Climate Viticulture Newsletter - 2020 December

Average temperatures last month were 2°F to 6°F warmer than the 1981-2010 normal for almost all of the region (orange areas on map). For comparison, temperatures in November 2019 were within 2°F of normal for much of the region, except for southern Arizona where temperatures were 2°F to 4°F above normal.

Monthly precipitation totals last month were less than 50% of normal for almost all of the region (red areas on map). In stark contrast, rain and snow amounts last year were above 200% of normal for many areas, as several storms passed through the region.

For many parts of the region like north-central and southeastern Arizona, fall 2020 ranked as one of the warmest and driest on record. After a record-setting hot and dry summer, conditions this autumn wouldn’t let us shake water stress and soil salinity issues – due to relatively high evaporation – from the fronts of our minds. Both can affect fruit composition, with effects of the latter dependent on choices in rootstock and scion, as well as irrigation water quality.

There is a slight increase in chances for above-normal temperatures across all of Arizona and New Mexico (orange area on map). As a comparison, temperatures in December last year were within 2°F of normal for much of the region, except for eastern New Mexico where temperatures were 2°F to 6°F above normal.

A slight increase in chances for below-normal precipitation exists for the region (tan area on map). Monthly precipitation totals in December 2019 were above the 1981-2010 normal for most of Arizona and the northern tier of New Mexico, while less than 50% of normal for the rest of the Land of Enchantment.

With an elevated potential for warm and dry conditions to continue, due in part to the ongoing La Niña event, soil moisture in the vineyard may need closer attention this winter relative to the past two. Adequate levels will help guard against damage from cold temperatures as well as delayed spring growth, which manifests as uneven bud break, stunted growth, reduced flower clusters, and other issues.

Based on your expressed interest, we’re beginning a new topical series this month that provides information on climate change and viticulture for the region. Up first is growing season temperature, the average temperature between April 1 and October 31. We looked last month at this simple measure, which is used to compare different winegrape growing regions and as a first guess of which grape varieties might do well in a new area, over the past four decades. Now, we’ll look at it over the coming three.

Once again using the Sonoita, Willcox, and proposed Verde Valley AVAs as examples, we’ve replotted from the figure last month the historical observations and 1981-2010 normals (thick dark gray and gray lines, respectively). These data overlay the range of growing season temperatures as estimated for these areas by 32 models used to project plausible future climatic conditions. Observations for all three AVAs lie almost entirely within the ‘modeled history’ (gray shading), which indicates that the models do well in representing past growing season temperatures. Observations continue to occur within the range of ‘modeled future scenarios’ through 2020 (yellow, orange, and red shading¹). We can expect growing season temperatures to continue over the next 30 years along the evident warming trend, such that values close to the 1981-2010 normals become unlikely during the 2030s and 2040s while values similar to and warmer than this past year become more likely.

For vineyard decisions that are long term in nature², such as site selection, row orientation and trellis design, rootstock and scion varieties, and irrigation capacity, climate information – both historical and projected future – plays a particularly important role. Knowing how climatic conditions are likely to change relative to past years can help one determine if and when changes from past long-term decisions – whether from one’s own vineyard or others in the same area that are serving as examples – are needed. This look at growing season temperature gets us started in developing that knowledge, something we’ll continue to do in coming newsletter issues in the context of some of the myriad effects that temperature has on vine growth and fruit quality and composition.

¹ ‘Modeled future scenarios’ are based on two different scenarios of future greenhouse-gas emissions. RCP4.5 (yellow shading) corresponds to a slower increase of global carbon emissions to 2050, relative to RCP8.5 (red shading). Global climate models use these scenarios to drive plausible trajectories of future climatic conditions. Orange shading represents the overlap of the ranges of growing season temperature based on the scenarios. RCP is an abbreviation for Representative Concentration Pathway.

² For a diagram of vineyard management considerations listed by long-term, between-season, and within-season timeframes, see Figure 12 by Webb and colleagues.