Melting of major Glaciers in the western Himalayas: evidence of climatic changes from long term MSU derived tropospheric temperature trend (1979–2008)

Abstract. Global warming or the increase of the surface and atmospheric temperatures of the Earth, is increasingly discernible in the polar, sub-polar and major land glacial areas. The Himalayan and Tibetan Plateau Glaciers, which are the largest glaciers outside of the Polar Regions, are showing a large-scale decrease of snow cover and an extensive glacial retreat. These glaciers such as Siachen and Gangotri are a major water resource for Asia as they feed major rivers such as the Indus, Ganga and Brahmaputra. Due to scarcity of ground measuring stations, the long-term observations of atmospheric temperatures acquired from the Microwave Sounding Unit (MSU) since 1979–2008 is highly useful. The lower and middle tropospheric temperature trend based on 30 years of MSU data shows warming of the Northern Hemisphere's mid-latitude regions. The mean month-to-month warming (up to 0.048±0.026°K/year or 1.44°K over 30 years) of the mid troposphere (near surface over the high altitude Himalayas and Tibetan Plateau) is prominent and statistically significant at a 95% confidence interval. Though the mean annual warming trend over the Himalayas (0.016±0.005°K/year), and Tibetan Plateau (0.008±0.006°K/year) is positive, the month to month warming trend is higher (by 2–3 times, positive and significant) only over a period of six months (December to May). The factors responsible for the reversal of this trend from June to November are discussed here. The inequality in the magnitude of the warming trends of the troposphere between the western and eastern Himalayas and the IG (Indo-Gangetic) plains is attributed to the differences in increased aerosol loading (due to dust storms) over these regions. The monthly mean lower-tropospheric MSU-derived temperature trend over the IG plains (dust sink region; up to 0.032±0.027°K/year) and dust source regions (Sahara desert, Middle East, Arabian region, Afghanistan-Iran-Pakistan and Thar Desert regions; up to 0.068±0.033°K/year) also shows a similar pattern of month-to-month oscillation and six months of enhanced and a statistically significant warming trend. The enhanced warming trend during the winter and pre-monsoon months (December–May) may accelerate glacial melt. The unequal distribution of the warming trend over the year is discussed in this study and is partially attributed to a number of controlling factors such as sunlight duration, CO₂ trends over the region (2003–2008), water vapor and aerosol distribution.