Using several simulation models, it is generally agreed that global mean surface temperature could rise between 1–4,5 °C depending on the future development of industrial emissions with a best estimate of a near 1,8–2,5 °C warming by the middle of the next century (Carter et al., 1991, Intergovernmental panels on climate change (IPCC) 1992, 1994, 1998, 2001). An increase in temperature of this magnitude in Europe would have profound effects on Viticulture.

In fact current temperature development in the northern hemisphere clearly shows a gradual warming trend in all seasons (fig. 1), especially over the last twenty years (Jones et al., 2001). This warming has been less pronounced than that in the southern hemisphere over the same time period. However, global circulation models (GCM) currently predict a more rapid warming in the northern hemisphere over the next 50 years (Evans, 1996). This would considerably change the margins of suitability for grape growing shifting the northern boundary for Viticulture under this scenario on the order of 10–30 km per decade up to 2020 with a predicted doubling of this rate between 2020 and 2050 (Kenny and Harrison, 1993). This would also profoundly change the distribution of suitable varieties within Europe. Calculations using the Huglin climate index classification (Huglin, 1986) for varietal suitability in Geisenheim, Germany (50 ° latitude North) shows that Riesling, Pinot gris and Pinot noir are suitable cultivars for this location (fig. 2) based on the average climatic conditions over the past 30 years. If suitability would be based on the average temperature development over the next 50 years, cultivars such as Merlot or Cabernet franc become a possible choice (fig. 2). The warming trend can also be demonstrated through the large decrease in the frequency of autumn (October) frost events over the past 100 years (Hoppmann and Hüster, 1988). Indirect estimates of the growth of terrestrial vegetation by use of satellite data from measurements of the normalised difference vegetation index (NDVI) have recently confirmed that plant growth and growing season length is significantly increasing especially in the northern high latitudes (> 45 °N) (Myneni et al., 1997).

Concurrent to global warming are predictions on altered (annual, seasonal) precipitation patterns. For central Europe higher incidence of strong precipitation events in winter, higher frequency of extreme temperatures and less precipitation in summer are among the expected effects (Schönwiese and Rapp, 1997). Significant changes have already occurred over all continents and are predicted to continue based on GCMs for a doubling in CO₂ concentration (Hulme et al., 1992). Even if actual precipitation rates and/or their distribution would be unaffected, water evaporation rates will increase due to higher surface temperatures of oceans and land masses. For land masses, changes in evaporation rates may reduce soil moisture on the long term. Some earlier predictions estimated 20-30% reduction in soil moisture in northern Europe, 30 to 50% in western Europe, 20 to 40% in central Europe, 20 to 30% for most of the Mediterranean region and even more for the Iberian peninsula (Stigliani and Salomons, 1992) but these estimates have been doubted recently based on past and current climatic trends (Schönwiese and Rapp, 1997). The impact on Viticulture and the socio-economic consequences would nevertheless be dramatic, especially in southern Europe where water is a scarce resource and irrigation is often not possible. Predictions based on past climatic trends between 1960 and 1990 (not models) show that coastal regions and western Europe in general will receive more precipitation in winter (fig. 3) but less in summer (fig. 4), but these scenarios are not uniform over Europe.

Shifts in precipitation patterns may necessitate introduction of cover crops over winter in order to minimise soil erosion and to maximise water and nutrient storage. Favis-Mortlock (1994)  has shown that an increase in 8-15% rainfall in winter will increase erosion by 27 to 35%. On the other hand, higher frequency of extreme temperatures in summer will automatically lead to increased evapo-transpiration and, coupled with reduced precipitation in summer rainfall areas, may render full or partial use of cover crops impossible in countries such as Germany, Switzerland, or northern Italy where this is currently common practice. In recent years, off-flavour problems in white wine have been linked to competition problems for water and nitrogen between cover crop and grapevines in dry years (Rapp et al., 1993). However, soil erosion and nitrogen leaching coupled to reduced water quality for human consumption may preclude going back to clean cultivation. In other viticultural areas with no summer rain, like the ‘La Mancha’ area south of Madrid in Spain, soil tillage systems also may have to be changed completely. Common practice in these and other comparable regions in Europe is very frequent tillage to avoid capillary rise of water from deeper soil layers and subsequent unproductive evaporation, yet at low soil water content going to a no-tillage system may actually conserve more water.

Changes in soil water is not the only ‘soil’-parameter affected through changes in temperature and precipitation. Soil respiration is positively correlated with an increase in these variables and soil organic matter content will be altered in the long term and may require different ways of soil management practices. Higher winter temperatures coupled with higher relative precipitation in winter as compared to summer will cause large increases in organic matter decay rates. It must also be kept in mind, that concomitantly to the increase in ambient temperature and CO₂, some sources predict a large additional deposition of nitrogen, particular in the northern hemisphere.

Although the CO₂ concentration may have been close to 20 times the current level at certain times in earth’s history, it remained relatively stable at around 270 ppm over the last 5 centuries (Ehleringer and Cerling, 1995), beginning to rise only after the industrial revolution when human caused CO₂ release became significant. The oldest continuous record of direct measurements of CO₂ comes from the top of Mauna Loa, Hawaii, which started in 1957 (fig. 5A). The current level of near 370 ppm is more than 30% higher since the industrial revolution 200 years ago and nearly 20% higher since direct measurements began (fig. 5A). Fossil fuel consumption will further increase through the increase in world population, land use, and higher energy consumption per capita. Currently, a person in the United States of America, on average, uses 22 tons of carbon per year, whereas a person in India, for example, uses only 0.7 tons of carbon per year (Bazzaz, 1998), yet it is the developing nations who will contribute dramatically to future CO₂ emissions through their development (for a complete list see internet data bank of Carbon Dioxide Information Analysis Centre at Oak Ridge National Laboratory, Tennessee, USA). The global increase in CO₂ is thought to occur much faster than plants are capable of genetically adapting to the change and photosynthesis will inevitably be affected. Predictions of how much atmospheric CO₂ concentration will rise during the next century differ between different model analyses but most agree on an approximate doubling by the end of this century (fig. 5B), with some scientists speculating that atmospheric CO₂ concentration could actually exceed 1000 ppm (Bazzaz, 1998). The direct response of grapevines to a rise in CO₂ concentration seems to be similar to the results obtained in most of the studies conducted on annual and perennial plants, where an increase in net photosynthesis, biomass, crop yield, light –nutrient– and water-use efficiency is found (Bindi et al., 1996a). In the short term, photosynthesis and water-use efficiency (ratio of photosynthesis to water consumption) is stimulated by increased CO₂ (Schultz, 2000).Grapes grown in arid regions like Spain may therefore be expected to benefit from increased CO₂ and may be able to at least partly overcome some of the adverse conditions created by increases in likelyhood and severity of drought events. But on the whole plant level, long term elevated CO₂ exposure may have very different effects. The initial increase in photosynthesis may be partly or completely down-regulated if sinks for the manufactured photosynthate are not sufficient so that over a period of days, weeks or months of growth in elevated CO₂, the acclimation response may be substantial enough that the photosynthetic rates of plants grown and measured in elevated CO₂ may even become equal to those grown at current ambient concentrations (Bazzaz, 1998).

There is regulation at the biochemical, physiological and the molecular level of photosynthesis, and judging from the large differences in vegetative and reproductive development between grape cultivars (Champagnol, 1984), it is likely that sink size and activity will largely affect the responses of different varieties to elevated CO₂. Bindi et al. (1996a) in a study using a FACE-system (free air carbon dioxide enrichment in the field) with the variety Sangiovese over several months, found a stronger increase in leaf area (+ 35%) and vegetative dry weight (+ 49%), than in reproductive dry weight (+21%) when CO₂ concentration was increased to 700 ppm. The increase in vegetative dry matter was confirmed when simultaneous changes in temperature and solar radiation were additionally considered in running several climate change scenarios and GCMs, but yield response was negative in most cases and more so for Cabernet Sauvignon than for Sangiovese (Bindi et al., 1996b). Faster development of larger leaf areas may in turn have important consequences for water consumption and canopy management, which just points to the difficulty in predicting the combined action of several changing environmental factors.

Table 1 Some known effects of UV-B radiation and their possible relevance for grape production

Additionally, on the molecular level, UV-B can destruct peptides and lipids and can photo-degrade the plant hormone auxin which absorbs in the UV-B range and may play a significant role in the formation of an off-flavour in white wines increasingly found over the last decade in Central Europe (Geßner et al., 1999) (table 1).

In a field trial with different levels of UV-radiation we found both amino acid (AA) and carotenoid concentration at harvest to be substantially reduced under current level ambient UV-B radiation in berry skins of the cultivar White Riesling (Schultz et al., 1998). Additionally, AA composition was altered under UV-B radiation, exhibiting lower levels of arginine and glutamine, the main sources of AA for yeast metabolism. Several authors suggested that the difference between the carotenoid level at the onset of ripening and that at harvest indicates the formation of norisoprenoids (Razungles et al., 1993; Bureau et al., 1998). These compounds have been linked to the ageing flavour of wines (vitispirane, 1,1,6-trimethyl-1,2-dihydronaphtalene, TDN) and the fruity character of must and wine (damascenone). Thus the response to UV-light may not be exclusively negative, but our current knowledge on this subject is still very limited. The great challenge in the future will be to predict the responses of grapevines to simultaneously changing climatic components and to develop adequate strategies to overcome potential problems.