Temps de lecture :4 minutes
The greenhouse effect is a well-known mechanism. It was discovered in the 19th Century : Joseph Fourier described it as early as 1824. Briefly: the gases in the atmosphere absorb some of the energy of sunlight and, in doing so, heat the planet. The more gases there are, the more they heat the planet. There are many gases: water vapour, CO2, methane, nitrous oxide, etc. Thanks to these gases, the Earth’s average temperature is compatible with our existence. Without them, it would go down to -18°C.
In 1896, the Swedish researcher Arrhenius calculated that if the amount of CO2 in the atmosphere doubled, the average temperature would go up by 4°C.
A historical modification
Human activity has considerably increased certain greenhouse gases. For CO2, the world atmospheric concentration went from about 280 ppm (parts per million) in 1750 to 380 ppm in 2005. This increase is mainly due to the use of fossil fuels (transport and heating…) and to the change in land use. The increases in methane and nitrous oxide are mainly due to agriculture.
The logical forecast is therefore that the Earth’s temperature will increase. This is indeed what has been observed. In a century, between 1906 and 2005, the average temperature went up by 0.74 °C. Eleven out of the past twelve years have been the hottest since they started being recorded (in 1850). This change can seem small but this is not the case if we consider that global glaciation is only equivalent to a 2 to 4 degree decrease.
The planet alternates between periods of heating up and cooling down (glaciation). These are due to the oscillation of the Earth’s orbit and changes in sunlight. But these changes correspond to cyclical phenomenon. This is not the case for the greenhouse effect. The gases build up and the effects add up. For the moment, there is no predictable natural regulation. Moreover, the changes that are observed now seem to be much faster than those in the past.
The existence of global warming only came about a step at a time. Over the years various personalities denied the existence of the phenomenon, its importance and the part Man played in it. Although they came from different walks of life and they advocated different points of view that could even be contradictory, these opponents were grouped together under the term « sceptics », after a book by the economist Bjorn Lomborg : The skeptical environmentalist [see debate]. In France, Claude Allègre carried on these ideas. Often, sceptics took advantage of what was not known on areas of the subject to call the whole phenomenon into question. Some sceptics were funded by the petrol industry and particularly by Exxon (Esso in France).
However the debate is now over. The scientific community has drawn together in a collective international effort around the Intergovernmental Panel on Climate Change (IPCC). It updated the different areas that were not known, the areas that were unclear and also, the points on which the scientists agreed. The IPCC published several reports of which the last was published in 2007. These clearly detailed the subject and are now references. The IPCC was even awarded the Nobel Peace Prize in 2007 for its work.
The IPCC’s projections
The IPCC scientists have, amongst other things, mapped out several scenarios on the possible ways in which the climate could evolve. These scenarios are divided into families, on the basis of hypothetical models of development – more or less globalised, faster or slower, more or less energy-consuming, etc. They estimate the amount of greenhouse gases that could be released into the atmosphere and the corresponding increase in temperatures. There are many divergences between these scenarios, between the underlying models and the ensuing forecasts. However, we can draw at least two conclusions from them.
The first is that if humanity does not change how it functions, the amount of greenhouse gas will reach levels that will deeply affect our climate: the level that is usually mentioned is 450 ppm of CO2 in the atmosphere (see above). Then, the average estimate for the temperature increase is +2°C in 2100, but more pessimistic scenarios (+6°C and even more) have not been excluded.
The IPCC has analysed the consequences, such temperature increases could have. [see the consequences sheet].
If we now understand how climate functions on the whole, scientists have admitted that many points still need to be explained. The different mechanisms that come and interact with the greenhouse effect and could amplify it are amongst these. They are called positive feedback.
One of these is melting ice caps. The ice reflects the sun’s rays and it sends many of them into space. However, water absorbs heat. The melting ice caps which are a consequence of global warming in turn reinforce this mechanism.
Another is the melting permafrost, the layer of frozen soil in Siberia and in America. When this melts, the microorganisms which are present start releasing greenhouse gases.
Finally, there are large amounts of CO2 stored in oceans in forests. It is possible that global warming changes how these two function by considerably lowering their storage capacity and by possibly causing stored gases to be released. But it is not clear to what extent.
The Kyoto Protocol
International mobilisation, especially around the IPCC scientists drew the attention of politicians who set up a first agreement, the UNFCC, then a second one which depends on it, the Kyoto Protocol. This set emission reduction targets for the six main greenhouse gases by 2012. These reductions only apply to certain developed countries and not to the USA which did not ratify the Protocol. However, this agreement remains moderate. The reduction targets are not enough (-5.2% compared to CO2 emissions in 1990) and some countries such as Canada, Spain or Japan do not even respect them.
The Protocol will expire soon so what will happen post-2012? Issues that are being negotiated include how to make emerging countries contribute to the effort – for now, China does not have any limits but it has become the highest producer of greenhouse gas – and how to make the United States join the protocol.