The greenhouse effect

The CO2 emitted in the atmosphere behaves like a greenhouse gas, according to the mechanism shown in Figure 3.1. The atmosphere is transparent to the incident solar radiation transmitted in the visible light spectrum, but some of the solar energy received by the Earth is reflected back as infrared radiation. This radiation can be partly stopped by some gases present in the atmosphere, the greenhouse gases, and returned to the Earth whose surface warms up accordingly.

Carbon dioxide is not the only greenhouse gas. Other gases such as methane, nitrous oxide and ozone produced by human activity contribute to the thermal imbalance of the Earth.

Agricultural activities are also responsible for emissions of greenhouse gases: in addition to CO2 emissions due to the use of fossil energies, they also produce methane (from cattle and paddy fields) and nitrous oxide. Some industrial gases also participate in global warming, especially halogenated gases such as carbon tetrafluoride and sulphur hexafluoride.

The contribution of these greenhouse gases depends on their global warming potential (GWP), which measures the absorption of infrared radiation emitted back into the atmosphere. Carbon dioxide, with a GWP of 1, acts as reference. Methane has a GWP of 22, nitrous oxide 310 and sulphur hexafluoride 23 900!

Figure 3.1 Greenhouse mechanism

The gas concentration and average atmospheric lifetime must also be taken into account. The effect of methane, with a lifetime of about 12 years, is therefore less damaging than one might expect in view of its GWP. We must nevertheless bear in mind that the mechanisms involved in atmospheric circulation of these gases and their exchanges with the soil and the oceans are complex. This leads to uncertainties regarding the long-term behaviour of CO2 in the atmosphere and on the time required to return to equilibrium, in case of disturbance. The return to equilibrium occurs initially through absorption over several centuries of most of the CO2 in the atmosphere by the oceans and the biosphere, followed by a very slow mineralisation reaction during which CO2 is incorporated into carbonated rock, over a timescale of up to 100 000 years. This timescale extends well beyond our current forecast capabilities, especially since a new ice age is predicted to occur in 10 000 years. Whatever the case, spontaneous return to equilibrium can only be very slow.

Water vapour is also a greenhouse gas, but it condenses as clouds and does not accumulate in the atmosphere.

Of all the greenhouse gases, CO2 is the one which contributes most to the greenhouse effect related to human activities, the anthropogenic sources, due to the considerable quantities emitted. In addition, since emissions of this gas are also increasing steadily, it plays a major role in the future risks of climate change.

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