The burgeoning global demand for food, feedstuff and biocombustible will require dramatic increases in wheat and other staple crop yields. According to Reynolds et al. (2009), increases of 50% in wheat yield are feasible, principally by improving potential yield (Hall and Richards 2013); however, in order to achieve this, it will be indispensable to reduce climate change and attenuate its effects upon crop yield (IPCC 2018, Asseng et al. 2015). This change, driven mainly by the anthropogenic emission of greenhouse gases (ONU 1998), leads to pronounced modification of the climate system, principally in temperature increase due to its effect on the balance between incoming solar radiation and outgoing infrared (thermal) radiation (IPCC 2007a). Such human influence appears to be the predominant long-term factor for global temperature rise (Medhaug et al. 2017), evident once short-term temperature changes caused by the El Niño Southern Oscillation (ENSO), volcanic aerosols and solar variation are taken into account (Foster and Rahmstorf 2011, De Saedeleer 2016).
Mean earth surface temperature has risen constantly since the twentieth century, with the decade of the 2000s being the hottest to date and where the number and period of sub-zero temperature days has reduced almost without exception in each country where the variable has been examined (Stocker et al. 2013); this, in spite of the hiatus reported by Chen and Tung (2014) though refuted by De Saedeleer (2016). Also, since 1980 increases have been observed for temperature and precipitation for the main wheat crop regions, amongst other crop regions (maize, rice and soybean) (Lobell et al. 2011).
The 2015 Paris Agreement includes the long-term global goal of promoting measures aimed at holding the increase in the global average temperature to well below 2 ºC (1.5°C, if possible) above pre-industrial levels (UNFCCC 2016). Future emissions are uncertain, since they depend upon complex dynamic systems determined by demographic change, socioeconomic development, technological advance and political will. IPCC scenarios have provided projections for mean global temperature rise of between 1.8 and 4°C for the decade 2090–2099 compared to 1980–1999, with ranges between 1.1 and 6.4°C (Nakicenovic et al. 2007, IPCC 2007b) and increments of 1°C in mean global temperatures in the period 2016–2035 versus1850-1900, under high greenhouse gas emission scenarios (Stocker et al. 2013). Furthermore, for each extra degree of air temperature rise, the atmosphere can retain an additional 7% of water vapour (Breón et al. 2013).
The majority of analyses of long-term climate change have focussed upon changes in mean values, with less emphasis on extremes (Alexander et al. 2006). According to Easterling et al. (2000), climatic extremes can be categorised into two groups: those simply based upon climate statistics (very low and very high daily temperatures or large quantities of daily or monthly precipitation occurring over a year) or more complex extremes that do not necessarily occur each year at a given site (for example, drought, flood or hurricane). Changes in temperature and precipitation extremes have been reported (Nicholls 1995, Karl and Knight 1998) and coincide with a world undergoing heating: diminution in cold extremes and a rise in hot extremes (Alexander et al. 2006, Easterling et al. 2000).
These changes would be expected to have a profound impact upon future crop yields, many of them deleterious (Worldbank 2013, Asseng et al. 2015, Zhao et al. 2017, IPCC 2018); in many areas of the world, climate change represents 32–39% of the annual global variability of yield for wheat, corn, soybeans and rice (Ray et al. 2015). Hence it is necessary to study the future environments each crop could face if current climate trends persist or even worsen. For the current work, we have taken wheat as a study model, given its historical importance in Argentina.
Wheat, one of the most important cereals globally, is widely used for human and animal consumption. Worldwide, more than 200 million hectares are harvested, with China and India currently being the largest producers (FAO 2019). Argentina is one of the producing countries, contributing 2.4% of world production (FAO 2019), and an exporter, since current annual production (19 million tonnes in 2017/2018) far outstrips internal market requirements of 6 to 7 million tonnes (Ministry of Agriculture, Argentina, 2018). This, added to the proximity of Brazil and Mercosur, configures the country as an important exporter of the crop.
Temperature and rainfall affect wheat grain yield and quality, and the difference between the potential number of grains and the number of grains finally obtained strongly depends upon the environment. Increases in mean temperature accelerate the accumulation of grain dry matter, with an important reduction in the grain filling period, resulting in reduced interception of radiation by the crop and hence reduced final grain weight (Slafer et al. 2003), Water stress, as well as frosts, are very damaging in the boot stage due to the occurrence of pollen grain meiosis (Slafer et al. 2003). Water deficit also lowers grain weight by reductions in the rate of fill (Martos Núñez 2003) and its duration. Regarding final grain weight, temperature and water availability during grain fill are two of the factors of major incidence.
Numerous studies have attempted to quantify wheat yield loss due to climate change. In Argentina, Abbate and Lázaro (2010) reported falls of 1.15 mg in potential grain weight (approximately 4% for a mean grain weight of 30 mg) for each degree Celsius increase in mean temperature during 35 days post-anthesis; while in Europe, Moore and Lobell (2015), reported yield losses of 2.5% for wheat, analysing long term trends of temperature and precipitation, and pointed to more detrimental effects in southern with respect to cooler regions (i.e. the United Kingdom and Ireland), which could not mitigate the impact with rainfall increase. Lobell et al. (2011), considering the whole growing season, reported a higher yield loss per degree increase, of 5.5%; furthermore, according to Zhao et al. (2017), without CO2 fertilization, effective adaptation and genetic improvement, each degree-Celsius increase in global mean temperature would, on average, reduce global yields of wheat by 6.0 ± 2.9% (Zhao et al. 2017), in concurrence with Asseng et al (2015), who also reported a figure of 6%. Wheat yield losses have been observed in spite of the beneficial effects of CO2 for C3 species (Lobell et al. 2011), and at low latitudes, the climate change effect will be more negative, specially at a high level of warming with nitrogen (N) stress, where there would be little true CO2 compensation for C3 species (Rosenzweig et al. 2014).
Schleussner et al. (2016) found substantial differences in impacts between 1.5 ºC and 2 ºC warming with local yield reduction at middle and low latitudes, especially for wheat and maize. Trends are particularly severe for temperature in wheat evaluated at regional and national level (Lobell et al. 2011), with 6 and 8% of yield losses for 1.5 ºC and 2 ºC respectively for wheat (Schleussner et al. 2016).
Many studies analysing the umbral and maximum temperature have found negative effects on wheat yield and/or quality, and there is a certain consensus that temperatures above 30°C lower yield and quality through reductions in the rate of starch deposition (Jenner 1994) and in dough strength (Randall and Mo 1990).
Added to the above, events between anthesis and maturity condition quality characteristics of the crop since during this stage the molecular weight of the storage proteins, which are major determinants of quality, is rising (Fraschina 2017). Water stress events after flowering modify the rate of deposition of distinct grain components, negatively affecting yield and quality.
In Argentina, the long-term tendency for 1940–2007 of various agroclimatic indices showed heating, principally due to increments in the minimum temperature (Fernández Long et al. 2008), although for the most recent period (1975–2007), the tendency was weaker and in the opposite direction in some meteorological stations (Fernández Long et al. 2008). In the Pampas region, rises in annual minimum temperatures of 2°C/century were reported during 1959–1998 (Rusticucci and Barrucand 2004). Additionally across this region, except for a small number of locations, the frost period decreased for the period 1940–2007, by a mean of 7 days per decade (Fernández Long et al. 2012).
Regarding precipitations, increases have been reported across the planet throughout the 20th century (Easterling et al. 2000). For example, the frequency of abundant precipitations has risen in South-Central USA and sectors of South America during 1950–2005, by 2 days per decade for the number of days/year, and the number of consecutive days without rain has reduced since 1960 (Alexander et al. 2006).
In the Pampas region of Argentina, Magrin et al. (2005) reported that mean precipitation rose by over 150 mm during the last thirty years of the 20th century compared to its beginning, especially between October and March. The changes varied over different months of the year, which could differentially affect different crop species. Marked precipitation increase has been observed in southern Brazil and northeastern Argentina from November to May (Berbery et al 2006; Re and Barros 2009). In north-eastern Argentina, the annual maximum amount of one- and five-day precipitation events increased from the 1970s to the 2000s; the higher frequencies of precipitation variability favoured extreme events post-2000 even during moderate extreme phases of the ENSO (Lovino et al 2018). Regarding the future, model projections have suggested that there could be an increase in the frequency of precipitation extremes over the La Plata Basin during future El Niño and La Niña events (Cavalcanti et al 2015).
In Argentina, climate studies have been carried out in the wheat region for mean temperatures (Fernández Long et al. 2008, 2012; Magrin et al. 2005, 2009), extreme temperatures in relation to frost (Fernández Long et al. 2012) and maximum and minimum temperatures (Rusticucci and Barrucand 2004), as well as on the impact of changes in temperature, radiation and precipitation on potential yields in wheat and other crops using simulators (Magrin et al. 2005, 2009). Although these studies provide a base for the current investigation, more exhaustive studies are necessary that also analyse changes in extreme rainfall variables, as proposed by Easterling et al. (2000), and temperature in relation to the umbral that prejudices wheat yield and quality, as well as the situation over recent years and differences within the wheat region. This type of study could serve as a model for the elaboration of perspectives for each cultivar and zone in particular.
The aim of the current work was to characterise the trend in the change in temperature and rainfall variables in two contrasting sites representative of the wheat growing region of Argentina. In contrast to Magrin et al. (2009), who studied changes in the centre of this region, we chose two sites, one towards the north of the region and the other towards the south, in order to discern whether there were differential effects of latitude, as has been suggested by modelling studies (Barros et al 1996).