Due to global warming, global temperatures are expected to rise faster over the next century than any 100 years period during the past 10000 years [1-2]. Recent reports from the Intergovernmental Panel on Climate Change [3-4] confirm that climate

Geothermal Energy Research and Application Center, Izmir Institute of Technology, Urla 35430, Izmir, Turkey e-mail: [email protected]

change is occurring at a larger and more rapid rate of change than was thought likely only 6 years ago. Eleven of the last 12 years (1995-2006) rank among the 12 warmest years in the instrumental record of global surface temperature (since 1850). The 100-year linear trend (1906-2005) of 0.74°C [0.56-0.92°C] is larger than the corresponding trend of 0.6°C [0.4-0.8°C] (1901-2000) [1, 2, 5]. Climate warming observed over the past several decades is consistently associated with changes in a number of components of the hydrological cycle and hydrological systems such as: changing precipitation patterns, intensity and extremes; widespread melting of snow and ice; increasing atmospheric water vapour; increasing evaporation; and changes in soil moisture and runoff. There is significant natural variability in all components of the hydrological cycle, often masking long-term trends. There is still substantial uncertainty in trends of hydrological variables because of large regional differences, and because of limitations in the spatial and temporal coverage of monitoring networks [6]. At present, documenting inter-annual variations and trends in precipitation over the oceans remains a challenge [3].

Greenhouse gases are considered to be the foremost parameter influencing the climate change. Carbon dioxide (CO2) is the most important anthropogenic greenhouse gas with annual emissions growing by about 80% between 1970 and 2004 [4]. The long-term trend of declining CO2 emissions per unit of energy supplied reversed after 2000. Global atmospheric concentrations of CO2, methane (CH4) and nitrous oxide (N2O) have increased markedly as a result of human activities since 1750 and now far exceed pre-industrial values determined from ice cores spanning many thousands of years. During the past 50 years, the sum of solar and volcanic forcing would likely have produced cooling. Observed patterns of warming and their changes are simulated only by models that include anthropogenic forcing [5]. Global carbon dioxide (CO2) emissions from residential, commercial, and institutional buildings are projected to grow from 1.9 Gt C/year in 1990 to 1.9-2.9 Gt C/year in 2010, 1.9-3.3 Gt C/year in 2020, and 1.9-5.3 Gt C/year in 2050. It must also be noted that 75% of the 1990 emissions are attributed to energy production [3].

Controlling greenhouse gases emission and adapting human settlements to withstand the extreme climatic conditions have become the most formidable challenges of our times. Geothermal energy development has thus great CO2 emission reduction potential when substituting fossil sources of energy. Geothermal energy is one of the contributors to any future energy mix. The advantages of geothermal energy are numerous. It is an environmentally friendly and economically rewarding resource, which is still only marginally developed. Its two main utilization categories power generation and direct use are already introduced in many countries around the globe [7]. Geothermal development estimates for 2050 indicate that CO2 emissions could be mitigated by 100s of Mt/year with power generation from geothermal resources and more than 300 Mt/year with direct use, most of which could be achieved by geothermal heat pumps [8]. Based on these fundamentals, the purpose of this study is to explain application of geothermal energy and its effect on climate change and to assess environmental impacts of the utilization of geothermal resources.

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