The Earth receives the overwhelming amount of its energy from the sun in the form of visible and near-infrared radiation; it is mainly this latter energy which warms its surface. Our planet is cooled by the emission of thermal infrared radiation into space. Treating the Earth as a blackbody radiator with an effective temperature Te, the balance of incoming radiation and outgoing radiation leads to the following relationship (e.g. Wallace and Hobbs 2006): ctT4 = S /4(1 - A), where s is the
Stefan-Boltzmann constant, where S is the solar flux at the distance Sun-Earth, and A is the planetary albedo. Using the appropriate numerical constants and quantities, solving the equation yields an effective emission temperature of 255 K. Reality is, however, slightly more complex, in that the Earth together with its atmosphere is not a perfect blackbody. The atmosphere warms the surface by the so-called greenhouse effect; parts of the infrared radiation emitted from the surface is selectively absorbed and re-emitted by infrared-active gases within the atmosphere. With the present-day incoming solar flux (~1,368 Wm-2), albedo (~0.3) and atmospheric composition, a global mean surface temperature of 288 K results. The difference between the effective emission temperature and this surface temperature of 33 K is the magnitude of the actual greenhouse effect. A more complete treatment of the involved energy fluxes with emphasis on the energy budget resolved for additional processes is given in Kiehl and Trenberth (1997). The most difficult factor in the quantification of the energy fluxes through the atmosphere is the planetary albedo, which is determined by ocean and land surface characteristics (soil type, soil moisture, vegetation) and the three-dimensional cloud distribution in the atmosphere, which is responsible for the largest fraction. The relevant cloud (and aerosol) properties are difficult to predict. Thus, calculations of past or future climates based on energy principles (radiative and turbulent fluxes) are subject to high uncertainties, i.e. since clouds are involved in several feedback loops (Stephens 2005).
In the present-day atmosphere the most important greenhouse gases are water vapour and carbon dioxide, of which the former contributes about two-thirds of the associated warming. Lesser contributions come from methane, nitrous oxide, ozone and various chlorofluorocarbons. It is important to distinguish between long-lived greenhouse gases, which are removed slowly from the atmosphere on a time scale of hundreds to thousands of years, and short-lived greenhouse gases, which are removed within weeks to a year by condensation or fast chemical reactions. The short-lived greenhouse gases act primarily as a feedback mechanism. In the eighteenth century, the atmospheric concentration of most of these gases (with the exception of water vapour) began to be significantly altered due to emissions from power plants, industry, agriculture and animal farming as well as the mobility sector. The ongoing greenhouse effect discussion is driven by the anthropogenic emissions of CO2, CH4, N2O, O3 and CFC which show strong increases, especially over the last decades. It has to be noted that all of these gases have a relatively long residence time in the atmosphere and therefore, a high greenhouse warming potential.
Water vapour as the major player in the Earth's energy budget is buffered by the huge oceans on a time scale of a few weeks. This gas adjusts its atmospheric concentration in response to climate changes, and it has a strong positive feedback in the climate system, thus amplifying global warming caused by other forcings (Held and Soden 2000). Water vapour, although an important greenhouse gas, is not a prime driver of modern climate change (it plays an essential role via its positive feedback). As there are no significant anthropogenic emissions, water vapour has not become the subject of political regulatory protocols.
Of course human beings started influencing the atmospheric composition, and greenhouse gas concentrations, well before the massive industrialisation began. About 11,000 years ago stone-age farmers may have already altered Earth's climate by clearing forests and irrigating fields to grow crops. Besides resulting in changes to the albedo, these activities may have led to considerable amounts of CO2 and CH4 being emitted. It should be mentioned that some scholars have even put forward the hypothesis that this early interference with the climate system could possibly have averted the start of a new ice age (Ruddiman 2003, 2005). This controversial hypothesis is discussed, for example, by Claussen et al. (2005), since the concentrations of CO2 during the Holcene could also be attributed to natural processes (e.g. Broecker and Stocker 2006; Steffen et al. 2007).
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