze data of 27 global climate models from the sixth phase of the Coupled Model Intercomparison Project (CMIP6), and examine projected changes in temperature and precipitation over the African continent during the twenty-first century. The temperature and precipitation changes are computed for two future time slices, 2030–2059 (near term) and 2070–2099 (long term), relative to the present climate (1981–2010), for the entire African continent and its eight subregions. The CMIP6 multi-model ensemble projected a continuous and significant increase in the mean annual temperature over all of Africa and its eight subregions during the twenty-first century. The mean annual temperature over Africa for the near (long)-term period is projected to increase by 1.2 °C (1.4 °C), 1.5 °C (2.3 °C), and 1.8 °C (4.4 °C) under the Shared Socioeconomic Pathways (SSPs) for weak, moderate, and strong forcing, referenced as SSP1-2.6, SSP2-4.5, and SSP5-8.5, respectively. The future warming is not uniform over Africa and varies regionally. By the end of the twenty-first century, the largest rise in mean annual temperature (5.6 °C) is projected over the Sahara, while the smallest rise (3.5 °C) is over Central East Africa, under the strong forcing SSP5-8.5 scenario. The projected boreal winter and summer temperature patterns for the twenty-first century show spatial distributions similar to the annual patterns. Uncertainty associated with projected temperature over Africa and its eight subregions increases with time and reaches a maximum by the end of the twenty-first century. On the other hand, the precipitation projections over Africa during the twenty-first century show large spatial variability and seasonal dependency. The northern and southern parts of Africa show a reduction in precipitation, while the central parts of Africa show an increase, in future climates under the three reference scenarios. For the near (long)-term periods, the area-averaged precipitation over Africa is projected to increase by 6.2 (4.8)%, 6.8 (8.5)%, and 9.5 (15.2)% under SSP1-2.6, SSP2-4.5, and SSP5-8.5, respectively. The median warming simulated by the CMIP6 model ensemble remains higher than the CMIP5 ensemble over most of Africa, reaching as high as 2.5 °C over some regions, while precipitation shows a mixed spatial pattern.

Published in partnership with CECCR at King Abdulaziz University large latitudinal extent, the climate of the African continent varies widely. Barkhordarian et al. (2012) found an increasing trend in the observed annual and seasonal mean surface temperatures over North Africa. Several other studies also identified a projected increase in the mean temperature over North Africa and adjacent areas of the Middle East and Arabian Peninsula during the twenty-first century (Bucchignani et al. 2018;Almazroui et al. 2017a, b;Lelieveld et al. 2012Lelieveld et al. , 2016Vizy and Cook 2012). According to the fifth assessment report of the Intergovernmental Panel on Climate Change (IPCC AR5, IPCC 2013), projected average air temperature over Africa could exceed the twentieth-century range of variability by 2047 and 2069, under Representative Concentration Pathways (RCPs) RCP8.5 and RCP4.5, respectively (Niang et al. 2015). The northern part of Africa and the Middle Eastern regions adjacent to the Mediterranean have been identified as hotspots for climate change (Giorgi and Lionello 2008). New et al. (2006) found fewer cold days and cold nights and more warm days and warm nights over West Africa during the period 1961-2006. The risk of deadly heat stress is likely to increase over West Africa, while heat waves are likely to be more frequent and longer under the effects of global warming (Diedhiou et al. 2018;Sylla et al. 2018). Anyah and Qiu (2012) similarly observed an increase in temperature over the southern and eastern parts of Africa in recent decades. Consistent with these results, most parts of southern Africa have also experienced an increasing trend in mean annual temperature in recent decades (Collins 2011;Kruger and Sekele 2013;MacKellar et al. 2014).
On the other hand, precipitation over the African continent is highly variable. Available observational datasets show large discrepancies over Africa (Kruger and Shongwe 2004;Nikulin et al. 2012;Kalognomou et al. 2013;Sylla et al. 2013), and therefore it is difficult to draw robust conclusions about precipitation trends during the past century. According to IPCC AR5, mean annual precipitation decreased over the western and eastern Sahel regions in northern Africa, while it increased over parts of eastern and southern Africa, in the past century (IPCC 2013). Observed precipitation over Africa also displays strong seasonal variability. The boreal winter and spring precipitation over northern Africa show a decreasing trend in recent decades (Barkhordarian et al. 2013). The Sahara desert in northern Africa receives very low mean annual precipitation and likewise shows little seasonal change in precipitation (Liebmann et al. 2012). The overall reduction in seasonal and annual precipitation over different parts of the African continent during the twentieth century is also noted by several other studies (Sylla et al. 2016(Sylla et al. , 2018Klutse et al. 2018) (Hoerling et al. 2006;Lebel and Ali 2009;Lyon and Dewitt 2012;Mohamed 2011;New et al. 2006).
Future climate projections over Africa for the twentyfirst century indicate that its land temperature will rise faster than the global mean temperature, particularly in the more arid regions (Niang et al. 2015). Under high-emission RCP8.5, the mean annual temperature over Africa could rise by 3-6 °C by the end of the twenty-first century. Precipitation projections are more uncertain than temperature projections and display larger spatial variability and seasonal dependence (Orlowsky and Seneviratne 2012). IPCC's fifth assessment report projected a reduction in precipitation over northern Africa and southwestern parts of South Africa by the end of the twenty-first century (Niang et al. 2015). The IPCC climate projections for Africa are based mainly on Coupled Model Intercomparison Project phase 3 and phase 5 (formally known as CMIP3 and CMIP5) datasets. Data from the more recent phase 6 of the Coupled Model Intercomparison Project are now available for analysis (Eyring et al. 2016). The CMIP6 dataset differs from CMIP5 and CMIP3 datasets in terms of forcing scenarios and carbon emissions. The CMIP6 project employed updated versions of the coupled global climate models, a new start year, and a new set of Shared Socioeconomic Pathways (SSP) scenarios of concentrations (Gidden et al. 2019). The present study, therefore, aims to examine the projected changes in future temperature and precipitation over Africa using the latest CMIP6 dataset. This paper further describes projected annual and seasonal mean changes in temperature and precipitation over eight African subregions (described in the next section).

Data and Methodology
First, observational temperature data from the Climatic Research Unit (CRU) (Harris et al. 2014) and precipitation data from the Global Precipitation Climatology Centre (GPCC) (Becker et al. 2013) were analyzed to depict temperature and precipitation climatology over Africa for the period 1981-2010. In a second step, we analyzed the multi-model ensemble from 27 CMIP6 models (listed in Table 1) and examined future changes in temperature and precipitation over Africa in the twenty-first century. Analyses were performed over the entire African continent as well as over eight subregions (Fig. 1). These eight subregions are as follows: North Africa (NAF), Sahara (SAH), West Africa (WAF), Central Africa (CAF), Central East Africa (CEAF), North East Africa (NEAF), South West Africa (SWAF), and South East Africa (SEAF) (see Iturbide et al. 2020). The subregions are defined for the IPCC's sixth assessment report (AR6), and the rationale behind selecting these subregions is explained in detail by Iturbide et al. (2020). The CMIP6 dataset was obtained from the CMIP6 database (https ://esgf-node.llnl.gov/searc h/cmip6 ). This CMIP6 dataset is available at different horizontal resolutions. Prior to the analysis, we re-gridded all CMIP6 data from original spatial resolutions to a 1° × 1° common grid resolution owing to the higher resolution of CMIP6 models, which is explained in detail by Iturbide et al. (2020). To accomplish this, a bilinear re-gridding method is applied to temperature data, and a conservative first-order re-gridding to precipitation data. Previous studies (Jones 1999;Saeed et al. 2017) also reported that a conservative re-gridding method is more appropriate for discontinuous variables like precipitation.
We first examined the performance of the CMIP6 models over Africa in the present climate (1981-2010) against the CRU observations. For this purpose, we computed the temperature and precipitation biases for each sub-region as well as for all of Africa, and examined the spread in bias for the 27 CMIP6 models. For future projections, the mean annual temperature and precipitation data were analyzed over 30-year time intervals for the base period , the near term , and the long term (2070-2099). Temperature and precipitation trends were then examined for a continuous period from 2030 to 2099. Since there is large seasonal variation over Africa, we also examined temperature and precipitation projections for boreal summer (JJA) and winter (DJF) seasons. The ensemble from the 27 available CMIP6 models for the present and future climates was analyzed for temperature and precipitation changes under three future SSP scenarios, namely a low forcing scenario SSP1-2.6, a medium forcing scenario SSP2-4.5, and a strong forcing scenario SSP5-8.5. All SSPs are based on five narratives that describe alternative socioeconomic developments (Gidden et al. 2019). These include sustainable development (SSP1), middle-of-the-road development (SSP2), regional rivalry (SSP3), inequality (SSP4), and fossil-fueled development (SSP5). Complete descriptions of SSPs are available in O'Neill et al. (2016). SSP1-2.6, SSP2-4.5, and SSP5-8.5

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Published in partnership with CECCR at King Abdulaziz University are taken to be representatives of low-, medium-. and highemission scenarios, respectively. For the evaluation of future temperature and precipitation under SSPs in the form of spatial plots, two criteria based on the robustness and significance of change were employed. The robustness of the projected signal is a measure based on the agreement of different model projections in the direction of changes (Solomon et al. 2007). The changes, whether an increase or a decrease in the projected signal, are robust if at least 66% of the models agree on the change direction (Haensler et al. 2013;Almazroui et al. 2017a, b). The significance of a change in the projected signal was measured by employing a two-tailed Student t test based on equal and unequal variances between future and historical data for each grid box. Equal and unequal variance was determined for each grid box using an F test. To assess the uncertainty associated with the temperature and precipitation over Africa, the 66% likely range (17th-83rd percentile) and the full range (results from all models) were also used along with median values. Finally, we examined the difference between CMIP6 and CMIP5 projections (Table S1) to assess the climate response to the anthropogenic forcings in both sets of CMIP model simulations over Africa.

Observed Climatology and Climate Models' Evaluation
The observed mean annual temperature and precipitation over Africa during the period 1981-2010 is shown in Fig. 2a, b. The mean annual temperature varies widely from 10 to 30 °C (Fig. 2a). The lowest values of mean annual temperature (< 15 °C) occur over NAF, SWAF, SEAF, and some hilly areas of CEAF and NEAF. The highest values of mean annual temperature (> 28 °C) occur in the western parts of SAH and WAF, northern parts of CAF, NEAF, and adjacent areas of SAH, and eastern boundaries of NEAF and CEAF (Fig. 2a). Unlike temperature, precipitation over Africa is highly variable. Most of the mean annual precipitation over Africa during 1981-2010 occurred between 10°N and 20°N. The NAF receives mean annual precipitation amounting to ~ 200-500 mm year −1 . The rainfall over NAF is highly variable and season-dependent. The smallest amount of precipitation occurs in SAH, and ranges from 25 to 100 mm year −1 . The coastal areas of WAF and central parts of CAF and NEAF and Madagascar receive the highest amount of annual rainfall, reaching 3000 mm year −1 . Rainfall over central parts of Africa is highly dependant on the seasonal March of the Intertropical Convergence Zone (ITCZ). During the boreal winter season, rain falls mostly in the Southern Hemisphere between 0 and 20°S, while during the boreal summer season the ITCZ moves to the Northern Hemisphere, and thus rain falls between 0 and 20°N (Fig. 2b). Figure 2c, d displays the performance of the CMIP6 models in the simulation of mean annual temperature and precipitation over Africa in the present climate with respect to the observations. The length of each bar shows the spread in temperature and precipitation biases simulated by the CMIP6 models over all regions and over the entire continent relative to CRU observations. For temperature, in each region except NEAF, the ensemble median tends to remain slightly below the zero line. However, the 66% range around the median spans the zero line. Precipitation results show a mixed pattern, where the ensemble median lies above and below the zero line for different regions. Similarly, the 66% range spans the zero line for all regions except SWAF, which has a large wet bias. A similar wet bias is observed over the SWAF region for the CMIP5 models, and is associated with the enhanced northeasterly moisture influx (Munday and Washington 2018). The SAH region is hyper-arid and has a poor density of observation stations, which results in a huge bias in relative plots for precipitation. Therefore, results for

Change in Multi-Model Mean Annual Temperature and Precipitation Projections Over Africa
The spatial distribution of projected changes in mean annual temperature and precipitation over Africa is shown in Figs. 3 and 4. The future temperature projections indicate enhanced warming over Africa, during both the near-and long-term periods during the twenty-first century relative to the reference climate, under all three future scenarios (Fig. 3). The warming increases toward the end of the twenty-first century, and the maximum warming occurs in the high-emission SSP5-8.5 scenario.
The warming over Africa for the near (long)-term period is projected to be 1.  5). The full range of temperature change slightly exceeds the 66% likely range. A detailed description of temperature trends in the twenty-first century over Africa and its eight subregions is given in Table 3. This shows a significant warming trend over Africa under the SSP1-2.6, SSP2-4.5, and SSP5-8.5 scenarios. During the period 2030-2099, temperature trends over Africa under SSP1-2.6, SSP2-4.5, and SSP5-8.5 are 0.03 °C decade −1 , and d precipitation (%) simulated by CMIP6 models over Africa and its subregions. The median bias is shown by the black horizontal line in each bar. The length of the bar shows the full range, while the dark shades represent the 66% range around the median bias 1 3 Published in partnership with CECCR at King Abdulaziz University 0.23 °C decade −1 , and 0.62 °C decade −1 , respectively. These temperature trends are significant at the 99% confidence level. The spatial distribution of changes in precipitation over Africa under three future scenarios for the near-and longterm periods is shown in Fig. 4. The projected precipitation shows a robust decrease over northern and southern Africa, with increases over Central Africa during both future periods. For the near (long)-term period, projected changes in precipitation over Africa under SSP1-2.6, SSP2-4.5, and SSP5-8.5 are 6.2 (4.8)%, 6.8 (8.5)%, and 9.5 (15.2)%, respectively, while the 66% likely range of the change in precipitation over Africa for the near (long)term period is 0.1-14% (−4.3 to 12.0%), 1.1-17% (−0.6 to 18.6%), and 0.5-28.4% (1.0-65.4%) for SSP1-2.6, SSP2-4.5, and SSP5-8.5, respectively (Table 2 and Fig. 6).
These changes in precipitation over Africa during the twenty-first century show large variability. The areaaveraged values of projected precipitation change over the entire African continent do not realistically represent projected precipitation change over different subregions. Therefore, we examined temperature and precipitation changes over each of the eight subregions of Africa (see Fig. 1).

Fig. 3
Future changes in mean annual temperature (°C) over Africa under three scenarios (SSP1-2.6, SSP2-4.5, SSP5-8.5) for the two time periods (2030-2059 and 2070-2099) as compared with the reference period . The backslash and forward slash represent the grid boxes having significant and robust change, respectively, while hatching represents the grid boxes having both significant and robust change. Significance is defined based on a two-tailed Student t test, while robustness is achieved when 66% of all models project a climate change signal in the same direction Published in partnership with CECCR at King Abdulaziz University
The spatial distribution of changes in the boreal summer precipitation over the African continent is shown in Fig. 12. The precipitation pattern shows a reduction over the northern, northwestern, and southern parts of Africa, along with an increase in the central and adjacent northeastern and northwestern parts, during both the near-and long-term periods under all three scenarios. The projected change in boreal summer precipitation is nearly opposite that of the winter pattern, where the northern parts show much lower precipitation as compared with the southern parts. During boreal summer, southern Africa displays a robust reduction in precipitation, while the northern parts show much less robust signals of reduced precipitation.

Differences Between CMIP5and CMIP6-Projected Signals Over Africa
We further investigated the difference between the CMIP5 and CMIP6 climate model responses to anthropogenic forcing over Africa. We computed the difference between the projected mean annual temperature for medium (SSP2-4.5 and RCP4.5) and strong (SSP5-8.5 and RCP8.5) scenarios between the median values of the two projects (Fig. 13). For both the scenarios and time periods, the CMIP6 climate models simulate higher temperatures over large parts of Africa. In the Southern Hemisphere, the CMIP6 temperatures are higher than those of the CMIP5 by 1 °C, whereas in the Northern Hemisphere this difference increases to as much as 2.5 °C over the region surrounding Sudan and Niger. The results for precipitation show a mixed signal which is more or less consistent across both the scenarios and time periods (Fig. 14). The CMIP6 model ensemble projects more precipitation over NAF, SAH, CEAF, and western WAF as compared with CMIP5, whereas CMIP6 projects reduced precipitation over SWAF, CAF, and especially NEAF.

Summary and Conclusions
We analyzed a multi-model ensemble based on 27 CMIP6 models and examined projected temperature and precipitation changes over Africa in the twenty-first century. These changes were computed for the African continent and its eight subregions during two future time slices (2030-2059 and 2070-2099) relative to the present climate . The CMIP6 multi-model ensemble projected a continuous increase in annual temperature over all of Africa and its eight subregions during the twenty-first century. Over that time, a significant increasing trend (at the 99% confidence level) in temperature is projected for the three SSP scenarios analyzed in this study (Table 3). The annual temperature over Africa for the near (long)-term period is projected to increase by 1.2 °C (1.4 °C), 1.5 °C (2.3 °C), and 1.8 °C (4.4 °C) under SSP1-2.6, SSP2-4.5, and SSP5-8.5, respectively. The uncertainty associated with projected temperature over Africa for the near (long)term period under SSP1-2.6, SSP2-4.5, and SSP5-8.5 ranged from 0.8 to 2.0 °C (0.7-2.4 °C), 1.0 to 2.3 °C (1.8-3.8 °C), and 1.1 to 2.7 °C (2.9-6.4 °C), respectively. The projected warming over Africa is not uniform and varies regionally. Furthermore, regional differences in projected warming are more pronounced for the high-emission scenario. Under SSP5-8.5, the largest warming is projected over the Sahara and adjacent parts of northern Africa. A robust increase in warming is also projected for southern Africa during the twenty-first century. In the near (long)term period, the temperature over the eight subregions of Fig. 13 Difference (°C) in the projected mean annual temperature over Africa between CMIP6 and CMIP5 models. The difference (CMIP6 minus CMIP5) in projected signals is obtained for median values for two scenarios (SSP2-4.5 and SSP5-8.5) as well as for two future (near-term and long-term) periods 1 3 Published in partnership with CECCR at King Abdulaziz University Africa, namely NAF, SAH, WAF, CAF, CEAF, NEAF, SWAF, and SEAF, is projected to increase by 2.2 (5.0) °C, 2.2 (5.3) °C, 1.7 (4.2) °C, 1.6 (3.7) °C, 1.5 (3.5) °C, 1.7 (4.1) °C,1.9 (4.7) °C, and 1.7 (4.0) °C, respectively, under the SSP5-8.5 scenario. The projected temperature during boreal winter and summer also shows greater warming over northern Africa as compared with other regions.
On the other hand, projected precipitation over Africa shows substantial spatial variability and seasonal dependency during the twenty-first century. The northern and southern parts of Africa show a robust reduction in annual precipitation while the central parts of Africa show an increase under the three SSP scenarios in both the nearand long-term periods. The area-averaged precipitation at the continental scale revealed an increasing trend in annual precipitation over Africa under all scenarios. For the near (long)-term period, the area-averaged precipitation over Africa is projected to increase by 6.2 (4.8)%, 6.8 (8.5)%, and 9.5 (15.2)% under SSP1-2.6, SSP2-4.5, and SSP5-8.5, respectively. The uncertainty associated with precipitation is also large relative to that in projected temperature. This uncertainty associated with projected precipitation under SSP1-2.6, SSP2-4.5, and SSP5-8.5 is −5.2 to 20.7% (−9.1 to 20.0%), −10.3 to 27.6% (−9.7 to 45.0%), and −4.9 to 47.1% (−7.8 to 200.3%), respectively, for the near (long)term period. The projected seasonal precipitation pattern shows a robust reduction in boreal winter over northern parts of Africa, while the boreal summer pattern shows the opposite of boreal winter, with even reduced precipitation over southern parts of Africa. Central Africa shows an increase in The comparison between the CMIP5 and CMIP6 model ensembles shows enhanced warming projected by CMIP6 more or less over the entire continent, with much higher warming over Northern Africa. Precipitation shows a mixed pattern, with the CMIP6 model ensemble simulating less precipitation over SWAF, CAF, and NEAF than the CMIP5 ensemble, while projecting a higher amount of precipitation over NAF, SAH, CEAF, and western WAF as compared with CMIP5.
This study is among the initial assessments of a CMIP6 climate model ensemble over the African continent in which a large number of climate models have been employed. It offers useful information about the regional patterns of projected temperature and precipitation and can serve as a reference, providing first-hand information based on the new generation of climate models over the continent.