Understanding trends in climatic hazards in Nepal

Submitted by Ben Smith 26th July 2011 18:23

Impacts of rising temperature

While the magnitude of change is uncertain, it is clear that the climate of Nepal will warm significantly in the coming century. The impact of this warming is expected to be most severe in the mountain regions of Nepal, and will impact on existing hazards such as flooding and landslides. It is estimated by the National Communication that with an increase of 3ï‚°C, predicted by most models for Nepal, the snow covered area will decrease by 58%. This melting will intensify the trend which is already underway towards an increased number and increased size of glacial lakes, and will thus increase the risk posed from Glacial Lake Outburst Floods (GLOFs) (Agrawala et al 2003). Rising temperatures will also mean that less precipitation is stored as snow, and may thus increase the risk of flooding by increasing run-off directly following precipitation events (Agrawala et al 2003).

A study by Fukui et al (2007) suggests that the rising temperatures in Nepal are also the reason for the increase of 100-300m in the lower boundary of permafrost on south facing slopes in the Khum-bu Himal, which rose from 5200m-5300m in 1973 to 5400m-5500m in 1991, although there has been no further rise in the last decade. This melting of permafrost has been observed elsewhere in the Himalayas, and is associated with an increase in the frequency of landslides and mud-flows as the ground is destabilised (IPCC 2007b). It is likely, therefore, that increasing temperatures in Nepal will lead to an increase in the number of landslides experienced, and will create landslides in areas which were previously stable.

Precipitation

The National Communication indicates that overall precipitation in Nepal is decreasing at the rate of 9.8mm/decade, using data from 1981-1999, and that monsoon precipitation increased at 8.4mm/decade over the same period (National Communication 2004). The IPCC however, cite stu-dies indicating that there has been no long term trend in precipitation in Nepal for the period 1948-1994 (IPCC 2007b). This highlights the fact that even for current trends, it is much harder to accu-rately assess precipitation. The IPCC states that the prediction of future precipitation in the Himalayan region is very complex and difficult due to the combination of complicated atmospheric teleconnections which modify the strength of the monsoon, and the complex topography of the region (IPCC 2007a). There are large biases (up to 2.5 times annual precipitation) present in the models currently used, and there is also a large inter-model variability. Only 3 of the 21 models used in the IPCC analysis, however, indicate an overall decrease in precipitation over the S. Asia region, and predictions for the Tibetan Plateau are also for increased precipitation (IPCC 2007a), as seen in Table. 4.

Data provided in the National Communication of Nepal also indicate the extreme variability in pre-dictions of changes in precipitation. The two GCM models (CCCM and GFD3) give predictions of changes in annual average precipitation under conditions of doubled CO2 which range from -400mm to + 1600mm and +150mm to + 1050mm respectively, and the RCM used (HAD RM2) gives a range of -1000mm to + 3000mm. Clearly the general trend appears to be an increase in precipita-tion, but specifics are too variable between models to be reliable.

There is a consensus, however, that there has been an increase in the intensity and frequency of extreme precipitation events across Asia (National Communication 2004, IPCC 2007b, Agrawala et al 2003). The models also agree that this increase will continue in the future as Asia warms, so that while there may be fewer precipitation days, there will be more days of extreme precipitation, and the intensity of this precipitation will increase (IPCC 2007a,b).

Impacts of changes in precipitation on natural hazards

In terms of the two major hazards, floods and landslides, what is more important than changes in average precipitation are the changes in extreme precipitation events. It is these events that trigger landslides and cause rivers to flood, and it is very likely therefore that there will be an increase in these events due to the increase in intensity and frequency of extreme rainfall (IPCCa,b, Agrawala et al 2003, National Communication 2004). Indeed there is already some evidence that there has been an increase in the number of flood days on some rivers in Nepal (Agrawala et al 2003). The large and serious floods which struck Nepal in 2002, 2003 and 2004 may be an indication that not only is the frequency of flooding increasing, the magnitude of flood events is also on the rise (IPCC 2007b). The increase in intensity, in particular during the monsoon period could also increase the risk of GLOFs by raising the water level in glacial lakes, as research shows that the majority of GLOFs in the Himalayas occur in the monsoon period (Richardson and Reynolds 2000).