ABSTRACT
In recent years, concern has risen about the effect of human activity on global climate, via an enhanced 'greenhouse effect'. The so-called 'greenhouse effect' is a natural phenomenon which acts to keep the Earth around 30°C warmer than its position in the Solar System suggests it should be. The Earth receives radiation from the Sun, and re-radiates this back to outer space. The Sun radiates energy at shorter wavelengths than the Earth does, due to its higher surface temperature (Figure 12.1). This difference in wavelength has important implications for the radiation budget of the Earth —the atmosphere is largely transparent to the incoming, short-wave radiation but absorbs some of the outgoing, longer-wavelength radiation. This absorption is caused by a range of greenhouse gases (especially CO2, CH4, N2O and water vapour) which show strong absorption at longer (infrared) wavelengths. This absorption of radiation causes the Earth to warm, maintaining the Earth's surface temperature at levels equable for life. The atmospheric concentrations of the major greenhouse gases (CO2, CH4 and N2O) have dramatically increased over the last 200 years or so, and this increase is generally thought to be a result of increased emissions due to human activity (particularly industrial and agricultural development). Large increases in the atmospheric concentrations of greenhouse gases are likely to result in a net warming of the Earth's surface (see Section 12.2), i.e. global warming.
