The greenhouse effect

A variety of gases collaborate to form a canopy over the Earth which causes some solar radiation to be reflected back from the atmosphere, thus warming the Earth's surface, hence the greenhouse analogy. The greenhouse effect is caused by long-wave radiation being reflected by the Earth back into the atmosphere and then reflected back by trace gases in the cooler upper atmosphere, thus causing additional warming of the Earth's surface (Figure 1.1).

The main greenhouse gases are water vapour, carbon dioxide, methane, nitrous oxide and tropospheric ozone (the troposphere is the lowest 10-15 kilometres of the atmosphere).

The sun provides the energy which drives weather and climate. Of the solar radiation which reaches the Earth, one third is reflected back into space and the remainder is absorbed by the land, biota, oceans, ice caps and the atmosphere. Under natural conditions the solar energy absorbed by these features is balanced by outgoing radiation from the Earth and atmosphere. This terrestrial radiation in the form of longwave, infra-red energy is determined by the temperature of the Earth-atmosphere system. The balance between radiation and absorption can change due to natural causes such as the 11-year solar cycle. Without the greenhouse shield the Earth would be 33°C cooler, with obvious consequences for life on the planet.

Figure 1.1

The greenhouse 'blanket'

Since the industrial revolution, the combustion of fossil fuels and deforestation has resulted in an increase of 26 per cent in carbon dioxide concentrations in the atmosphere. In addition, rising population in the less developed countries has led to a doubling of methane emissions from rice fields, cattle and the burning of biomass. Methane is a much more powerful greenhouse gas than carbon dioxide. Nitrous oxide emissions have increased by 8 per cent since pre-industrial times (IPCC 1992).

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