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which indicate that both
T
max
and
T
min
are moving in opposite direction.
Around 46% and 60% of the season-stations experienced significant warming
trend of
T
max
at F = 0.05 and 0.10 levels, respectively. The significant warming
trends of
T
max
were more concentrated in the non-monsoon seasons. For
T
min
,
around 39 and 48% of the season-stations experienced significant cooling
trend at D = 0.05 and 0.10 levels, respectively. Increasing number of cooling
trends was observed in the pre-monsoon summer season. The Kendall slopes
for trend quantification indicated that the
T
max
exhibited an overall warming
trend at the rate 0.37
o
C yr
-1
(standard deviation 0.18). The seasonal distribution
of the Kendall slope indicated that the pre-monsoon summer months
experienced more warming trends. The
T
min
exhibited a cooling trend at
the rate 0.32
o
C yr
-1
(standard deviation 0.23). Comparatively higher rate of
cooling was experienced in the pre-monsoon summer months. The test
of homogeneity (Table 11.4) indicates that the overall trend (F
2
Trend, 1
) for
both
T
max
and
T
min
are significant at F = 0.05 level (critical F
2
0.995, 1
= 3.84).
This result indicates an overall warming trend for
T
max
(
—
= 1.85) and cooling
trend for
T
min
(
—
= -1.62) without any significant spatial and seasonal
heterogeneity.
The opposite trends of
T
max
and
T
min
indicate that the governing factors
are also different. Greenhouse gases in the atmosphere accelerate the warming
process. The observed pronounced warming trend of
T
max
in the study area
may be attributed to the build-up of greenhouse gases due to coal combustion,
land-use change, and other anthropogenic factors (Garg et al., 2001; CSE,
2006). Earlier, Rao (1993) attributed the warming trend in the Mahanadi river
basin of Orissa to the increase in the greenhouse gases, specially CO
2
and
CH
4
, and the changes in the land-use pattern. Several other researchers have
also observed the warming trends of seasonal and annual temperatures for
different parts of India (e.g., Hingane et al., 1985; Arora et al., 2005; Kothawale
and Kumar, 2005; Flower and Archer, 2006). More interesting result is the
simultaneous cooling trend of the night temperatures i.e.
T
min
, and consequent
widening trend of the MTR. The aerosol loads may have influenced the
observed cooling trends by reducing the surface receipt of solar radiation and
rainfall efficiency as discussed in the previous section. Cooling trends have
also been observed at other parts of India (Hingane et al., 1985; Yadav et al.,
2004; Flower and Archer, 2006). The deforestation and drying rainfall trend
may have partially contributed to the cooling trend of Orissa.
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