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A perspective on climate change II
Implications for global and South African viticulture
John Wooldridge
Summary
Introduction
As outlined by in the previous article in this series, world climates are currently changing. Temperatures are slowly increasing everywhere, except for Antarctica. In the Western Cape, rainfall is likely to become less reliable, leading to a probable increase in aridity, notably in the lowlands. Of the many environmental factors that potentially affect grapevine performance, and the characteristics of must and wine, climate is probably the most important, mainly because it affects grapevine physiology (Hunter & Bonnardot, 2004). The aim of this article is, therefore, to speculate on the effects of the climate changes discussed by Wooldridge on global and South African viticulture.
Effects of climate on grapevines and wine
Since most biochemical reactions are temperature sensitive, any change in the temperature regime experienced in a defined viticultural area (a ’terroir’, according to Vaudour & Shaw, 2005) is likely to affect the grapevine, the juice and the wine. Cultural practices such as canopy management may help to maintain quality (Hunter et al., 2004; Marais, 2005a,b,c). In other cases, chiefly in areas in which temperatures are already close to the upper temperature limits for the existing wine grape varieties, such forms of mitigation (IPCC, 2007b) are likely to be of limited effectiveness necessitating more drastic actions, such as the substitution of tolerant for sensitive varieties. Such major changes are nevertheless likely to alter the character of the products of a region, possibly eliciting negative responses from established consumers.
Within limits, warming promotes earlier ripening, an increased rate of vegetative growth in grapevines, and a faster progression through the phenological stages. However, a survey of the worlds’ viticultural areas indicates that, provided the climate is sufficiently warm to ripen any given grape variety, wine quality will decrease with increasing warmth and length of summer (Jackson & Lombard, 1993). This stems from the fact that such physiological processes as photosynthesis, sugar and potassium accumulation, and organic acid formation proceed at near peak efficiencies at day temperatures between 20°C and 30°C (Hunter & Bonnardot, 2004), although some processes have different ranges. A consequence is that the effects of both supra and of suboptimal temperatures will be negative, as in the case of anthocyanin synthesis where colour is impaired at temperatures below and above an optimum range of 17 to 26°C. In Pinotage, colour was darker, and total antioxidant capacity (TAC) higher, in wine from trellised vines from cool, than from warm regions (De Beer et al., 2006). These researchers also found that Pinotage from bush vines was darker, with slightly higher TAC, than from trellised vines, perhaps because the bush canopies were denser and more shaded than the trellised canopies. Wine quality in white varieties, such as Sauvignon blanc, is also significantly affected by canopy density and management (Marais, 2005a). Flavour and aroma components are temperature sensitive. Conradie & Bonnardot (2004) showed that, in Sauvignon blanc wines from the Breede River Valley, aroma intensity was higher from cool than from the warm localities whereas, in Cabernet Sauvignon from the cool vineyards possessed a marked grass character, although berry character dominated under warm conditions. In Cabernet Sauvignon from the warm vineyards, both berry character and aroma intensity were lower in sandy, relative to clay soils. In the Stellenbosch area, Sauvignon blanc grapes from warm vineyards were harvested 14 days earlier than from cool vineyards (Conradie & Olivier, 2004). Whereas the warm vineyard wines were characterised by tropical fruit character, irrespective of soil conditions, the wines from the cooler vineyards differed according to the water holding capacity of the soil, the wetter soils producing wines with fresh vegetative character, whilst the drier soils produced wines in which cooked vegetative character was dominant. Temperature and soil/water effects are therefore linked. Water stress, a common accompaniment to heat, low humidity and wind in Cape vineyards, tends to promote early ripening, but reduces yield and berry weight, and may suppress malic acid formation (Jackson & Lombard, 1993). Aspects of grape berry ripening and juice composition that are affected by temperature and stress include colour, acidity, sugar content, sugar to acid ratio and the type and abundance of flavour and aroma compounds. In Sauvignon blanc, Marais (2005a) identified two dominant wine styles: a cool climate style, characterised by a green pepper/asparagus, and a fruity/tropical warm climate style. The most likely effect of warming is that, although sugar content will increase rapidly, acidity may be lost through respiration whilst the winemaker is waiting for flavours to develop. Wines produced from such grapes will tend to be unbalanced; high in alcohol, with too little acidity to produce the desired freshness (Jones & Davis, 2000; Jones, 2004). Wines produced in Western Europe during 2003, when temperatures over a three month period were 3 to 5°C higher than normal were, indeed, high in alcohol, low in acidity and highly variable in quality (Seguin & De Cortazar, 2005). Soluble solids also increase with temperature (Jackson & Lombard, 1993). Temperature regimes in which warm (20 to 25°C) days contrast with cool (10 to 15°C) nights tend to be more beneficial than warm day and warm night temperature regimes, mainly because acidity is retained whilst pH declines (Jackson & Lombard, 1993; Hunter & Bonnardot, 2002, 2004). The trend over the past 50 years has nevertheless been for cold days and cold nights to become less frequent while hot days and nights have become more frequent, as have heat waves. Since day and night temperatures have increased by the same amount, the diurnal temperature range has remained the same, although there is variability between regions (IPCC, 2007a). High temperatures have a variety of negative effects. Both photosynthesis and rate of dry matter accumulation in grapevine decrease with increasing temperature and at ? 35°C are much reduced, which is consistent with findings that dry matter accumulation correlates with average rate of photosynthesis. In Chenin blanc and Chardonnay, one of the first effects of heat stress is reduced stomatal conductance (Sep?lveda & Kliewer, 1986). Reduced photosynthesis at high temperatures may nevertheless be less a consequence of stomatal closure than of enzymatic malfunction (Ferrini et al., 1995).
Defining precisely what constitutes an ideal vineyard climate is the subject of ongoing research. Hunter & Bonnardot (2002, 2004) differentiated between viticultural regions in terms of their relative abilities to promote physiological functioning (photosynthesis) on the basis of climate profiles compiled from pre and post véraison hourly mean temperature, relative humidity and wind speed data. Hours during which these parameters fell within optimal range limits for photosynthesis (between 09:00 and 16:00 (Greenwich Mean Time + 2); temperature range 20 to 25°C; relativity humidity 60 to 70%; wind speeds ? 4 m/s) were compared with hours above or below these limits. Marked variation in the number of hours available for optimal physiological functioning was observed between regions and between areas within regions. Hunter & Bonnardot (2002, 2004) hold that mean climatic data alone are insufficient to understand climatic impacts on grapevine physiology, and provide little information about conditions within the canopy and between the vine rows. In Chenin blanc, light intensity within the canopy appears to be a critical quality determining factor, necessitating careful management to maximize indirect radiation around the ripening berries (Marais, 2005b).
Implications of climate change for global viticulture
The effects of increased temperatures are likely to be positive in some viticultural areas. Jones et al. (2005) evaluated international vintage ratings to determine the effects of climate on vintage rating over time and showed that the warming trend between 1950 and 1999 was linked with an increase in vintage quality and a decline in year-to-year variation. This trend could not be fully explained by the improvements in viticultural methods that occurred during the same period. The cooler climatic regions benefited most from the warming, some improving by 13 points on a 100 point scale per 1°C rise in temperature (Jones, 2004). The impacts of climate change were nevertheless not uniform across all regions and varieties, with poleward locations potentially becoming more suitable for grape growing and wine production, as in the case of England where growing conditions in the past decade have been likened to those of the 13th century (Jones, 2004). The early 13th century was the latter part of a period of anomalously warm weather (possibly associated with a peak in solar activity) that preceded the Little Ice Age. In Bordeaux, France, Jones & Davis (2000) showed that incoming solar radiation levels during the budburst interval were positively linked with vintage quality, probably by promoting higher rates of photosynthesis. Further, an increase in the number of days with maximum temperatures over 30°C during floraison and véraison promoted early growth and full maturation.
Other viticultural areas will not benefit from global warming. Deleterious effects will be most marked in regions that are already at, or near, the optimum temperature range for the cultivars grown, and for the wine styles produced. Wine production will become more difficult under these changed circumstances, and may even prove unviable. White et al. (2006) consider that periods of extreme heat, amongst other factors, will result in premium wine production in the United States becoming largely restricted to parts of the west coast, the Northwest and the Northeast, with a potential 81% decrease in premium wine grape production by the end of the 21st century. In Australia, temperatures in most viticultural areas are predicted to increase by between 0.3 and 1.7°C by 2030 and 0.8 to 5.2°C by 2070 (Webb et al., 2005). Although these increases, coupled with higher atmospheric CO2 concentrations, could conceivably enable larger crops to be ripened, the increases will have negative effects on grape production and quality within regions. Costs, in terms of diminished quality, will range from zero to 25% by 2030, and there will be a progressive southward shift in suitability within each region, value decreasing in the inland, and increasing in the more southerly regions.
The longevity of vineyards makes them particularly sensitive to climate change, and problems could arise if heat intolerant varieties are planted in localities which, though cool at present, grow warmer within the production life of the vineyard. Wine grape varieties that are likely to be negatively affected by warming include Pinot Noir and Riesling, which perform well in coastal valleys and in such cool areas as the slopes of Burgundy in France and the Rhine River Valley in Germany. In South Africa, aroma intensity and overall quality are higher in cool, relative to warm localities in Sauvignon blanc (Conradie & Bonnardot, 2004), implying that this variety must also be regarded as sensitive to warming. More emphatically, Marais (2005a) maintains that Sauvignon blanc should only be grown in selected cool localities. On the other hand, some varieties, such as Cabernet Sauvignon, and Zinfandel, seem to tolerate a range of climatic conditions.
On a wider agricultural basis, the potential for food production at mid to high latitudes should increase with increasing local average temperature over a range of 1 to 3°C, then decrease. Even small increases in temperature (1 to 2°C) may reduce production at low latitudes and in seasonally dry areas (IPCC, 2007b).
Implications of climate change and viticulture in South Africa
South Africa will not be exempt from the effects of climate change. However, whereas changes in surface temperature over the period 1970 to 2004 ranged from 0.2 to 1.0°C over most of South Africa, the corresponding increase for much of Spain and France was 1.0 to 2.0°C (IPCC, 2007b). If projections that Antarctica will remain too cold to permit much melting of the continental ice sheet are accurate (IPCC, 2007a), it seems likely that the cold circum-Antarctic ocean circulation and climate will remain more stable than the climates of the continental land masses of mid to high northern latitudes. Recent reports of warming of the southern ocean surface water, and of destabilization and thinning of ice sheets leading to increased rates of glacial discharge from the Antarctic continent have been refuted by IPCC (2007a) which states that, although the Antarctic sea ice varies in extent between seasons and localities, there is no statistically significant trend. This is consistent with the absence of a warming trend in average atmospheric temperatures across the region. In contrast, average annual sea ice coverage in the Arctic has shrunk by 2.7% (2.2 to 3.3%) per decade since 1978 (IPCC, 2007a).
Much will depend on the effect of warming on the strength and latitude range of the southern Hadley convection cell which, in summer, brings dry, descending, deflection- (Coriolis) driven air masses to the Cape in the form of the South Easter, and which generates the South Atlantic and Indian Ocean high pressure cells. Strengthening of the Hadley Cells could reduce the extent to which the South Hadley Cell moves north in winter. This would lead to an increased percentage of the storm front-bearing north westerly winds of the south convective Ferrel Cell (which parallel the southern edge of the Hadley Cell), following a more southerly path in winter, resulting in an increased percentage of cold fronts missing the Western Cape altogether. Patterns of evaporation and precipitation over the oceans are already changing, as is evidenced by the fact that the ocean water at mid to high latitudes is becoming fresher, whilst that at low latitudes is becoming more saline (IPCC, 2007a). By mid century, water availability in dry regions at mid latitudes is projected (with ‘high confidence’, ie, about an 8 out of 10 chance) to decrease by 10 to 30% (IPCC, 2007b)
According to Jones (2004), projected temperature changes for the northern hemisphere are greater than for the southern hemisphere. Of the world’s wine grape growing regions, South Africa will be the least affected by warming (Portugal will be the most seriously affected). This is reassuring in view of the fact that, because viticulture is already taking place close to South Africa’s southern coastline, the option of relocating further south, as in parts of Australia, is limited. Because of the rugged topography of the Western Cape, there is the possibility of moving to higher land where, because air temperature decreases by almost 1°C per 100 m increase in altitude, cooler conditions may be expected as, in some of the higher locations, may an increased incidence of orographic cloud. The relationship between altitude and height is nevertheless affected by a range of physical parameters, such as aspect, slope and the varying effects of topography on slope (Wooldridge & Beukes, 2005). Based on data from the Coastal Region of the Western Cape, Myburgh (2005a) found that February mean temperatures declined at the rate of 0.5°C per 100 m increase in altitude, but increased by around 0.6°C for every 10 km increase in distance from the ocean. The strategy of moving to higher ground suffers from the obvious disadvantages that the higher the land, the less there is of it, the greater the difficulty of access, working, and water supply, and the greater the risk of erosion and, perhaps, fire. Since the viticultural areas of the Cape are located in fairly close proximity to the ocean with its locally pronounced cooling effect, temperatures will continue to be more moderate than those experienced in intra-continental settings, as in Europe and on the North American continent. Vineyards along the Orange River near Upington, where summer temperatures frequently exceed 40°C, are nevertheless likely to be severely affected by warming
Offsetting the relative coolness induced by the closeness of the Western Cape to the ocean is that the vineyards of the Western Cape are situated 10 or more degrees of latitude closer to the equator than most of the European viticultural areas. Because light intensity increases with decreasing latitude, they therefore receive appreciably higher levels of incoming solar radiation, even though summer day length is shorter (Jackson & Lombard, 1993; Wooldridge & Beukes, 2005). The range of climatic variables between, and even within individual wine producing regions and areas may nevertheless be wide (Conradie et al., 2002; Hunter & Bonnardot, 2002; 2004). This is especially the case in the Western and Southern Cape where there are three distinct gradients (Midgley et al., 2005). These are: an increase in aridity from south to north, a west to east transition from winter to summer rainfall, and a tendency for the higher mountain ranges to receive more rain in summer than the surrounding lowlands. A further variable is that the rugged topography conferred by the fold mountain ranges, and by the contrasting, erosion-resistant sandstone and soft, easily weathered shale formations has led to the formation of a variety of habitats, each with its own mesoclimate. These habitats range from low to high altitude, from north to south facing, from cool to hot, and from arid to relatively moist. They also differ in terms of distance from the coast, and exposure to coastal and other winds. The soils also vary, from sandy or gravelly with low water holding capacities, to clay soils with high water holding capacities. In some areas, such as the Breede River Valley (Conradie & Bonnardot, 2004) and Stellenbosch (Conradie & Olivier, 2004) wine style is significantly affected by soil form, as well as by climate.
The Elgin, Grabouw and Villiersdorp areas, amongst others, may soon become too warm for effective commercial apple production (Midgley et al., 2005). In these areas it is likely that deciduous fruit having high chilling requirements will progressively give way to stone fruit, fruit of Mediterranean origin or wine grapes. Viticulture is also likely to expand in the cooler areas inland of Cape Agulhas, wherever suitable land is available without impinging on the fynbos. Only 149 000 ha of land in South Africa are suitable for the production of high quality wines under present conditions (Knight, 2006). Since 101 607 ha were occupied by wine grapes in 2005 (SAWIS, 2006), there would seem to be little room for expansion. This situation will worsen if sections of the land identified by Knight (2006), which mainly occupies a narrow strip along the Western Cape coast and between Natures’ Valley and East London, are rendered unusable for viticulture by construction or by erosion. Coastal erosion could become a problem in some areas towards the end of the 21st century (IPCC, 2007b).
The climatic variable that will probably have the greatest influence over the future success of viticulture in the Western Cape in a warming world is the amount, intra-seasonal timing, geographic distribution and season-to-season variation in rainfall. In lowland areas, where water is already limiting, any further reduction may result in too little water being available to the vine to offset heat stress induced by stomatal closure, and to prevent desiccation of the berries. Stomatal closure is likely to offset any gains that may result from the increased CO2 content of the atmosphere (CO2 fertilization). Increases in summer rainfall, as predicted by Midgley et al. (2005) could also have negative effects on quality, notably through berry enlargement, elevated juice pH and acid content, and reduced anthocyanins due to shading induced by prolonged growth (Jackson & Lombard, 1993; Jones & Davis, 2000). In practice, the Western Cape is already experiencing water shortages, exacerbated by increasing demands from burgeoning urban populations. In view of the fact that irrigation currently uses 60% of South Africa’s available water (Midgley et al., 2005), these demands are likely to be acceded to. Seasons in which the region’s largest storage dams fill to capacity are infrequent, and may become even less frequent in future. The new Berg River dam will ease short term supply problems but by 2015 demand is again expected to exceed supply. Another factor to take into account in low rainfall areas is the progressive tendency for salts to accumulate in the root zone, particularly where evaporation rates are high, water quality marginal and drainage poor.
Possibly offsetting the negative effects of water shortage is the refined state in South Africa of the science and practice of maximizing wine grape yield and quality per unit of irrigation water consumed. Myburgh (2005b), for example, showed that a single supplementary irrigation event when the berries were pea sized was sufficient to sustain vegetative growth and yield in Sauvignon blanc and Chenin blanc in the Stellenbosch district, provided that the soils were well prepared. Indeed, further irrigation, during berry ripening, tended to reduce wine quality in both cultivars (Myburgh, 2006). Under semi-arid conditions irrigation may be used to influence berry size (Myburgh, 2003). Irrigation techniques that suppressed berry size in Chenin blanc resulted in wine with more intense fruitiness and less intense bottle ageing character than grapes from treatments which increased grape size Marais (2005c). Small Chenin blanc berries seem to contain higher concentrations of flavourants and their precursors than large berries (Marais, 2005b).
A cause for cautious optimism is legislation that permits the removal of alcohol from wine. Further, because consumer tastes change over time, it is probable that tastes will adapt in step with warming-induced changes in wine character and style (Schulz, in Joubert, 2006), provided that the changes continue to be gradual. It is reassuring that, during the late Cenozoic interglacial periods, with their rising or high sea levels and elevated atmospheric CO2 concentrations, conditions in the Western Cape appear to have been equable, with moderate to warm temperatures, reasonable rainfall and an extensive vegetation cover. In contrast, conditions during glacial periods were far less pleasant. Falling sea levels were accompanied by relatively low temperatures, reduced rainfall, incision of watercourses, exposure of the continental shelves, increased aridity, the spread of the Kalahari desert, and abundant wind-driven dust (Hendey, 1983). Of these alternatives, a limited amount of warming is preferable to a reversion to glacial conditions. Taking optimism to its limit, it is conceivable, though unlikely, that the premises on which the warming predictions are based are erroneous, and that the current warming trend will prove to be nothing more than a late stage of recovery from the Little Ice Age. Only time will tell.
From a demarcation viewpoint it is pertinent that, in many old-established European viticultural areas, zoning is strongly influenced by historical factors and by tradition, and may even be based on a single qualitative variable (Vaudour & Shaw, 2005). If warming renders the designated geographic area less able to produce the traditional product, a new area will need to be designated, established and marketed. However, because history and tradition will have no predictive, or market value in the warmer world, it will be necessary to develop a new set of criteria before rezoning can take place. In contrast, zoning in South Africa is based on records of observations and measurements, mostly obtained from research programs of which some span decades. The ongoing nature of this process means that any changes in demarcation or cultural practice that further climate change may make desirable will be routine in terms of research and development.
On balance, given the diversity of potential viticultural sites, efficient irrigation, an excellent knowledge base concerning viticulture and soils and a field research based system of demarcation, fine wines are likely to be produced in South Africa for a considerable time to come. If, perhaps, on a reduced scale. Expansion into new areas, and the development of new methods, will certainly lead to a considerably increased diversity of wine styles. Carbon efficiency in wine production may soon become a plus point in marketing, as is the environmentally-friendly treatment of winery wastes. As ever, the key to success in changing times will be timely adaptation, based on sound information. (Low adaptive capacity was identified by IPCC (2007b) one of the main reasons why Africa is amongst the most vulnerable continents to climate change). Relative to viticulture, the future of the wider deciduous fruit industry in the Cape is less certain (Midgley et al., 2005). Fruit trees are generally less resilient than grapevines. Of greater importance, though, is the reality that temperatures across much of the Western Cape are already too high to enable sensitive fruit varieties to obtain sufficient winter chill, and to reliably break dormancy, without chemical stimulation. Future prosperity, particularly for apple growers, growers will therefore hinge on the ability of plant breeders to produce cultivars which have ultra-low winter chill requirements (Labuschagné et al., 2000), are resistant to sunburn. The trees must also be able to produce economic yields of high quality fruit on limited amounts of water of less than optimal quality due to increased salinity.
For further information contact John Wooldridge at wooldridgej@arc.agric.za.
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