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Sunday, November 11, 2012

Indian monsoon more likely to fail as global warming accelerates - Climate tipping point

A new predictive study for future monsoon failure in India says that full season failure will become much more likely in the next two hundred years. Failure of the Indian Seasonal Monsoon (ISM) has been identified as a climate change tipping point by climate scientists.

The study highlighted that monsoon rains could fail about one year in every five between 2150 and 2200 with continued global warming due to continued human burning of fossil fuels, and related shifts in tropical air flows.

More than a billion people are dependent on the reliability of the Indian summer monsoon (ISM) for agricultural productivity. A small variability in rainfall on the Indian sub-continent has large impacts on agriculture. Lower rainfall can reduce crop yield, while excessive rain causes flooding damaging to crops and disruption to peoples' lives. India's monsoon lasts from June to September each year.

“Monsoon rainfall is vital for thousands and thousands of farmers in India and thus for feeding the people in the world’s second-most populous country,” lead author Jacob Schewe said. “So possible changes of precipitation patterns can have important implications for long-term adaptation planning of the Indian economy.”

Full monsoon failure in a season is possible but unlikely in present climatic conditions, but abrupt and strong shifts have ocurred in both the Indian and the East Asian monsoon during the last two glacial cycles and the Holocene, reports the study.

The study authors defined monsoon failure as a fall in rainfall of between 40 and 70 percent below normal levels. Monsoon rainfall has been relatively stable since accurate records started to be kept in 1870, but with a trend for more extreme rain events. There are signs that Cyclone intensity in the Northern Indian Ocean is increasing causing more damage to crops and infrastructure and lives, such as Cyclone Thane which hit the sub-continent in late December 2011.

"In this study, we show that more severe failure of ISM rainfall than ever observed in the past century is possible but unlikely under present climatic conditions, according to a realistic coupled climate model; and that such failure is projected to become much more frequent over the next 200 years under a climate change scenario." says the study.

An earlier study from the Potsdam Institute for Climate Impact Research in 2009 looked at the self-amplifying effect presently that sustains monsoon winds and the potential for abrupt transitions. “Our analysis shows on the basis of observations that there could be two stable states for monsoon systems and the possibility of abrupt transitions from one to the other,” said study co-author Anders Levermann.

The modelling shows that "there is a critical value of irradiation from the sun for a monsoon circulation to start. If irradiation falls below the critical value, for instance due to high air pollution, no conventional monsoon can develop. Above the critical value, one state with and one state without a monsoon circulation exist." The bistable state may manifest as a month of heavy rain followed by a month of extraordinary weak rainfall such as exhibted in the extremely weak rainfall in July and September in 2002, while the rest of the season had average rainfall.

This 2009 study - Basic mechanism for abrupt monsoon transitions (Full Paper) - outlines the important of studying monsoon dynamics:

Monsoon rainfall shapes regional culture and the livelihoods of hundreds of millions of people. The future evolution of monsoon rainfall under increasing levels of atmospheric CO2 and aerosol pollution is highly uncertain. Although greenhouse gas abundance tends to increase monsoon rainfall strength, the situation is more complex with changing aerosol distribution. Given this large uncertainty in the future forcing of monsoons, it is crucial to understand internal monsoon dynamics, especially with respect to self-amplifying feedbacks, which might result in potentially strong responses to small perturbations. Zickfeld et al. (2005) found two stable states in a simple model of the Indian summer monsoon, which in principle allows for rapid transition between radically different monsoon circulations and thereby identified the Indian monsoon as a potential tipping element of the climate system. Evidence for such behavior is found in paleodata that show rapid and strong variations in Indian and East Asian monsoon rainfall. These abrupt changes have been linked to climatic events in the North Atlantic for the last glacial period as well as for the Holocene. Though a physical mechanism for this teleconnection has been suggested, relevant climatic signals of the North Atlantic events in Asia (such as temperature and moisture anomalies) are very small indicating that internal feedbacks in monsoon dynamics may have amplified the weak external forcing.

Anders Levermann told Reuters "In the past century the Indian monsoon has been very stable. It is already a catastrophe with 10 percent less rain than the average,"

“From a risk assessment perspective, our modeling results combined with observation data suggest that Indian monsoon changes are something we urgently need to investigate further,” second author Levermann says. “We need additional studies to confirm whether the monsoon changes found in our study are robust across a number of different climate models.” said Anders Levermann, a Professor of Dynamics of the Climate System at the Potsdam Institure for Climate Impact Research (PIK).

The Indian Summer Monsoon (ISM) has been identified as a major climate tipping point in the 2008 paper - Tipping elements in the Earth's climate system - along with Arctic Sea-ice, Greenland Ice Sheet, West Antarctic Ice Sheet, Atlantic Thermohaline Circulation (THC), El NiƱo–Southern Oscillation (ENSO), Sahara/Sahel and West African Monsoon (WAM), Amazon Rainforest, and the Boreal Forest.


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