'The atmosphere is heated from below’. Explain what is meant by this statement and how changes in the gaseous composition of the atmosphere may result in climate change.'
⇒ The Earth is heated primarily by radiation from the Sun, ranging in frequency from ultra-violet at (<)250 nm to infrared at (>)2500 nm, but concentrated in the visible light spectrum at approx. 400-700 nm (Danny Harvey, 2000). Of the incoming solar energy (approx. 340 w/m2 ) around 30% is reflected back into space by clouds, aerosols, the atmosphere and the Earth’s surface. The remaining energy is absorbed, mostly by the surface but to a small extent by the atmosphere. It is then reradiated as long-wave energy in the infrared (IR) frequency; this reradiation is primarily responsible for the heating of the atmosphere. As a result, writes Cockell (1995: 18), “the atmosphere is heated primarily from below and the overall energy budget of the Earth system is balanced by the longwave radiation re-emitted into space”. The heating of the atmosphere is augmented by the presence of so-called greenhouse gases (GHGs). These are atmospheric gases which are not transparent to the outgoing longwave radiation and therefore trap and reradiate it – principally water vapour, carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) and ozone (O3). (It is notable that this is quite a different process to that in an actual greenhouse, in which the glass prevents hot air from rising and escaping as opposed to absorbing and reradiating heat). Because of the energy conserved as a result of GHGs the average surface temperature of the Earth is 33° c hotter than it would otherwise be (15° c instead of the Earth’s effective temperature of -18° c) (Danny Harvey, 2000). According to the IPCC (1995: 75), “a change in the concentration of an atmospheric constituent can cause a radiative forcing by perturbing the balance between the net incoming radiation and the outgoing terrestrial radiation”. Radiative forcings can occur in two ways: a change in GHG concentration altering the magnitude of the greenhouse effect, or a change in atmospheric aerosol levels altering the amount of solar energy initially reflected. These changes can occur as a result of both natural and human causes. At present the large increases in GHG concentrations due to human activity are responsible for an additional radiative forcing of around 2.4 w/m2 compared with before the Industrial Revolution, resulting in the overall warming of the climate (IPCC, 1995). Radiative forcings have a direct impact on climate by altering the energy budget of the atmosphere; however they can also affect the climate through indirect or feedback effects. Feedbacks may be in the form of chemical processes which further affect gas concentrations, or effects resulting from the warming of the climate itself. They can either amplify the initial trend (positive feedback) or counteract it (negative