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The Earth’s climate responds to external phenomena such as variations in the planet’s orbit around the Sun, volcanic eruptions and atmospheric concentrations of greenhouse gases.
The precise causes of the recent warming trend remain the subject of discussion, but there is a virtually unanimous scientific consensus that the high levels of greenhouse gases produced by human activity (anthropogenic) are the principal cause. The data and the conclusions are particularly clear for the last 50 years, for which period a great deal of data is available.
None of the phenomena influencing the climate have an instantaneous effect. The thermal inertia of the oceans (the long time they take to warm up or to cool down) and the indirect character of many of the reactions in the global system, are such that the system does not reach equilibrium immediately. Studies show that, even if the current levels of greenhouse gases were to be stabilised, significant increases in temperature would continue for many years.
Greenhouse gases in the atmosphere
The greenhouse effect was discovered by Joseph Fourier (see picture 1) in 1824 and studied quantitatively by Svante Arrhenius (see picture 2) in 1896. The absorption and emission of infrared radiation by gases in the atmosphere heat both the atmosphere and the planet’s surface, a bit like the sun’s rays, partially trapped by the water vapour inside a greenhouse, lead to the heating of the atmosphere of the greenhouse.
The greenhouse effect due to the gases naturally present in the atmosphere has a heating effect of + 33 °C. Its effect is therefore very positive, since, without it, the Earth would be much less comfortable, with an average atmospheric temperature of– 18 °C ! The problem is that the intensity of the natural greenhouse effect is magnified by human activity which increases the atmospheric concentration of certain greenhouse gases.
The principal greenhouse gases are water vapour (H2O), carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) and ozone (O3). The atmospheric concentrations of carbon dioxide and methane have increased by 30 % and 150 % respectively since the start of the industrial era in 1750. Current concentration levels are significantly higher than at any other time in the last 650 000 years, a period for which reliable data can be extracted from the icecaps. Other geological indicators tend to show that current CO2 concentrations are higher than at any time for at least 20 millions years. (see picture 3)
The current concentration of CO2 , in volumetric terms, is about 400 ppm (parts per million). This level should go on increasing, because of the continued use of fossil fuels and the pursuit of deforestation. The rate of this increase depends on many factors and is subject to significant uncertainty, but exhaustion of fossil fuels fixes a limit. The 2001 IPCC report on emission scenarios gives a wide range of future scenarios for CO2 concentrations, from 541 to 970 ppm in 2100. Fossil fuel reserves would be sufficient to reach this level and to continue emissions after that date, provided that coal, bituminous sands and methane hydrates were widely consumed.
Secondary phenomena resulting from warming, such as the liberation of 70 000 million tonnes of methane after the melting of the Siberian permafrost, could contribute to an enormous increase in greenhouse gas emissions. Conversely, trapping all or part of the atmospheric carbon, using natural phenomena (the so-called «carbon sinks»), or by human intervention, could reduce the impact of emissions.
More about the greenhouse effect
Complex interactions
An increase in the concentration of greenhouse gases leads to a rise in average temperature and that rise has different effects that, in the end, can produce either an increase or decrease in the initial temperature.
The most important effect is linked to water vapour. When CO2 concentration increases, the temperature rises and evaporation increases. But … water vapour is itself a greenhouse gas, so … the greenhouse effect increases, the temperature rises, evaporation increases … a new state of equilibrium is reached, with a new average temperature significantly higher than that which would have resulted from the CO2 effect alone.
The clouds might also have an impact. Seen from below, they reflect infrared radiation onto the Earth’s surface and therefore contribute to the rise in temperature. Seen from above, they reflect part of the sunlight and radiate the infrared into space. It is not yet clearly understood whether the net result of these two phenomena is a warming (the most likely) or cooling effect. One of the difficulties is the low resolution of climate models : their basic grid, of the order of a hundred kilometres, is far too large compared to the size of clouds.
Another phenomenon is linked to the albedo (reflecting power) of ice. When the temperature rises, the polar icepack melts and is replaced by an exposed surface of water or land. Both of these are surfaces that are less reflective than ice and absorb more of the solar radiation, resulting in more warming, followed by more melting, and so on. The discharge of methane from the oceanic ice beds might also amplify the phenomenon. (see picture 4)
Lastly, emissions by Man of various pollutants, particularly sulphates contained in aerosols, can have a cooling effect, since they reflect incident sunlight.
Variations in solar emissions
The energy radiated by the sun varies cyclically and these variations might, at certain times, contribute to global warming. Most studies, however, conclude that any such contribution, like that from volcanic action, is negligible compared to that from greenhouse gases emitted by Man’s activities. (see picture 5)
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