INFORMACIÓN CUANTITATIVA

The Intergovernmental Panel on Climate Change (IPCC) has been analyzing the historical temperature data available from the late of 19th _{century [32]. The IPCC reported that the global}

mean temperature has been rising and the last three decades have been warmer than any previous decades during the historical record. In addition, the recent paleoclimatology data showed that the air temperatures and rate of warming during the course of the twentieth century have been higher than any other time within the previous 1000 years [33,34]. The analyses of trends in the global temperature shows that the global temperature has increased by 0.13˚ C per decade in the last fifty years, which is double of the temperature trend during the last 100 years [35]. In another study, Nicholls et al. reported that the mean surface temperature of the globe increased by around 0.3˚ C and 0.6˚ C in the late 19th _{century and between 0.2 and 0.3 per decade over the last four decades}

[36]. Other than mean temperature, there are some studies focusing on the changes in maximum and minimum temperatures. Nicholls et al. found that the minimum temperature increases with higher magnitude than maximum temperature for several regions of world during the 19th _century

[36]. In its third assessment report, the IPCC mentioned that both minimum and maximum temperature have increased between 1950 to 2004 globally over the Planet Earth’s surface [37]. The rate of increase of the minimum temperature is greater (0.204˚C/decade) than that of the maximum (0.141˚C/decade). These rates of warming are progressively increasing in time and are more intensified over the land surface. For example, Vose et al. performed a trend analysis on the maximum and minimum temperature data obtained from a large number of climate stations for the period of 1979 to 2004. The estimated magnitude of trends showed that the rate of warming in minimum temperature is more than maximum temperature [38].

The changes in temperature can make a wide range of domino effects in other climate variables. Most importantly, the spatial and temporal characteristics of precipitation, rainfall, runoff, and evaporation can be impacted by increasing global temperature [14]. In brief, precipitation is expected to increase globally due to the expected increase in temperature, yet the magnitude of increase can be subject to geographic differences [14]. The physics behind the phenomena is quite straightforward: The amount of rainfall is strongly dependent on the level of atmospheric moisture. Therefore, given increases in temperature, the atmospheric moisture will also increase due to enhanced evaporation [39]. Such an intensification of water cycle can lead to increase precipitation [40]. Empirical data confirm this. Jones et al. mentioned that the world total levels of precipitation have increased by approximately 2% since the start of the twentieth century [41,42]. Kunkel et al. argued that increases in temperature will not only cause increases in total precipitation, but also render extreme events of precipitation more frequent [43]. Solomon argued that the precipitation trend has increased in some parts of the globe like northern Europe, Asia, and the eastern region of America. On the other hand, the southern regions of Africa and Asia, as well as the Mediterranean, have experienced a decrease in precipitation over the past fifty years [35]. After synthesizing a large literature review focused on precipitation patterns changes around the world, Dore et al concluded that precipitation in the middle and high latitude of the northern hemisphere – except for the eastern region of Asia – has kept on increasing between 0.5 - 1% per decade. The subtropical region has experienced a decrease in precipitation about 0.3% per decade. Average precipitation increased between 0.2 - 0.3% per decade in tropical regions over the past century[44].

The warming of air temperature and its consequent effects on other climate variables are expected to be more intensified under future conditions. The IPCC provided a complete assessment of

climate change projections with large numbers of simulations under different scenarios. In general, among climate models, the changes in direction and magnitude of temperature are more consistent than those of precipitation [45]. These analyses showed that the global average surface temperature increases in all simulations for the period of 2090-2099 relative to 1980-1999 climate conditions. The changes are supposed to be greater in northern latitudes and parts of the North Atlantic Ocean. During the same period, increases in precipitation for the higher latitudes and decreases in the tropical regions are expected – see Figure 2.2. In the same report, the IPCC projected about a 0.2˚C increase in temperature per decade under several Greenhouse Gas (GHG) emission scenarios for the next twenty years [46]. In terms of precipitation, Emori and Brown observed that the global mean rainfall increased by 6% from 1981 to 2000, and projected that it would increase by 13% from 2081 to 2100 [47]. Kharin and Zwiers projected that the mean annual precipitation would increase by 1% from 2040-2060, and 4% from 2080-2100 [48]. Bates et al mentioned that precipitation increments of at least 20% are to be expected in the higher latitudes of both hemispheres, while mid-latitude regions will experience dryer climates [49].

Figure 2.3 Total hydropower generation trend [53]