Climate change issues are discussed widely around the world. Many scientists (Treut et al., 2007) relate global warming and its consequences to human activities and not to natural fluctuations. The reasoning of this approach is the timescale of climate change. Recent warming of the Earth is considered to be abrupt compared to the timescale usually accompanied with natural climate change episodes. Earth's natural climate changes happen gradually in a long period of time (tens of thousands to millions of years), but we are witnessing an abrupt change over the past 200 years. The industrial revolution with fossil fuels as its main source of energy is setting a steady emission increase of carbon dioxide and other greenhouse gases which trap heat causing an increase of temperature in the lower atmosphere. Climate change is recognized as an important issue, and international communities through the United Nations created special groups to focus on climate change effects and initiated protocols to organize a global response to deal with its consequences. Unusually strong tropical storms, heavy precipitations causing devastating floods, more frequent heat waves, frequent droughts, and other similar events are connected to a modern climate change. The government of Saudi Arabia has recognized this by signing the Kyoto Protocol. This calls, among others, for implementation of commitment to stabilize greenhouse emissions and furnish a report about the current status of climate change to the UN Framework Convention on Climate Change (UNFCCC) on the status of greenhouse gases and climate change impacts and mitigation.
Climate is a complex system as it involves air, water, ice, land, and various interactions like water cycle and greenhouse effects. Water is the bond that brings climate and hydrology together. Any changes in solar inputs, the atmosphere, or the hydrological cycle will affect the interactions among the atmosphere, the hydrosphere, the cryosphere, and the biosphere. The impacts of climate change will be devastating to rivers, lakes, sea level, vegetation, ecosystems, and many others.
Latest studies point to a steady increase in temperature during the 20th century and unprecedented increases in the last 50 years (Nakicenovic et al., 2000).
The mean Earth temperature has been on the rise for the past 150 years in an abrupt manner and expected to increase further by the end of the 21st century with the doubling of the current atmospheric carbon dioxide. Climate change interacts with various natural processes in the atmosphere, the hydrosphere, the cryosphere, and the biosphere. There is an increasing evidence that anthropogenic (caused by human's activities) gases are to be blamed in causing the climate change (Treut et al., 2007).
Studying the future state of water resources in a changing climate requires a common ground of approximation. The hydrologist would like to get specific information about future precipitation, temperature, evaporation, runoff, and others on a specific location like an aquifer or a watershed which is mostly on a scale of less than 105 km2 (Loaiciga, 1997). On the other hand, climatologists run global climate models (GCMs) with a resolution around 300 km to predict the future. From here the need to run a regional climate model (RCM) arises, to include topographical and geographical details to produce high-resolution results.
Climate change is a global issue, but knowing its effect on water resources in a small area requires a great deal of information and analysis. The climate of Saudi Arabia is arid with insignificant contribution to water recharging. Overexploitation of fossil groundwater is apparent everywhere in the Kingdom. No matter what the future brings, being aware of the state of climate change and its impacts on water resources can be of a great benefit to the nation and consist of essential information to aid in protecting and managing the water resources.
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