Deluges after the deluge

Catastrophic floods in Pakistan are likely to recur as global warming combines with El Niño

The Pakistani crisis is already one of the very first order. Some 20 million people have been left homeless, along a path of destruction of more than 600 miles. Prime Minister Yousaf Raza Gilani has even compared the challenges the country now faces to those during the 1947 partition of the subcontinent in which around half a million people were killed in mass violence.

It is small wonder that Pakistani president Asif Ali Zadari has said that it will take at least three years for the country to recover from the disaster, and that he is thinking ahead to “prepare the capabilities and capacity” for the “next monsoon”. Zadari’s comments highlight the fact that one of the key questions arising from the crisis is whether the floods, the worst for at least 80 years (with one fifth of the country estimated to be under water), are linked to global warming and are thus likely to happen again.

The danger is that Pakistan, and the Indian subcontinent in general, will become the focus of much more regular catastrophic flooding with the problems this would bring for a state at the centre of the campaign against terrorism. This is not just therefore a question of better protecting against natural hazards, but also one with profound implications for geopolitics and international security.

Heavy monsoon precipitation has increased in frequency in Pakistan and western India in recent years. In July 2005, Mumbai was deluged by almost 950mm 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.

This trend is fuelled both by global warming (which means extremes of rainfall are a growing worldwide trend) and potentially by any intensification and alteration of the El Niño/La Niño cycle. To understand the reasons why global warming is playing a role, one needs to look at the main climatic trends in South Asia. In addition to more extreme rainfall, there is also a reduction of ice over the Tibetan plateau and changing precipitation patterns, 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. 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 the north-east. In 2006 there was an unusually intense drought in Assam and rain in north-west India. This year with the strong rainfall in the north-west, there is no pronounced decrease in the north-east. Recent US studies have also concluded that the mountain meteorology is changing, but as a result of the aerosols emitted from urban areas of south Asia.

The biggest question is whether the El Niño southern oscillation (Enso), that determines the 10-year oscillations of weather across the Pacific basin and into South Asia and Africa, will change.

Although there is no scientific consensus, it seems likely that if the Amazon rainforest 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 become comparable with the areas of the Pacific where the temperature fluctuates over a few degrees – which is now better monitored and computer modelled.

The reason for concern about changing Enso is that depending on its periodic strength, it greatly affects magnitudes and locations of floods, droughts and 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, not least because of the sizable proportion of the world population affected, these issues need urgent study and also preparations on the ground by the affected countries. Unless this happens, including better flood warning systems and water management infrastructure put in place, societies and governments in the region will be unable to respond to the devastating combination of changing environmental stresses, growing population and geopolitical instability.

Lord Hunt is visiting professor at Delft University and a former director-general of the Met Office

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