Climate Change : Contrasted Trends?
p. 78-79
Texte intégral
1The lack of meteorological records makes the study of the climatic trends experienced by Lebanon in the medium and long term extremely difficult. The measures dating back to before the 1950s are rare and many stations stopped working or were damaged during the Civil War. The impact of climate change, which has become a major environmental issue worldwide, is thus hard to determine. However, existing data series reveal several climatic trends, certainly partly intertwined and linked to regional and global tendencies.
Contrasted climate trends nationwide
2Overall, average rainfall recorded for the Lebanese territory has not known any major change over the past 50 years. It has strongly declined in regions that were already among the driest, especially in northern parts of the Bekaa Valley, while it has increased in the North and South-East of the country. The main change observed on a national scale is the multiplication of torrential rains. In addition, temperatures have distinctly evolved. The average temperature has increased by 2 °C in 30 years and the difference between the minimum and maximum temperatures has also risen, a trend which contributes to desertification but the impact of which still has to be ascertained.
3These escalating temperatures participate in the acceleration of the melting rate, but the records gathered these past few decades do not show whether the average surface area of the snow cover has significantly evolved or not. Combined with an insufficiently regulated exploitation of water resources, these trends lead to a depletion of the aquifers and a dramatic decline in the discharge of wells and rivers: the average annual discharge recorded for the Litani has thus decreased from 275 to 125 million cubic meters between 1965 and 2011.
Figure V-2: Rainfall in Lebanon between 1950 and 2013
Figure V-3: Temperatures in Lebanon between 1974 and 2011
Figure V-4: Discharge recorded for several wells in Lebanon in 1984 and 2013
A growing urban heat island effect?
4One of the most noticeable trends observed in Lebanon is the elevation of the temperatures recorded in Beirut. The average temperature has likely increased by 4 °C since 1964. The CIRCE program has established that the records of the Beirut airport reveal a rise equivalent to 0.1 °C per decade in the maximum temperatures and to 2 days per decade in the number of summer hot days. These changes have major consequences on the urban environment and its inhabitants, for example on the pollution levels and in terms of public health. They result in an intensive use of air conditioning, thus in an increase in energy consumption and gas emissions, including from power plants.
5These changes seem mostly related to the urban heat island effect. Urban areas are darker than others: they reflect less sunlight and absorb more solar radiation, which leads to an increase in temperature. On the contrary, vegetation cover, which is very limited in the capital, helps maintaining lower temperatures, thanks, among others, to the evapotranspiration process. The link between these trends and the ones observed at a smaller scale has yet to be established but an increase in average temperatures reinforces this phenomenon. The latter seems particularly marked in Beirut, as the temperature in Tripoli does not seem to have increased by more than 2 °C between 1964 and 2004. In the absence of constraining legislation and regulation, the proliferation and concentration of newly built structures in the country’s biggest urban center has certainly played a significant role in this.
Figure V-5: Temperatures recorded at the airport of Beirut between 1971 and 2000
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