Climate Change and Distribution of Date Palms Figure 5. The climate (EI) for P. dactylifera in current time and projected using CLIMEX under the CSIRO-Mk3.0 GCM running the SRES A2 scenario and for 2030, 2050, 2070 and 2100 for the north and south of Africa and the Middle East. doi:10.1371/journal.pone.0048021.g005 compared to the other scenarios, such as A1F1, A1B, B2, A1T, B1 by 2100 [47]. No scenarios from the B family of SRES scenarios were included in this paper, mainly because of the observation that some parameters such as global temperature and sea level rise are presently increasing at a much greater rate than predicted by the hottest SRES scenarios [48]. Fitting CLIMEX Parameters Using both native habitat range and agricultural distribution data in parameter fitting is highly recommended because it produces a model that approximates the potential distribution of the taxa being modeled [49]. This is because the limitations imposed by biotic influences in the species’ native range may be absent in non-native locations, thus allowing it to expand its range beyond its realized Hutchinsonian niche [49,50]. In this study, parameters were fitted using the native range and the global agricultural distribution of date palms. However, the distribution data of P. dactylifera from North America, Mexico, and the Caribbean were not used in parameter fitting as this was set aside for model validation. The parameters were iteratively adjusted depending on satisfactory agreement between the potential and known worldwide distribution of P. dactylifera. The parameters were subsequently verified to ensure that they were biologically reasonable. Model validation was conducted using North Amer- ican, Mexican, and Caribbean distribution data. It should be highlighted that the wet stress threshold parameter does not have a unit, while the stress accumulation rate uses the week21 unit. The heat and cold stress thresholds use degrees Celsius (uC) unit. Cold Stress The cold stress temperature threshold (TTCS) mechanism was used to describe the species’ response to frost. Generally, the minimum winter temperature that can be tolerated by P. dactylifera is 10uC [11]. However, date palms have been recorded in locations as low as 4uC [28]. Therefore, intolerance to frost was incorporated by accumulating stress when the average monthly minimum temperature fell below 4uC, with the frost stress accumulation rate (THCS) set at –0.01 week21. This cold-stress mechanism allowed the species to survive in Spain (39u 6359 N and 2u 5239 W) [28]. Additionally, this value provided an appropriate fit to the observed distribution in South America, South Africa and Asia. Heat Stress The heat stress parameter (TTHS) was set at 46uC because it was reported that P. dactylifera is able to persist up to this temperature in eastern Pakistan [28]. The heat stress accumula- tion rate (THHS) was set at 0.9 week21, which allowed P. dactylifera to persist along eastern Pakistan [37,38,42] and southern Iran [8,28,31]. PLOS ONE | www.plosone.org 5 October 2012 | Volume 7 | Issue 10 | e48021