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Long-term trends in daily temperature extremes in Iraq [1]

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Date: 2017-12-01

Nowadays climate change has become one of the most severe environmental problems in the world (Wang et al., 2016). Increased concentration of atmospheric greenhouse gases (GHG) due to massive industrialization and widespread use of fossil fuels in the last century has changed the global energy balance (Wentz et al., 2007, Chu et al., 2010, Huang et al., 2011). The alternation of Earth's energy balance due to the changes in Earth's atmospheric composition has caused a change in global climate. It has been reported that the Earth's temperature has increased by 0.74 °C in the last hundred years (1906–2005). The increase became much significant after 1970, with a rate of 0.15 °C/decade (IPCC, 2013).

Temperature at the regional scale is also increasing in the line of global warming (Shahid, 2010, Mayowa et al., 2015, Ghasemi, 2015, Abatan et al., 2015). Changes in temperature get involved in temperature variability as well temperature extremes (Song et al., 2014, Matiu et al., 2016). The implications of these changes are particularly significant for areas already under stress, such as in arid region where temperature extremes is a common phenomenon. Arid regions are often predicted one of the most vulnerable regions to environmental changes (Salguero-Gómez et al., 2012, Dutta and Chaudhuri, 2015). According to IPCC (2013), the increased temperature in arid region of Asia could cause an increase in potential evapotranspiration by few folds, which may lead to severe water-stress conditions, deterioration of food security and more people at the risk of hunger.

Iraq predominantly has an arid climate and thus has frequently experienced temperature extremes. A strong ridge of high pressure usually persists over the Middle East during the month of July, resulting in the extreme heat wave (Nasrallah et al., 2004). As the land heats up around the Persian Gulf, the air rises up quickly and rushes inland, creating an onshore wind that transports humid air from sea contributing to the high humidity, which causes some of the highest combinations of heat and humidity called wet bulb temperature (WBT) in the world. The heating of the sea due to global warming may produce high humidity and increase WBT in Middle East. Therefore, the region is generally considered vulnerable to climate change. Iraq has been ranked as one of the most vulnerable counties in Arab region to climate change (UNDP, 2010). The country has been experiencing more summer heat waves in recent years. On July 22, 2016, temperature in the southern city of Basrah reached to 54 °C which is one of the hottest ever recorded in the Eastern Hemisphere. Temperature extremes are closely related to public health (Xiang et al., 2014), agriculture (Niero et al., 2015, Prasad et al., 2008), environment (El-Sharkawy, 2014, Cardil et al., 2014), energy demands (Shahid, 2012, Liu and Sweeney, 2012), economy (Barros et al., 2015), refugees (WMO, 2016) and social unrest (Reich et al., 2014, Tencer et al., 2014). Therefore, it is essential to study the trends in extreme temperatures in the context of global climatic change.

Despite this enormous importance, studies related to temperature and temperature extremes have been very limited for Iraq. Only two studies have been significantly conducted so far to assess the trends in temperature in Iraq. Robaa and Al-Barazanji (2015) assessed the trends in surface air temperatures at eleven stations for the period 1972–2011 using Mann-Kendall test, Sen's slope estimator and linear regression. They reported the increasing trend in annual minimum and maximum temperatures at most stations of Iraq. Muslih and Błażejczyk (2016) assessed the inter-annual variations and the long-term trends of monthly temperatures at seven observation stations using linear regression and Mann-Kendall (MK) test. They also reported the increasing trends with the strongest warming trends in the summer months. Monthly mean temperature data were used in both studies to assess only the seasonal and annual trends in temperature of Iraq for different time periods. No study has been conducted so far to assess the changes in temperature related extremes in Iraq. Furthermore, the trends in DTR which is considered as the fingerprint of climate change (Braganza et al., 2003) has not been conducted in Iraq so far. However, studies in neighboring countries indicate increase in temperature related extremes in the region (Tabari and Talaee, 2011, AlSarmi and Washington, 2014, Zhang et al., 2005, Al Senafi and Anis, 2015, Hereher, 2016, Sarli et al., 2016). A summary of those studies is given in Table 1. This emphasizes the need for assessment of temperature extremes in Iraq.

Non-parametric Mann-Kendall (MK) test are most widely used for hydrological trend analysis as they are robust against outliers, distribution free and low sensitivity to abrupt breaks in time series (Yue et al., 2002, Yue and Wang, 2004, Shahid, 2010, Zamani et al., 2016). However, it has been found that the positive serial autocorrelation in the data increases the chance for significance in trend in MK test. Yue and Wang, 2004used “pre-whitening” of the data, where serial correlation was first removed and then the trend test was performed on the uncorrelated residuals. On the other hand, Hamed and Rao (1998) and Yue and Wang (2004) introduced modified Mann–Kendall trend test to account for the effect of serial correlation.

The significance of the hydroclimatic trends over time is very sensitive to the assumptions of whether the underlying data have short-term or long-term persistence. Generally, if there is a significant correlation at long lags then a process is considered to exhibit long-term persistence (LTP) (Fathian et al., 2016). This LTP can be estimated by a coefficient “H” known as the Hurst coefficient and the strength of the LTP can be exhibited in the time series when 0.5 < H < 1 while a random process has H = 0.5 (Karagiannis et al., 2004). The existence of LTP in time series affect the significance of the MK test (Koutsoyiannis, 2003). Hamed, 2008, Hamed, 2009 modified the MK method to account for the scaling effect, thus enhancing the ability of the test to differentiate the multi-decadal oscillatory variations from long-term persistence or secular trends. Thus, the modified Mann-Kendall (m-MK) test is also used in the present study to confirm the significance of MK test for temperature extremes previously observed in the Middle East.

Thus, the objectives of this study are to assess the trends in (i) annual and seasonal daily average minimum and maximum temperatures; (ii) annual and seasonal DTR; and (iii) a matrix of temperature related extremes in Iraq for the period 1965–2015 using both MK and m-MK tests. Total 14 temperature extremes are considered in the present study which includes number of summer hot days and nights, winter warm days and cold nights, and hot and cold spells in a year. It is expected that the assessment of trends in temperature and temperature related extremes will be helpful for adaptation in public health, agriculture and water resources management as well as people's well-being and development of Iraq.

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[1] Url: https://www.sciencedirect.com/science/article/abs/pii/S0169809517303678?via%3Dihub

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