After the devastating effects of Tropical storm Washi in the Phillipines earlier this week, I thought I would look further into any links between monsoons and anthropogenic climate change. It is well acknowledged that land use changes and human impacts through mining and illegal logging were a major cause in the flash floods, but 12 hours of continuous rain, in an area of the Philippines which storm tracks do not normally take is somewhat unusual. In an earlier post where I stated some of the latest finding form SREX, the tropical cyclone statement was one that particularly shocked me:
"It is likely that the global frequency of tropical cyclones will either decrease or remain essentially unchanged."
As a result I have done some digging to try and find other sources to these claims. I have come across two papers: one which looks at the evolution of Monsoons through the Holocene (12,000 years before present to modern day) using climate modelling, and another which assesses monsoon precipitation changes over the past 100 years.
The first paper deals with changes on the millennial time-scale and focuses on insolation forcing and feedbacks. It is found that the responses of the monsoons to the insolation forcing and oceanic feedback differ substantially among regions, because of regional features of ocean - atmosphere interaction. In the Northern Hemisphere, 'the models show a significant enhancement of all of the monsoons in the early Holocene and a gradual weakening toward the present'. The monsoons are enhanced in the Holocene by a positive oceanic feedback in North Africa and North America but are suppressed by a negative overall feedback in Asia.
The more recent second paper (July 2011) deals with changes on the decadal time-scale, over the past 100 years. Changes of global land monsoon precipitation are assessed by using three sets of rain-gauge precipitation data for the period of 1901-2002. It has been found that during 1901–2001, global land monsoon precipitation exhibits multi-decadal variations, with an overall increasing trend from 1901 to 1955, followed by a decreasing trend up to 2001. See the image below for the regional trends and Northern Hemisphere (NH) mean. There is uncertainty over changes in the 2000s.
Time series of precipitation anomalies for Northern Hemisphere land monsoon mean. (a) NH Mean (b) North African Monsoon (c) Indian Monsoon (d) East Asian Monsoon.
What does this all mean in brief?
The papers describe an overall weakening of monsoons on the millennial and decadal time scales, suggesting that anthropogenic warming of the past 200 years has not intensified them. I have looked in previous posts about how the intensification of the hydrological system can lead to more precipitation in some areas and decline in others, but I was not expecting such a significant downward trend of monsoons in the Northern Hemisphere over the past 40 years. It is a gentle reminder that ocean-atmosphere interactions are far more complex than we can imagine, and that despite SREX stating that heavy precipitation events will become more likely, it is unlikely that they will come through more extreme monsoon and tropical storm events.
Moreover, it shows all the more clearly that land use changes can have such devastating effects on our planet, even without significant changes in weather patterns. The removal of the natural environmental buffer in the case of the Philippines has played a major role in the devastating effects and death toll, rather than an intensification of weather patterns.
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