The global mean surface temperature has increased by 0.6 °C over the last 100 years, with 1998 being the warmest year. Most of the increase in global mean temperature has been observed in two distinct periods: 1910–1940 (0.35 °C) and since 1970 (0.55 °C) (IPCC, 2007). Anthropogenic forcing accounts for almost all of the global warming observed between 1946 and 1995, whereas warming between 1896 and 1945 is explained by a combination of anthropogenic and natural forcing and internal variability (Braganza et al., 2004). The warming is very likely the response of the main anthropogenic drivers, such as the population growth, deforestation, industrialization, changes in land use and increasing atmospheric concentrations of greenhouse gases. Atmospheric brown clouds, which are mostly formed due to biomass burning and fossil fuel consumption, also contribute to lower atmospheric heating and surface cooling (Ganguly and Iyer, 2009).
Air temperature is a good indicator of the state of climate globally because of its ability to represent the energy exchange process over the earth's surface with reasonable accuracy. The ease of measurement of temperature with highly standardized thermometers and availability of long, homogeneous records further underline its importance. Braganza et al. (2004) also emphasized that changes in mean temperature are an indicator of climate variability, but changes in maximum and minimum temperatures provide more useful information than the mean temperature alone. The changes in mean temperature can be due to trends in either maximum or minimum temperatures, or relative trends in both. The surface warming in recent years is associated with much larger increases in minimum temperature than in maximum temperature (Karl et al., 1993; Easterling et al., 1997; Vose et al., 2005). As a result, the diurnal temperature range (DTR), i.e. the difference between maximum and minimum temperatures, has been decreasing over vast land areas of the world. Therefore, DTR is an important index of climate change and is receiving considerable attention over various regions of the globe in recent times. The mean, maximum and minimum temperatures hereafter will be referred as Tmean, Tmax and Tmin, respectively.
Very little information is available on the analysis of DTR, Tmean, Tmax and Tmin over the biodiversity-rich north-eastern (NE) region of India. The objective of the present study, therefore, was to investigate trends in DTR, minimum and maximum temperatures and sunshine duration over eight sites of NE India on annual, seasonal and monthly time scales. Since cloud cover data are not available at any of the eight sites, the sunshine duration was used as a proxy to cloudiness. The relationship between DTR and other variables was also explored using Spearman's Rho correlation technique to better understand the origins of observed DTR trends and variability.
1.2. Review of the literature
Temperature changes have not been uniform globally, but have varied over regions and different parts of the Earth's atmosphere. Karl et al. (1993) detected increases in Tmax and Tmin by 0.28 and 0.84 °C, respectively, over a large portion of the earth's surface area, and attributed the decrease of about 0.5 °C in DTR to the possible increase in cloud cover. In the Eastern Mediterranean, Turkes et al. (1996) have shown significant decreases in DTR in Turkey due to significant increases in Tmin. Significant reductions of DTR were also reported by Cohen and Stanhill (1996) in the Jordan Valley, by Ben-Gai et al. (1999) in Israel and Price et al. (1999) in Cyprus. Easterling et al. (1997) and Jones et al. (1999) also reported the cases of differential changes in Tmax and Tmin, resulting in both a narrowing of DTR and an increase in Tmean over many parts of the world. Rebetez and Beniston (1998) reported that the decrease in DTR at lower elevation sites was found to be associated with a corresponding decrease in the sunshine duration in Swiss Alps, which was inferred to be a consequence of an increase in low-level cloudiness. Recently, Vose et al. (2005) reported that historical observations have revealed a substantial decreasing trend in globally averaged DTR for 1950–1990, as Tmin has increased at a faster rate than has Tmax.
Increases in cloudiness, as per the proposition of Easterling et al. (1997), are possibly one of the main contributing factors for the observed increases in nighttime temperature in the Southern Hemisphere, because the atmospheric aerosol loading in this hemisphere is comparatively much less than that in the Northern Hemisphere. However, the stronger trace-gas-induced warming observed during the last quarter of the 20th century in the Philippines was one of the main reasons for the larger increase in the annual Tmax and Tmin than the global trends (Peng et al., 2004). The projected DTR changes are often positively correlated with projections of average temperature changes, because increased cloud cover and soil moisture are negatively correlated with both quantities (Lobell, 2007). However, at higher elevation sites, the DTR trends may be poorly correlated with cloud cover or the sunshine duration. Rebetez and Beniston (1998) put forward the probable reason for the absence of such a relationship between DTR and the sunshine duration as due to the fact that the higher elevation sites lie above the low-level cloud layers and the moisture-laden lower atmospheric boundary layer. Thus, the reported studies recognize the shift in the importance established so far in the Tmean changes from the greenhouse gases to the cloud cover. Also, attention is being paid to the signs of global warming in terms of DTR, because the changes in both maximum and minimum temperatures can possibly be linked to the changes in humidity and cloud cover.
In India and other neighbouring countries, several investigators have reported both increasing and decreasing trends in temperature and mostly increasing trends in DTR. Hingane et al. (1985) reported an increase of 0.4 °C in annual Tmean in India over the past century. However, decreasing trends in temperature were reported from 1950 to 1970 and increasing trends after 1970 in the Qinghai-Xizang (Tibetan) Plateau (Li and Tang, 1986). Srivastava et al. (1992) gave the first indications that the DTR decadal trends over India are quite different from those observed over other parts of the globe because of the comparatively large increase in trends in Tmax than Tmin over a major part of India. Rupa Kumar et al. (1994) reported that increases in DTR over the Indian subcontinent are contributed solely by increases (0.6 °C) in Tmax, with Tmin remaining practically trendless, and these trends do not show any urban and altitude bias. They observed that increase in Tmax over a major part of India was predominant in winter and post-monsoon seasons.
Kothyari and Singh (1996) and Ahmad and Warrick (1996) observed increasing trends in temperature over the Ganga basin in India and a large region encompassing Bangladesh, respectively. Shrestha et al. (1999) found warming trends ranging from 0.06 to 0.12 °C/year in most of the Middle Mountain and Himalayan regions in Nepal. The greatest warming trend in the all-Nepal temperature was observed in the post-monsoon season over all the five regions of Nepal. Yadav et al. (2004) observed cooling in pre-monsoon Tmean and warming in DTR due to a relatively high decrease in Tmin and an increase in Tmax in the western Himalayas during the later part of the 20th century.
Roy and Balling (2005) reported significant increases in Tmax and Tmin over the Deccan plateau; but, in general, DTR trends were not significant except for a decrease in Northwest Kashmir in summer for the time period 1931–2002. Large increases in DTR were observed in all the seasons over the Karakoram and Hindu Kush Mountains of the Upper Indus basin during the period 1961–2000 (Fowler and Archer, 2006). Bhutiyani et al. (2007) and Singh et al. (2008) observed increasing trends in the temperature range due to a higher rate of increase in Tmax than Tmin across the northwestern Himalayan region over the majority of river basins of Northwest and Central India, respectively.