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Why Pakistan monsoons support evidence of global warming

This article by Lord Julian Hunt first appeared on Reuters The Great Debate UK on August 20, 2010.  Lord Julian Hunt is visiting Professor at Delft University of Technology, and former Director-General of the UK Met Office.

The unusually large rainfall from this year’s monsoon has caused the most catastrophic flooding in Pakistan for 80 years, with the U.N. estimating that around one fifth of the country is underwater.  This is thus truly a crisis of the very first order.

Heavy monsoon precipitation has increased in frequency in Pakistan and Western India in recent years.  For instance, in July 2005, Mumbai was deluged by almost 950 mm (37 inches) of rain in just one day, and more than 1,000 people were killed in floods in the state of Maharashtra. Last year, deadly flash floods hit Northwestern Pakistan, and Karachi was also flooded.

It is my clear view that this trend is being fueled both by global warming (which also means extremes of rainfall are also a growing world-wide trend), and indeed potentially by any intensification of the El-Nino/La-Nino cycle.

To understand the reasons why global warming is playing a role here, one needs to look at the main climatic trends in South Asia.  In addition to more extreme rainfall events, there is also a decreasing thickness of ice over the Tibetan plateau and changing patterns of precipitation, with less snow at higher levels, plus more rapid run off from mountains.

How does climate change help explain this?

First, the warming in temperatures leads to less snow.

Second, the less stable atmosphere causes deeper convection and intense rainfall events.

The less stable atmosphere also leads to more airflow over mountains and less lateral deviation — so that the monsoon winds and precipitation can be higher in North West India and Pakistan and weaker in North East.  In 2006, there was an unusually intense drought in Assam and rain in North West India.  This year with the strong precipitation in North West, there is no pronounced decrease in rains in North East.

Recent U.S. studies have also concluded that the mountain meteorology is changing but as a result of the aerosols emitted into the atmosphere from urban areas of South Asia.

The biggest question going forwards is whether the El-Nino southern oscillation, that determines the large 10 year oscillations of weather across the whole Pacific basin and into South Asia and Africa, will change.

Although there is no scientific consensus on this, it seems likely to me that if the Amazon rain forest continues to disappear, and snow/ice melt significantly increases over the Tibetan plateau, there will be significant changes in enso climatic fluctuations as rises in temperature over land areas become comparable with the areas of the Pacific where currently the temperature fluctuates over a few degrees — which is now better monitored and computer modeled.
The reason for concern about changing enso is that depending on its periodic strength, it greatly affects magnitudes and locations of floods, droughts, hurricanes.  Until about 2020-2030, these natural fluctuations are expected to be greater than man-made changes (as was pointed out by many scientists in the 1990s).

Given the massive stakes in play, not least because of the sizeable proportion of the world population impacted, these issues need urgent study and also preparations on the ground by the affected countries.

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