Editing IPCC:AR6/WGII/Chapter-5 (section)

==== 5.4.3.2 Projected impacts on major crop production ====

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AR5 [[IPCC:Wg2:Chapter:Chapter-7|Chapter 7]] estimated global crop yield reduction due to climate change to be about 1% per decade ( [[#Porter--2014|Porter et al., 2014]] ), similar to the previous assessment reports ( [[#Porter--2019|Porter et al., 2019]] ). Additional research confirms that climate change will disproportionately affect crop yields among regions, with more negative than positive effects being expected in most areas, especially in currently warm regions, including Africa and Central and South America ( ''high confidence'' ).

A systematic literature search between 2014 and 2020 resulted in about 100 peer-reviewed papers that simulated crop yields of four major crops (maize, rice, soybean and wheat) using Coupled Model Intercomparison Project Phase 5 (CMIP5) data ( [[#Hasegawa--2021b|Hasegawa et al., 2021b]] ). Most studies focus on the relative change in crop yields due to climate change but do not consider technological advances. Nevertheless, they provide useful insights into time-, scenario- and warming-degree-dependent impacts of climate change.

The impact of climate change on crop yield without adaptation projected in the 21st century is generally negative even with the CO 2 fertilisation effects, with the overall median per-decade effect being −2.3% for maize, −3.3% for soybean, −0.7% for rice and −1.3% for wheat, which is consistent with previous IPCC assessments ( [[#Porter--2014|Porter et al., 2014]] ). The effects vary greatly within each crop, timeframe and RCP, but show a few common features across crops (Figure 5.6a ''')''' . Differences in the projected impacts between RCPs are not pronounced by mid-century. From then onward, the negative effect becomes more pronounced under RCP8.5, notably in maize. Rice yields show less variation across models than other crops presumably because simulations are mostly under irrigated conditions. A part of the uncertainty in the projection is due to regional differences (Figure 5. 6b). Negative impacts on cereals are projected in Africa and Central and South America at the end of the century, which agrees with the previous studies ( [[#Aggarwal--2019|Aggarwal et al., 2019]] ; [[#Porter--2019|Porter et al., 2019]] ).

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'''Figure 5.6 |''' '''Projected yield changes relative to the baseline period (2001–2010) without adaptation and with CO''' '''2''' '''fertilisation effects (Hasegawa et al.''' , '''2021b).''' The box is the interquartile range (IQR), and the middle line in the box represents the median. The upper and lower end of whiskers are median 1.5 × IQR ± median. Open circles are values outside the 1.5 × IQR. '''(a)''' At different time periods (near future, NF, baseline to 2039; mid-century, MC, 2040–2069; end-century, EC, 2070–2100) under three RCPs, and '''(b)''' at different regions at EC.

The differences due to regions, RCPs and timeframes are related to the current temperature level and degree of warming (Figure 5.7). The projected effects of climate change are positive where current annual mean temperatures ( ''T'' ave ) are below 10°C, but they become negative with ''T'' ave above around 15°C. At ''T'' ave &gt; 20°C, even a small degree of warming could result in adverse effects. In maize, negative effects are apparent at almost all temperature zones. A new study using the latest climate scenarios (Coupled Model Intercomparison Project Phase 6, CMIP6) and global gridded crop model ensemble projected that climate change impacts on major crop yields appear sooner than previously anticipated, mainly because of warmer climate projections and improved crop model sensitivities ( [[#Jägermeyr--2021|Jägermeyr et al., 2021]] ).

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'''Figure 5.7 |''' '''Projected yield changes relative to the baseline period (2001–2010) without adaptation and with CO''' '''2''' '''fertilisation effects (Hasegawa et al''' '''.''' ''', 2021b).''' '''(a)''' Mid-century (MC, 2040–2069) and end-century (EC, 2070–2100) projections under three RCP scenarios as a function of current annual temperature ( ''T'' ave ), '''(b)''' as a function of global temperature rise from the baseline period by three ''T'' ave levels. See Figure. 5.6 for legends.

As noted in [[#5.3.1|Section 5.3.1]] , most simulations do not fully account for responses to pests, diseases, long-term change in soil, and some climate extremes ( [[#Rosenzweig--2014|Rosenzweig et al., 2014]] ), but studies are emerging to include some of these effects. For example, based on the temperature response of insect pest population and metabolic process, global yield losses of rice, maize and wheat are projected to increase by 10–25% per degree Celsius of warming ( [[#Deutsch--2018|Deutsch et al., 2018]] ). Rising temperatures reduce soil carbon and nitrogen, which in turn exacerbate the negative effects of +3°C warming on yield from 9% to 13% in wheat and from 14% to 19% in maize ( [[#Basso--2018|Basso et al., 2018]] ).

A few studies have examined possible occurrences of tele-connected yield losses (5.4.1.2) using future climate scenarios. Tigchelaar (2018) estimated that, for the top four maize-exporting countries, the probability that simultaneous production losses greater than 10% occur in any given year increases from 0% to 7% under 2°C warming and to 86% under 4°C warming. Gaupp (2019) estimated that risks of simultaneous failure in maize would increase from 6% to 40% at 1.5°C and to 54% at 2°C warming, relative to the historical baseline climate. Large-scale changes in SST are the major factors causing simultaneous variation in climate extremes, which are projected to intensify under global warming ( [[#Cai--2014|Cai et al., 2014]] ; [[#Perry--2017|Perry et al., 2017]] ). Consequently, risks of simultaneous yield losses in major food-producing regions will also increase with global warming levels above 1.5°C ( ''medium confidence'' ). Further examination is needed for the effects of spatial patterns of these extremes on breadbaskets in relation to SST anomalies under more extreme climate scenarios.

Future surface ozone concentration is highly uncertain ( [[#Fiore--2012|Fiore et al., 2012]] ; [[#Turnock--2018|Turnock et al., 2018]] ); it is projected to increase under RCP8.5 and decrease under other RCPs depending largely on different methane emission trajectories because methane is an important precursor of ozone. Methane, therefore, reduces crop yield both from climate warming and ozone increase ( [[#Avnery--2013|Avnery et al., 2013]] ). [[#Shindell--2016|Shindell (2016)]] estimated yield losses of four major crops (to be 25±11% by 2100 under RCP8.5, as a net balance of the positive effect of CO 2 (15±2%) and negative effects of warming (35±10%) and ozone (4.0±1.3%), and that 62% of the yield loss was attributable to methane. This points to the importance of reducing methane and other precursors of ozone as an effective adaptation strategy ( ''medium evidence'' , ''high agreement'' ).

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