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Future Temperature in Southwest Asia Projected to Exceed a Threshold for Human Adaptability
Supplementary Information
Jeremy S Pal1, 2 and Elfatih A B Eltahir2
1. Department of Civil Engineering and Environmental Science, Loyola Marymount
University, Los Angeles, CA 90045
2. Ralph M. Parsons Laboratory, Massachusetts Institute of Technology, Cambridge, MA
02139.
Methods In this study, the Massachusetts Institute of Technology Regional Climate Model
(MRCM) is used1, which is based on the ICTP Regional Climate Model version 3
(RegCM3)2 with several significant enhancements3-‐6. 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 (Figure SI1). 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
integrations7. 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
target9. RCP 4.5, which represents 4.5 W/m2 of radiative forcing, is considered a
mitigation scenario10.
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 region11-‐15. As much of the land surface in Southwest Asia is
characterized as desert and semi-‐desert, it is essential that the soil albedo and
Future temperature in southwest Asia projected to exceed a threshold for human adaptability
Future Temperature in Southwest Asia Projected to Exceed a Threshold for Human Adaptability
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 (Figure SI2)16. 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 data17. 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 Reanalysis18 (ERA-‐Interim) data, except in
areas of complex topography (Figure SI3). The model includes a representation for the
emission, transport and deposition of mineral aerosols and their direct radiative
effects11,14. 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 (Table SI1 and Figure SI4). 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-‐Interim18 and Climate Research Unit19 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)20, Max-‐Planck-‐Institute Earth System Model
(MPI-‐ESM)21 and Norwegian community Earth System Model (NorESM)22.
Future Temperature in Southwest Asia Projected to Exceed a Threshold for Human Adaptability
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