Friday, 5 April 2013

Climate change

My post on "Freshness" in wine reminded me about climate change and specifically evidence for it. The Observatoire Viticol site for the Department de l'Herault is an excellent source of all sorts of statistics related to matters vinous. Original material can be reproduced providing credit is given.

The following chart is from the 'Documents' section under 'Viticulture' sub-menu 'Milieu Physique' and uses data from Meteo France. It plots the average annual temperature over 62 years in Montpellier.

Clearly the 1980s saw a transition and since then the average temperature has risen by pushing one degree C in relation to the long-term average since 1950. 1987 is the most recent below average year, although 2010 got close mainly due to a cool summer.

Further analysis of this quickly becomes subsumed into other variables, a reminder that statistics can be used to prove anything. This chart plots average winter temperatures with the interesting observation being that colder winters tend to come in 3 year clutches.

Other charts show that most of the warming since the 1980s is down to warmer May to August weather. This fits in with the winter temperatures chart above, along with 2010 having a cooler summer than recent years resulting in a lower average temperature than its peers.

I found some Eurostat and DataMarket data on Languedoc-Roussillon Degree-Days from 1980 to 2009. Degree-days are used as a guide to how much heating a house will need, so the lower the figure the warmer it is. On days when the temperature exceeds 18 deg. C that day scores zero. Below that, the colder it is the bigger the number so the data is more a measure of winter temperature than summer highs.

As expected, Degree-Days shows a correlation with the average winter temperatures chart above. What my chart displays is the moving average (purple line) which is clearly in decline.

When it comes to wine then cooler nights in the growing and especially ripening season will normally be more desirable for growers than the absolute average temperature.

I also looked at rainfall data for the Hérault. Because of the mountainous hinterland rainfall amounts vary dramatically but one conclusion that can be drawn is that since 1999 below average winter rainfall featured in the majority of years, especially on the coastal plains. Most climatologists seem to agree that we must expect more weather extremes going forward. March 2013 has been the wettest since 1960 in most places (Montpellier had 191 mm) and in some areas, e.g. close to Pic St Loup, the wettest on record. On a positive note the water table, having been in decline due to too many dry winters, has recovered and is good news for most vineyards.

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