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Exceptional drought in Corsica


For those who like J numbers, I have summarized the precipitation over the last three years in Corsica. The last three years, the eastern part of Corsica has experienced an exceptional drought. I will explain the table based on two topics: the duration of the summer drought, and the absolute amount of precipitation.

I used data from the Bastia-Poretta station (in the plain).

The average annual precipitation for this station is 799 mm (for the period 1981-2010) and this corresponds to a fairly high average. For example, the city of Paris has an average of 637 (Montsouris station).

Even the Netherlands, famous for its rain, experiences an annual rainfall of +-800 mm.

It is therefore this high precipitation which allows for the exceptional greenery of eastern Corsica (and in particular the Costa Verde, Casinca and Castagniccia).

Duration of summer drought

Our climate is characterized by a summer drought that lasts two to three months (with negligible precipitation). Another characteristic is the interannual variability of the climate: rainy years are alternated with dry years so it is completely normal to sometimes have a dry year.

The table shows that for the years 2021-2023 we observe a dry period (defined by me as monthly precipitation less than 50% of normal) which lasts 4-5 months instead of the classic 2-3 months. The year 2022 was especially exceptional: from June to the end of October no significant precipitation at all!

Hydraulic stress is becoming very important for crops and natural plants: it is hotter in summer than before because of global warming & and summer drought lasts much longer. Evapotranspiration is then much higher and this combination of factors is responsible for the drastic change in mountain vegetation that scientists predict in the coming decades for Corsica.

Absolute amount of precipitation

Knowing that the average annual precipitation in Bastia-Poretta is 799 mm, it is remarkable to see the figure 320 mm for 2022... This is 40% of normal and it is the lowest value ever recorded by this weather station in its 50 years of records. It is logical that many villages faced cuts in their drinking water because the flow of springs in summer was very low.

What is also remarkable is that the three consecutive years of 2021, 2022, 2023 (for the moment) are very dry. It is rare (or even non-existent) to find three consecutive years as dry as these three years in history.

Effects on vegetation

The vegetation can probably cope with a year of extreme drought, provided that this year is followed by a rainy year to compensate. Three years of extreme drought exceeds the limits of several species found in Corsican vegetation, particularly in the mountains where we find species of trees that are not used to the true Mediterranean climate (such as the chestnut tree, the apple tree for example). The question is: how much stress can these sensitive species tolerate before they start to die? From what I see I think that apple and chestnut trees in difficult locations in particular (south orientation, poor soil) are dying on a large scale. Where the soil is richer it is better.

Scientists predict a radical change in mountainous vegetation in Corsica in the coming decades and I think that in the Campile valley we can already see this change; a large part of the deciduous species (chestnut trees in particular) are disappearing (and not only because of diseases) and holm oaks are taking their place. According to my estimates, 20-30% of the trees (especially chestnut trees) in our valley are dying or have already died. What complicates things are the diseases and oriental gall wasp which have weakened the chestnut trees. Droughts are another stress but not the only one so it is not easy to separate the effects of droughts from the effects of diseases and oriental gall wasp. We'll see how it develops over the next few years.

The average annual temperature in recent decades in Bastia Poretta is 16.0 C. The temperature in 2022 is 17.4 C. According to forecasts, at the end of the century the average temperature can reach 19 or even 20 degrees depending on the fossil fuel emissions. How will this increase impact water availability and vegetation in Corsica?

We could look to areas further south that already have a temperature of 19 degrees to see what vegetation they have. The problem is that Corsica will probably not have a significant drop in annual precipitation (according to the majority of studies), despite the fact that this precipitation will be more extreme and over a shorter period. When we look for example at a city like Athens, which has an average temperature of 18.2 for the period 1991-2020 (and even higher in recent years), we note that the vegetation around Athens is very different than the vegetation that found in a large part of eastern Corsica (much lower, less dense, fewer trees). The problem with this comparison is that Athens has much less annual rainfall than Corsica. Then there are also differences in soil fertility if we compare one region with another.

But what is certain is that we are not going to have more trees in the future. Generally, the further south you go, the more the vegetation is subject to the effects of drought and heat. Corsica is lucky to be a mountain in the sea because the mountain range is responsible for copious rainfall, even in the future (one problem is that as the warm atmosphere contains more water, the rainfall become more extreme too).

Lots of speculation then, but one thing is certain, if climate change causes more and more dry years like the last 3 years, Corsica will experience drastic changes in its water availability and vegetation .

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