Does climate change, even if we’re only talking about a temperature increase of 0.81°C, make a difference to what ends up in your bottle – or is it that simple?

The letter below was published in the Irish Times letters page of 24th January 2012. It was written in response to another letter, printed in the same pages on 16th January, which in turn was prompted by an article by John Wilson in the Irish Times Magazine of 7th January, 2012:


In response to Jim Ryan’s (January 16th) letter on John Wilson’s article “Now for something different”, I would strongly disagree with his claim that a 0.81 increase of a degree Celsius is “hardly enough for…plants to notice.”

Whilst the increase seems insignificant in the greater scheme of things, it is actually the hotter temperatures during the growing season and in particular more frequent spikes in temperature that are causing problems for grape producers. Grape growing conditions are a complex subject and are affected by numerous macroclimatic conditions. Sunlight, rainfall, altitude and continentality as well as what is referred to as heat summation, all have a role to play with heat being responsible for 75% of the grape growth and ripening. Grape growers refer to the amount of heat generated during the growth season as growing degree days (GDDs). Added to this, mesoclimatic factors such as aspect, soil, wind exposure and bodies of water also have an influence on the ripening of grapes on the vine.

GDDs are calculated by taking the mean temperature for a month, subtracting ten and multiplying this figure by the number of days in the month. The GDDs for each month are then added together across the growing season (April-October in the Northern Hemisphere) to get the heat summation figure for the whole year. Typical GGD readings for well known grape growing areas are: Bordeaux 1440, the Barossa Valley, Australia 1680 and Burgundy 1100. These figures are used by the grape growing industry to help assign the correct grape variety to the correct area. For example the Syrah grape requires 1250 GDDs to ripen to a stage where it can be used to make dry table wine and therefore whilst not suited to Burgundy, it would easily ripen in the Barossa Valley.

Having obtained even fairly recent historical information from the weather station at Frankfurt airport in Germany it is clear to see that Mr. Ryan’s assertion that a small overall temperature increase has no affect on grape production is wrong. In 1997 the GDDs recorded at the station were 1317 and yet for the following fourteen years they were recorded at an average of 1474. In fact only 2010 had a lower reading with 1287 whilst the memorable heatwave summer of 2003 came in at 1700 GDDs. Under the 1997 reading a grape grower would have been recommended to plant varieties such as Chenin Blanc for whites and Merlot or Syrah for reds. In the intervening period that same grape grower would have regretted their decision as the advice today would be to plant traditional Mediterranean grape varieties such as Grenache Blanc and Mourvedre for whites and reds respectively such is the climatic change in the growing season. As for the traditional Riesling grape so commonly found in German vineyards, has Mr. Ryan not noticed that these wines are no longer found to be at the previous alcohol levels of nine or ten percent but more likely at twelve to thirteen percent? Riper grapes equals more sugar which equals more alcohol.

In addition, I don’t think any wine lover would dispute Mr. Ryan’s claim that Germany has been making good Pinot Noir for a long time but with the change in growing conditions I think it is safe to say that they are now doing so more regularly!

Yours faithfully,

Simon Tyrrell

And we love the reply in the paper today, 25th January 2012:

Sir, – I have just calculated the GDD (growing degree days) of my “Château Garage” 2011 utilising Simon Tyrell’s very helpful guide (January 24th) and using the mean temperatures from the Malin Head Weather Station, April to October 2011. It comes out at 656.1. I have one question: will it improve with age? – Yours, etc,


Main Street,

Bundoran, Co Donegal.