Meteorological Cooperative › Weather and climate › Observed change

Observed change

The national series shows a mean warming of about 1.4 degrees since 1960, greater in winter than in summer and greater inland than on the coast. Three consequences have passed from measurement into administration: the failure of harvests in the Quintorum basin, the retreat of the eastern shore at Ravelluri, and the concentration of southern rainfall into fewer and heavier days.

The temperature series

The homogenised national series begins in 1901 and runs from twelve stations with continuous record. Warming is distributed unevenly through the year and across the territory. Winter has warmed by about 1.9 degrees since 1960 and summer by about 1.1. The continental interior has warmed by about 1.7 degrees and the maritime coast by about 1.1.

Change in mean temperature since 1960, by local climate
Local climate	Annual	Winter	Summer	Frost days a year	Growing season
Maritime coast	+1.1 °C	+1.5 °C	+0.9 °C	-19	+21 days
Western upland	+1.5 °C	+2.1 °C	+1.1 °C	-27	+24 days
Continental interior	+1.7 °C	+2.2 °C	+1.4 °C	-24	+29 days
Sheltered south-west	+1.3 °C	+1.6 °C	+1.2 °C	-16	+26 days
National mean	+1.4 °C	+1.9 °C	+1.1 °C	-22	+25 days

Homogenised series, twelve stations of continuous record, referenced to the 1961 to 1990 base. Frost days and growing season are areal means.

Snow and the upland

Snow cover on the Vandorum high ground lay for a mean of 84 days a year in the 1961 to 1990 period and lies for 61 days in the current normals. The lower limit of reliable cover has risen by about 180 m. Communes above that limit organised their winter access on the old figure, and the schedule for clearing the upland roads was rewritten in 2016 on the current one.

The Quintorum basin

The basin is the driest district of the Republic and its soils are fertile and shallow-rooted. Warming raises evaporative demand there faster than anywhere else, and rainfall has held steady while demand has risen. The basin failed three harvests in the seven years to 2019, a frequency without precedent in the record, and the abstraction regime under which the basin is irrigated has been tightened twice since.

The Ecological Budget Authority records the basin as in local deficit and applies a stricter water ceiling there than the national one. The physical series behind that decision are the ones published here: rainfall, soil moisture at four depths, reference evapotranspiration and river stage on the upper Marnaqui.

The eastern shore

Ravelluri and Marendo

The shore at Ravelluri has retreated by roughly nine hundred metres since 2013. Retreat is driven by sea level, by the loss of sediment supply to the dune barrier, and by the increased frequency of surge events on the eastern seaboard.

The commune of Marendo is being moved inland over forty years. A sea wall was considered and set aside, on the finding that a wall would hold the settlement and destroy the dune, the lagoon behind it and the seagrass and weed beds inside the lagoon.

Mean sea level at the Marendo gauge has risen 3.9 mm a year over the last three decades, against 2.1 mm a year in the three decades before.

Rainfall intensity and the Sornorum floods

Rainfall in the south has become more intense while remaining no more plentiful. Annual totals at Sornorum have moved by less than three per cent since 1960. The share of the annual total falling in the wettest five days has risen from 14 per cent to 21 per cent, and the one-hour intensity exceeded once in ten years has risen from 28 mm to 37 mm.

That pattern produced the floods of 2017. Two convective systems crossed the southern crossroads within thirty hours and delivered 191 mm at the Sornorum station, of which 106 mm fell in four hours. The drainage of the town was designed against a one-hour intensity of 26 mm. The REP-FEW system was specified in response, and the design intensity for urban drainage in the southern sub-regions was revised in 2019.

Rainfall intensity, Sornorum station, change since 1960
Measure	1961 to 1990	1991 to 2020	Change
Annual total	624 mm	641 mm	+2.7 %
Share falling in the wettest five days	14 %	21 %	+7 points
One-hour intensity, ten-year return	28 mm	37 mm	+32 %
Days above 40 mm a year	1.8	3.4	+89 %
Consecutive dry days, summer maximum	17	24	+7 days

Method and attribution

Series are homogenised against station metadata, and every break arising from a move, a screen change or an instrument change is documented and corrected. The correction record is published with the series, so a user may work from the raw values where the correction is disputed.

The cooperative attributes trends and abstains from attributing single events. A statement that one storm was caused by warming exceeds what the method supports. A statement that storms of that intensity have become more frequent by a measured factor is what the record supports, and it is the form in which findings are given to the ecological organs and to the courts.