Grape harvest dates and year-to-year climate variability (global climate change)

What we now call “climate change” used to be called “global warming”. The experts introduced the change in name in order to emphasize that there are actually two things that we need to be aware of:

1. the atmospheric temperature is increasing at an unprecedented rate
2. the weather is getting increasingly variable from year to year.

Both of these things are happening simultaneously.

Much of the evidence for both of these phenomena actually comes from agriculture, because we have good written records about harvest dates of various crops, often over centuries. These records show both of the patterns: harvest dates getting earlier and earlier on average, but also with erratic harvest dates from year to year.

I have written before about Grape harvest dates and the evidence for global warming. In that post I discussed several datasets from around the world, which all show the same patterns. In particular, the grape harvest dates for Burgundy are valuable, because of the 700 years over which they have been collected, from 1350 CE. I noted: “there has been a dramatic change in harvest date in recent years, with the earlier and earlier harvests since 1984 being attributed to global warming.”

The other weather pattern has also been emphasized in the wine industry recently (The dirt on wine):

“These days, you can’t say hot, cool, wet or dry vintage any longer. Weather has become totally unpredictable, with extremes even within one growing season,” began Diego Tomasi, director of the Centro di Ricerca per la Viticoltura e Enologia di Conegliano (CRA-VIT) in Italy. To emphasise his point, he projected onto a screen several graphs; one showing the increased variability of harvest dates in Burgundy in the past 15 years compared to the previous two centuries.

So, the obvious thing for me to do here is re-visit the Burgundy data, to show you the big picture.

I have used exactly the same dataset as in my previous post, containing a complete record of the official start of the Burgundy grape harvest for every year from 1370 to 2018 CE, inclusive. Last time, I calculated a running average of 9-year blocks of harvest dates, but this time I calculated the standard deviation of the harvest dates, instead. This calculation describes how variable the harvest dates were, across each 9-year period — a larger standard deviation indicates more variation from year to year (see my post on Statistical variance and global warming).

I have graphed the data above. Each dot represents one 9-year period. The horizontal line is simply the median value — half of the standard deviations are below the line and half are above it. The dashed lines show the inter-quartile range — half of the values are between the two dashed lines.

For our purposes here, I have highlighted the final 32 values — the final 16 are shown in red (since 2000) and the 16 before that in green. Note that red values are almost all above the upper dashed line (ie. in the top 25% of the variation) while the green ones are all below the lower dashed lines (ie. in the bottom 25% of the variation). This means that the recent harvest dates (this century) have been much more variable from year to year than were the ones immediately before that (the end of last century).

This emphasizes the quote above from Diego Tomasi — the Burgundy grape harvests are now much more variable than they have been within living memory.

However, as I noted in my previous post on the Burgundy harvests, the graph also illustrates that there have been recordings of previous large variations in the weather; indeed, on occasion even more extreme than we are observing now. In that sense, the current change in the weather is not necessarily unheard of, although it is definitely unusual.

This is what the climate-change skeptics are on about, and they are right when they point out that rapid changes in long-term weather have occurred before in our recorded history. This does not mean that the effects of the climate change will be any less, or that we do not need to respond to them. Our recent agricultural practices will have to change, irrespective of whether the current weather patterns have occurred before or not.

This point is emphasized by a recent study of soil moisture over the past 1,200 years (see Climate change: US megadrought ‘already under way’). The recent 20 years of relatively dry conditions is the fourth such period found by the study, so in that sense this is not unexpected (the previous megadrought ran from 1575–1603 CE). However, the authors also note that, while the current drought may be a natural event, it is being made much worse by climate change. That is, the effects of the natural event are being exacerbated.