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Wednesday, April 17, 2013

What the climate study actually said...

Last week, the wine writing world was ablaze with abject proclamations and naive denials about the predicted changes of the spatial distribution of viticulture suitability put forth by Hannah et al. (2013) in the Proceedings of the National Academy of Sciences. The idea that the Earth's climate is changing is indisputable. That those changes will affect viticulture is also incontrovertible. How those changes will manifest is up for debate. But all the writers and bloggers that had to put there two cents in about the article are missing the point of the study., as well as failing to critically analyze what was actually described.

First of all, the article was not focused on what will become of the world's wine regions. The study's goal was to estimate conservation conflicts in land use in areas currently not altered by viticulture. The researchers used a variety of environmental variables critical to viticulture to assess the spatial suitability under predicted conditions in 2050 (actually, the mean of 2041-2060) using 17 global climate models. The authors found that the areas currently suitable for viticulture may decrease and new areas at higher latitudes and altitudes may became suitable for production of Vitis vinifera winegrapes. They concluded that such a redistribution of wine production may result in substantial economic and conservation consequences in both areas.

Hannah et al. were not trying to sound any alarm bells in Napa. They were, however, trying to raise awareness for conservationists in the Yellowstone to Yukon Conservation Planning Area and other places potentially affected by future viticulture. They mentioned that investment in new varieties and changes in vineyard management could lead to innovative adaptions in extant wine production regions. While they mentioned that, they were not overly concerned with what may happen in Napa or Sonoma because they sought to estimate where grapevines might be planted in the future so that conservation strategies can be addressed in areas likely to be affected by potential indirect impacts of climate change (i.e., wine production).

Several other facts overlooked by many of the previous commentators include the parameters used to estimate suitability. First, the authors only used 21 common vinifera varieties. Now, we all know that there are way more than 21 varieties of vinifera grapes with varying degrees of environmental tolerance. And when you throw in the countless hybrids and non-vinifera species suitable for wine production, the potential for viticulture suitability is increased exponentially. This means that the loss of suitability in current regions is greatly exaggerated. Likewise, this also means that potential areas suitable for viticulture are also underestimated.

Second, the scale of the models is something others have alluded to. The grid resolution in the viticulture suitability portion of the study was only 2.5 arc-minutes. Depending on where the grid is located on the geoid, that equates to about 2.5 - 3-mile resolution. Now, obviously that is not fine enough resolution to make the sweeping statements that so many have about the death of California viticulture. Yet, tt is enough to make general conclusions about changes global viticulture. All the authors said, was that a changing climate will significantly affect viticulture and that vintners and conservationists should consider as many adaptations as possible to mitigate the changes as much as possible.

Finally, a third thing that I have not seen mentioned in any other review of the study is of the minimum temperature constraint. Wisely, Hannah et al. did address the fact that overwinter minimum temperature is an important limiting factor in viticulture suitability. Napa vintners don't have to fear the killing winter freezes that make viticulture in Colorado so extreme. Sure, frost at the wrong time of the year is a problem, but no where near the issue of getting to -15 C. However, what they did used for this parameter was the mean minimum temperature in the coldest month. A vine does not care what the mean minimum temperature is. The maximum minimum temperature is what is going to kill a vine. If it it gets to -35 C for even a few hours a grapevine will die. This vital information is lost if the mean minimum temperature for that month is only -12. Their model will assess that area as being suitable when in fact it is not. So, they may have overestimated the new area suitable for viticulture in 2050. As often is the case, using means when extremes are the important parameter can change the story.

Am I saying that California viticulture is going to be just fine in 35 years? I am not. I am also not saying their will be no more vineyards in Napa as many others are incorrectly assuming. What I am saying is wine writers failing to see the whole picture and that may come from failing to critically analyze a scientific study that is more about conservation than it is about predicting the future for Napa Valley viticulture. Hell, when one prominent writer claims that El Niño is the biggest driver of the ocean's temperature I know not the take what he says seriously...

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