cycles, which are related to the Earth's orbital patterns (Milankovitch 1941). The heat balance is the basis for the temperatures that we have in the atmosphere and at ground level.
In general, the gases present in the atmosphere convert radiation energy into thermal energy – by absorption1 of electromagnetic radiation, and vice versa – by radiation. The mechanism for absorption of radiation in a gas is that the gas molecules absorb the radiation energy by increasing its kinetic energy through molecular translation, rotation, and vibration, as well as electron translation and spin and nuclear spin. The increase in thermal energy of a gas translates into increased temperature. The longer the radiation travels through a gas, the more energy is converted. The radiation is at various wavelengths. The solar radiation is at rather low wavelengths (0.2–3 μm), either in the visible (0.4–0.8 μm) or in the near-visible (e.g. ultraviolet <0.4 μm) range. Radiation from the ground and from the atmospheric gases is at higher wavelengths (0.7–300 μm), which is known as infrared radiation.
The incoming solar radiation is about 342 W/m2 (IPCC-WG1 2007). Some of this is reflected back into space by clouds, aerosols2, and atmospheric gases, and some is reflected by the Earth's surface. About 240 W/m2 is absorbed by the Earth's surface and atmospheric gases. In order to have a heat balance with constant temperature, the radiation from the Earth must be the same. Most of the outgoing radiation energy is at wavelengths in the range of 7–15 μm. Radiation from a surface is temperature dependent (∝T4), and a radiation of 240 W/m2 would require an average surface temperature of −19 °C, which is much lower than the actual average Earth surface temperature of +14 °C. Because some of the outgoing radiation is absorbed by clouds or gases in the atmosphere, the temperature at which radiation takes place is forced from −19 °C to +14 °C to maintain the average flux of 240 W/m2. This absorption of radiation and the related increase in temperature is known as the atmospheric greenhouse effect. This is different from the effect observed in greenhouses, where the temperature increase is caused by suppression of convection.
A key issue is that the gases in the atmosphere have different properties with respect to absorption of radiation and radiation from the gases themselves. The absorption of radiation in a gas depends on the wavelength. Ozone (O3) is a gas that absorbs ultraviolet radiation very well, whereas CO2 absorbs at wavelengths around 3–5, and 12–20 μm. Water vapour absorbs at various wavelength ranges, including that of 7–15 μm. Visible light from the sun is absorbed by atmospheric gases only to a minor extent. The main bulk of infrared radiation, at wavelength 7–15 μm, is absorbed only to some extent. For some wavelength ranges, the absorption is about 100%.
The most important gases for the absorption of infrared radiation are water vapour (H2O), carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), halocarbons (gases containing fluorine, chlorine, and bromine), and ozone (O3).
Water vapour is the most important greenhouse gas in the atmosphere, accounting for about 60% of the natural greenhouse effect for clear skies. Human activities influence the atmospheric water vapour content to only a small extent; it depends much more on the temperature. The relation between temperature and water vapour content in the atmosphere is approximately a constant relative to humidity. The greenhouse effect of water vapour is much stronger in humid areas around the equator compared to that in polar areas where the air humidity is very low. Consequently, the importance of CO2 as a greenhouse gas is more evident in polar regions, and changes in the concentration of CO2 have a larger impact on the temperature in these regions.
The two most abundant gases in the atmosphere – nitrogen and oxygen – contribute almost nothing to the greenhouse effect. Homonuclear diatomic molecules such as N2, O2, and H2 neither absorb nor emit infrared radiation.
The greenhouse effect was discovered by Joseph Fourier (1768–1830) in 1824. He was followed by John Tyndall (1820–1893), who did important work on the radiative properties of gases by verifying, through experiments, the absorption of radiation in gases and that emissions vary with wavelength and type of gas. In 1896, Svante Arrhenius (1859–1927) was the first to publish work on a quantitative investigation of the greenhouse effect, which he believed could explain the ice ages. At that time, the link between man-made emission of CO2 and climate change was already established. Even though the calculations by Arrhenius were shown to be erroneous, he cleverly managed to collect information from a large number of sources and to make predictions not so very different from those recently made by the IPCC. Guy Stewart Callendar (1898–1964) made a very important contribution with a publication (Callendar 1938) presenting a comprehensive global temperature time series and a model, linking greenhouse gases and climate change (Fleming 2007). He found that a doubling of atmospheric CO2 concentration resulted in an increase in the mean global temperature of 2 °C, with considerably more warming at the poles.
The effect of greenhouse gases in the atmosphere can be quantified on two different scales. One is the atmospheric lifetime, which describes how long it takes to restore the atmospheric system to equilibrium following a small increase in the concentration of the gas in the atmosphere. Individual molecules may interchange with the soil, oceans, and biological systems, but the mean lifetime refers to the net concentration change towards equilibrium by all sources and sinks. The other scale is the global warming potential (GWP), which is defined as the ratio of the time-integrated radiative forcing from a sudden release of 1 kg of a substance g relative to that of 1 kg of a reference gas, CO2 (IPCC-WG1 2007):
(1.1)
rg is the radiative forcing per unit mass increase in atmospheric abundance of component g, and dg(t) is the time-dependent abundance of g, and the corresponding quantities for the reference gas (CO2) in the denominator. Radiative forcing is defined as the change in net irradiance at the tropopause. Net irradiance is the difference between the incoming radiation energy and the outgoing radiation energy in a given climate system and is measured in W/m2. The GWP definition is time dependent (tx), but, for any time horizon, the GWP of CO2 is unity by definition.
In Table 1.1, a list of some selected greenhouse gases and their GWP and atmospheric mean lifetime is given. Water vapour is not included in the list even though it is an important greenhouse gas because the presence of water vapour in the atmosphere is mainly determined by the temperature. The short atmospheric lifetime of tropospheric ozone (hours to days) precludes a globally homogeneous distribution and is consequently not included in Table 1.1. Ozone concentrations, and associated radiative effects, are highest near their sources. CO2 has an atmospheric lifetime that is difficult to specify precisely because CO2 is exchanged with reservoirs having a wide range of turnover times: 5–200 years or even much longer than that.
1.2 Atmospheric CO2
The concentrations of carbon dioxide, methane, and other greenhouse gases are currently increasing over time. The carbon dioxide concentration, measured as the mole fraction in dry air, on Mauna Loa, Hawaii, constitutes the longest record of direct measurements of CO2 in the atmosphere. The average Mauna Loa CO2 level for 2017 was 407 ppmvd (based on the monthly averages) compared to 316 ppmvd in 1959. The measurements were started by C. David Keeling of the Scripps Institution of Oceanography in March of 1958 at a facility of the National Oceanic and Atmospheric Administration (Keeling et al. 1976). NOAA started its own CO2 measurements in May 1974, and they have run in parallel with those
