4. RESULTS AND DISCUSSION
4.1 Temperature trend analysis
4.1.1 Discussion
An analysis of global climate data from 1950 - 2004 has shown an increasing annual maximum air temperature by 0.20 oC/decade, minimum air temperature by 0.14 oC/decade
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and a DTR of -0.07 oC/decade (Vose et al., 2005). The magnitude of rate of increase or decrease was compared to the global rates in order to quantify the of significance in trends obtained in this study.
Kruger and Shongwe (2004) found an increase in both mean annual Tx and Tn after investigating data from 26 climate stations across South Africa for 1960 to 2003. An earlier similar study done by Hughes and Ballings (1996) for the period 1960 – 1990 demonstrated similar results. None of the areas studied in either the western (Bothaville and Lichtenburg) or the eastern (Bronkhorstspruit and Marble Hall) part of the Highveld showed simultaneous increase in both Tx and Tn. The western part showed negative Tx, positive Tn and negative DTR trends. The negative Tx trends contradict the global trends as well as various findings from similar previous studies conducted in South Africa. Positive Tn and DTR trends agree to the global trends revealed by Easterling et al., (1997); Vose et al., (2006), who studied global historical climate data (1950 – 1999) and found a substantial decreasing trend in global averaged DTR. Many other models have predicted further significant changes (Stone and Weaver, 2004).
The eastern part showed trends opposite to that of the western part (postive Tx, negative Tn
and positive DTR trends). Notably, both the eastern and western annual Tx trends are less than the global annual Tx trend of 0.20 oC/decade whilst the annual Tn trends from the western part are above the global Tn trends of 0.14 oC/decade (see Table 4.1) (Vose et al., 2005). This implies that temperatures are increasing at a faster rate in the western half than the eastern part of the Highveld Eco-region.
Results obtained in the study are dependent on the time frame of the available data. The record lengths analysed for all the areas of the study are a subset to that analysed by Kruger and Shongwe (2004) which included data from 1960 – 2003. Data from all the locations exclude the possible influence of the period 1960 – 1970, during which South Africa experienced a cooling trend (Hughes and Balling, 1996). This was followed by a relatively large increase in mean air temperature during the early 80s. Exclusion of either of these periods could have influenced results obtained by (Kruger and Shongwe, 2004). Data from locations such as Bronkhostspruit and Marble Hall included data beyond 2003, the last year Kruger and Shongwe (2004) included in their study. With reference from Table 4.2,
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exclusion of the years beyond 2003 reduced trends made them more negative except Cedara while for Tn, Bothaville and Cedara showed more positive trends. Bronkhorstspruit and Marble Hall showed more negative trends. Therefore, inclusion of the years beyond 2003 in this study cannot be considered to be a factor that influenced trends to deviate from other trends from previous studies. However, when compared to Northen and Southern Hemisphere trends (Table 2.1), only DTR from Bothaville exceeded that of the Southern Hemisphere.
Both minimum and maximum air temperature trends fell short of those from both Northern and Southern Hemispheres.
Furthermore, another possible factor to have influenced the results could have been the quality of the data. Past methods of data collections were done manually and thus relied heavily on human effort of which greater uncertainty exists. Presently, AWS systemshave replaced their manual equivalent. Well maintained AWS systems are now well equiped to capture and collect accurately various kinds of climate, soil and plant data. The uncertainty involved in data treatments could have also affected the observed outcomes.
There is a general increase in DTR in the eastern part (Bronkhorstspruit and Marble Hall) of the Highveld and a decrease in the western part (Bothaville and Lichtenburg). The decrease in DTR in the western half resulted in increased Tn temperature trends as opposed to decreased Tx temperatures. Karl et al., (1993) also found that DTR generally decreased in South African conditions. Therefore, a decrease in DTR (Tx - Tn) reduces solar irradiance which implies more cloud and possibly more rain. Even though more rainfall could benefit yields, the influence of decreased solar irradiance could mask the benefits of increased rainfall thus posing a major risk to the development of the maize crops by decreasing photosynthetic function, sugar and starch content (particularly as Tn increases faster than Tx) (Loka and Oosterhuis, 2010). Other effects include: supressed floral bud development, male sterility and low pollen viability hastening maturity (Ahmed and Hall, 1993). As a result, crop yields decrease.
Air temperature controls the rate of growth as well as mediates various biochemical reactions in plants. Increased Tx temperatures can impede crop development in various ways. One area of concern is the sensitivity of cereal crops during the grain filling processes. Studies on wheat have shown that air temperatures beyond 31 oC decrease the rate of grain filling (Al-
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Khatib and Paulsen, 1990). Also, temperatures well above the plant cells optimum functional temperatures affects cell activity and if plant development is around the anthesis period, pollination may be inhibited. In fact, the transfer of pollen to stigma, germination of pollen grains and growth of pollen tubes down the style, fertilization and development of the zygote are all temperature sensitive. If affected, yields can be reduced significantly.
Ideally, DTR should neither increase nor decrease significantly enough to affect agricultural production. Global night time (Tn) have been found to increase twice as much as Tx. This is due to the enhanced greenhouse effect which retains more infra-red radiation due to increase water vapour, carbon dioxide, methane, nitrous oxide, ozone, aerosols halocarbons (a group of gases containing fluorine, chlorine and bromine) and sulphur hexafluoride (Boadi et al., 2002). The accumulation of these gases due to anthropogenic activities increase the heat retaining capacity of the atmosphere causing warming. Also, re-radiation of the infra-red that was absorbed during the day as shortwave radiaiton, occurs in mostly in the night time.
Therefore, with higher degrees of warmth experienced with time, more warmer night time temperatures will be experienced (Rasul et al., 2011) .
For Cedara, decreasing nature of trends for Tx agreed with that of the western part whilst of Tn
trends agreed to that found from the eastern part. The difference in rates of decrease, i.e. Tx
decreasing twice as much as the Tn rates is verified by the negative annual DTR. This shows that warming is taking place gradually. Kruger and Shongwe (2004) showed annual Tx and Tn trends to be positive for Cedara. They showed Tx annual rates to increase by about 0.14
oC/decade and Tn annual rates to increase by 0.18 oC/decade. Findings of this study contradicts this finding as negative trends were obtained (see Table 4.2). However, inclusion of the years 2003 – 2011 altered Tx and Tn trends such that they became similar to those found by Kruger and Shongwe (2004).
Even though the were variations in the annual trends for both Tx and Tn it was noted that trends at all locations except Tn for Cedara were considered statistically insignificant.
Therefore, the general DTR decrease and increase in the western and eastern parts respectively, have not reached levels that can potential cause major reductions in yields.
However, it is evident that temperatures are on the rise especially in the western part of the
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Highveld. Therefore, modeling possible future air temperature changes in order to predict future crop losses is necessary.