• In this study, the Massachusetts Institute of Technology Regional Climate Model (MRCM) is used, which is based on the ICTP Regional Climate Model version 3 (RegCM3) with several significant enhancements. The MRCM grid, centered at 24°N and 47°E on a Lambert Conformal projection, consists of 144 points in the x-direction and 130 points in the y-direction. The grid cells are separated by 25-km in the horizontal, and 18 sigma levels are prescribed in vertical. Output from three global climate models (GCMs) from the Coupled model Intercomparison Project Phase 5 (CMIP5) database is used as atmospheric boundary conditions for the MRCM integrations. Present-day conditions are represented with historical greenhouse gas (GHG) concentrations for the period 1975 through 20058. To consider the impacts of climate change, two future GHG scenarios are considered based on the IPCC Representative Concentration Pathway (RCP) trajectories for the period 2070 through 2100: RCP 8.5 and RCP 4.5. RCP 8.5, which represents 8.5 W/m2 of radiative forcing values in the year 2100 relative to pre-industrial values, is considered a high (or business as usual) GHG concentration scenario that does not consider any mitigation target. RCP 4.5, which represents 4.5 W/m² of radiative forcing, is considered a mitigation scenario.
• Recently, work has been carried out focusing on improving understanding of the regional climate of Southwest Asia and on improving the skill of MRCM in simulating the key processes in this arid region. As much of the land surface in Southwest Asia is characterized as desert and semi-desert, it is essential that the soil albedo and emissivity be accurately characterized. To correct the high soil albedo bias (0.06 over land) present in the default version of MRCM, albedo is prescribed based on the NASA/GEWEX Surface Radiation Budget (SRB) Project. In addition, emissivities over desert and semi-desert are reduced from 0.95 to 0.91 and 0.93, respectively, according to the NASA MODIS surface emissivity data. These two improvements significantly reduce an overall T and TW cold biases of approximately 1.5°C to less than 0.5°C over Southwest Asia when compared to the European Centre for Medium-Range Weather Forecasts Interim Reanalysis (ERA-Interim) data, except in areas of complex topography. The model includes a representation for the emission, transport and deposition of mineral aerosols and their direct radiative effects. Lastly, irrigated crop and marshland land cover types are included to better represent the surface conditions in southern Iraq.
• In order to objectively compare and select GCMs for use as boundary conditions for MRCM, we apply the following criteria: (1) The GCM provide representations of the Red Sea and Persian Gulf by use of an ocean model of adequate resolution. Each of the GCMs selected represents ocean processes between 0.4° and 1.11° horizontal resolution. While these resolutions are less than optimal to simulate some of the key processes in the Red Sea and Persian Gulf, they represent the best available from the CMIP5 archive. (2) The GCM accurately simulate surface T, TW and relative humidity over the Red Sea and Persian Gulf surrounding coastal regions, as well as over all of Southwest Asia. Output from more than 30 GCMs used in CMIP5 are objectively analyzed and compared to both the ERA-Interim and Climate Research Unit datasets. To assess the performance of each GCM, the normalized root mean square error for each variable (T, TW, and relative humidity) is averaged separately over each region (Persian Gulf, Red Sea, and Arabian Peninsula).
• As a result of applying the above objective criteria, the three GCMs with the lowest total sum of root mean square errors for each variable and region are selected: Community Climate System Model version 4 (CCSM4), Max-Planck-Institute Earth System Model (MPI-ESM) and Norwegian community Earth System Model (NorESM).
• The accuracy of the simulations of future T and TW conditions by MRCM in Southwest Asia reflects to a significant degree the accuracy of the SSTs projected by the coupled GCMs used as boundary conditions. This is particularly true for SSTs in the Persian Gulf and Red Sea. Although the CMIP5 GCMs used in this study are screened to resolve the Persian Gulf and Red Sea, their representations are nevertheless spatially limited, especially CCSM4 and NorESM1. The associated limited representation of the SSTs in the Persian Gulf and Red Sea may pose shortcomings that should be evaluated and addressed in future studies.
• TW is computed by the formulation developed by Davies-Jones 23. The ERA-Interim reanalysis data are considered the best available combined spatial and temporal representation of observations for the region, and are therefore used for the following bias correction procedure: (1)The maximum 6-hour average TW and T for each day are computed for both the MRCM hourly output and the ERA-Interim Reanalysis 3-hourly 0.75° x 0.75° data, denoted by TWmax and Tmax, respectively. (2)The ERA-Interim Tmax and TWmax data are interpolated from the 0.75° x 0.75° horizontal grid to the 25-km MRCM grid. (3)Consistent MRCM and ERA-Interim climatologies of Tmax and TWmax are computed for each day of the year on the MRCM 25-km grid. (4)The magnitude of the bias for each day of the year is estimated by the difference between 30-day running means of the two climatologies. (5)The daily bias is finally applied to the MRCM daily values of Tmax and TWmax for the present-day and future climates.
• The corrections are on the order of 1-2°C for TWmax. It assumed that the bias in the present day is the same as the bias in the future, which is commonly done in impacts studies.
• The adjusted values are in turn used to compute annual maxima and histograms at each grid point. Since maximum values of TW in the region occur in July, August, and September (JAS), histograms of TWmax and Tmax are computed for this period. The JAS TWmax and Tmax values are additionally sorted to determine the 50th (median) and 95th percentile values. The 50th percentile value is on average exceeded half of the days and provides a measure of the mean TWmax, while the 95th percentile is exceeded on average every 4.6 days of the three-month period suggestive of a typical hot TWmax summer event.
• To verify the quality of the ERA-Interim Reanalysis data, the observed annual maximum TWmax is estimated for six stations from the region: two stations adjacent to the Persian Gulf; two adjacent to the Red Sea; and two in the desert interior. Present- day TWmax values exceeding 31°C are observed adjacent to the Persian Gulf and the Red Sea, and lower values are observed in the desert interior, both of which are consistent with our simulation results. Furthermore, an increasing trend significant at the 95% level is observed for each of the stations, also consistent with the simulations. It is important to note that the station data represent point values, while the ERA-Interim reanalysis data represent spatial averages over a large area (1,000s km²). As a result, there may be some inconsistencies between the station data and ERA- Interim data. The latter is the dataset used in the bias correction procedure.

• A human body may be able to adapt to extremes of dry-bulb temperature (commonly referred to as simply temperature) through perspiration and associated evaporative cooling provided that the wet-bulb temperature (a combined measure of temperature and humidity or degree of “mugginess”) remains below a threshold of 35°C. This threshold defines a limit of survivability for a fit human under well-ventilated outdoor conditions and is lower for most people. We project using an ensemble of high-resolution regional climate model simulations that extremes of wet-bulb temperature in the region around the Persian Gulf are likely to approach and exceed this critical threshold under the business-as-usual scenario of future greenhouse gas concentrations. Our results expose a specific regional hotspot where climate change, in absence of significant mitigation, is likely to severely impact human habitability in the future.
• The geologic formations beneath and around the Persian Gulf in Southwest Asia, commonly referred to as the Middle East, are a major source for the oil and gas consumed locally and around the world, contributing greatly to the past and current emissions of carbon dioxide. Here, we show that by the end of the century certain population centers in the same region are likely to experience temperature levels that are intolerable to humans due to consequences of increasing concentrations of anthropogenic greenhouse gases (GHGs).
• The 5th Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) presents substantial evidence that increasing anthropogenic GHG concentrations are responsible for much of Earth’s warming in recent decades. While observations and model simulations largely support this global climate change hypothesis, more research efforts are needed to improve understanding of impacts at regional and local scales. Some important limitations to the accuracy of global climate model (GCM) projections of these impacts stem from the lack of sufficient resolution needed to resolve regional processes and understand societal impacts; and the inadequate treatment of physical processes of regional importance. To investigate dangers to human health of extreme heat and humidity in Southwest Asia, we apply a regional climate model (RCM) at a 25-km grid spacing specifically customized for the region forced by three IPCC Global Climate Models (GCMs) objectively selected based on performance (See Methods). By conducting high-resolution RCM simulations, we resolve approximately 30 grid-points for each GCM grid-point allowing for a more detailed representation of topography, coastlines, extreme climatic events, and physical processes.
• We consider both dry-bulb temperature (T) and wet-bulb temperature (TW), specifically their daily maxima averaged over 6 hours, denoted by Tmax and TWmax, respectively. While the general public can easily relate to the concept of T, TW is not a widely used and understood concept. It is the temperature an air parcel would attain if cooled at constant pressure by evaporating water within it until saturation. It is a combined measure of temperature and humidity, or “mugginess”.
• Like all living species, human survival is partially a function of the environmental temperature. 35°C is the threshold value of TW beyond which any exposure for more than six hours would likely be intolerable even for the fittest of humans resulting in hyperthermia. In current climate, TW rarely exceeds 31°C. While other dry bulb temperature and combined empirical temperature and humidity indices have been used to investigate the impacts of climate change on heat stress1, TW provides a physically based relationship to the human body’s core temperature.
• For extreme temperature, we somewhat arbitrarily select 60°C, a value close to the highest temperature ever reported on Earth. In dry heat conditions, the human body is at high risk of heat stroke at temperatures well below 60°C if not well hydrated and exposed to the sun. In addition, when T approaches such extremes, much machinery designed for the current climate may malfunction. For example, aircraft may not operate properly during takeoff and landing and rail lines can buckle at extreme temperatures, even at temperatures around 40°C.
• Under recent climate conditions (1976-2005) with historical GHG concentrations, the ensemble average of the largest TWmax event exceeds 31°C primarily in the Persian Gulf and surrounding coastal regions. These regions are located in low elevation areas close to sea level allowing for high T, and near the coast allowing for high humidity. Interior desert regions have lower values of TW and TWmax due to drier air. While the 35°C threshold is approached in many locations, it is not exceeded anywhere in the domain. In contrast, the ensemble average of the largest Tmax events displays values exceeding 50°C in some interior desert regions and in coastal areas, but relatively low values over the Persian Gulf and Red Sea. These severe heat-related conditions located in relatively low areas located next water bodies are consistent with projected heat-wave conditions in southern Europe and Mediterranean coasts.
• The high values of TWmax over the Red sea and Persian Gulf are due to a combination of physical processes. First, the entire region experiences virtually clear sky conditions due to subsidence during summer associated with the rising air motion over the monsoon region to the east. The reason higher surface TWmax in this region fails to trigger deep convection is explained by persistent regional scale subsidence, involving adiabatic and diabatic descent, which suppresses deep convection. Subsidence over this region results in absence of clouds and high incoming solar radiation. Second, unlike the surrounding deserts, the surface albedo of the Red sea and Persian Gulf is relatively low, yielding strong absorption of solar radiation and increased total heat flux. Third, the high evaporation rate increases water vapor and heat retained at the surface. The boundary layer is relatively shallow over these water bodies concentrating water vapor and heat close to the surface. All these factors taken together maximize the total flux of heat into a relatively shallow boundary layer and hence maximize the near-surface TW over these water bodies. Coastal locations surrounding these water bodies are thus susceptible to high TW via air transport (e.g., sea breeze circulations).
• To predict impacts of future climate change towards the end of century (2071-2100), two GHG concentration scenarios are assumed based on the IPCC Representative Concentration Pathway (RCP) trajectories: RCP4.5 and RCP8.5. RCP8.5 represents a business as usual scenario while RCP 4.5 considers mitigation. Under RCP8.5, the area characterized by TWmax exceeding 31°C expands to include most of the Southwest Asian coastal regions adjacent to the Persian Gulf, Red Sea and Arabian Sea. Furthermore, several regions over the Persian Gulf and surrounding coasts exceed the 35°C threshold.
• Annual TWmax increases monotonically in the different locations surrounding the Gulf. By the end of the century, annual TWmax in Abu Dhabi, Dubai, Doha, Dhahran and Bandar Abbas exceeds 35°C several times in the 30 years, and the present-day 95th percentile summer (July, August, and September; JAS) event becomes approximately a normal summer day. During the summer warm northwesterly (Shamal) winds frequently blow from Turkey and Iraq across the Gulf where they gain moisture and transport high TW to most of the cities in the gulf. The primary exceptions are Kuwait City and Bandar-e Mahshahr, which are protected from such extreme TW conditions due to their geographic position to the north of the Gulf.
• Extreme Tmax events exceeding 45°C become the norm in most low-lying cities during JAS. While being protected to extreme TWmax events, annual Tmax is projected to exceed 60°C in Kuwait City during some years. Annual Tmax values exceeding 60°C are also projected in Al Ain, which is somewhat isolated from the Gulf coast but still low in elevation. Doha is uniquely geographically positioned to receive hot dry air from the desert interior to the west and hot moist air from the Persian Gulf. As a result, it is vulnerable both T and TW extremes.
• On the coast of the Red Sea, milder conditions, but still quite severe, are projected compared to the Persian Gulf. In Jeddah and nearby Mecca, for example, annual TWmax is projected to reach values as high as 33°C and 32°C, respectively, with annual Tmax approaching and exceeding 55°C. These extreme conditions are of severe consequence to the Muslim rituals of Hajj when Muslim pilgrims (~2 million) pray outdoors from dawn to dusk near Mecca. The exact date for this ritual is fixed according to the lunar calendar and can therefore occur during the boreal summer for several consecutive years. This necessary outdoor Muslim ritual is likely to become hazardous to human health, especially for the many elderly pilgrims, when the Hajj occurs during the boreal summer.
• As the population in Southwest Asia continues to rapidly increase, cities will likely expand and new cities may emerge. The rise in annual Tmax as a result of climate change would make currently harsh desert environments even harsher, while the rise in annual TWmax would likely constrain development along the coasts. The countries in Southwest Asia stand to gain considerable benefits by supporting the global mitigation efforts implied in the RCP4.5 scenario. Such efforts applied at the global scale would significantly reduce the severity of the projected impacts as annual TWmax does not breach the 35°C threshold in any of the locations considered. Tmax would not likely exceed 55°C except at a couple of locations where the current temperature is already severe. Near Jeddah and Mecca, where the rituals of Hajj take place, TWmax under this scenario would be only about 2°C warmer than the current climate.
• While much of the oil produced in this region eventually ends up in the atmosphere and contributes to global climate change, the same oil brings significant financial benefits to the region. These same benefits enhance the capacity of the region to adapt to climate change. Electricity demands for air conditioner use, for example, would considerably increase in the future in order to adapt to projected changes in climate and population. While it may be feasible to adapt indoor activities in the rich oil countries of the region, even the most basic outdoor activities are likely to be severely impacted. In contrast, the relatively poor countries of Southwest Asia with limited financial resources and declining or non-existent oil production will likely suffer both indoors and outdoors. For example, TWmax in the coastal region of Yemen in the area around Al-Hudaydah and Aden is projected to reach about 33°C in extreme years. Under such conditions climate change would possibly lead to premature death of the weaker, namely children and elderly. A plausible analogy of future climate for many locations in Southwest Asia is the current climate of the desert of Northern Afar on the African side of the Red Sea, a region with no permanent human settlements due to its extreme climate.

This interactive web application has been developed to provide public access to predicted future temperature data. The data and variations can be visualized in both spatial and temporal dimensions.

Eltahir Research Group

The current research focuses on two general areas: (1) Regional climate studies of Africa, Southwest Asia and Southeast Asia; and (2) Development of a new class of environmental, hydrological and entomological models of infectious disease transmission (including malaria and dengue fever), which can be used for planning of environmental management interventions and prediction of the impact of climate change on disease transmission.

Research Team

• Prof. Elfatih Eltahir  

  Professor and Associate Department Head of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA

• Prof. Jeremy Pal

  Associate Professor, Loyola Marymount University, Los Angeles, CA

• Xin Qiu

  Graduate Student, Massachusetts Institute of Technology, Cambridge, MA