The distribution of observed temperature and precipitation climatology for the period 1981 to 2010 is shown in Fig. 2. The time period 1981–2010 is selected for the climatology because it is used as the reference (base) period to obtain the changes in simulated climate from the CMIP6 models. The observed data shows that the temperature in the northwest and southwest mountain areas is lower than in the central to eastern regions, as seen in both the CRU and UoD datasets. The annual mean temperature is below 20 °C in the northwestern and southwestern regions, while it exceeds 30 °C in the central to eastern regions. The CRU data shows large precipitation in the southwest Peninsula, which can exceed 400 mm annually. The precipitation is low (below 60 mm) in the northwest part, and over the Rub Al-Khali desert areas. The GPCC data shows a similar distribution of precipitation to the CRU data. There is little variation between the datasets, and overall distributions of temperature and precipitation remain almost the same. The next sections describe the change in temperature over the Arabian Peninsula for the near and far future periods.
The historical trends of temperature and precipitation over Saudi Arabia (which represents 80% of the Peninsula) using the quality controlled and updated data for the period 1978–2019 indicate that the region continues to warm and to dry (Fig. 3). The temperature was slightly below normal (24.39 ˚C) before 1997 and above normal afterward. The sharp peak in 2010 indicates the warmest year in the historical record. Regression anaylysis shows that the temperature increased at a rate of 0.63 °C decade–1 for the period 1978–2019, which is significant at the 99% confidence level. Almazroui (2020a) also found a similar increasing rate in observed annual mean temperatures over the Arabian Peninsula. Annual precipitation is highly variable, however, it tends to be slightly below normal (96.92 mm) after 1998. Precipitation is decreasing at a rate of 6.3 mm decade–1, which is statistically insignificant. Almazroui (2020b) also found a decreasing trend in observed annual precipitation over Saudi Arabia. Details of the temperature and precipitation trends at all 25 station locations across Saudi Arabia are provided in Table 2. Overall, a larger rate of increase in temperature is observed in the northern Peninsula relative to the southern Peninsula, which ranges from 0.24 ˚C decade–1 (at Gizan) to 0.81 ˚C decade–1 (et al.–Jouf), and are all significant at the 99% confidence level. Precipitation trends range from – 30 mm decade–1 (at Hail) to 16 mm decade–1 (at Dammam). Except for four stations, precipitation trends are statistically insignificant. The increasing temperature and decreasing precipitation trends are in line with previous findings obtained for a relatively shorter period (1978–2009) by Almazroui et al. (2012), who reported temperature increase at a rate of 0.60 °C decade–1 and precipitation decrease at a rate of 6.20 mm decade–1. Note that the non-parametric Mann–Kendall trend analysis for the same data period over Saudi Arabia show almost similar trends for temperature and precipitation, however, to compare results with available literature and IPCC repot, linear trends are considered in this analysis (Almazroui 2020b).
Changes in temperature for the near and far futures
The spatial distribution of changes in projected temperature for the near and far futures under all three future scenarios are shown in Figs. 4, 5, and 6. The robust changes in temperature at annual scale over most parts of the Arabian Peninsula are projected to be below 2.0 °C in the near future under the low emission scenario SSP1–2.6 (top panels, Fig. 4). Very similar patterns of changes in annual temperature are obtained for the far future. Robust changes in temperature are also noted for the medium emission scenario SSP2–4.5, where most parts of the Peninsula will be 2.0 °C warmer than the present climate in the near future period, and 3.0 to 4.05 °C warmer in the far future (middle panels, Fig. 4). The high emission scenario SSP5–8.5 indicates a rise in temperature by 3.0 °C compared to the present climate over the entire Peninsula for the near future and more than 4.5 °C over most of the Arabian Peninsula. Under the SSP5–8.5 scenario, some parts of the Peninsula in the north-central region may exceed 6 °C in the far future (lower panels, Fig. 4). This rise of temperature is consistent with CMIP5 data analysis over South Asia under RCP8.5, which is in the range 3.2–5.1 °C for the period 2070–2099 (Alamgir et al. 2019). Overall, the change in the future temperature over the northern Peninsula is bigger (exceeding 6.0 °C) than over the southern Peninsula, which is in line with the CMIP3 and CMIP5 projections, as shown in Almazroui et al. (2016, 2017b). Summer temperatures over the Arabian Peninsula show a robust increase with a similar pattern to that of annual temperature. However, the magnitude of the temperature increase is more than 4.5 °C in the northern Peninsula under the medium emission scenario (Fig. 5). In this season, the area with a temperature increase above 6.0 °C expands towards the south, east, and west. In the case of winter temperature, the increase is lower than the annual and summer seasons in the northern Peninsula under a low emission scenario for the far future (Fig. 6). The higher increase in temperature over the northern Peninsula compared to the southern Peninsula observed in the summer season is not observed in the winter season. This feature is in line with the results from CMIP3 and CMIP5, as reported in Almazroui et al. (2016, 2017b). Note that the summer temperature of the Peninsula is highly correlated with the quasi-stationary mid-latitude Eurasian Rossby wave train pattern and the Atlantic Ocean sea surface temperature (Attada et al. 2018b; Ehsan et al. 2020; Rashid et al. 2020).
Changes in Precipitation for the Near and Far Futures
The spatial distribution of changes in projected precipitation for the near and far futures under all three SSPs are shown in Figs. 7 and 8. The projected precipitation shows a robust increase in annual mean precipitation over most of the Peninsula by the end of the twenty-first century under the three future scenarios. A decreasing signal is found over Jordan and adjacent areas of Saudi Arabia which get further intensified under high emission scenario SSP5-8.5. Annual precipitation is projected to decrease in the northwest and increase in the southern region (Fig. 7). The maximum deficit will be about 30%, and the surplus will exceed 50% under SSP5–8.5 in the far future. It is important to mention that because of its hyper-arid climate, the amount of precipitation in the southeastern region is very small. Therefore, a small increase in absolute precipitation may result in a large increase in relative (percentage) terms (Almazroui et al. 2017a; Almazroui and Saeed 2020). Note that the large increase over southwest Saudi Arabia is significant because the annual rainfall is highest in this region. The projected precipitation shows a decrease in the northwest region, which is more pronounced in the far future compared to the near future period, especially for the SSP5–8.5 scenario. This is in agreement with previous findings over the Arabian Peninsula based on CMIP3 and CMIP5 datasets (Almazroui et al. 2017a, b).
Over the Arabian Peninsula, the wet season (Oct–May) precipitation contributes the most to the annual total rainfall. The wet season precipitation shows a similar spatial distribution of future precipitation signals as compared to the annual patterns. The southern parts of the Peninsula display an increase, while the northwestern parts show a decrease in the projected precipitation (Fig. 8). The northwestern area of precipitation decrease is smaller in CMIP6 data as compared to CMIP5 data. Interestingly, the projected precipitation change during the wet season displays an identical spatial pattern, with a decrease over northwestern parts and an increase over the southern Arabian Peninsula for different scenarios. For precipitation projections, there is inconsistency among the CMIP6 models over a large part of the Arabian Peninsula. For the wet season, the agreement between CMIP6 model projections improves towards the end of the twenty-first century in both the middle (SSP2–4.5) and high emission scenarios. The signal of projected precipitation in CMIP6 models over the Arabian Peninsula is in agreement with CMIP5 model projections over the same region. The precipitation variability is reduced in the central Peninsula for the CMIP6 ensemble, as compared to CMIP3 and CMIP5.
Likelihood of Projected Temperature and Precipitation Signal over the Arabian Peninsula
Projected changes in temperature over the Arabian Peninsula show a large increase in the annual and summer season, which remains a bit higher as compared to the winter (upper panels, Fig. 9). Over the AP, the annual increase of temperature is projected to be 1.56 (1.63) ˚C with a likely range of 1.16 (1.24)–1.89 (2.1) ˚C for SSP1–2.6 in the near (far) future (Table 3). The increase will be 1.73 (2.7) ˚C with a likely range of 1.41 (2.27)–2.08 (3.39) ˚C under SSP2–4.5, and 2.17 (5.03) ˚C with a likely range from 1.81 (4.13) to 2.67 (5.81) ˚C under SSP5–8.5, in the near (far) future. It is worth mentioning that while likely ranges remain similar in the high emission scenario, the full ranges of temperatures are higher in CMIP6 as compared to CMIP5. This points towards the higher sensitivity of CMIP6 climate models to the greenhouse gas (GHG) emissions.
In the summer season, the increase of temperature over the Peninsula will be 1.64 (1.78) ˚C, 1.82 (2.92) ˚C, and 2.37 (5.21) ˚C, under SSP1–2.6, SSP2–4.5, and SSP5–8.5, respectively, for the near (far) future. In the winter season, the increase in the projected temperature is expected to be 1.43 (1. 35) ˚C, 1.57 (2.52) ˚C, and 1.9 (4.68) ˚C, under SSP1–2.6, SSP2–4.5, and SSP5–8.5, respectively, for the near (far) future. The projected increase in the summer temperature is larger than the projected increase in the winter temperature. The NAP is also warming faster than the SAP. For example, the increase in NAP summertime temperature is projected to be 5.87 ˚C, but only 4.71 ˚C for the SAP at the end of the twenty-first century under SSP–8.5. Similarly, the increase in the annual (winter) projected temperature over NAP is 5.32 (4.74) ˚C, compared with 4.77 (4.70) ˚C for SAP under SSP5–8.5. A detailed description of the likely and full ranges associated with projected temperatures over AP, NAP, and SAP is available in Table 3. Under the SSP5–8.5 scenario, NAP warming is projected to be faster than SAP warming in both summer and winter. Irrespective of the season, region, and scenario, the temperature over the Arabian Peninsula is projected to rise in the twenty-first century. Moreover, the warming over the northern Peninsula will be greater compared to the southern Peninsula in the future. These results are in agreement with the previous studies based on CMIP3 and CMIP5 datasets over Arabian Peninsula (see Almazroui et al. 2016, 2017b).
The projected precipitation also shows robust changes, either increasing or decreasing, over the Arabian Peninsula in the future period for the annual and wet season (Fig. 10). A detailed description of the precipitation changes over AP, NAP, and SAP during the wet season and annual time scale is given in Table 4. By the end of the twenty-first century, the wet season precipitation is projected to increase by 5.31/12.23/25.05% under SSP1–2.6, SSP 2–4.5, and SSP5–8.5, respectively, over the Arabian Peninsula (Table 4). For the same period and under the same scenarios, annually, the precipitation averaged over the Arabian Peninsula is likely to increase by 3.76/16.87/31.83%, respectively. Moreover, the annual precipitation is projected to increase more over SAP (10–51%) as compared to NAP (2–19%). Precipitation projections for the wet season are similar (see Table 4). A detailed description of changes in precipitation with associated likely and full ranges under different SSP scenarios for the near and far future with reference to the base period are tabulated in Tables 4. Hence, it is clear that the southern Peninsula is likely to receive more precipitation than the northern Peninsula at the end of the twenty-first century, which is in line with findings obtained from the CMIP3 and CMIP5 datasets (Almazroui et al. 2016, 2017b).
Trends in Projected Temperature and Precipitation Changes over the Arabian Peninsula
The changes in temperature and precipitation under three SSP scenarios (SSP1–2.6, SSP2–4.5 and SSP5–8.5) for the entire Arabian Peninsula (AP), northern Arabian Peninsula (NAP), and southern Arabian Peninsula (SAP), for the period 2030–2099 with reference to the base period 1981–2010, are shown in Fig. 11. The median values indicate the trends, and the likely ranges (66% around the median) represent the uncertainty ranges. The increase in temperature with reference to the base period will attain maximum value in the 2050s under the SSP1–2.6 scenario, and afterward, the increase is projected to be a bit slower for AP, NAP, and SAP. Over the AP, the rate of temperature increase is projected to be 0.05 ˚C decade–1 for the entire period, which includes changes of 0.21 ˚C decade–1 in the near future, and – 0.05 ˚C decade–1 in the far future. For the NAP (SAP), the rate of increase of temperature is projected to be 0.05 (0.04) ˚C decade–1 for the entire period, which results from 0.23 (0.22) ˚C decade–1 in the near future and –0.03 (–0.08) ˚C decade–1 in the far future. For the SSP2–4.5 scenario, the increase in temperature is higher than that in the SSP1–2.6 scenario. Hence the increase will be at the rate of 0.26 ˚C decade–1 for the entire period, or 0.34 ˚C decade–1 for the near future and 0. 16 ˚C decade–1 for the far future over the AP. The NAP shows a large increase rate (0.29, 0.34, and 0.18 ˚C decade–1 for the entire period, near future and far future, respectively) as compared to the SAP (0.27, 0.33, and 0.17 ˚C decade–1 for the entire period, near future and far future, respectively). The SSP5–8.5 scenario shows a gradual increase in temperature, but still higher than in the other two scenarios. The increase rate is projected to be 0.70, 0.58, and 0.87 ˚C decade–1 for the entire period, near future, and far future, respectively, over the AP. Over the NAP (SAP), the rise of temperature will be 0.79 (0.68), 0.64 (0.56), and 0.98 (0.75) ˚C decade–1 for the entire period, near future and far future, respectively. All the trends are significant at the 99% confidence level. There is a clear indication that NAP is warming at a higher rate than the SAP for the SSP5–8.5 scenario, and the increase at the end of the century will be higher compared to the previous decades. These results are in line with the CMIP3 and CMIP5 results over the Peninsula (Almazroui et al. 2016, 2017b).
The changes in precipitation for the future climate clearly indicate an increase for the SAP, which is most prominent in SSP5–8.5, while it gradually becomes less pronounced for the other two scenarios. This behavior is also reflected over the full AP. However, NAP does not show a clear trend in any of the scenarios. Over the AP, precipitation will decrease at the rate of 0.83, 1.27, and 2.18% decade–1 under the SSP1–2.6 scenario for the entire period, near future and far future, respectively. In the case of the SSP2–4.5 (SSP5–8.5) scenario, the projected rate of change is 1.12 (4.27), 2.77 (2.93), and 0.32 (4.17) % decade–1 for the entire period, near future, and far future, respectively. Over NAP, it is projected at – 0.73/ – 0.36/2.16, – 0.73/ – 0.34/2.16, and – 1.26/1.02/6.46% decade–1 for the SSP1–2.6/SSP2–4.5/SSP5–8.5 scenarios for the entire period, near future, and far future, respectively. Over the SAP, this rate is projected at – 1.26/1.02/6.46, 0.77/4.25/6.73, and – 4.90/–3.96/8.56% decade–1 for the SSP1–2.6/SSP2–4.5/SSP5–8.5 scenarios for the entire period, near future and far future, respectively. All the trends are significant at the 99% confidence level, except for the SSP2–4.6 scenario over NAP and AP. It is important to note that the likely ranges are very wide for the precipitation, which implies that there is a large variation in the simulation of precipitation from model to model and the uncertainty is very high, at least from the 2050s onwards during the twenty-first century.