“There is substantial evidence that trends in change in climate variables can negatively or positively affect crop yield trends despite advances in technology and other yield-influencing variables. These changes can exhibit substantial deviation for local/regional scales than global values. Thus, trends and magnitudes of changes in these primary climate variables must be quantified for local scales so that relevant mitigation and adoption strategies and best agricultural practices can be researched, developed and implemented to enhance agricultural productivity for a local region. The associated policy and decision-making should also engage in developing these strategies so that research and development and policy and decision-making processes can be established simultaneously.”

Some of the primary drivers of agricultural productivity and their variability include technology, genetics, climate, soil characteristics, diseases, stresses, field management practices and associated decision-making such as fertilizer applications (timing, type and amount), tillage (timing and type), hybrid selection, irrigation method and management, planting row spacing, planting date and depth, planting population density, and other factors. Among these variables, climate is one of the most-grain productivity-influencing factors in many parts of the world.

There is substantial evidence that climate variables are changing and they are changing with significant amount in some regions. For example, The Intergovernmental Panel on Climate Change Fifth Assessment Report (Stocker et al., IPCC, 2013) stated that the last century experienced an increase of 0.74°C globally in air temperatures due to increased greenhouse gas emission concentrations and the period of 1983–2012 was the warmest 30-year span over the last 800 years for the Northern Hemisphere. Also, it reports evidence of increasing precipitation, especially in mid latitudes of the Northern Hemisphere with medium confidence since 1901, but high confidence after 1951. The global mean land-surface air temperature has risen by about 1°C over the past 100 years (1906-2015) and is predicted to increase even more by 1.5-2.0°C to 6.4°C by 2100 (IPCC, 2018). Furthermore, the atmospheric CO2 concentration (CO2 mole fractions) measured at Mauna Loa, Hawaii (a location where atmospheric contamination from greenhouse gas emissions is minimal) has increased significantly from 315.71 parts per million (ppm) in March 1958 to 412 ppm in December 2017 (24% increase) with a rate of 1.468 ppm per year since 1958.

Skaggs and Irmak (2012) studied the air temperature trends of long-term data for five agricultural locations, ranging from the subhumid eastern to the semiarid western parts of Nebraska, to determine local temperature changes and their potential effects on agricultural practices. The study quantified trends in annual and monthly average maximum and minimum air temperature (Tmax and Tmin), daily temperature range (DTR), total growing degree-days, extreme temperatures, growing-season dates and lengths, and temperature distributions for five heavily agricultural areas of Nebraska: Alliance (semi-arid), Central City (transition zone between sub-humid and semi-arid), Culbertson (semi-arid), Fremont (humid), and Hastings (transition zone between sub-humid and semi-arid). July and August were the months with the greatest decreases in Tmax for the central part of Nebraska-Culbertson, Hastings, and Central City. Alliance, Culbertson, and Fremont had year-round decreases in DTR. Central City and Hastings experienced growing-season decreases in DTR. Increases in growing-season length occurred at rates of 14.3, 16.7, and 11.9 days per century for Alliance, Central City, and Fremont, respectively. At Hastings, moderately earlier last spring frost (LS) at a rate of 6.6 days per century was offset by an earlier (2.7 days per century) first fall frost (FF), resulting in only a 3.8 days per century longer growing season. There were only slight changes in LS and FF dates of around 2 days earlier and 1 day later per century, respectively, for Culbertson.

Evidence strongly suggests that global average surface temperature increased by 0.74 °C±0.18 °C from 1906 to 2005, a large portion of which occurred at a rate of 0.13°C±0.03°C per decade during the latter half of the century (Solomon et al., 2007). Global surface temperature is expected to continue to increase by 0.4°C by 2025 (Solomon et al., 2007). Solomon et al. (2007) suggest, with the increase in average temperature, that cold days and nights and frosts have become less frequent, while hot days and nights and heat waves have become more frequent. Irmak et al. (2012) found increases of 3.8°C and 1.9°C in daily minimum and average air temperatures, respectively, from 1893 to 2008 at Central City in central Nebraska. Widespread decreases in daily temperature range have also been observed (Karl et al., 1984; Easterling et al., 1997; Bonan, 2001). Several studies have quantified the variation in the length of the frost-free season, also known as the climatological growing season. Kunkel et al. (2004) found an average of a 2-week increase in the frost-free season for the United States from 1895 to 2000 with a greater increase observed in the western part than in the eastern part of the country.

There have been significant changes in climate variables in Turkiye as well and these changes can, and will, have significant implications to country’s agricultural productivity. For example, Irmak (2018) has shown that maximum air temperature, minimum air temperature, and as a result, average air temperature, incoming shortwave radiation received at the earth surface and net radiation intercepted at the surface, vapor pressure deficit (atmospheric evaporative demand) have been increasing substantially in Turkiye. The relative humidity on the other hand, has been decreasing. All these variables are the primary drivers of surface evaporative losses (evapotranspiration) and since these variables have been increasing, evapotranspiration rates have been increasing significantly as well. When country average values are considered, the maximum air temperature in Turkiye has increased by 1.6°C in the last several decades and minimum air temperature has increased by 1.5°C. However, when specific regions within the region are considered, the changes are more pronounced. For example, in one of the agriculturally most productive regions in Cukurova Region, the maximum and minimum air temperatures have increased more than the country average values by 2.1 and 2.2°C during the crop growing seasons, respectively. It is also critical to note that while in some areas of the country, there has been increasing trends in precipitation; the rate of increase in evapotranspiration has exceeded the rate of increase in precipitation, resulting in water deficit conditions. For example, in Cukurova Region, the precipitation has increased by 42 mm in the last several decades on an annual basis (January 1-December 31), but evapotranspiration has increased by a much larger amount by 133 mm during the same time step. The evapotranspiration has increased by 91 mm more than precipitation in the last several decades. Also, the timing of the precipitation during the growing season is critical for meeting crop water requirement. It is desirable to have a uniformly distributed and slow intensity precipitation during the growing season to obtain maximum infiltration of precipitation into the soil profile and crop root zone for crop uptake. However, when the precipitation intensity and distribution are considered, precipitation events are getting more intense with larger and unevenly distributed precipitation events that would reduce the infiltration and increase surface run-off and erosion, which would be less beneficial for meeting crop water requirements during the growing season. Thus, changes in all these climate variables indicate that the conditions have been getting drier in Turkiye, which may negatively impact country’s agricultural productivity and water resources, if necessary effective precautions are not developed and implemented immediately.

There is substantial evidence that trends in change in climate variables can negatively or positively affect crop yield trends despite advances in technology and other yield-influencing variables. In a study conducted with a climate dataset from 1895 to 1998, Hu and Buyanovsky (2003) found that within-season precipitation and temperature variations explained differences in central Missouri maize yield. Ferris et al. (1998) found declines in spring wheat yield when plants were exposed to extreme maximum temperatures during the anthesis stage. Hu et al. (2005) found that earlier winter wheat heading dates were associated with warmer spring minimum temperatures. Matsui et al. (2001) found that variety selection was crucial to minimizing the effects of extreme heat stress on japonica rice during flowering on grain yield. In addition to other climate variables, increase in minimum air temperature has specific and critical implications to yield productions. Daily Tmin usually occurs during nighttime. For most agronomic crops, Skaggs and Irmak (2012) suggested that Increases in Tmin, which usually occur during the night, may have significant implications in crop productivity. Nighttime increases in temperature can result in greater plant respiration, which is a physiological process opposite to transpiration. Transpiration is mainly driven by sunlight, air temperature, and soil/plant water availability during the daytime and results in dry matter production and accumulation by the plant. Since Tmin is the main driver of respiration during nighttime, increases in Tmin can accelerate respiration, increase dry matter consumption by the plants at night, and result in reduction in crop yields. In the literature, there are numerous other data and information related to negative impacts of aforementioned changes in climate variables on agricultural yields.

Suggestions Remarks: Since agriculture is highly sensitive to weather and climate change, agricultural resource management practices and policy decisions require detailed information of trends in temperature, precipitation, and other variables, such as carbon dioxide concentration and cloudiness, on relevant spatial and temporal scales (Skaggs and Irmak, 2012). Furthermore, these changes can exhibit substantial deviation for local/regional scales than global values. Thus, trends and magnitudes of changes in these primary climate variables must be quantified for local scales so that relevant mitigation and adoption strategies and best agricultural practices can be researched, developed and implemented to enhance agricultural productivity for a local region. The associated policy and decision-making should also engage in developing these strategies so that research and development and policy and decision-making processes can be established simultaneously.

• While so much discussion and analyses take place on global climate change, it is imperative that the analyses are conducted for local/regional conditions so that local changes can be documented and local best agricultural and water resources management practices can be developed in response to changes in climatic variables. While these increases in air temperature and CO2 concentration may seem to be small for humans, the implications of these small increases in air temperatures for plant physiological functions and, in turn, their impact(s) on agricultural practices and productivity can be significant.

• Whether it is “man-made” or “natural occurrence,” climate variables are changing and they are having significant impacts on our agro-ecosystems. This message must be communicated through good quality science and research-based data and information.

• Potential changes in climate variables must be quantified and studied for all major agriculturally important regions with finer resolution.

• Crop water use (evapotranspiration) and crop response to water and other environmental factors must be quantified for most, if not all, cropping systems.

• Climate variables must be measured with good quality weather stations/instrumentation with finer resolutions and spatio-temporally.

• Agricultural impacts of magnitude and trends if change in climate variables must be economically quantified.

• Impact(s) of climate variables on water resources must be quantified with finer resolutions. and spatio-temporally.

• Effective agricultural practices that can aid in encountering some of the negative impacts of change in climate variables must be researched, demonstrated and education programs must be developed to enable adoption of these strategies in production fields.

• Technology implementation in agriculture and natural resources and water resources must be accomplished to adopt climate impacts on agriculture to enhance productivity. It is a difficult task, but can be done.

References/Sources *Bonan, G. B., 2001: Observational evidence for reduction of daily maximum temperature by croplands in the Midwest United States. J. Climate, 14, 2430–2442.Easterling, D. R., 2002: Recent changes in frost days and the frostfree season in the United States. Bull. Amer. Meteor. Soc., 83, 1327–332. *Intergovernmental Panel on Climate Change (IPCC). Stocker, T. F. vd. 2013: Climate Change 2013: the physical science basis. Contribution of working group I to the fifth assessment report of the intergovernmental panel on climate change (2013). *Intergovernmental Panel on Climate Change (IPCC), Allen, M. vd. 2018. Global Warming of 1.5oC. *Irmak, S., I. Kabenge, K. Skaggs ve D. Mutiibwa. 2012. Trend and magnitude of changes in climate variables and reference evapotranspiration over 116-year period in the Platte River Basin, central Nebraska-USA. Journal of Hydrology 420-421: 228-244.Irmak, S. 2013. Long-term (1893-2012) changes in air temperature, relative humidity and vapor pressure deficit (atmospheric evaporative demand) in central Nebraska. UNL Extension Circular EC716. *Irmak, S. 2018. İklim değişkenleri ve bunların tarım, su kaynakları ve ilgili politikalar üzerindeki etkisine dair yayınlanmamış araştırma verileri. 2. Uluslararası Tarım, Gıda ve Beslenme Politikaları Konferansı’nda sunulmuştur. 6-7 Kasım 2018. Swiss Hotel, Ankara, Türkiye *Ferris, R., R. H. Ellis, T. R.Wheeler ve P. Hadley, 1998: Effect of high temperature stress at anthesis on grain yield and biomass of field grown crops of wheat. Plant Cell Environ., 34, 67–78. *Hu, Q., A. Weiss, S. Feng ve P. S. Baenzinger, 2005: Earlier winter wheat heading dates and warmer spring in the U.S. Great Plains. Agric. For. Meteor., 135, 284–290. *Hu, Q. ve G. Buyanovsky, 2003: Climate effects on corn yield in Missouri. J. Appl. Meteor., 42, 1626–1635. *Karl, T. R., G. Kukla ve J. Gavin, 1984: Decreasing diurnal temperature range in the United States and Canada from 1941 through 1980. J. Climate Appl. Meteor., 23, 1489–1504. *Kunkel, K., D. R. Easterling, K. Hubbard ve K. Redmond, 2004: Temporal variations in frost-free season in the United States: 1895–2000. Geophys. Res. Lett., 31, L03201, doi:10.1029/ 2003GL018624. *Matsui, T., K. Omasa ve T. Horie, 2001: The difference in sterility due to high temperatures during the flowering period among Japonica–rice varieties. Plant Prod. Sci., 4, 90–93. *Skaggs, K.E., ve S. Irmak. 2012. Long-term trends in air temperature distribution and extremes, growing degree days, and spring and fall frosts for climate impact assessments on agricultural practices in Nebraska, USA. J. Applied Meteorology and Climatology 51:2060–2073. doi:dx.doi.org/10.1175/JAMC-D-11-0146.1. *Solomon, S., D. Qin, M. Manning,M. Marquis, K. Averyt,M.M. B. Tignor, H. L. Miller Jr. Ve Z. Chen, Eds., 2007: Climate Change 2007: The Physical Science Basis. Cambridge University Press, 996 pp.