Editing IPCC:AR6/SR15/Chapter-3 (section)

==== 3.4.6.1 Crop production ====

<div id="section-3-4-6-1-block-1"></div>

Quantifying the observed impacts of climate change on food security and food production systems requires assumptions about the many non-climate variables that interact with climate change variables. Implementing specific strategies can partly or greatly alleviate the climate change impacts on these systems (Wei et al., 2017) <sup>[[#fn:r852|852]]</sup> , whilst the degree of compensation is mainly dependent on the geographical area and crop type (Rose et al., 2016) <sup>[[#fn:r853|853]]</sup> . Despite these uncertainties, recent studies confirm that observed climate change has already affected crop suitability in many areas, resulting in changes in the production levels of the main agricultural crops. These impacts are evident in many areas of the world, ranging from Asia (C. Chen et al., 2014; Sun et al., 2015; He and Zhou, 2016) <sup>[[#fn:r854|854]]</sup> to America (Cho and McCarl, 2017) <sup>[[#fn:r855|855]]</sup> and Europe (Ramirez-Cabral et al., 2016) <sup>[[#fn:r856|856]]</sup> , and they particularly affect the typical local crops cultivated in specific climate conditions (e.g., Mediterranean crops like olive and grapevine, Moriondo et al., 2013a, b) <sup>[[#fn:r857|857]]</sup> .

Temperature and precipitation trends have reduced crop production and yields, with the most negative impacts being on wheat and maize (Lobell et al., 2011) <sup>[[#fn:r858|858]]</sup> , whilst the effects on rice and soybean yields are less clear and may be positive or negative (Kim et al., 2013; van Oort and Zwart, 2018) <sup>[[#fn:r859|859]]</sup> . Warming has resulted in positive effects on crop yield in some high-latitude areas (Jaggard et al., 2007; Supit et al., 2010; Gregory and Marshall, 2012; C. Chen et al., 2014; Sun et al., 2015; He and Zhou, 2016; Daliakopoulos et al., 2017) <sup>[[#fn:r860|860]]</sup> , and may make it possible to have more than one harvest per year (B. Chen et al., 2014; Sun et al., 2015) <sup>[[#fn:r861|861]]</sup> . Climate variability has been found to explain more than 60% of the of maize, rice, wheat and soybean yield variations in the main global breadbaskets areas (Ray et al., 2015) <sup>[[#fn:r862|862]]</sup> , with the percentage varying according to crop type and scale (Moore and Lobell, 2015; Kent et al., 2017) <sup>[[#fn:r863|863]]</sup> . Climate trends also explain changes in the length of the growing season, with greater modifications found in the northern high-latitude areas (Qian et al., 2010; Mueller et al., 2015) <sup>[[#fn:r864|864]]</sup> .

The rise in tropospheric ozone has already reduced yields of wheat, rice, maize and soybean by 3–16% globally (Van Dingenen et al., 2009) <sup>[[#fn:r865|865]]</sup> . In some studies, increases in atmospheric CO <sub>2</sub> concentrations were found to increase yields by enhancing radiation and water use efficiencies (Elliott et al., 2014; Durand et al., 2018) <sup>[[#fn:r866|866]]</sup> . In open-top chamber experiments with a combination of elevated CO <sub>2</sub> and 1.5°C of warming, maize and potato yields were observed to increase by 45.7% and 11%, respectively (Singh et al., 2013; Abebe et al., 2016) <sup>[[#fn:r867|867]]</sup> . However, observations of trends in actual crop yields indicate that reductions as a result of climate change remain more common than crop yield increases, despite increased atmospheric CO <sub>2</sub> concentrations (Porter et al., 2014) <sup>[[#fn:r868|868]]</sup> . For instance, McGrath and Lobell (2013) <sup>[[#fn:r869|869]]</sup> indicated that production stimulation at increased atmospheric CO <sub>2</sub> concentrations was mostly driven by differences in climate and crop species, whilst yield variability due to elevated CO <sub>2</sub> was only about 50–70% of the variability due to climate. Importantly, the faster growth rates induced by elevated CO <sub>2</sub> have been found to coincide with lower protein content in several important C3 cereal grains (Myers et al., 2014) <sup>[[#fn:r870|870]]</sup> , although this may not always be the case for C4 grains, such as sorghum, under drought conditions (De Souza et al., 2015) <sup>[[#fn:r871|871]]</sup> . Elevated CO <sub>2</sub> concentrations of 568–590 ppm (a range that corresponds approximately to RCP6 in the 2080s and hence a warming of 2.3°C–3.3°C (van Vuuren et al., 2011a <sup>[[#fn:r872|872]]</sup> , AR5 WGI Table 12.2 ) alone reduced the protein, micronutrient and B vitamin content of the 18 rice cultivars grown most widely in Southeast Asia, where it is a staple food source, by an amount sufficient to create nutrition-related health risks for 600 million people (Zhu et al., 2018) <sup>[[#fn:r873|873]]</sup> . Overall, the effects of increased CO <sub>2</sub> concentrations alone during the 21st century are therefore expected to have a negative impact on global food security ( ''medium confidence'' ).

Crop yields in the future will also be affected by projected changes in temperature and precipitation. Studies of major cereals showed that maize and wheat yields begin to decline with 1°C–2°C of local warming and under nitrogen stress conditions at low latitudes ( ''high confidence'' ) (Porter et al., 2014; Rosenzweig et al., 2014) <sup>[[#fn:r874|874]]</sup> . A few studies since AR5 have focused on the impacts on cropping systems for scenarios where the global mean temperature increase is within 1.5°C. Schleussner et al. (2016b) <sup>[[#fn:r875|875]]</sup> projected that constraining warming to 1.5°C rather than 2°C would avoid significant risks of declining tropical crop yield in West Africa, Southeast Asia, and Central and South America. Ricke et al. (2016) <sup>[[#fn:r876|876]]</sup> highlighted that cropland stability declines rapidly between 1°C and 3°C of warming, whilst Bassu et al. (2014) <sup>[[#fn:r877|877]]</sup> found that an increase in air temperature negatively influences the modelled maize yield response by –0.5 t ha−1°C–1 and Challinor et al. (2014) <sup>[[#fn:r878|878]]</sup> reported similar effect for tropical regions. Niang et al. (2014) <sup>[[#fn:r879|879]]</sup> projected significantly lower risks to crop productivity in Africa at 1.5°C compared to 2°C of warming. Lana et al. (2017) <sup>[[#fn:r880|880]]</sup> indicated that the impact of temperature increases on crop failure of maize hybrids would be much greater as temperatures increase by 2°C compared to 1.5°C ( ''high confidence'' ). J. Huang et al. (2017) <sup>[[#fn:r881|881]]</sup> found that limiting warming to 1.5°C compared to 2°C would reduce maize yield losses over drylands. Although Rosenzweig et al. (2017, 2018) <sup>[[#fn:r882|882]]</sup> did not find a clear distinction between yield declines or increases in some breadbasket regions between the two temperature levels, they generally did find projections of decreasing yields in breadbasket regions when the effects of CO <sub>2</sub> fertilization were excluded. Iizumi et al. (2017) <sup>[[#fn:r883|883]]</sup> found smaller reductions in maize and soybean yields at 1.5°C than at 2°C of projected warming, higher rice production at 2°C than at 1.5°C, and no clear differences for wheat on a global mean basis. These results are largely consistent with those of other studies (Faye et al., 2018; Ruane et al., 2018) <sup>[[#fn:r884|884]]</sup> . In the western Sahel and southern Africa, moving from 1.5°C to 2°C of warming has been projected to result in a further reduction of the suitability of maize, sorghum and cocoa cropping areas and yield losses, especially for C3 crops, with rainfall change only partially compensating these impacts (Läderach et al., 2013; World Bank, 2013; Sultan and Gaetani, 2016) <sup>[[#fn:r885|885]]</sup> .

A significant reduction has been projected for the global production of wheat (by 6.0 ± 2.9%), rice (by 3.2 ± 3.7%), maize (by 7.4 ± 4.5%), and soybean, (by 3.1%) for each degree Celsius increase in global mean temperature (Asseng et al., 2015; C. Zhao et al., 2017) <sup>[[#fn:r886|886]]</sup> . Similarly, Li et al. (2017) <sup>[[#fn:r887|887]]</sup> indicated a significant reduction in rice yields for each degree Celsius increase, by about 10.3%, in the greater Mekong subregion ( ''medium confidence'' ; Cross-Chapter Box 6: Food Security in this chapter). Large rice and maize yield losses are to be expected in China, owing to climate extremes ( ''medium confidence'' ) (Wei et al., 2017; Zhang et al., 2017) <sup>[[#fn:r888|888]]</sup> .

While not often considered, crop production is also negatively affected by the increase in both direct and indirect climate extremes. Direct extremes include changes in rainfall extremes (Rosenzweig et al., 2014) <sup>[[#fn:r889|889]]</sup> , increases in hot nights (Welch et al., 2010; Okada et al., 2011) <sup>[[#fn:r890|890]]</sup> , extremely high daytime temperatures (Schlenker and Roberts, 2009; Jiao et al., 2016; Lesk et al., 2016) <sup>[[#fn:r891|891]]</sup> , drought (Jiao et al., 2016; Lesk et al., 2016) <sup>[[#fn:r892|892]]</sup> , heat stress (Deryng et al., 2014 <sup>[[#fn:r893|893]]</sup> , Betts et al., 2018) <sup>[[#fn:r894|894]]</sup> , flooding (Betts et al., 2018; Byers et al., 2018) <sup>[[#fn:r895|895]]</sup> , and chilling damage (Jiao et al., 2016) <sup>[[#fn:r896|896]]</sup> , while indirect effects include the spread of pests and diseases (Jiao et al., 2014; van Bruggen et al., 2015) <sup>[[#fn:r897|897]]</sup> , which can also have detrimental effects on cropping systems.

Taken together, the findings of studies on the effects of changes in temperature, precipitation, CO <sub>2</sub> concentration and extreme weather events indicate that a global warming of 2°C is projected to result in a greater reduction in global crop yields and global nutrition than global warming of 1.5°C ( ''high confidence'' ; Section 3.6).

<div id="section-3-4-6-2"></div>

<span id="livestock-production"></span>