Agriculture Reference
In-Depth Information
to the present, the trend in global mean surface temperature over this period has
been approximately flat (see figure 6.9A).
These uncertainties in climate change trends are more marked at a regional
scale, which is the scale of direct interest to winegrowers. In particular, they are
concerned about the seasonality of any future temperature changes (e.g., summer
versus winter), changes in the diurnal temperature range, the number of frosts
and when they occur, the incidence of extreme weather events, and the amount
and distribution (temporal and spatial) of rainfall. In a comprehensive review
of wine geography, Jones et al. (2012) reveal the marked regional variations in
temperature change during the growing season (the period of greatest change)
that occurred between 1950 and 1999. For example, the observed warming in
the Rhine Valley and Bordeaux regions was 0.7ºC and 1.8ºC, respectively, while
in wine regions of northeast Spain it ranged from 1.0 o C to 2.2 o C. In California,
Oregon, and Washington, the mean growing season temperature (GST) increased
by 0.9 o C, driven mostly by higher night temperatures. Given that high-quality
wine production is limited to a GST in the range 13 o to 21 o C, some regions are
now at or approaching the upper limit for their traditional varieties, as shown in
figure 2.3 in chapter 2. Nevertheless, Jones et al. (2005) found that vintage quality
ratings for the best wine regions increased significantly between 1950 and 1999
and that interannual variation decreased. These regional differences in past GST
changes are reflected in model projections of future warming, which means that
if the projections become reality, those varieties with a narrow optimum tempera-
ture range are less likely to flourish in their present regions in the future.
Global circulation or “climate” models are least reliable for projecting trends
in the amounts and distribution of rainfall at regional scales. For example, for the
U.S. West Coast, Lobell et al. (2006) found for a range of climate models, the
projected changes in annual precipitation out to 2050 were −40 to +40% relative
to the period 1960 to 1990. Simulation modeling of soil water changes can also
be misleading if the spatial scale is inappropriate (White, 2013). Generalizing,
Jones et al. (2012) concluded that the observed warming over the last 50 years has
been largely beneficial for viticulture in many regions through longer and warmer
growing seasons with less risk of frost. The unanswered question is—will the cur-
rent “pause” in warming (see figure 6.9) continue, or will mean surface temper-
atures rise or fall over the next 10 years and more? The answer will determine
whether further adaptive measures should be taken in the wine industry if the
warming trend resumes, with the extension of wine-producing areas to higher lati-
tudes and higher elevations. Other important considerations are the consequences
for vine growth and balance of (a) further increases in atmospheric CO 2 concen-
trations and (b) increased competition for water, especially in the inland regions
of California, Australia, and Spain, where irrigation is essential for viticulture to
be viable. The latter consideration underlines the importance of water conserva-
tion measures such as practicing efficient irrigation, reducing soil evaporation,
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