An interesting study; I pulled out some excerpts below for those who don’t care to read the whole paper or enjoy econometric models (bold type was added by me).
http://vinecon.ucdavis.edu/spaw2/uploads/files/cwe1001.pdf
Excerpts from section on hypotheses tested:
We propose two main hypotheses about the sources of the rise in sugar content of wine grapes in California. In each case the increase in sugar content of grapes is seen as an unsought consequence of other factors.
The first hypothesis is that exogenous changes in the weather, with generally rising average temperatures, implied increases in sugar content of grapes even without any changes in management of the vineyard by growers. Profit-maximizing responses of growers and wineries to such changes could mitigate the implications for sugar content of grapes but should not be expected entirely to eliminate their impact.
The second hypothesis is that the trend was caused by a market demand (perceived or real) for wines with ripe flavors and lower tannin levels, attributes associated with grapes that are picked at higher degrees Brix. Under this hypothesis, profit-maximizing responses of wineries and growers to changes in demand for quality characteristics of wine required changes in viticultural practices that resulted in unsought increases in sugar content of grapes. For instance, extending the ―hang time‖ and picking the grapes later than they would do otherwise is likely to result in higher sugar content if only because the grapes are more dehydrated. To some extent vignerons can independently manage the sugar content of grapes and other quality characteristics, but an increase in intensity and ripeness of fruit is likely to come to some extent at the expense of a reduction in tons per hectare and an increase in degrees Brix.
One contrast between the two theories is that warming temperatures would tend to imply increases in Brix correlated with earlier harvest dates, whereas later harvest dates would be correlated with demand-driven increases in Brix. There may be corollary differences in implications for yield (if climate change implies higher yields while delayed harvest implies lower yields).
Excerpts from conclusions section
Previous studies have shown some increase in measures of temperature in California over the longer term, which may have contributed to changes in winegrape characteristics including sugar content at harvest. We used a measure of heat during the growing season for wine grapes to attempt to account for any direct effect of climate change. This measure itself exhibits large year-to-year swings making it difficult to discern clear trends in it. The variable contributed statistically significantly to the models, and showed that an increase in heat during the growing season would contribute to an increase in the sugar content of grapes. However, the heat index did not exhibit any statistically significant growth during the growing season and, in any event, its coefficient was small. Hence, this variable did not account for much of the growth of the average sugar content of grapes, compared with the other variables in the model.
Sugar content of grapes at harvest was relatively high for red varieties and premium varieties, and for grapes from ultra-premium and premium regions. The same categories tended to show evidence of faster growth rates in sugar content as well, but here the story is a little mixed, depending on the details of the model.
In all of the models, however, the analysis shows a higher propensity for growth in sugar content for premium varieties, compared with non-premium varieties, even though premium varieties had higher sugar content to begin with. This feature and the patterns of the level of sugar content among regions and varieties could be consistent with a --Parker effect-- where higher sugar content is an unintended consequence of wineries responding to market demand and seeking riper flavoured more intense wines through longer hang times.
