Indian Summer Monsoon
The Indian Summer Monsoon has attracted most concern and is of importance to India's economy which is particularly reliant on the monsoon precipitation. It accounts for 80% of mean precipitation of India but it has decreased 4-5% in the last five decades. A regime shift from Indian summer monsoon with mean and regular precipitation within the season towards a weakened monsoon precipitation may be occurring in response to changes in vegetation driven by agricultural change. Changes in vegetation cover are leading to a decrease in the temperature difference between land and ocean that is essential for the creation of monsoon precipitation. A weaker monsoon reduces rainfalls with strong consequences for agricultural productivity in one of the densely populated areas of the world. The local and regional vegetation cover therefore needs to be managed sustainably to prevent a weakening of the Indian Summer Monsoon circulation system. This requires redusing greenhouse gas emissions, decreasing deforestation and cropland area planning.
Key direct drivers
- Vegetation conversion and habitat fragmentation
- Harvest and resource consumption
- Soil erosion & land degradation
- Small-scale subsistence crop cultivation
- Large-scale commercial crop cultivation
- Extensive livestock production (rangelands)
- Timber production
- Moist savannas & woodlands
- Drylands & deserts
Key Ecosystem Processes
- Water cycling
- Food crops
- Wild animal and plant products
- Fuel and fiber crops
- Air quality regulation
- Climate regulation
- Water regulation
- Aesthetic values
- Food and nutrition
- Health (eg toxins, disease)
- Livelihoods and economic activity
- Cultural, aesthetic and recreational values
- Social conflict
Typical spatial scale
Typical time scale
- Contemporary observations
Confidence: Existence of RS
- Speculative – Regime shift has been proposed, but little evidence as yet
Confidence: Mechanism underlying RS
- Contested – Multiple proposed mechanisms, reasonable evidence both for and against different mechanisms
Links to other regime shifts
- Forest to Savannas
- Dryland degradation
The Indian Monsoon system is solar heating driven and starts in the southernmost point of the Indian Peninsula where it splits into two parts. One branch moves along western part of India northwards and the other flows over the Bay of Bengal heading towards North-East India and Bengal, picking up moisture from the Bay of Bengal. The winds then arrive at the Eastern Himalayas along with heavy rainfall. After the arrival at the Eastern Himalayas, the winds turn West, covering northern and eastern India, Bangladesh, parts of Pakistan, and southern Nepal pouring rain all along its way. There are several changes in monsoon intensity that have been documented on centennial to millennial time scales, when the East Asian Monsoon rapidly strengthened while the Indian Summer Monsoon rapidly weakened (Bing et al. 2006). Although several studies have indicated that the East Asian summer monsoon has also become weaker after the end of the 1970s (Wang et. al 2001). Therefore, the end of 1970s may be viewed as an abrupt and significant change point in the inter-decadal variability of the East Asian summer monsoon (Ding et al. 2008). The change of both monsoon systems weakening at the same time might therefore point to a shift in the interconnected monsoon system.
Indian summer monsoon with mean and regular precipitation within the season
In this regime precipitation reaches its annual mean and ensures that agriculture receives enough water to sustain the production of crops and other goods. The rapid change in increase of CO2 concentrations in the atmosphere has supported the existing regime increasing the sea surface temperature thus increasing the precipitation. The Indian Summer monsoon circulation is characterized by monsoon rains arriving at the start of June as warm air converges and ascends in the low pressure over the continent, leading to clouds and heavy rainfall. Monsoon rains advance northward as summer progresses, enhanced by forced ascent as the flow reaches the Himalayan foothills. In winter, this thermally direct circulation reverses as the land surface cools relative to the oceans. Rains continue till the end of September when the season ends. (Rickenbach et al. 2009).
Indian summer monsoon with weak and irregular precipitation within the season
This regime is characterized by weak precipitation causing droughts with the rain arriving much later than expected in June. The increasing emissions of fossil fuel SO2 and black carbon result in large atmospheric concentrations of black carbon and other aerosols generating atmospheric Brown clouds. This induces strong negative trends in surface solar radiation, surface evaporation, and summer monsoon rainfall (Ramanathan 2005). As the monsoon season starts, precipitation can be irregular - raining in June and then having weak or no precipitation in July and returning in August.
Drivers and causes of the regime shift
The main direct driver of changing monsoon rains is deforestation associated with the indirect driver of increased food production. As a consequence of the vegetation loss due to deforestation the surface albedo is increased. Therefore the amount of reflected solar radiation increases due to high albedo, thus decreasing the temperature difference between ocean and land. All this leads to change in the main monsoon circulation mechanism that is responsible for precipitation in the region.
The increasing concentration of CO2 in the atmosphere is a driver, although its not sure as to whether it maintains the current regime or pushes the system towards a regime with weaker precipitation. Researchers are still arguing which case is most likely and it depends on the source to present the influence of this driver. IPCC report (2007) predicts that carbon release from anthropogenic sources will continue increasing during the coming decades. There are studies indicating that emissions of greenhouse gases that alter the heat budget of the system and therefore the land-sea temperature contrast, could increase the monsoon intensity and/or variability (Knopf et al 2008; Kripalani et al. 2007). Nevertheless Palmer et al. (1992) pointed out that enhanced convection (the transfer of heat by the actual movement of the warmed matter) associated with the warm SST anomalies suppresses the monsoon rainfall.
How the regime shift works
For the monsoon precipitation to occur the main atmospheric processes have to be present (latent heat, land to ocean pressure gradient, advection etc.). The release of latent heat from precipitation over land ensures the increase in the temperature difference between land and ocean. This enhances the land to ocean pressure gradient that determine in which direction and at what rate the pressure changes around the Indian peninsula. The increased pressure gradient leads to stronger winds and pushes more moist air northward from the ocean onto the continent. The stronger flows on shore increases landward advection of moisture eventually forming rain clouds which leads to increased precipitation and associated release of latent heat.
However the increasing deforestation in regional scale reduces vegetation cover therefore decreasing rainfall and increasing surface albedo. The monsoon circulation is thereby weakened as the amount of reflected solar radiation increases due to high albedo, decreasing the temperature difference between ocean and land.
As a result weaker pressure gradient leads to weaker winds pushing less moist air onto the continent. As the flows onshore are weaker landward advection of moisture decreases, which leads to decreased precipitation. This reinforces the lack of vegetation cover as the soil moisture decreases resulting in droughts and increased biomass burning occurs which decreases vegetation cover even further.
Impacts on ecosystem services and human well-being
Due to irregular precipitation one of the most essential and recognized provisioning services that could be lost is freshwater as the groundwater levels are becoming deeper thus drying up the soil. Food crops and livestock are two other provisioning services that are directly affected by this regime shift and linked with the freshwater service. Losing these services would mean poor harvests and food shortages from lack of cattle among the rural population, which constitutes two thirds of India's total population (Knopf et al. 2008). Timber production would also be affected, as barren and dried soils can no longer support the growth of trees. Also the fire frequency would increase damaging timber productivity even more. Regulating services would be lost, such as climate regulation as a weak summer monsoon can change climate variability (Gordon et al. 2008). Air quality regulation as a service is endangered because of increased aerosol/dust and Brown cloud concentrations caused from biomass burning and industrial pollution. Regulation of water and soil erosion would be lost as monsoon rains ensure that soils are moist and inhabited with flora and fauna enough not to lose the fertile topsoil due to wind or other type of erosions. Water regulation would also be in danger, as lack of precipitation would alter the water cycle changing the typical water distribution in it. Biodiversity would rapidly decrease in the case of weak precipitation in the long term.
Human wellbeing would also be affected, as monsoon rains are critical to the functioning of hydroelectric power plants. The lack of precipitation would therefore disrupt energy supply which may cause delays in productions or increase product costs. The loss of food crops may results in food crisis, leading to rapid inflation on food prices. In turn this can lead to large number of people suffering from hunger, as any adverse effect on farming will affect the purchasing power of the people as well. The hunger and rise of poverty could result in large numbers of people emigrating from the region (Barnet & Adger 2007). Lack of freshwater also could aggravate the sanitation and health issues that already haven't been completely solved. Crime levels could potentially rise as the depression among society would increase and the necessity for food would drive the people to support their families in any circumstances (Barnet & Adger 2007).
Options for preventing a weakening of the Indian summer monsoon circulation system primary relates to the sustainable management of the local and regional vegetation cover. The area of complete deforestation should be decreased and cropland area planning has to be in place. That has to be done in order to avoid rapid changes in surface albedo in large areas that can change the existing feedback mechanisms. Sustainable water management planning has to be practised to avoid significant water due to irrigation. New irrigation practises has to be considered such as the installation of drip irrigation and low pressure pivots to get more yield with less use of water. It has also been suggested that a better weather forecasting system for India would help people to better adapt strategies in times of droughts and floods induced by the monsoon variability (Nature News Blog, Aug-2011). Greenhouse gas emissions also need to be managed to reduce air pollution and the amount of black carbon in the atmosphere tht increases the formation of brown clouds that negatively influence the monsoon rainfall.
Technology transfer could be a good initiative from developed countries as they can provide more advanced technological solutions and funding to developing countries to help accelerate reduction of greenhouse gas emissions and irrigation management. Investing in sustainable irrigation tools and supporting the industrial production industry for example with filters that decrease the amount of pollutants entering the atmosphere, techniques that are energy efficient. Overall the shift in the Indian summer monsoon circulation is considered to be irreversible if the changes in vegetation cover due to food production continue to increase.
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