The purpose of this chapter is to summarize global and French climate patterns during the past century, most notably in what concerns temperature regimes change, and extreme weather and climate events and changes in the annual number of cold/hot days.
During the 20th century the observed frequency of natural disasters in the world has risen significantly (Fig.1, source EM-DAT), with most intense heat-related events between 2000 and 2010.
Warming of the climate system is unequivocal, and since the 1950s, many of the observed changes are unprecedented over decades to millennia. The atmosphere and ocean have warmed, the amounts of snow and ice have diminished, sea level has risen, and the concentrations of greenhouse gases have increased (IPCC 2013)
Changes in many extreme weather and climate events have been observed since about 1950. It is very likely that the number of cold days and nights has decreased and the number of warm days and nights has increased on the global scale. It is likely that the frequency of heat waves has increased in large parts of Europe, Asia and Australia. (IPCC 2013)
Fig. 1 Number of natural disasters reported 1900 – 2012 (EM-DAT)
In France, the patterns of the twentieth century show a warming more intensive than global. The average annual temperature has increased by 0.95 °C over the French territory (+ 0.6 ° C globally) between 1901 and 2000.
The analysis of climate patterns between 1951-2000 performed by Météo- France identified the main trends in the evolution of temperature extremes, including: higher daytime temperatures, higher night temperatures, increased frequency and intensity of heat waves, decreased frequency of cold waves and increased variability of summer temperatures (fig.2,3). Since 1950 several notable heat waves are observed:
– An episode of exceptional intensity of 2 to 14 August 2003;
– Intensive heat wave episodes from June 23 to July 7, 1976 , from 9 to 31 July 1983 and 10 to 28 July 2006;
– Episodes of moderate intensity in 1952, 1957, 1975 , 1990, 1994 , 1998 and 2005;
– Episodes of low intensity in 1989, 1992 and 1995.
Fig. 2 Deviation from average summer temperature in metropolitan France 1900 – 2011 (average summer temperature calculated for the period 1981- 2010)
Fig. 3 Heat waves in France (1947- 2013) The surface of spheres reflects the intensity of heat wave
Climate change is a measured reality: observations provided by Météo-France climate parameters has enabled to highlight significant trends in climate parameters for temperature.
The graphs below (Fig. 4 and 5) provide an overview of this evolution, the available data for the station Bonneuil-en-France – Le Bourget on infoclimat.fr site. The trends in evidence are similar to those obtained in the framework of the Regional Plan for Climate, from the data station Paris-Montsouris (Annex 2b, Météo-France), as shown in the two graphs below.
Fig.4. Evolution of the observed minimum and maximum temperatures (Artelia, according to data available on the site for infoclimat.fr Bonneuil-en-France station – Le Bourget, 2014, as cited in Artelia, RCT, Plaine Commune 2014).
Fig. 5. Evolution of the observed minimum and maximum temperatures (Regional Plan for Climate, Météo-France, according to data from the station Paris-Montsouris, 2010, as cited in Artelia, RCT, Plaine Commune 2014)
The average minimum and maximum temperatures have increased by about 1.3 ° C over the last 50 years in Plaine Commune. This average trend comes despite a strong interannual variability, a trend increase in exposure to heat waves and a downward trend in exposure to cold episodes (Fig.6 and 7).
Fig. 6. Evolution of the observed annual number of very hot days: maximum temperature above 30 ° C (Artelia, according to data available on the website for infoclimat.fr Bonneuil-en-France station – Le Bourget, 2014)
Fig. 7. Evolution of the observed annual number of days without thawing: maximum temperature at or below 0 ° C (Artelia, according to data available on the site for infoclimat.fr Bonneuil-en-France station – Le Bourget, 2014)
 This increase is questionable, as it’s difficult to distinguish between the increase in instances and increase in the number of reported events. With the amelioration of technology, it became easier to report the events than it was at the beginning of the century.
 Notethat further in text the terms of probability will be used according to the scale developed by IPCC, which means: “exceptionally unlikely” – 0-1% probability, “very unlikely” – 0-10%, “unlikely” -0- 33%, “about as likely as not” – 33 – 60%, “likely” – 66 – 100%, “very likely” – 90 -100%, “virtually certain” – 99- 100% probability.
IPCC (2010) Guidance Note for Lead Authors of the IPCC Fifth Assessment Report on Consistent Treatment of Uncertainties. Retrieved from: http://www.ipcc.ch/pdf/supporting-material/uncertainty-guidance-note.pdf, p.3