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Data Insight

Feeling the heat – the effects of higher temperatures on people, crops and livestock

Introduction

Heat is a multi-faceted problem for agriculture. It affects the agricultural sector through a variety of mechanisms, differing in time and place.

Data Insight, Featured Image: Feeling the heat

500 million people live in rural areas suffering from heat stress

There is now an abundance of evidence that heat has effects on people (Raymond et al., 2020; Sun et al., 2019; Tigchelaar et al., 2020). Heat stress occurs when the heat received by the body is greater than the body can tolerate. This causes physiological impairment, which can lead to heat stroke and, in extreme cases, death. It affects the health of farm workers directly, reducing labor productivity which, in turn, has an impact on the income generated by farms. We define heat stress on people as occurring when daytime temperatures exceed 32°C. We find that over 500 million people live in rural areas suffering from heat stress at some point during the year, with the greatest concentration in the Sahel.

Data Insight 1, Map 1: Combined effects of heat on people, crops and livestock (cattle)

Map 1 Combined effects of heat on people, crops and livestock (cattle). It shows the areas where multiple heat-related hazards coincide, using projected temperature data for 20301.

Around 46 million hectares of crops are projected to be heat stressed in 2030

Heat also affects crops at a range of physiological stages (from early growth through to flowering and seeding/fruiting) and different times of the day (with nighttime temperatures affecting crops just as much as those in daytime). The effect varies depending on the crop and variety, with heat-tolerant species such as sorghum and cassava being much less affected than heat-sensitive ones such as beans, wheat, maize or potato (Gourdji et al., 2013; Siebert and Ewert, 2014; Teixeira et al., 2013).

We calculated the effect of heat stress on a range of crops by calculating the number of days above a crop-specific temperature threshold per month, with more than five days of excess heat signifying moderate-to-high heat stress. Around 46 million hectares of crops are projected to be heat stressed in 2030, with the greatest effects in the northern Sahel2.

809 million hectares have moderate-to-high heat stress, accounting for 116 million head of cattle

Finally, heat affects livestock. When ambient temperatures, humidity and radiant energy soar, animals are impaired in multiple ways. Initially, heat stress causes animals to divert energy away from producing meat, milk or other products to maintaining homeostasis. Progressively higher heat stress affects behaviour and causes metabolic variations, which impacts animal health and may cause mortality (Rojas-Downing et al., 2017).

As with crops, some livestock species and breeds are more resistant than others. We calculated the heat stress on livestock using the temperature-humidity index, classifying moderate-to-high heat stress as occurring with scores over 78. Results show that heat stress is a problem across many livestock-producing regions of sub-Saharan Africa (SSA), from southern Africa to the Sahel. A total of 809 million hectares have moderate-to-high heat stress, accounting for 116 million head of cattle worth $92.8 billion International Dollars today.

When heat stress simultaneously affects people, crops and livestock, the livelihoods and health of farmers and their families are at risk

Data Insight 1, Maps 2, 3, 4: Heat stress in crops, people and livestock in 2030

Map 2 (left) Heat stress in crops in 2030, Map 3 (center) Heat stress in people in 2030, Map 4 (right) Heat stress in livestock 2030.

When heat stress simultaneously affects people, crops and livestock, the livelihoods and health of farmers and their families are at risk. In many areas of the Sahel, heat stress occurs around springtime, impairing livestock production and affecting farmers’ health and ability to prepare the land. Later in the year (towards August), heat stress affects crops, reducing food availability and farmers’ capacity to harvest.

Heat-stressed crops require greater upkeep by farmers, while at the same time livestock need continued supplies of feed (from crops). Thus, the effects of heat stress interact with each other, creating significant challenges to maintaining agricultural livelihoods.

45 million hectares of crops worth $18.4 billion International Dollars will be located in areas of high heat stress in people, crops and livestock

By 2030, we found that 45 million hectares of crops worth $18.4 billion International Dollars will be located in areas of high heat stress in people, crops and livestock, with 51 million smallholder farmers affected3. For these areas, a range of adaptation options must be promoted, including heat-resistant crop varieties and livestock breeds, shade-based production systems to reduce field temperatures, and mechanization to reduce drudgery and the risks of heat stress to human health.

Methods

1 We used three different indicators to represent the level of heat stress on people, livestock, and crops. These were the Heat Index (HI), Temperature-Humidity Index, and the number of heat stress days per month, respectively. We used 2030 projections under the RCP 8.5 scenario and classified each hazard into four heat stress categories (0: mild or no stress; 1: moderate; 2: severe; 3: extreme). Pixels with either severe or extreme heat stress were identified for each index independently and combined to produce a map of total heat stress.

2 The number of people, crops and livestock at risk of heat stress was then calculated by combining the aforementioned heat stress maps with WorldPop 2020, MapSPAM and FAO–GLW (cattle only, in this instance).

3 Household surveys were used to calculate the share of smallholder farmers by admin unit for 20 countries across SSA. These were then averaged and applied to those countries for which household surveys were not available. All values were then applied to the total population using WorldPop 2020. This data was then merged with the GADM admin 1 level shapefile and rasterized, dividing the total number of smallholders in each admin unit by the number of pixels in that unit to give the average number of smallholder farmers per pixel. This raster file was overlaid with the aforementioned heat stress maps to calculate an approximation of the total number of smallholder farmers at risk of severe or extreme heat stress by 2030 under the RCP scenario 8.5.