Agriculture and forestry

Although agriculture and forestry are known to be highly dependent on climate, little evidence of observed changes related to regional climate changes was noted in the TAR. This is probably due to the strong influence of non-climate factors on agriculture and, to a lesser extent, on forestry, especially management practices and technological changes, as well as market prices and policies related to subsidies (Easterling, 2003). The worldwide trends in increasing productivity (yield per hectare) of most crops over the last 40 years, primarily due to technological improvements in breeding, pest and disease control, fertilisation and mechanisation, also make identifying climate-change signals difficult (Hafner, 2003).

13.6.1 Crops and livestock

Changes in crop phenology provide important evidence of responses to recent regional climate change (Table 1.10). Such changes are apparent in perennial crops, such as fruit trees and wine-making varieties of grapes, which are less dependent on yearly management decisions by farmers than annual crops and are also often easier to observe. Phenological changes are often observed in tandem with changes in management practices by farmers. A study in Germany (Menzel et al., 2006c) has revealed that between 1951 and 2004 the advance for agricultural crops (2.1 days/decade) has been significantly less marked than for wild plants or fruit trees (4.4 to 7.1 days/decade). All the reported studies concern Europe, where recent warming has clearly advanced a significant part of the agricultural calendar.

Since the TAR, there has been evidence of recent trends in agro-climatic indices, particularly those with a direct relationship to temperature, such as increases in growing season length and in growing-degree-days during the crop cycle. These increases, associated with earlier last spring frost and delayed autumn frost dates, are clearly apparent in temperate regions of Eurasia (Moonen et al., 2002; Menzel et al., 2003; Genovese et al., 2005; Semenov et al., 2006) and a major part of North America (Robeson, 2002; Feng and Hu, 2004). They are especially detectable in indices applicable to wine-grape cultivation (Box 1.2). In Sahelian countries, increasing temperature in combination with rainfall reduction has led to a reduced length of vegetative period, no longer allowing present varieties to complete their cycle (Ben Mohamed et al., 2002).

However, no detectable change in crop yield directly attributable to climate change has been reported for Europe. For

Table 1.10. Observed changes in agricultural crop and livestock.

Agricultural metric

Observed change




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