A Dionysian indicator of climate change

By Rachel Berkowitz

“The sun, with all those planets revolving around it and dependent on it, can still ripen a bunch of grapes as if it had nothing else in the universe to do,” said Renaissance astronomer Galileo Galilei in his Dialogue Concerning the Two Chief World Systems.

It’s the same old Sun, but we humans are changing how it goes about ripening a bunch of grapes.

Wine grapes in Australia have been ripening early in recent years. That trend often has undesirable effects on wine quality, which motivated researchers based in Aspendale to investigate the particular attributes that have been causing grape ripening to advance by about eight days per decade since 1985.

Viticulturists match Dionysus’s favorite fruit with unique combinations of climate and soil to produce specific qualities in wines. Records of wine-grape vintages through time are “an excellent showcase to analyse the fingerprints of climatic changes,” according to a recent paper in Nature Climate Change.

Time series of wine-grape maturity and climate trends suggest that two variables—warming and declines in soil water content—are driving forces behind that maturity, indicated by sugar concentration levels in the grapes. And higher average temperatures and recent drying trends in Australia have been attributed to anthropogenic influence such as greenhouse gas emissions.

A statistical model built on the basis of interannual fluctuations separately assessed temperature, rainfall, incident solar radiation, and soil wetness for their influence on how long grapes took to mature at 10 sites across southern Australia. The indices were assessed individually and then in combination and showed that variation in maturity date was best represented by growing-season average temperatures, lower-layer soil moisture, and crop yield.

It’s not surprising that higher growing-season average temperatures lead to earlier maturation, but soil moisture content is a new idea. Changes to moisture content may affect the temperature of the soil profile. Some greenhouse studies have associated warmer soils with earlier ripening. Additionally, drier soils are associated with higher levels of abscisic acid, a plant hormone in vines. The hormone is related to the timing of grapevine ripening.

“We have made suggestions that explain our results but realize that more detailed physiological work will need to be done to confirm some of our theories,” says the University of Melbourne’s Leanne Webb, the lead author on the Nature Climate Change paper.

In addition to the climatic effects, variation in crop yield affects maturity date. The grower targets a certain yield by cutting away bunches at the earlier stages of growth. Sugar accumulation is a function of photosynthetic capacity, and distribution of sugars is a function of the volume of grapes. Therefore, a lower crop yield will mature faster.

Observed shorter-term shifts in maturation date agree with shifts calculated by the model. But longer-term shifts in maturation date did not agree for many sites.

“We discovered that growers had introduced changes to management that coincided with periods we could not explain with our modeling,” says Webb. New agricultural practices, such as using different fertilizers to improve vine health, would have inadvertently affected the ripening rate.

Grapes are a perennial crop. Penny Whetton, senior principal research scientist with the Climate Projections Science Team at CSIRO (Commonwealth Scientific and Industrial Research Organisation) Marine and Atmospheric Research, notes that growers need to make a decision about what to plant in a given year while considering what will be the ideal vine to grow in 20 years’ time. That also requires considering all the climate effects that could be changing over the lifetime of the vine.

Earlier maturation is undesirable, and growers may intervene by managing soil moisture, crop yield, or vine response. Identifying the factors that drive shifts in phenological phases will make it possible to improve the adaptive capacity of vineyards. The methodology may be applied to other biological systems that are showing trends in response to human-driven climate change.

If this Dionysian effect of anthropogenic climate change isn’t enough to scare society into the realization that something needs to change, then perhaps nothing is.