Yes, while the human population keeps growing, climate change is likely to cause various detrimental effects on global agriculture and thereby food security. Like other climate change impacts these effects will increase with the magnitude of the warming, and most likely exponentially so. But what if we manage to limit warming to no more than 1.5 degrees? Here we take a closer look at what IPCC’s recent special report SR15 (‘Special Report on Global Warming of 1.5 °C’) has to say on that topic.

If the above picture on threshold effects of climate warming, derived from IPCC’s special report SR15, on the effects of global warming of 1.5 degrees, tells you one thing, it should be that many other impacts of climate change are even more urgent. But if you look closely at the bar for agricultural crops, the topic we discuss in great detail below, you see agriculture is already quite seriously affected at the current warming (grey range) and these effects will steadily increase towards 1.5 degrees and especially beyond 2 degrees. It’s yet another exclamation mark, emphasizing the importance of the global ambition to limit average warming to below 1.5 degrees…

It’s well documented that climate change can lead to net declines in agricultural productivity for major food staples across different climate zones. But why actually? Well, there’s a multitude of possible routes through which climate change exerts an influence. Let’s try to categorise:

Drier & wetter

Perhaps what comes to mind first by most people are simultaneously increasing droughts and extreme precipitation events (possibly leading to floods), both of which can lead to declining yields and even crop failures, and both of which are caused by shifts in the positioning of Earth’s general circulation and a general increase in evaporation and the water transportation capacity of the atmosphere (let’s say an intensification of this general circulation). It can carry a lot of water away from your land, or bring an even larger amount from lands far beyond. Hence, paradoxically, many farmers will experience a simultaneous increase of dry and wet events – at least in temperate climate zones, and also locally in the tropics. Enough on that for now, because water is just part of the story, and possibly not the largest.

Climate change affects the distribution of weather extremes. Although reality does not listen to statistical simplifications the top graph is largely true for temperature (shifting mean resulting in large increase in heat waves and therefore heat stress events) while the lower graph is a proper illustration to visualise a simultaneous increase in dry and wet extremes, that is also observed in many regions, including temperate zones and the tropics. All these extremes affect agriculture, and often more so than the changing average climate.

Ah, yes, ecology

Then, before we again forget the fact that humans, apart from that single pinch of sea salt and perhaps a rock-derived mineral supplement, exclusively feed on flora and fauna, ecological disturbances can have great effects. Like the changing occurrence and spread of fungal and other plagues & plant diseases, various pollinator declines and the increase of other mismatches between interdependent species [please read part 38-42 in our series on ecological climate mismatches], and ‘of course’ degradation of soil micro-ecosystems (worms, springtails – and the millions of other hard-working friends you and I don’t even have words for) that we are all fully unaware of that our daily dining depends on.

A benefit or two

And then there are also possible agricultural benefits of a higher average temperatures in cold regions and a locally lengthened growing season. This is not in areas where there is also a dry season (because there the growing season may actually decline), but in areas that used to have long winters – so in continental high latitudes. And possibly even ‘CO2 fertilisation’ can bring benefits to crop growth, although in agriculture, due to soil depletion, almost always the availability of other nutrients will be the growth limiting factor – or at least a nutrition-determining factor.

Heat stress

The most important detrimental effect of climate change on agriculture however seems quite mundane – the direct influence of the temperature rise itself, manifesting as heat stress – that crops are particularly vulnerable for during a specific time period in the growth cycle (the crop’s ‘thermal sensitive period’, for wheat for instance the ‘filling period’ of the grains). Increasing heat stress appears to be the major agricultural problem in the tropics – but more surprisingly may be of equal concern in temperate latitudes. And worst off, when it comes to heat stress on agriculture and anything else that lives, are probably the subtropics (for instance North Africa and the Middle East), where water availability is already low and further decreases, climate change is amplified and temperatures soar.

Summarising the above mechanisms: there are probably local agricultural benefits from climate change – but the net picture is probably a decline, especially under high emissions and warming scenarios. Because that’s what most of the impact studies that we just referred to are based on. Let’s say an on average 3 degrees warmer world (so twice that amount where strong local amplification occurs!).

With all that neatly sorted let’s take a look at what the latest IPCC Special Report on Global Warming of 1.5°C [coincidentally numbered IPCC SR15, it’s the fifteenth ‘special report’].

IPCC on wider context surrounding agriculture

Let’s first start with a collection of quoted text fragments that we think are relevant to understand the wider context around agricultural impacts of climate chance. These all come from chapter 3 on climate impacts.

Human migration away from agricultural-dependent communities as consequence of climate change:

“Poverty and disadvantage have increased with recent warming (about 1°C) and are expected to increase for many populations as average global temperatures increase from 1°C to 1.5°C and higher (medium confidence). Outmigration in agricultural-dependent communities is positively and statistically significantly associated with global temperature (medium confidence). Our understanding of the links of 1.5°C and 2°C of global warming to human migration are limited and represent an important knowledge gap.”

As the above statement shows the impacts of climate change on African agriculture deserve special attention, as most sub-Saharan countries are agriculture-dependent. There however attributing regional effects to specific global temperature thresholds remains difficult:

“Sub-Saharan Africa has experienced the dramatic consequences of climate extremes becoming more frequent and more intense over the past decades (Paeth et al., 2010; Taylor et al., 2017). In order to join international efforts to reduce climate change, all African countries signed the Paris Agreement. In particular, through their nationally determined contributions (NDCs), they committed to contribute to the global effort to mitigate greenhouse gas (GHG) emissions with the aim to constrain global temperature increases to ‘well below 2°C’ and to pursue efforts to limit warming to ‘1.5°C above pre-industrial levels’. The target of limiting global warming to 1.5°C above pre- industrial levels is useful for conveying the urgency of the situation. However, it focuses the climate change debate on a temperature threshold (Section 3.3.2), while the potential impacts of these global warming levels on key sectors at local to regional scales, such as agriculture, energy and health, remain uncertain in most regions and countries of Africa (Sections 3.3.3, 3.3.4, 3.3.5 and 3.3.6).

In specific African regions projected climatic changes do however create rather clear-cut backgrounds trends, at least beyond a global-average warming of 2 degrees, and sometimes also already at 1.5 degrees, that will undoubtedly have major consequences for regional agricultural productivity. Drying of the western Sahel region [in line with eastern Sahel greening] is a very notable example. Apart from a risk of water shortages here too increasing heat stress may be the main concern for crops:

“Western Sahel is projected by most models (80%) to experience the strongest drying, with a significant increase in the maximum length of dry spells (Diedhiou et al., 2018). Above 2°C, this region could become more vulnerable to drought and could face serious food security issues (Cross-Chapter Box 6 and Section 3.4.6 in this chapter; Salem et al., 2017; Parkes et al.,2018). West Africa has thus been identified as a climate-change hotspot with negative impacts from climate change on crop yields and production (Cross-Chapter Box 6 and Section 3.4.6; Sultan and Gaetani, 2016; Palazzo et al., 2017). Despite uncertainty in projections for precipitation in West Africa, which is essential for rain-fed agriculture, robust evidence of yield loss might emerge. This yield loss is expected to be mainly driven by increased mean temperature, while potential wetter or drier conditions – as well as elevated CO2 concentrations – could modulate this effect (Roudier et al., 2011; see also Cross-Chapter Box 6 and Section 3.4.6). Using Representative Concentration Pathway (RCP) 8.5 Coordinated Regional Climate Downscaling Experiment (CORDEX) scenarios from 25 regional climate models (RCMs) forced with different general circulation models (GCMs), Klutse et al. (2018) noted a decrease in mean rainfall over West Africa in models with stronger warming for this region at 1.5°C of global warming (Section 3.3.4).”

In some ways expected climate impacts for southwestern Africa are similar to those for the western Sahel, as both directly north and south of the tropics in western Africa increasing droughts are expected, for instance in Namibia and bordering countries. Here too increasing heat (stress) and drought are combined expected phenomena. Although drought is often a consequence of an increase in evaporation, locally in southwestern Africa a decrease in precipitation can exacerbate water shortages, already from 1.5 degrees of global average temperature rise. Droughts will increase further and spread across a wider region under 2 degrees of global average temperature rise:

“Areas in the south-western region, especially in South Africa and parts of Namibia and Botswana, are expected to experience the largest increases in temperature (Section 3.3.2; Engelbrecht et al., 2015; Maúre et al.,2018). The western part of southern Africa is projected to become drier with increasing drought frequency and number of heat waves towards the end of the 21st century (Section 3.3.4; Engelbrecht et al., 2015; Dosio, 2017; Maúre et al., 2018). At 1.5°C, a robust signal of precipitation reduction is found over the Limpopo basin and smaller areas of the Zambezi basin in Zambia, as well as over parts of Western Cape in South Africa, while an increase is projected over central and western South Africa, as well as in southern Namibia(Section 3.3.4). At 2°C, the region is projected to face robust precipitation decreases of about 10–20% and increases in the number of CDD [consecutive dry days], with longer dry spells projected over Namibia, Botswana, northern Zimbabwe and southern Zambia.”

Eastern Africa, including the eastern Sahel and as far south as Tanzania can expect a more capricious monsoon, with a combined increase of extreme precipitation events (and possible subsequent floods) and dry spells. On average, while temperature rises and therefore evaporation also increases, for East Africa a nett increase in precipitation is expected, notably for the eastern Sahel (Sudan) and the Horn of Africa – albeit with local exceptions, like central Ethiopia. Due to this mixed signal, the possible increased regional spread of the monsoon and the increase of evaporation, it is notable that projections show the number of consecutive wet days will decrease, while the consecutive dry days will increase. Yes, it is a paradox, but regions that may become wetter, can still become drier:

“For Eastern Africa, Osima et al. (2018) found that annual rainfall projections show a robust increase in precipitation over Somalia and a less robust decrease over central and northern Ethiopia (Section 3.3.3). The number of CDD and CWD are projected to increase and decrease, respectively (Section 3.3.4). These projected changes could impact the agricultural and water sectors in the region (Cross-Chapter Box 6 in this chapter and Section 3.4.6).”

Increasing drought risk is an even larger concern for the Mediterranean Basin (both North Africa and southern Europa) and the Middle East, due to combined effects of amplified temperature rise and therefore increasing evaporation and local decreases in precipitation, due to an intensification and northward shifts of the Hadley Cell, with its northern border of dominant high pressure systems. These combined trends leading to water shortages are already underway for decades, indicating important effects for this region are measurable at a global average warming well below 1.5 degrees. Uncertainties are increased when trying to assess regional adaptation capacity:

“Droughts in the Mediterranean Basin and the Middle East Human society has developed in tandem with the natural environment of the Mediterranean basin over several millennia, laying the groundwork for diverse and culturally rich communities. Even if advances in technology may offer some protection from climatic hazards, the consequences of climatic change for inhabitants of this region continue to depend on the long-term interplay between an array of societal and environmental factors (Holmgren et al., 2016). As a result, the Mediterranean is an example of a region with high vulnerability where various adaptation responses have emerged. Previous IPCC assessments and recent publications project regional changes in climate under increased temperatures, including consistent climate model projections of increased precipitation deficit amplified by strong regional warming (Section 3.3.3; Seneviratne et al., 2012; Christensen et al., 2013; Collins et al., 2013; Greve and Seneviratne, 2015). The long history of resilience to climatic change is especially apparent in the eastern Mediterranean region, which has experienced a strong negative trend in precipitation since 1960 (Mathbout et al., 2017) and an intense and prolonged drought episode between 2007 and 2010 (Kelley et al., 2015). This drought was the longest and most intense in the last 900 years (Cook et al., 2016). Some authors (e.g., Trigo et al., 2010; Kelley et al., 2015) assert that very low precipitation levels have driven a steep decline in agricultural productivity in the Euphrates and Tigris catchment basins, and displaced hundreds of thousands of people, mainly in Syria. Impacts on the water resources (Yazdanpanah et al., 2016) and crop performance in Iran have also been reported (Saeidi et al., 2017). Many historical periods of turmoil have coincided with severe droughts, for example the drought which occurred at the end of the Bronze Age approximately 3200 years ago (Kaniewski et al., 2015). In this instance, a number of flourishing eastern Mediterranean civilizations collapsed, and ruralsettlements re-emerged with agro-pastoral activities and limited long-distance trade. This illustrates how some vulnerable regions are forced to pursue drastic adaptive responses, including migration and societal structure changes. The potential evolution of drought conditions under 1.5°C or 2°C of global warming (Section 3.3.4) can be analysed by comparing the 2008 drought (high temperature, low precipitation) with the 1960 drought (low temperature, low precipitation) (Kelley et al., 2015). Though the precipitation deficits were comparable, the 2008 drought was amplified by increased evapotranspiration induced by much higher temperatures (a mean increase of 1°C compared with the 1931–2008 period in Syria) and a large population increase (from 5 million in 1960 to 22 million in 2008). Koutroulis et al. (2016) reported that only 6% out of the total 18% decrease in water availability projected for Crete under 2°C of global warming at the end of the 21st century would be due to decreased precipitation, with the remaining 12% due to an increase in evapotranspiration. This study and others like it confirm an important risk of extreme drought conditions for the Middle East under 1.5°C of global warming (Jacob et al., 2018), with risks being even higher in continental locations than on islands; these projections are consistent with current observed changes (Section 3.3.4; Greve et al., 2014). Risks of drying in the Mediterranean region could be substantially reduced if global warming is limited to 1.5°C compared to 2°C or higher levels of warming (Section 3.4.3; Guiot and Cramer, 2016). Higher warming levels may induce high levels of vulnerability exacerbated by large changes in demography.”

Summarising the context

All right. We’ve assessed some important context to possible climate change impacts on agriculture. Again it appears temperature rise is the most important factor, as evaporation is a dominant driver of droughts and even regions with increasing net precipitation can experience an increase in dry spells. Factors increasing vulnerability are projected precipitation declines, local amplification of the temperature rise and both current and future trends of increasing food demand due to population growth and possibly already existing water shortages due to current extraction and depletion. Especially in Africa, the Mediterranean and the Middle East large impacts on agriculture can already be expected at a global average temperature rise of 1.5 degrees.

That one crucial factor for all of life of Earth: water.

Now of course as drought is a major risk in many regions the general water availability is a major underlying factor, that is also influenced by climate change. So a good thing, as part of the general context, to also take a quick look at some relevant statements from paragraph 3.4.2., about freshwater resources and water availability.

It’s important to note that our general starting point here is that we already have an enormous problem, even without future climate change impacts:

“Over the past century, substantial growth in populations, industrial and agricultural activities, and living standards have exacerbated water stress in many parts of the world, especially in semi-arid and arid regions such as California in the USA (AghaKouchak et al., 2015; Mehran et al., 2015). Owing to changes in climate and water consumption behaviour, and particularly effects of the spatial distribution of population growth relative to water resources, the population under water scarcity increased from 0.24 billion (14% of the global population) in the 1900s to 3.8 billion (58%) in the 2000s. In that last period (2000s), 1.1 billion people (17% of the global population) who mostly live in South and East Asia, North Africa and the Middle East faced serious water shortage and high water stress (Kummu et al., 2016).”

Again, important to stress, when it comes to both water and food security the continuous high population growth in worst affected regions is a major factor, one that is locally larger than climate change – at least in the near future (decades timescale) and under relatively constrained warming scenarios, with temperature below 2 degrees:

“Over the next few decades, and for increases in global mean temperature less than about 2°C, AR5 concluded that changes in population will generally have a greater effect on water resource availability than changes in climate. Climate change, however, will regionally exacerbate or offset the effects of population pressure (Jiménez Cisneros et al., 2014).”

However, if we keep out human demography, climate change will also, further increase general water shortage for the human population. These impacts will double between 1.5 degrees warming to 2 degrees warming, from a 4 percent increase in general water shortage under 1.5 degrees warming to 8 percent under 2 degrees warming – which means increasing the climate ambition from 2 to 1.5 degrees prevents water shortage for an additional 184-270 million people:

“Assuming a constant population in the models used in his study, Gerten et al. (2013) determined that an additional 8% of the world population in 2000 would be exposed to new or aggravated water scarcity at 2°C of global warming. This value was almost halved – with 50% greater reliability – when warming was constrained to 1.5°C. People inhabiting river basins, particularly in the Middle East and Near East, are projected to become newly exposed to chronic water scarcity even if global warming is constrained to less than 2°C. Many regions, especially those in Europe, Australia and southern Africa, appear to be affected at 1.5°C if the reduction in water availability is computed for non-water-scarce basins as well as for water-scarce regions. Out of a contemporary population of approximately 1.3 billion exposed to water scarcity, about 3% (North America) to 9% (Europe) are expected to be prone to aggravated scarcity at 2°C of global warming (Gerten et al., 2013). Under the Shared Socio-Economic Pathway (SSP)2 population scenario, about 8% of the global population is projected to experience a severe reduction in water resources under warming of 1.7°C in 2021–2040, increasing to 14% of the population under 2.7°C in 2043–2071, based on the criteria of discharge reduction of either >20% or >1 standard deviation (Schewe et al., 2014). Depending on the scenarios of SSP1–5, exposure to the increase in water scarcity in 2050 will be globally reduced by 184–270 million people at about 1.5°C of warming compared to the impacts at about 2°C.”

Now the main problem with water, climate change and agriculture, could well be ground water. Ground water depletion is already taking place on all continents, again due to an increasing human population, leading to increasing food and water demand and agricultural intensification. Problem is, it is relatively difficult to monitor – as many aquifers are very deep and irrigation often started before any baseline measurements were executed. Additional problem: while irrigation demand is likely to further increase, climate change will probably also have direct effects on ground water – and of course on irrigation demand:

“Working Group II of AR5 concluded that the detection of changes in groundwater systems, and attribution of those changes to climatic changes, are rare, owing to a lack of appropriate observation wells and an overall small number of studies (Jiménez Cisneros et al., 2014).”

“In some regions, groundwater is often intensively used to supplement the excess demand, often leading to groundwater depletion. Climate change adds further pressure on water resources and exaggerates human water demands by increasing temperatures over agricultural lands (Wada et al., 2017). Very few studies have projected the risks of groundwater depletion under 1.5°C and 2°C of global warming. Under 2°C of warming, impacts posed on groundwater are projected to be greater than at 1.5°C ( low confidence ) (Portmann et al., 2013; Salem et al., 2017).”

“Portmann et al. (2013) indicated that 2% (range 1.1–2.6%) of the global land area is projected to suffer from an extreme decrease in renewable groundwater resources of more than 70% at 2°C, with a clear mitigation at 1.5°C. These authors also projected that 20% of the global land surface would be affected by a groundwater reduction of more than 10% at 1.5°C of warming, with the percentage of land impacted increasing at 2°C.

IPCC SR15’s direct quotes about agricultural impacts

Probably a good point to take a look at what IPCC SR15 has to say about agriculture specifically. That should be addressed in paragraph 3.4.6: ‘Food, Nutrition Security, and Food Production Systems’.

Although the paragraph of course deals with other food production systems (fisheries to name one) there are a lot of relevant quotes concerning agricultural impacts. Below these are presented in order of appearance.

Again, an important starting point is noting that agricultural effects of climate change are (and have been) already felt, at a global average temperature rise of approximately 1 degree (and earlier):

“[…]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) to America (Cho and McCarl, 2017) and Europe (Ramirez-Cabral et al., 2016), 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).

Overall the already witnessed effects are detrimental:

“Temperature and precipitation trends have reduced crop production and yields, with the most negative impacts being on wheat and maize (Lobell et al., 2011), 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).”

…while locally beneficial effects have been observed:

“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), and may make it possible to have more than one harvest per year (B. Chen et al., 2014; Sun et al., 2015).”

A very interesting aspect is the direct influence of increased atmospheric concentrations of greenhouse gases, not just CO2, but also (tropospheric, anthropogenic) ozone. Ozone decreases crop yield, while CO2 – due to ‘CO2 fertilisation’ – may itself actually lead to increased crop yields, however (as stated in the introduction of this article) with a major downside: a relative decrease of the crop’s nutritional value. In addition CO2’s crop yield increasing effect is smaller than and thereby overcompensated by the detrimental effect of the associated temperature rise:

“The rise in tropospheric ozone has already reduced yields of wheat, rice, maize and soybean by 3–16% globally (Van Dingenen et al., 2009). In some studies, increases in atmospheric CO2 concentrations were found to increase yields by enhancing radiation and water use efficiencies (Elliott et al., 2014; Durand et al., 2017). In open-top chamber experiments with a combination of elevated CO2 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). 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 CO2 concentrations (Porter et al., 2014). For instance, McGrath and Lobell (2013) indicated that production stimulation at increased atmospheric CO2 concentrations was mostly driven by differences in climate and crop species, whilst yield variability due to elevated CO2 was only about 50–70% of the variability due to climate. Importantly, the faster growth rates induced by elevated CO2 have been found to coincide with lower protein content in several important C3 cereal grains (Myers et al., 2014), although this may not always be the case for C4 grains, such as sorghum, under drought conditions (De Souza et al., 2015). Elevated CO2 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, 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). Overall, the effects of increased CO2 concentrations alone during the 21st century are therefore expected to have a negative impact on global food security (medium confidence).”

Projected changes in temperature and precipitation are also likely to have further negative effects on global agriculture. Again these effects occur at warming well below global average of 1.5 degrees, as for instance in the below impact study a 1 to 2 degrees local warming is evaluated. As almost all of Earth’s land masses have above-average warming, these values have already been reached in many places, including much of Europe and Africa.

“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).”

Studies that focus on the associated global average temperature thresholds project similar effects, often with a significant decrease in expected agricultural losses comparing 2 degrees to 1.5 degrees global average warming:

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) 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) highlighted that cropland stability declines rapidly between 1°C and 3°C of warming, whilst Bassu et al. (2014) 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) reported similar effect for tropical regions. Niang et al. (2014) projected significantly lower risks to crop productivity in Africa at 1.5°C compared to 2°C of warming. Lana et al. (2017) 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) found that limiting warming to 1.5°C compared to 2°C would reduce maize yield losses over drylands.

“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).”

In a broader overview all major food staples seem to be affected in a temperature-dependent manner, suggesting a global net decline in average agricultural productivity, increasing with global average warming:

“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). Similarly, Li et al. (2017) 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).”

These increasing climate extremes, as mentioned in the introduction of this article, deserve special attention – as possible more specific crop yield declining factors:

“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), increases in hot nights (Welch et al., 2010; Okada et al., 2011), extremely high daytime temperatures (Schlenker and Roberts, 2009; Jiao et al., 2016, Lesk et al., 2016), drought (Jiao et al., 2016; Lesk et al., 2016), heat stress (Deryng et al., 2014, Betts et al., 2018), flooding (Betts et al., 2018; Byers et al., 2018), and chilling damage (Jiao et al., 2016), while indirect effects include the spread of pests and diseases (Jiao et al., 2014; van Bruggen et al., 2015), which can also have detrimental effects on cropping systems.”

On cropland impacts of climate change in the range of 1.5 to 2 degrees warming the IPCC concludes the following:

“Taken together, the findings of studies on the effects of changes in temperature, precipitation, CO2 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).”

Although we focus here on cropland agriculture, being the most crucial factor for human food security, to further emphasize the combined risk on other food systems we add this final citation about livestock, that due to various climate-induced mechanisms is expected to be almost equally detrimentally affected by a global average warming in the range up to 2 degrees:

“Globally, a decline in livestock of 7–10% is expected at about 2°C of warming, with associated economic losses between $9.7 and $12.6 billion (Boone et al., 2018).”

Should we conclude anything in addition to all the above? We think the report speaks for itself. There’s just one thing we should not forget. Clearly there are major effects already witnessed, and clearly these effects will grow larger as the world warms further to 1.5 degrees and then again (significantly!) further until it warms to 2 degrees Celsius. But this is the reality of ‘the global ambition’. The reality of the combined national emission targets for 2030 is still beyond 3 degrees and actual policy is even worse – still uncomfortably close to the business as usual emissions scenario.

So, yes, apart from the ecological damage, apart from major ice sheet thresholds and sea level rise and all the other climate impacts, also considering the agricultural impacts we should definitely aim for 1.5 degrees Celsius in stead of 2 degrees [and IPCC SR15 actually shows this is still quite feasible!]. But all these scenarios require one thing: emissions reductions. Work. If we do work, we have a world to win – comparing 1.5 degrees to everything that is worse. Many millions of people will be able to eat, people that would otherwise face hunger, with all the other ill effects that hunger can trigger. Stabilising the climate is stabilising an entire planet.

© Rolf Schuttenhelm | www.bitsofscience.org