The greenhouse effect itself occurs when short-wave solar radiation (which is not impeded by the greenhouse gases) heats the surface of the Earth, and the energy is radiated back through the Earth's atmosphere as heat, with a longer wavelength. In the wavelengths 5-30µm a lot of this thermal radiation is absorbed by water vapour and carbon dioxide, which in turn radiate it, thus heating the atmosphere. This is what keeps the Earth habitable. Without the greenhouse effect overnight temperatures would plunge and the average surface temperature would be about minus 18oC, about the same as on the moon, which lacks the shroud of our atmosphere.
The particular problem arises in the 8-18µm band where water vapour is a weak absorber of radiation and where the Earth's thermal radiation is greatest.* Increased concentrations of CO2 and other radiative gases here mean that less heat is lost to space from the Earth's lower atmosphere, and temperatures at the Earth's surface are therefore likely to increase.
A number of indicators suggest that warming due to increased levels of greenhouse gases is indeed observable since 1980, despite some masking by aerosols (see below). One problem is that while global air temperatures do appear to have risen about 0.6oC over the last century, this has been irregular rather than steady, and does not correlate well with the steady increase in greenhouse gas concentrations. While the amount is consistent with natural climate variability, some of the warmest years on record have been in the last decade. However, the climate is a complex system and other factors influence global temperatures.
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