Climate change, natural resources and effects on crop productivity

Climate change (CC). There is broad consensus in the global scientific community that the increasing accumulation of greenhouse gases in the atmosphere (mainly CO₂) is associated with the observed increase in global average atmospheric temperature. This increase is projected to vary emission scenarios of greenhouse gases during the century XXI. In RCP2.6 benign scenario, the average increase in air temperature is + 1 °C, while for the most severe scenario RCP8.5, is projected increased +3.7 °C (^{IPCC Working Group I, 2013}). The models also predict the intensification of extreme temperature and precipitation compared to the historically observed between regions, years, seasons and daily cycles (^{Easterling et al., 2000}; ^{Ahmed et al., 2009}).

The literature review of ^{Tubiello and Rosenzweig (2008)} led to conclude that a moderate heating (up to 2 °C) in the early part of the century, could benefit agricultural production and pasture in temperate regions of the world while reducing agricultural production in semi-arid and tropical regions. Instead, the additional heating the second half of the century reduce production in all regions.

Warming increases the vapor pressure of water, so that the global rainfall will increase, although subject to the back marked variability.

The global warming associated with higher content of CO₂ and water vapor, and its variations between regions, years, seasons and days, it has profound implications agronomic mostly negative. They are affected significantly, a) photosynthesis (^{Ort et al., 2011}); b) the phenology of crops (^{Ainsworth and Ort, 2010}); c) abiotic stresses (^{Jenks et al., 2007}) and biotic (^{Zavala et al., 2008}); d) the soil moisture regime due to increased evapotranspiration demand of the atmosphere, and to the changing dynamics of surface and infiltration of rainwater runoff; e) water erosion; and f) the crop yields.

It has made copious research in the world to design adaptation strategies for agricultural ia. 1) studies on the evolution of the genetic susceptibility of improved varieties corn and soybean to drought and extreme heat in the US (^{Roberts and Schlenker, 2010}); 2) projected maize yield irrigated or temporary regional level (Wang et al., 2011; ^{Tinoco-Rueda et al., 2011}); 3) risk of impact on crop yields (^{Conde et al., 2004}); and 4) conceptual developments (^{Ainsworth and Ort, 2010}). Most Third World countries deficient food production -Mexico Among them- and where 75 percent of the total population, is located in the semiarid and tropical region of the world where the effect of global warming will be more severe than in the countries of the temperate region (^{Rosenzweig and Liverman, 1992}). There is also consensus that underdeveloped character there will be less able to adopt effective strategies to reduce the acute effects on food production (^{Morton, 2007}; ^{Hertel and Rosch, 2010}).

Soil and freshwater resources. The Mexican countryside has just over 31 million hectares of arable land, discreetly distributed to across the country, which in itself is of rugged terrain. Half north of the country is arid or semiarid climate, while the rest is semiarid climates, subhumid or humid, warm temperate thermal regimes. For historical reasons, not all the land of existing work corresponds to quality agricultural land and vice versa, not all quality agricultural lands of the country are used as arable land. The 33 percent of the land of current work under temporary agricultural quality not being part of the agricultural provinces of marginal land and low productivity (^{Gonzalez et al., 1991}; Turrent et al., 2014). Its soils are thin and when deep, have sharply limited availability of rainwater, which confers high risk of drought.

In addition to the amount of land under temporary work, the field has equipped 6.3 million hectares of irrigation infrastructure (^{Montesillo, 2006}). Historically, the use of arable land of Mexico (rainfed and irrigated), has been largely "extractivist" having significantly decreased its agricultural quality. The country has accumulated large deferred investment management and conditioning of its 31 million hectares of arable land. At present, most of the more than 13 million hectares of arable land located on slopes is handled without protection against water erosion. Nor it has protected agricultural soil against accelerated its organic matter decline, or been driven crop rotation.

The 425 of the best arable land and 57% temporary land of lesser quality are located on slopes (^{Turrent 1986}), being largely unprotected against loss by water erosion. After the disappearance of the Directorate of Soil Conservation of SAGARPA in the middle-1980s, the State's efforts to protect the cultivated hillsides has been minimal. The SAGARPA held in 2012 an agreement with CIMMYT, for Sustainable Improvement Traditional Agriculture program (MasAgro), which will last 10 years (^{Del Toro, 2012}). This program involves changing paradigm of traditional agriculture by conservation agriculture, including protection against erosion. However, this change has been analyzed and questioned as a solution for sustainable hillside farming, small (Turrent et al., 2014).

The country receives 1 530 km³ of water in the form of annual rainfall. The water infrastructure retains 147 km³ (Anónimo, 1988); 410 km³ drained to the sea, almost consumptive use and the rest, infiltrates and evapotranspires. The 19% of the national annual average runoff occurs in northern and central highlands, which make up half of the national territory, which has built most of the country's irrigation infrastructure. The 67% of the average sea surface runoff occurs in the southeast, which corresponds to a quarter of the national territory (Anónimo, 1988), and where the irrigation infrastructure is underdeveloped.

There is a significant "deferred conditioning" in the fraction of irrigated land. Most of the dams built in the last century lacks the necessary drainage works to protect the lands of their progressive salinization. The 10% of these agricultural land has developed salinity problems. Irrigation efficiency at national level is just 46 -average 36.6 percent in irrigation districts and units 56.5 in irrigation- (^{Arreguin-Cortes et al., 2004}). The low efficiency is due to several factors ia, conduction losses from the dam and water application on the plot, the lack of investment to condition the pressurized irrigation system.

Agricultural strategies to address climate change

Adaptations in the infrastructure and operation of the field

Increase the availability of water for crops. There is global consensus that the best adaptation to climate change for countries that have reserves of fresh water and arable land is the increase in the area under irrigation (^{Morton, 2007}). This maxim must be extended to increase the efficiency of irrigation water in the existing irrigation infrastructure and also to promote the cycle of rainwater basins and micro-managed agriculturally approach as possible to the efficiency of the ecosystem undisturbed. Investment is needed to correct the "deferred conditioning" in the existing irrigation infrastructure and management, to significantly increase the efficiency of irrigation water.

The functionality of the larger fraction of the irrigation infrastructure of Mexico is challenged by the projection of lower precipitation in the northern half during this century, with its concomitant restriction of water in dams. It also predicts that by warming, crops demand higher irrigation levels, which may lead to the abandonment of irrigation areas where the resource is limited and inefficiently exploited. Significantly increase irrigation efficiency is the top priority strategy to protect the functionality of this resource.

Additionally, Mexico has in its south-east, significant reserves of fresh water, agricultural land quality region, belonging to agroecosystems and mild weather in the autumn-winter (^{Turrent et al., 2004a}) agricultural cycle. In it 67% of the total runoff occurs, almost unused for irrigation (Anónimo, 1988). The authors of this study estimate that the use of 60% of those runoffs, applied in 2/3 of the arable land currently underutilized agricultural quality, double the total area under irrigation in the country.

In point of time, there are options to enhance the availability of water for crops ia, a) protect the soil against water erosion; b) increasing infiltration of rainwater; c) perform works for "rainwater harvesting"; and d) protect the contents of soil organic matter. Such actions would recognize agricultural typological differences of Mexico. For its limitation on land resources, peasant agriculture requires multiobjective technologies, incorporating at least a) the significant increase in household income; b) protection against soil erosion; c) the perennial-annual crop diversification and annual turnover; and d) access to services for production and marketing.

For its social significance and its relative backwardness, peasant agriculture should receive the highest priority. Intercropping fruit trees and cornfield technology is an example that meets those objectives (^{Cortes et al., 2007}). By its scale of operations, business type requires intensive technologies in capital rather than labor, with Conservation Agriculture appropriate paradigm (^{Kassam et al., 2009}).

Protect and exploit plant genetic diversity. The rich plant genetic diversity (crops and their wild relatives) that Mexico has is a powerful tool for adaptation to CC and as such should be protected and exploited. This diversity is associated with food security and the multicultural richness of its cuisine. Conventional types of germplasm conservation in situ and ex situ although necessary, may not be sufficient for adaptation to CC in Mexico. It requires support, improve and stimulate peasant agriculture with practices of indigenous genetic improvement (MGA) (^{Turrent and Serratos, 2004b}) has identified and exploited the useful evolutionary polymorphism of species domesticated in Mexico. Peasant agriculture grows every year between 10 and 10 different genotypes of maize, nearly 4.5 million hectares. This area includes all agro-climatic conditions in the country. Equivalent to a mega genetic experiment "in parallel" in which recombination of 50 000 genes possessed the maize genome, which can only happen in Mexico occurs.

This is because the character of maize domestication center and uninterrupted process MGA made since prehistoric times by its 62 ethnic groups. Currently, this mega experiment is executed and observed by more than 2 million farmers experts in practices that make the MGA. There are probably genotypes (some unborn) combining in its genome to optimal alleles to address climate change in Mexico, which are expressed in extreme conditions. High priority is supporting this system, but above all, it is to preserve the agroecosystem occupied by native corn and agricultural farming activity in dynamic and steady progress. Finally, high priority is to protect the genetic integrity of native corn against contamination of transgenic DNA, the ban on planting in the open, the only possible protective action (^{Álvarez and Piñeyro, 2014}).

Adaptations agricultural research in short and intermediate terms

These adjustments do not require new scientific knowledge; they fall within the scope of applied research "downhill". The scope provided are transitional utility; however, its merit is that it can be addressed immediately and resolved in the short or intermediate term.

Adjustments to breeding programs. The generation and release of improved varieties basic grains in Mexico has relied so far in the progressive development of elite germplasm, whose genetic diversities are subsets of the total biodiversity of each of the species. In improved varieties obtained from those, it further enhances the uniformity of genetic diversity to overcome agronomically varieties that must be replaced. This has worked well in the relatively favorable historical agro-climatic conditions for which those varieties were developed. However, the CC impose environmental efforts for not be adapted. It is necessary to methodological adjustments to the current plant breeding, which should benefit from the diversity between and within each species elite germplasm.

In these settings will be weighted differently to historically ia, a) the genetic diversity of cultivars front uniformity; b) performance under extreme conditions restricting vs performance in benign conditions; c) environments selection-alternation environmental-assessment in the process of plant breeding, where the gametic selection, the response of germplasm and their genetic diversity and environmental genetic interaction, make robust varieties and attributes necessary to cushion the CC. A procedure could be to recycle the elite germplasm each crop in various agro-climatic regions and within each other to get in the shortest possible time, improved genetic material that can tolerate moderate environmental stresses CC. The use of more radical and costly for advanced conditions CC of the second half of the century adaptations most likely be necessary.

Adjustments to the agronomic management programs. Should be considered ia, a) reintroducing more tolerant of environmental stresses earlier crops of CC; b) new cropping patterns; and c) changes in agricultural paradigms. These actions will complement the investment to correct conditioning so far delayed on irrigation and temporary, previously cited in this essay, and to condition the reserves of arable land and freshwater in the country. For its social importance, of agriculture in small hillside it is of utmost importance for food security in Mexico. That is mainly engaged in planting basic grains under monoculture, being unprotected from water erosion.

According Turrent et al. (2014), nearly 6 million hectares (mdha) cultivated with rainfed maize in Mexico, 1.4 mdha are slopes with deep soils and more than 2.3 mdha are slopes with shallow soils. Obviously, this resource of the Nation must be protected against erosion, while being managed sustainably for food production in the CC. The INIFAP and COLPOS are developing a multi-objective technology for sustainable agricultural management slopes foster benign, intended for small production units. By design, this technology protects the soil from erosion, reduces rainwater runoff, protect soil organic matter, and significantly increase household income and fixation of atmospheric carbon. All these attributes are central to face the CC. This technology, known as interleaved cornfields in fruit trees (MIAF) (^{Cortés et al., 2007}) must be further adapted to the total agro-climatic diversity.

Adaptations to long-term agricultural technology

The list of new technological knowledge required, is a complex task because it involves scientific knowledge not yet available. Your solution will be expensive and long-term (25-50 years) (^{Cox et al, 2006}) and requires a significant concentration of specialized human resources and infrastructure. This path of development of knowledge, treading unexplored scientific territory, is typical strategic research "uphill". We discuss two examples of corn, one on genetic tolerance to environmental stress and another on the character of perennity.

^{Murray (2014)} continued for 9 generations seeking perennity, without success fully. The combination of summers and winters prevented the survival of their materials, while both wild relatives themselves behaved as perennials. Murray and Jessup (2014) also report progress on the genetics pioneers of the sustainability of the wild relatives of maize made by Westerbergh and Doebley, two QTL (Quantitative Trait locii) involved in the development of the rhizomes of Z. diploperennis. Finally, ^{Murray and Jessup (2014)} authors point out that the conditions for sustainability of corn include 1) not senescer the end of the cycle; 2) accumulate energy in structures resistant to degradation outside the green cycle; 3) ability to remobilize accumulated at the beginning of green energy cycle; 4) be able to switch between vegetative and reproductive development; 5) disease resistance; and 6) efficiency in the use of nutrients and water.

The extractive model of land use labor of Mexico, in particular arable land in hillside, could be significantly mitigated by technology perennial perennial corn and other staple crops. It is necessary for agricultural research in Mexico will join international efforts. Both the native corn as their wild relatives are native to Mexico and corn is and will be for many years, our main food.