Miller Magazine Issue 109 / January 2019

53 COVER STORY MILLER / JANUARY 2019 population density, and other factors. Among these vari- ables, climate is one of the most-grain productivity-influ- encing 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 (Stoc- ker et al., IPCC, 2013) stated that the last century expe- rienced 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 Hemisphe- re. Also, it reports evidence of increasing precipitation, especially in mid latitudes of the Northern Hemisphere with medium confidence since 1901, but high confiden- ce after 1951. The global mean land-surface air tempe- rature 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 CO 2 concentration (CO 2 mole fractions) measured at Mauna Loa, Hawaii (a location where at- mospheric contamination from greenhouse gas emissions is minimal) has increased significantly from 315.71 parts per million (ppm) in March 1958 to 412 ppm in Decem- ber 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 wes- tern parts of Nebraska, to determine local temperature changes and their potential effects on agricultural prac- tices. 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 da- tes and lengths, and temperature distributions for five heavily agricultural areas of Nebraska: Alliance (semi-a- rid), 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 Nebras- ka-Culbertson, Hastings, and Central City. Alliance, Cul- bertson, and Fremont had year-round decreases in DTR. Central City and Hastings experienced growing-season decreases in DTR. Increases in growing-season length oc- curred 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 tempera- ture 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 temperatu- res, respectively, from 1893 to 2008 at Central City in central Nebraska. Widespread decreases in daily tempe- rature 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 se- ason. 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 va- riables in Turkiye as well and these changes can, and will, have significant implications to country’s agricultu- ral productivity. For example, Irmak (2018) has shown that maximum air temperature, minimum air tempera- ture, and as a result, average air temperature, incoming shortwave radiation received at the earth surface and net radiation intercepted at the surface, vapor pressu- re deficit (atmospheric evaporative demand) have been increasing substantially in Turkiye. The relative humidity on the other hand, has been decreasing. All these variab- les 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 mini- mum 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 Cukuro- va Region, the maximum and minimum air temperatu- res have increased more than the country average va- lues 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 evapotranspira- tion has exceeded the rate of increase in precipitation, resulting in water deficit conditions. For example, in Cu- kurova 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 in-