The impact of greenhouse gas emissions on climate change

We have only recently become aware of the risks involved in climate change. In 1988, a decisive step was taken with the creation of the Intergovernmental Panel on Climate Change (IPCC), on the joint initiative of the World Meteorological Organisation (WMO) and the United Nations Environment Programme (UNEP).

The first IPCC report led in 1992 to the adoption of the 'climate' convention, currently ratified by 189 countries. The second report was published in 1996, before the signing of the Kyoto Protocol. The third report, published in 2001, led to the adoption of control measures and sanctions designed to ensure effective application of the Kyoto Protocol [19]. The fourth report, published in 2007, confirms and improves the main conclusions of the previous report.

Over 2000 scientists from 154 countries participate in the IPCC studies. Wishing to reach a consensus of opinion, the IPCC has remained prudent and it is therefore quite possible that the consequences of climate change will turn out to be more dramatic than those announced.

There is more and more evidence of the correlation between an increase in CO2 emissions and an increase in the average temperature observed over the entire planet. Since 1860, the start of the industrial era, the average temperature on the surface of the Earth has increased by 0.8 °C.

Temperature fluctuations have already occurred in the past. Long-term climatic cycles are due to variations in the terrestrial orbit around the sun. This is the case in particular of the glacial-interglacial cycles, which have a frequency of about 100 000 years. We are currently experiencing an interglacial period and the trend towards a slight drop in average temperature, observed over the first eight hundred years of the past millennium, indicates a drift into a new glacial age in 10 000 years time.

The sudden rise in average temperature since the start of the industrial era seems to be abnormal compared with past trends, both in view of its relative amplitude and the speed of the change, on a geological timescale. Other phenomena of shorter frequency also occur, especially variations in solar activity.

This activity, which has increased over the past centuries, could partly explain the current warming, but certainly not the amplitude of the variation observed, nor the fact that since 1980 global warming has accelerated while solar activity has remained stable.

Some scientists, although very few, still express doubts as to the relation between climate change and greenhouse gas emissions related to human activities. The most vehement sceptics are in the USA, where the positions taken on climate change remain a political stake. Since it is becoming more and more difficult to deny the reality of climate change, the sceptics are now trying to denounce the way it is presented which, in their opinion, is excessively alarmist. This is the stance adopted by Richard S. Lindzen, a professor at MIT, who is the most frequently quoted advocate of this minority group [20].

The trend over the last few years has strengthened the conviction of most scientists that the rise in the average temperature observed is indeed correlated with human activities and, in particular, with the rise in atmospheric CO2 content, which has changed from 270 ppm1 around 1850 to 380 ppm in 2005. Figure 3.2 shows the trend in atmospheric CO2 content.

This content has fluctuated over the last 400 000 years, just like temperature, whose variations are correlated with the atmospheric CO2 content. The current variation nevertheless lies outside the fluctuation

1 Parts per million.

Figure 3.2 Trend in atmospheric CO2 content (Sources: IPCC reports)

interval observed during previous geological eras. If the trend continues, the atmospheric CO2 content could double by the end of the century, reaching a value of 750 ppm, and possibly exceeding 1000 ppm, causing disastrous climate transformations.

According to the models produced, the average temperature increase could reach 2-4 °C by the end of the century, possibly even 6 °C [9-12], while an increase of more than 2 °C is considered unbearable.

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