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This chapter considers the most typical of the expected impacts of climate change on crop production, and the opportunities that exist for adapting to these changes and mitigating climate change through the sustainable intensification of crop production. 

Intensifying crop production and addressing climate change must be done in an integrated and sustainable way. Although crop production and climate are deeply interconnected, the module deals with these subjects in separate chapters as a way of breaking down this complex issue and addressing it comprehensively. Chapter B1-1 discusses the interlinkages among the response actions to sustainably intensify and diversify crop productions, to adapt crop systems to the changing climate and to mitigate climate change through sustainable practices. Chapter B1-2 summarizes the impacts of climate change on crop production. Since crop production and climate change adaptation are strongly related to the specific local agro-ecological endowments and the natural resource base, readers are referred to modules B6, B7, B8 and B9 that provide a comprehensive analysis of the relationships between climate change and water, soil, genetic resources and energy. Chapter B1-3 discusses the impact of crop production on climate change.

The most important ecosystem service delivered by agriculture is the provision of food, feed and fibres. 

The extent to which this provisioning depends on external production inputs is a fundamental issue. Agricultural ecosystems have evolved under human management. To obtain greatest possible production from the landscape, agricultural communities have developed and maintained ecosystems at their early succession state. The human selection pressure has favoured readably harvestable crops with high net production and it has penalized biomass production and accumulation on the landscape. 

Since the Green Revolution, mainstreamed agriculture has mainly involved controlling crop varieties and their genetics; soil fertility through the application of chemical fertilizers; and pests with chemical pesticides. The impact of this form of agriculture on the environment has been severe. There has been a significant simplification and homogenization of the world’s ecosystems. Maize, wheat, rice and barley, which were once rare plants, have become the dominant crops on earth and staples in human diets (FAOSTAT, 2014). Soil degradation is another critical concern. In agricultural ecosystems depleted of soil organic carbon, it will be increasingly difficult to produce higher yields. Each year, soil erosion destroys 10 million hectares of cropland. Forty percent of this loss is due to tillage erosion (Pimentel, 2006). In soils that have already experienced significant losses of soil organic matter, increased fertilization does not usually generate a net sink for carbon, because the production, transport and application of fertilizer releases higher amounts of carbon dioxide (Corsi et al., 2012). 

The FAO 'Save and Grow' model of sustainable crop production intensification calls for a 'greening' of the Green Revolution to achieve the highest possible productivity by unit of input within the ecosystem's carrying capacity. This can be achieved through the use of good quality seeds and planting materials of well-adapted varieties; a diverse range of crop species and varieties grown in associations, intercrops or rotations; the control of pests through integrated pest management; and the use of conservation agriculture and sustainable mechanization to maintain healthy soils and manage water efficiently (FAO, 2011). Greater access to technological innovations and a sound understanding of agricultural ecosystems will allow farmers to work 'smarter not harder' and work in tandem with biogeochemical processes inherent in diverse and complex ecosystems. Chapter B1-2 presents agronomic management practices that reflect these principles. 

The FAO model for the sustainable intensification of crop production is the cornerstone of climate-smart agriculture. It guides all climate-smart strategies aimed at overcoming the inefficiencies that are responsible for yield and productivity gaps. In each crop system, there exist many climate change adaptation and mitigation options to close yield gaps and minimize the harmful environmental impacts of crop production. Options will vary among farmers and will depend on each farmer's coping and adaptive mechanisms, and the degree to which each specific climate factor is responsible for the yield and productivity gap. The solutions identified should always be cost-effective and profitable for farmers and responsive to markets. Since most technologies have both advantages and disadvantages, trade-offs will need to be made. Ensuring that these trade-offs are properly assessed demands comprehensive capacity development for all stakeholders (see module C1). In particular, farmers must manage the foreseen business risks of changing their production practices (e.g. costs, investments and future value of the investments); consider the financial returns related to adapting to changes in local climate; evaluate the implications of local climate on local prices and markets; and anticipate the consequences climate change may have on crop prices in international markets. 

For more information, consult module B8 on plant genetic resources, module B7 on sustainable soil and land management and module A3 on integrated landscape management.

Crop production is highly sensitive to climate. It is affected by long-term trends in average rainfall and temperature, interannual climate variability, shocks during specific phenologicalⁱⁱ stages, and extreme weather events (IPCC, 2012). Some crops are more tolerant than others to certain types of stresses, and at each phenological stage, different types of stresses affect each crop species in different ways (Simpson, 2017). 

As climate changes, crop production strategies must change too. There will always be some uncertainty associated with modelling the complex relationships between agricultural yields and future climate scenarios. This chapter summarizes the most universally accepted effects of climate change on crop production. Chapter B8-3 addresses how climatic changes can disrupt the interactions among plants and pollinators (Kjøhl et al., 2011), Chapter B1-2 presents the management practices and technologies for climate change adaptation, and Chapter B1-3 presents these management practices in the context of specific crop systems.

A higher concentration of carbon dioxide in the atmosphere will have different effects on different crops.

In C₃ plantsⁱⁱⁱ, the photosynthesis relies on the concentration of carbon dioxide that is naturally available in the atmosphere. A higher concentration of carbon dioxide in the atmosphere will have a small fertilizing effect on these crops, if all other factors remain favorable. Adverse moisture conditions during the growing season, insufficient nitrogen availability or temperatures above the optimum range may offset this effect. However, the nutritional content of leaves, stems, roots, fruits and tubers of C₃plants grown at elevated carbon dioxide levels is expected to be lower particularly in protein, minerals and trace elements, such as zinc and iron (Taub et al., 2008; Loladze, 2014). Plants grown at higher concentrations of carbon dioxide have lower stomatal conductance^iv and transpiration. This means that plants absorb less water and nutrients and that their biomass becomes less nutritious. One insidious aspect associated with the nutritional quality of crops is that, in addition to humans, also insect pests will have to compensate by eating more to meet their nutritional needs (Hatfield et al., 2011).

C₄ plants^v have the capacity to increase the carbon dioxide concentration within their leaves before the photosynthesis begins. This is why increased concentrations of carbon dioxide in the atmosphere will not provide benefits to C₄ plants under normal conditions. Under moisture stress conditions, however, most C₄ crops will lose less moisture, and their yield will be affected less (Simpson, 2017).

Temperature alterations can take many forms: changes in average temperature; changes in daytime high and nighttime low temperatures; and changes in the timing, intensity and duration of extremely hot or cold weather.

In general, crops are most sensitive to high temperatures at the reproductive stage and grain-filling/fruit maturation stage (Hatfield et al., 2011). However, plant responses to each type of temperature alteration is species-specific and mediated through both photosynthetic activity for biomass accumulation, which is responsible for plant growth, and the phenological and morphological changes, which occur during plant development. Each type of temperature stress has a different effect on crop duration  and overall plant productivity. The effect will depend on how sensitive each species is at their stage of development when the temperature alteration occurs. Adapting to these effects will require different types of responses. 

To predict the responses of species to new temperature alterations, it is necessary (although not sufficient) to know how the same species have responded in the past to similar changes. Below are some of the findings from the few phenological studies of sufficient length on annual crops.

• The increase in average temperature during the growing season typically causes plants to use more energy for respiration for their maintenance and less to support their growth. With a 1°C increase in average temperatures, yields of the major food and cash crop species can decrease by 5 to 10 percent (Lobell and Field, 2007; Hatfield et al., 2009).
  • The increase in average temperature during the growing season typically causes plants to use more energy for respiration for their maintenance and less to support their growth. With a 1°C increase in average temperatures, yields of the major food and cash crop species can decrease by 5 to 10 percent (Lobell and Field, 2007; Hatfield et al., 2009).
  • With higher average temperatures plants also complete their growing cycle more rapidly (Hatfield et al., 2011). With less time to reproduce, reproductive failures are more likely and this will also lower yields (Craufurd and Wheeler, 2009). 
  • In general, photosynthesis in C₃ plants is more sensitive to higher temperatures compared with C₄ crops (Lipiec et al., 2013). 
  • Variations in the length of the thermal growing season^vii will generally affect temperate perennial species (e.g. apples, cherry and grapes). Most temperate perennials require an adequate period of chilling hours^viii during dormancy before they can resume active growth. Inadequate chilling impairs the development and/or expansion of vegetative and reproductive organs, which will affect fruiting.
  • Higher temperatures can also affect the marketability of fruits and vegetables. The increased rates of respiration caused by higher temperatures lead to a greater use of sugars by the plants. As a result less sugar remains in the harvested product, and this can reduce its market value (Hatfield and Prueger, 2015). These effects become more serious as temperatures continue to rise during the grain-filling or fruit maturation stage (Simpson, 2017).
  • Higher nighttime temperatures may increase respiration at night causing declines in yield (e.g. rice) and flowering or reproduction (e.g. beans).
  • Most crops can tolerate higher daytime temperatures during vegetative growth, with photosynthesis reaching an optimum at between 20°C and 30°C (Wahid et al., 2007). During the reproductive stage, yields decline when daytime high temperatures exceed 30°C to 34°C (FAO, 2016b). 
  • Extremely high temperatures above 30°C can do permanent physical damage to plants and, when they exceed 37°C, can even damage seeds during storage. The type of damage depends on the temperature, its persistence, and the rapidity of its increase or plants’ capacity to adjust (Wahid et al., 2007). It also depends on the species, the stage of plant development. As the climate changes, the frequency of periods when temperatures rise above critical thresholds for maize, rice and wheat is predicted to increase worldwide (Gourdji et al., 2013).

Changes in precipitation regimes include changes in seasonal mean, the timing and intensity of individual rainfall events, and the frequency and length of droughts. Each of these factors is critical to crop productivity. The impact of changes in precipitation will be particularly marked when they are combined with temperature alterations that affect the crop's evaporative demands. This may lead to different forms of moisture stress depending on the phenological stage the crop has reached. 

The specific impacts of changes in precipitation regimes on crops vary significantly because around 80 percent of the cropped area is rainfed and produces 60 percent of world's food (Tubiello et al., 2007). The levels and distribution of precipitation determine whether a crop can be grown without irrigation and/or drainage, or whether investments in this area are necessary. 

The general prediction is that, with climate change, areas that already receive high levels of rainfall will receive more, and those that are dry will become drier (Liu and Allan, 2013). The reduction in seasonal mean precipitation will have a greater impact on areas with degraded soils. Soils with lower levels of organic carbon retain less water at low moisture potentials. Furthermore, crops grown in nutrient-poor soils, especially those lacking potassium, recover less quickly from drought stress once water is again available (Lipiec et al., 2013). To help their crops use water more efficiently, farmers must pay attention to improving and maintaining soil fertility (see module B7 on sustainable soil and land management for climate-smart agriculture, Chapter B1-2 on sustainable soil management for increased crop productivity and Box B1.3 on crop residue management for soil carbon conservation and sequestration). 

As rainfall becomes more variable, farmers may no longer be able to rely on their knowledge of the seasonality of climatic variables. Shifting planting seasons and weather patterns will make it harder for farmers to plan and manage production. For example, a later start of the rainy season or an earlier end, or both, reduces the time that crops have to complete their growth cycle and, ultimately, causes yield losses (Linderholm, 2005). For photosensitive species, a change in the duration of the rainy season may cause a mismatch between their reproductive cycle, which is determined by day length, and the availability of sufficient soil moisture to produce good yields.

Another expected impact of climate change is an increased occurrence of extreme weather events. Even where mean values for precipitation are not projected to change, there are likely to be more significant extreme weather events that will reduce crop yields. Heavy rain, hail storms and flooding can physically damage crops. Extremely wet conditions in the field can delay planting or harvesting. Prolonged droughts can cause complete crop failure (Tubiello and van der Velde, 2010).

As discussed in Chapter B8-4, climate change modifies the interactions between plants and their pests  in space and over time. Plants weakened by the direct effects of weather stresses are generally more vulnerable to indirect stresses. For example, plants suffering from waterlogging are less resilient to viruses, and plants affected by drought are less able to outcompete weeds for soil moisture and nutrients (Simpson, 2017). In addition, if pests shift into regions outside the distribution of their natural enemies, the effectiveness of biocontrol will decrease unless a new community of enemies will provide some level of control.

The distribution of insect pests is influenced by temperatures. With global warming, insects, whose body temperature varies with the temperature of the surrounding environment (poikilothermic^x) are most likely to move polewards and to higher elevations (Bebber et al., 2013). Pest distribution will also respond to changes in cropping patterns to cope with climate change. Major insect pests of cereals, pulses, vegetables, and fruit crops, which may move to temperate regions, include cereal stem borers (Chilo, Sesamia, and Scirpophaga spp.), pod borers (Helicoverpa, Maruca, and Spodoptera spp.), aphids, and whiteflies (Sharma, 2014). The extent of crop losses will depend on geographical distribution of insect pests; the dynamics of the insect population; insect biotypes; the alterations in the diversity and abundance of arthropods; changes in herbivore-plant interactions; the activities and abundance of natural enemies; species extinctions; and the efficacy of crop protection technologies. 

Weeds will also be affected by climate change. For invasive plants with tolerances for higher temperatures, which are currently restricted by low temperatures, such as Vallisneria spp. and those intolerant to freezing such as Pistia stratiotes, Eichhornia crassipes and Salvinia auriculata, increasing temperatures could trigger the northward migration (Hussner et al., 2010). The species with higher mobility would be favoured. Traits that promote seed dispersal over long distances are common in invasive plants (Patterson, 1995a; Rejmanek, 1996; Dukes and Mooney, 2000; Malcom et al., 2002), such as Imperata cylindrica (cogon grass), Pueraria lobata (Kudzu) and Striga asiatica (witchweed). Pueraria lobata, whose range is restricted by low winter temperatures of -15°C, may spread with increases in minimum temperatures (Ziska et al., 2010). This may also be the case for the parasitic weed Striga, which grows well in temperatures of 30°C to 35°C in semi-arid environments, and spreads under conditions where soil fertility is poorly managed and cereal monoculture is practiced. The specific influence of climate change on Striga species and recommended control management are addressed in Box B1.2. 

The competition between crops and invasive weeds could also be influenced by the effects of rising temperatures on plant physiology (Ziska and Reunion, 2007). For example, higher mean annual temperatures have been shown to favour the assimilate partitioning towards root biomass in the shrub Parthenium juliflora. Greater root biomass of this species aids in the rapid and robust regeneration of branches after lopping for fuelwood (Kathiresan, 2006a). 

Competition between C₄ weed species and C₃ crops under different climate conditions and carbon dioxide concentrations may significantly alter crop productivity (Patterson, 1993). For example, a 3°C increase in the average temperature would favour the perennial invasive C₄ weed Rottboellia cochinchinensis (itch grass), which would cause significant yield reductions in various important C₃ crop systems (Patterson et al., 1979; Lencse and Griffin, 1991; Lejeune et al., 1994).

Soil and water management for crop production has a strong impact, both negative and positive, on the drivers of climate change. 

A large number of crop production practices contribute to emissions of greenhouse gas, and in particular to carbon dioxide, methane and nitrous oxide. Soil degradation, for example, is a major driver of climate change. Changes in land cover that leave the soil less protected hasten the mineralization of soil organic carbon, a process that releases carbon dioxide, nitrous oxide and methane (Bullock et al., 1995). The global warming potential for methane is approximately 20 times higher than it is for carbon dioxide; for nitrous oxide, it is 310 times higher. The many factors involved in crop production make the accurate measurement of greenhouse gas emissions from agriculture sectors more difficult than it is for other economic sectors. Despite this complexity, the processes causing greenhouse gas emissions from crop production have been described qualitatively.

• Carbon dioxide emission sources include the burning of crop residues in the fields, which also releases methane and nitrous oxide; the energy used in field operations, mainly for the mechanical tilling of the soil and pumping for irrigation; the production, transport and application of crop production inputs; and the mineralization of soil organic carbon. Tillage deserves special mention, as it is the field operation that produces the maximum carbon dioxide flux and has many direct effects on the carbon dioxide exchange between the soil surface and the atmosphere. Ploughing is the farm operation that disturbs the highest volume of soil and requires the most energy and fuel. Direct seeding requires the least amount of energy. Tillage also undermines the processes (the stabilization of microaggregates within macroaggregates) and damages the organisms (mainly fungi) responsible for the formation of soil organic carbon (Grandy et al., 2006; Six et al., 1998). Tillage accelerates the mineralization of the soil by speeding up the oxidative breakdown of soil organic carbon. In upland landscapes, tillage is the major cause of severe soil erosion and losses in soil organic carbon (Lobb et al., 1995; Lobb and Lindstrom, 1999; Reicosky et al., 2005).
• Methane flux from soil to atmosphere is the net result of two bacterial processes that are strongly influenced by land use, land management and the type of soil: methane production in strictly anoxic micro-environments (methanogenesis); and methane consumption and oxidation in aerobic micro-environments by methane-oxidizing bacteria (methanotrophs). Most emissions of methane from crop production are related to methanogenic bacteria living in flooded soils under rice cultivation (see Chapter B1-3.1) and the anaerobic decomposition of animal manure (e.g. liquid or slurry) and crop residues under very wet conditions.
• Most nitrous oxide emissions are generated from manure during its storage and from humid and compacted soils (asphyctic soils) in which nitrogen is present (IPCC, 2007). Microbial transformations of nitrogen, which can be caused by the application of synthetic fertilizers, animal waste, sewage sludge and crop residues, are responsible for nitrous oxide emissions through nitrification (the biological process by which ammonia is converted to nitrites and then nitrates) and denitrification (the biological process by which nitrate is converted to nitrogen gas). Denitrification is the most important of these two processes. The main factors controlling the speed of these processes is the presence of ammonia in the case of nitrification and oxidized nitrogen forms (nitrates and nitrites) in the case of denitrification. Nitrification is enhanced in microaerophilic soil conditions^xi, and specifically at values for water-filled pore space from 20 to 80 percent. Denitrification is enhanced under anaerobic soil conditions (i.e. in soils that are permanently or seasonally saturated by water). Nitrification is also favoured at high temperatures and is inhibited when the soil's pH values are acidic.

Some crop production practices (Chapter B1-2) enhance the capacity of the soil to conserve and accumulate soil organic carbon. These practices can reduce greenhouse gas emissions at the source and, at the same time, maintain or improve yields and enable crop systems to adapt to the projected impacts of climate change. The broader social benefits include enhanced wildlife habitats and the reduction of sediments, nutrients and pathogens in the runoff from agricultural fields (Box B7.3). The goal of sequestering carbon in the soil received renewed attention at the United Nations conference on climate change held in Paris in 2015 through the '4 per thousand' initiative'. This was launched with the objective to increase the existing soil organic carbon by 0.4 percent each year globally as a compensation for the global emissions of greenhouse gases. Lal (2004) estimates that the world’s cropland has the potential to store 0.4 to 1.2 gigatonnes of carbon equivalents per year (see module B7 for further details).

In addition to enhancing the sinks of greenhouse gases, a number of crop production practices can also reduce greenhouse gases while maintaining or improving yields and adapting the crop system towards the future projected climate change impacts.

• Emissions can be reduced by improving the efficiency of farm machinery in terms of its productivity, operating times and fuel usage. This can be done by using equipment that is best suited for the given farm type. The right machinery, such as two-wheel tractors, combined with agronomic innovations, such as direct seeding technology, can contribute to climate change adaptation and mitigation. Small tractors using tined equipment instead of disc ploughs, or modern direct seeding equipment are more productive than tillage-based systems. Smaller equipment may also be more accessible and affordable to smallholder producers, farmer organizations and service providers. In mechanized systems, decisions regarding the trade-offs between increased farm productivity, including energy efficiency, and affordability are especially important to the rural poor. Impoverished farmers have a particularly acute need to produce more and become more efficient but they are less likely to have access to the resources needed to make the required improvements, and to pay for them.
• Shifting production from one crop variety to another or to different locally adapted annual or perennial species may be another option for reducing emissions.
• Any changes in output, particularly changes in the primary crops grown in a given community, present technical and economic challenges with broad social implications. Decisions to implement changes of this nature must take into consideration broader strategies for rural development, and specifically the impact these changes will have on diets and employment (see module C7). Given the low labour costs in some countries and the generally low labour absorption capacity of perennial crop production, a change in the orientation toward perennial crops would need to include effective risk management activities. This would involve, for example, employment policies and programmes to increase opportunities for the rural poor, especially vulnerable groups, such as women and young people, in obtaining decent jobs (Blowfield, 1993; Devi, 2006).