Source: REUTERS/Mike Hutchings

Agricultural production is affected by seasonal and sub-seasonal temperature and precipitation patterns, as well as solar radiation (the amount of incoming radiation from the sun) and evapotranspiration (the rate at which water evaporates from the soil and leaves of plants). In addition to the direct effects of climate change, the productivity of agricultural systems is also affected by the indirect effects of socially and ecologically-mediated responses to climate change, including changing soil properties, pest and disease outbreaks, and planting decisions. More broadly, the effects of climate change on agricultural systems occur within a context of other large-scale trends, such as a shift towards increased use of irrigation. Although a review of the impacts of these non-climate-related trends is beyond the scope of this manual, it is important to consider that their effects may be substantial, and may amplify or mitigate the effects of climate change in complex and non-linear ways. 

Changes in precipitation, temperature, and evapotranspiration can affect the quantity and quality of crop yields. In the tropics, many crops show decreased productivity with increasing temperatures (Figure 2.1). Temperature data, in contrast to precipitation data, show less spatial variability. Agricultural planners concerned with general temperature trends may find relatively coarse projections useful. Those concerned with identifying specific thresholds, especially in areas with varied orography, may need to consider tradeoffs between higher resolution models and increased uncertainty.

Figure 2.1 (Reprinted from IPCC AR5 WGII) The percentage change in yield for three crops in tropical and temperate regions, as a function of local temperature change. Note that adaptation measures mitigate some, but not all, losses.

Agricultural impact assessments should consider not only the total quantity of precipitation, but also the variability of precipitation. Precipitation indices such as the length of dry spells, the intensity of rainfall on rainy days, and seasonal patterns can help to inform adaptation practices within the agricultural sector. However, daily precipitation is poorly modeled by both GCMs and RCMs. Models tend to project more days with light rain, while observations show fewer rainy days but more intense precipitation. This discrepancy makes accurate projections of precipitation variability indices difficult, and accounting for this uncertainty in crop models is crucial.  

Changes in cloud cover can also affect the amount of solar insolation an area receives. Whether or how clouds will change in the future remains one of the greatest outstanding sources of uncertainty in climate projections. While projections of future solar radiation in the region remain unsettled, agriculturalists should consider the consequences of this uncertainty.  

Evapotranspiration rates are determined by temperature, surface winds, plant characteristics, and air moisture content. Higher evapotranspiration rates decrease available soil moisture and crops’ water use efficiency, affecting agricultural productivity. Even in cases where models project increases in precipitation, soil moisture may remain unchanged or even decline if the increased evaporation expected with rising temperatures outweighs the effects of increased precipitation. Planting decisions, including what crops to plant, should consider the effect of evapotranspiration rates on water availability.  Changes in temperature and soil moisture content can also affect the rate of plant decomposition, which may alter soil nutrient composition. Increases in intense rainfall episodes can also cause increased runoff and soil erosion, which can affect crop productivity as well as the quality of neighboring water resources

The frequency and intensity of insect and disease outbreaks can also be affected by changes in temperature and precipitation. In some locations, pest outbreaks are tempered by the exceedance of certain low-temperature extremes; many pest populations cannot survive below specific temperature thresholds, and so an increase in mean temperatures could increase the probability that insect outbreaks occur. Agriculturalists concerned with pest and disease incidence may find tools focusing on climate extremes particularly useful. 

Livestock productivity is affected by the availability of grazing land, and the presence of parasites and pathogens. In general, higher temperature and humidity leads to increased rates of parasitism and pathogenesis. Indirect climate impacts, such as the disruption of traditional trade pathways, may also increase the risk of the introduction of new diseases to an area. Alterations to local vegetation from changes in precipitation, temperature, solar radiation, and evapotranspiration affect the availability of suitable grazing land, which in turn affects livestock productivity. 

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