Climate change is not new. The present phenomenon, however, has two new characteristics that quite fundamentally distinguish it from changes observed in the past. This time it is human activity that is the cause of change, essentially due to massive use of fossil hydrocarbons. The second difference is the rate of the current process, 50 to 100 times more rapid than what has occurred in the observable past (we can assess probable conditions going back several hundred thousand years, and determine them with certainty for the last 10 000 years). Large-scale climate change in such a short lapse of time (two or three human generations!) is likely to be a factor of widespread disruption of society, not only in terms of the environment but also economically, creating conditions conducive to the resurgence of political troubles of all sorts (totalitarian regimes, wars, etc.). Acting to avoid these outcomes will under any circumstances be less expensive than paying the consequences (the Stern Review is one study that substantiates this argument).
Fossil fuel consumption leads to an accumulation of greenhouse gases in the Earth’s atmosphere, carbon dioxide or CO2 being the most well-known of these gases. Even in infinitesimal quantities these gases have a first-order impact on the functioning of the climate system, just as a few milligrams of the active ingredient in medicines have determining effects on the functioning of the human organism.
These gases, transparent to visible light, are opaque to the Earth’s infrared radiation. The strong increase in the atmospheric concentration of greenhouse gases has upset the radiative equilibrium of the Earth’s atmosphere. To return to a state of equilibrium in an atmosphere that has become more opaque to infrared wavelengths, the temperature of the ground-level atmosphere must rise in order to increase radiation outwards to space.
The greenhouse gases that we have emitted in huge quantities since the beginning of the industrial revolution have set off a climatic shift the beginnings of which we have observed since the 1970s. Measurements recorded over the past several decades show that the global warming trend is accelerating.
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