2. What causes Earth’s climate change?
Current climate change is mainly caused by the “greenhouse effect”, where heat-trapping gases (water, carbon monoxide, carbon dioxide, methane, ozone and nitrous oxide) in the atmosphere cause an increase in global temperature. The effect occurs due to the atmosphere being almost completely permeable for solar short-wave radiation, but less transparent for the long-wave infrared radiation. The short-wave radiation is absorbed by the surface and emitted as infrared radiation, which then can be absorbed by the greenhouse gases. The gases then release the radiation once more towards space and towards the Earth’s surface. This effect traps part of the solar energy in between the atmosphere and the surface and contributes to an increase in global temperature.
Effectiveness of greenhouse gases
The intensity of the greenhouse effect depends on the atmosphere’s temperature and on the amount of greenhouse gases that the atmosphere contains. Without incorporating the effects of clouds, the largest portion of the greenhouse gases is made up of water vapor in the atmosphere with a percentage of 36 to 70%. Carbon dioxide contributes with 9 to 26%, methane with 4 to 9 % and tropospheric ozone with around 3 to 7% to the global greenhouse effect. It is not possible to assign exact percentages for the different greenhouse gases, because the absorption and emission bands of the gases overlap. Clouds also absorb and emit infrared radiation and thus affect the radiative properties of the atmosphere.
Greenhouse gases throughout history
Greenhouse gases have been around since the formation of Earth’s atmosphere. These naturally arising greenhouse gases add to a comfortable global mean temperature of +15°C and contributed to evolution of life as we know it. Without an atmosphere, Earth would be a much colder place, with a mean temperature of around -18°C. Since the industrial revolution, however, the anthropogenic input of greenhouse gases into the atmosphere has increased significantly. Anthropogenic influences, like the burning of fossil fuels, deforestation, production of cements and agriculture emit long-lived greenhouse gases (carbon dioxide, methane and nitrous oxide) as well as tropospheric ozone, which amplify the greenhouse effect. Measurements of CO2 from the Mauna Loa observatory show that the concentrations have increased from about 313 parts per million (ppm) in 1960 to 400 ppm in May 9, 2013 (figure 1). The global concentration of greenhouse gases has increased constantly since.
Figure 1: The Keeling Curve of atmospheric CO2 concentrations measured at Mauna Loa Observatory. Figure adopted from wikipedia.org.
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