Last friday, Slate.com posted an article by Joel Waldfogel called "Go North, Young Grapes: The effect of global warming on the world's vineyards." I was excited to see it, since I'm really interested in both global warming and wine. However, after reading it, I find several major deficiencies, some of which are obvious errors in understanding what global warming is and how plants, specifically grape vines, work.
The article reports on something called a "working paper" by Orley Ashenfelter and Karl Storchmann, who I think are economists. The paper is called "Using a Hedonic Model of Solar Radiation to Assess the Economic Effect of Climate Change: The Case of Mosel Valley Vineyards," written for the National Bureau of Economic Research, Inc., whatever that is. I have looked at this paper, which you can find through the RePEc (Research Papers in Economics) web site [LINK,pdf], and so I will let Joel Waldfogel off the hook for a time while we discuss the paper itself.
Section 2B of the paper states, "it is apparent that
total solar radiation is highly dependent on the amount, kind and density of clouds, and varies
with time and place. For the sake of simplicity engineers often calculate the so-called
extraterrestrial radiation, that is, the radiation that would be available if there were no
atmosphere (Duffie and Beckman, 1991)." What this means to me is that they don't want to account for variations in the atmosphere (weather and such, you know, that's not important), so they are going to use what I would call solar insolation. However, that varies only with latitude and time of year, and they are calculating it at the ground, so they ignore the atmosphere but take into account the slope of the ground. Okay, well, I'll tell you why that is a poor assumption shortly.
Let me now quote from section 2D:
"D. Other Factors that Affect Vineyard Sites Gladstones (1992) provides a detailed analysis of several other factors that make specific geographic sites more or less suitable for the production of high quality grapes. Important factors include those that reduce diurnal (night-day) temperature differences. Nearness to a body of water and, especially, soil type are important determinants of diurnal fluctuations..."
Hold on to this for our discussion below.
Before going on to the analysis, the authors discuss the data for vineyard prices, and how they take into account non-south-facing slopes, altitude, and soil characteristics. There are gross assumptions built into these choices, which I will ignore here. However, let's just say that vineyards don't necessarily suffer from being farther away from bodies of water, despite the authors' assumption. One aspect that might be worth mentioning here is that the authors state that they think vineyards far from large bodies of water will be hurt because they don't have smaller diurnal temperature fluctuations; as I understand it, grapes do extremely well in conditions where there is large diurnal variation.... the hot days and cool nights of California's Napa, Sonoma, and Mendocino counties come to mind.
So how do they do global warming without an atmosphere? Well, they don't. They do a very simple energy flux calculation using blackbody radiation, albedo (reflectivity) and an "emissivity." Fair, except that instead of actually considering something resembling an emissivity, the authors choose to assume that the energy emitted from the surface is half of that emitted from the atmosphere. Crude to say the least, especially when it would have been easy to do much better. So they are sort of taking account of the greenhouse effect, since they'll get temperatures that are way too cold if they don't. They then plug in a temperature change associated with global warming, and get the amount of "radiation energy" that must be associated with that change, and they continue to assert this is "solar radiation" (actually in their figure they say "positive net radiation" which is correct).
Here's the thing. They set up their model using solar insolation, or atmosphere-free radiative flux at the surface, but then they try to apply a climate change that relies on a crude assumption about the atmosphere. This is inconsistent. They could have done better, but let us accept it. A greater problem is that they are making a model based on how agriculture should use incident solar radiation, which is visible light. Yes, there is a connection between sunshine and temperature, but plants are highly dependent on the actual sunshine for photosynthesis, not temperature alone. This is a complex biological relationship the authors fail to take into account.
They mention that there are other factors that affect vineyards, as quoted above. A critical one is the night-day temperature variation. The model punishes vineyards for having a large/larger diurnal variation, including an assumption that higher altitude vineyards are farther from water, must have larger day-night temperature variations, and therefore suffer more from "global warming." I'm just not sure why they do that, as I've learned that wine grapes are better with large diurnal cycles, and also wines made from mountainside vineyards are among the most prized/collected wines in the world. This is actually going to be important too, because in global warming scenarios, the diurnal variation is often affected more than the actual maximum temperature. That is because the effect is in the infrared, not the visible light, so after the sun goes down the surface can't cool as efficiently because the atmosphere is warmed. That means minimum temperatures get higher, and they change more than daytime maximum temperatures, which reduces the diurnal variation. The authors ignore this fact.
The major deficiency of the paper is the assumption that plants will thrive under warmer conditions based on energy input arguments. While it is true that there will be a larger energy flux into the surface under global warming, this energy will be in the infrared, which does not necessarily benefit plants. In marginal growing areas where occasional freezing conditions damage crops during the growing season, increases in daily minimum temperatures might reduce the occurence of these freezes, but the increased energy flux will not increase photosynthetic activity. Vineyards will not benefit directly from global warming by absorbing more radiant energy.
A more appropriate hypothesis to test is whether the changes in growing season length might affect vineyards. Since "spring" will start earlier, plants might respond by starting their growth cycle earlier. Autumn-like temperatures will come slightly later, so the growing season my be extended further. In the case of vineyards, this might allow grapes to ripen more, which increases the sugar content of the berries and increases the alcohol content of the wine. More importantly, different grape varieties might benefit by a longer growing season, so areas that only grow grapes with a short "hang time" now might be able to expand to other longer "hang time" varieties. Regions that don't have a long enough growing season to properly ripen grapes might get a boost and obtain growing seasons long enough to produce them (thinking especially of regions of Oregon and Washington).
Existing vineyards are unlikely to be affected by global warming, especially in established regions with strong control on growing practices (e.g., Bordeaux, Burgundy). It is possible that the nature of the wine will change, as warmer days and nights might change the sugar levels of grapes, or various other aspects of the fruit. It is also possible that changes in rainfall patterns will significantly alter the agricultural practices, and the possibility of severe droughts and floods putting more vintages in jeopardy in the future is a distinct threat.