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Climatic risks

Climatic risks will be highly dependent on the expected pattern of time variability for weather variables. Increasing temperatures will promote the development rate of all winter crops, which therefore will face extreme events (cold spells) at a later stage (i.e., when they are more sensitive). Consequences depend as a whole on the probabilities of such extreme events and a higher intra-annual variability of minimum temperatures-yielding a higher probability of crop failure from frost damage. The same problem arises with winter cereals, which face extreme temperature maxima in early summer during the grain-filling period. Recent investigations show that the probabilities of extreme temperatures increase under all climate scenarios (CLAIRE, 1996) and that thermal shocks on poorly adapted genotypes lead to losses in grain yield and quality.

Higher temperatures in summer should not be a real challenge to summer crops (except spring cereals, if subjected to elevated temperatures during the grain-filling period) because they are more resilient than winter crops. Drought could be a major concern in the future, however, particularly in the Mediterranean zone and in central Europe. This is a genuinely complex problem; GCM-based scenarios do not agree on the magnitude of changes in space of at least one component of the water budget (precipitation), and changes in another component-potential evapotranspiration (PET)-are extremely dependent on calculation methods. Le Houérou (1995) states that a 1°C increase in air temperature will induce 37 mm more PET south of 40°N (ECRASE, 1996)-an enormous 60% increase in PET in southern European countries. If simulated changes of -10% to -20% in summer precipitation for western, southern, and central Europe are reliable, fully irrigated crops may become even larger competitors to domestic and industrial users for water resources stored in aquifers and rivers (a 20% reduction represents a significant loss in currently moist areas).

Adaptive responses

Adaptive responses could be facilitated by increased knowledge of weather patterns and climate-related variability through the use of climate forecast information.

To abate the shortening effect of temperatures on crop cycles, changed sowing dates and later-maturing genotypes could be used whatever the type of crop. This approach, however, may cause a problem with winter cereals whose cycle length often is linked with cold temperature requirements (vernalization) that may be not completely fulfilled during warmer winters. Later-maturing crops also should face climatic risks in the early summer. On the other hand, the sowing dates of winter crops cannot be postponed to early fall because of the much higher probability of experiencing low temperatures at a sensitive stage and because of the cost of fungal disease control during periods in the early fall.

For summer crops, using earlier sowing dates or longer-maturing varieties would counteract the detrimental effects of climate change in all cases-as was demonstrated for sunflowers throughout Europe in CLAIRE (1996), for spring wheat in Finland in Saarikko and Carter (1996), and for maize in Spain in CLAIRE (1996). Choosing adequate sowing dates also could help to synchronize full canopy development and maximum radiation availability on maize-type crops in northern European regions (Delécolle et al., 1996), which would enhance final production. Similarly, earlier sowing dates would allow the crop to develop during periods of lower PET demand, implying an improvement in global water-use efficiency and a reduction in irrigation demand-as simulated in Spain by CLAIRE (1996).

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