Climate change and water in south Asia - Overview and literature review

[摘要] South Asia is characterised by strong climate gradients from the south to north and the west to east, together with high seasonal and year to year variability. Two main climate drivers, the westerlies (western disturbances) and the monsoon (Indian summer monsoon and to a lesser extent eastern monsoon), interact with the extreme orography of the Himalayas to produce the observed spatial and temporal climatological variability. Thus the hot, wet and tropical south contrasts with the cold, arid and mountainous north. The monsoon impacts the whole region, providing a larger contribution to annual precipitation in the east whilst the north‐west is dominated by westerlies where precipitation is predominantly snowfall in the winter and spring.The interaction between climate drivers and orography results in variation in hydrological processes, with the rivers of the north and north‐west dominated by glacier and snowmelt sources whereas the southern and eastern rivers are rainfall dominated. Thus hydrological response to climate variability and trends varies from west to east and with elevation. Changes in the flow of the glacier and snowmelt dominated north‐western rivers (e.g. Indus) will be impacted by changes in western disturbances, and their interactions with the monsoon. Temperature changes influence increased contribution from glacier melt and the changing partition of precipitation between snow and rain, resulting to changes in both magnitude and seasonality of flow. In contrast, flow changes in the southern and eastern rivers (e.g. Brahmaputra) will be impacted predominantly by changes to the Indian summer monsoon strength, length and seasonality. Additional stresses will result from future increases in the intensity of extreme precipitation in a warming climate.Historical warming trends are evident across South Asia. Studies have found winter temperatures to be increasing faster than summer temperatures. Warming rates are also greater at higher elevations (elevation dependant warming) with a positive feedback as warming reduces snow cover, reducing surface albedo and hence warming more. All future projections indicate warming will continue.Observed precipitation trends are not consistent, varying from region to region and between seasons. Long term decreasing total monsoon rainfall trends are evident over recent decades with several studies suggesting the role of aerosols (localised cooling due to the ‘Asian brown cloud’). However, the large majority of GCM projections (CMIP3 and CMIP5) suggest increased South Asiantotal monsoon rainfall as the century progresses, although on a regional basis projections are less consistent. A caveat relates to the reduction of aerosol concentrations in CMIP5 scenarios, asincreasing precipitation may not occur if aerosol emissions do not decline as assumed in the driving scenarios. Practically all studies indicate that extreme rainfall will be more intense enhancing flood risks.Hydrological modelling of the flow response to projected climate changes varies west to east as the dominance of glacier contribution to flow decreases. The non‐linear flow response to warmingand precipitation changes are complex as increased glacier melt can compensate for reduced snow melt and increased monsoon season precipitation can compensate for reduced snow accumulation in winter and spring. Several studies indicate increased flow in eastern basins due to increased rainfall and in north‐western basins due to accelerated glacier melt. Glacier retreat, while initially increasing flow (for several decades) will in the long term result in decreased flow,increasing the importance of precipitation derived flow. Timing of maximum discharge is projected to move earlier in the year due to reduced snowpack combined with earlier glacier melt, due towarmer conditions both enhancing melting and resulting in more precipitation falling as rain rather than snow.Downscaling has been promoted as providing bett...

[发布日期] 2016-05-24 [发布机构] CSIRO

[学科分类] 地球科学(综合)