The Climate System and Relevant Processes

The usual definition of climate is that it encompasses the slowly varying aspects of the atmosphere-hydrosphere-land surface system. In some sense, climate is the average condition of the weather over several years to tens of years (averaging times need to be carefully chosen), as exemplified by the parameters viz., temperature, wind velocity, relative humidity, cloudiness and the amount of precipitation. Modern climate definitions include higher order statistics beyond mean values, such as the magnitudes of day-to-day or year-to-year variations, standard deviations or measures of shapes of parameter distributions.

Climate depends not only on atmospheric processes and composition, but also physical, chemical, and biological processes involving other components of the Earth system play a crucial role. In order to understand what the factors are which control the evolution of climate, the interactions among the different components of the Earth system need to be assessed (e.g. Brasseur et al. 1999). This adds to the complexity of the topic since the evolution and feedbacks involved run on a variety of different time scales. The atmosphere, the hydrosphere, the biosphere, the cryosphere and the lithosphere are the five different Earth system regimes with widely varying impacts and time scales which make up the climate system.

The abundance of water in its three states of aggregation and a functioning global water cycle is of utmost importance for the climate system (e.g. Pagano and Sorooshian 2006; Quante and Matthias 2006). The phase of water depends on the temperature and pressure it is exposed to. At the normal range of atmospheric pressures and temperatures on Earth water can exist in all three of its basic states, as is evident from its phase diagram in Fig. 1, which shows the phase transition curves as a function of temperature and partial pressure (see Webster 1994). The Earth's trajectory in Fig. 1, driven by an increasing water vapour greenhouse effect, intercepts the phase curves in the vicinity of the triple point of water (273.16 K), allowing the formation of a complex hydrological cycle. In contrast, because Venus is a star with a considerably warmer primitive surface temperature, the curve for Venus does not intercept any of the water phase transition lines at all. Water stays in the gaseous phase on this planet. The state curve for Mars starts at a relatively low temperature (~240 K; not shown here) and rapidly intercepts the vapour-ice phase transition.

All subcomponents of the climate system are involved in or maintain processes which can have a huge impact on climate. The interplay between radiation and convection in the atmosphere regulates the temperature at the Earth's surface. The oceans, which cover about 72% of the surface area of our planet, influence climate by their large thermal inertia and their important role in taking up carbon dioxide from the atmosphere. If present, the cryosphere with extensive snow and

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Fig. 1 Phase diagram of water illustrating the possible occurrence of the three states of water for the range of temperatures observed at the Earth's surface and in the lower atmosphere; ~190 K-325 K and 0-50 hPa partial pressure range at the surface. Figure adapted from Quante and Matthias (2006)

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Fig. 1 Phase diagram of water illustrating the possible occurrence of the three states of water for the range of temperatures observed at the Earth's surface and in the lower atmosphere; ~190 K-325 K and 0-50 hPa partial pressure range at the surface. Figure adapted from Quante and Matthias (2006)

ice covered areas has a strong influence on the planetary albedo (Parkinson 2006). Besides their large influence on the Earth's albedo with a corresponding net cooling effect, the clouds are also contributing to a warming of the surface by absorbing infrared radiation and emitting it partly back towards the ground (e.g. Quante 2004). Clouds cover at any given time between 60 and 70% of the globe. Living organisms on land and in the oceans are involved in liberating oxygen and sequestering carbon in the Earth's crust and thereby reducing the CO2 concentration of the atmosphere. The evolution of vegetation is strongly coupled with that of soil and climate, and there is a myriad of interactions involved (e.g. Berry et al. 2005; Barth et al. 2005). Plate tectonics exerts an influence on climate on time scales of more than millions of years through continental drift, creation of mountains (Turcotte and Schubert 2002) and volcanism (Robock 2000). Of all the biogeochemical cycles, the hydrological (e.g. Quante and Matthias 2006; Oki and Kanae 2006) and the carbon cycle (e.g. Houghton 2007; Doney and Schimel 2007) are the most relevant for climate and its evolution.

The major external forcing of the climate system comes from the sun. Everything on Earth relies on a steady energy flow provided by our central star. The amount of radiation produced by the sun is not constant, especially in the short, ultraviolet wavelengths. Due to changes in the magnetic structure of the gaseous sun, the solar activity shows variations, which are manifested in an 11-year cycle. Although attempts have been made, a firm theoretical coupling of this short-term solar activity fluctuations with climate changes could not be found. A slightly enhanced energy deposition in the stratosphere is among the most recognized effects. Some evidence for the influence of solar activity variations on the lower atmosphere and climate is critically assessed by Bard and Frank (2006), Foukal et al. (2006) and Haigh (2007). These variations play some role in the discussion on modern global warming, since if climate changes due to the sun were significantly large, it would be more difficult to extract the anthropogenic signal from the climate record (see Sect. 4.4). The story is different, however, when dealing with the long-term evolution of the sun; in its infancy, the sun's intensity was about 30% less than what is observed today (Gough 1981), the relatively moderate climate under these conditions is generally referred to as "faint young sun paradox" (e.g. Sagan and Chyba 1997). A further possible external forcing of Earth's climate might come via galactic cosmic rays and their influence on clouds (Marsh and Svensmark 2000; Carslaw et al. 2002; Kristjansson et al. 2004; Kirkby 2008). The related science and the potential magnitude of postulated effects is currently being debated and planned experiments at the Conseil Européen pour la Recherche Nucléaire (CERN) should, at least, provide some insight into underlying cloud microphysical processes.

In summary, Fig. 2 sketches the Earth system and its interactions, encompassing the physical climate system, biogeochemical cycles, external forcing, and the effects of human activities. For a more rigorous treatment of the different aspects concerning the climate system and underlying processes, the books by Peixoto and Oort 1992; Graedel and Crutzen 1993; Hartmann 1994; Brasseur et al. 1999; Seinfeld and Pandis 2006; Ruddiman 2008; and Pierrehumbert 2009 are recommended.

Fig. 2 Schematic diagram of the climate system and its interactions among different components of the Earth system (from Earth System Science: Overview, NASA)

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Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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