The BL Interview. Coastal cities prone to storm surges, flooding

The adverse impacts of climate change are expected to get much more pronounced than what we have witnessed thus far. In the backdrop of recent floods in Chennai, Ancha Srinivasan, Principal Climate Specialist and Climate Change Focal Point for South East Asia, Asian Development Bank, explains how the climate change could have far-reaching consequences.

The unpredictability of El Nino and its adverse impact on seasonal conditions, monsoons and agricultural operations and hotter summers are all the grim realities of climate change. In 2006, Mumbai received 956 mm rain on a single day.

There are predications that the climate change could lead to temperatures going up by 2.7 to 3 degrees C by 2100. Some have even predicted it to be as high as 7 degrees C. That is scary. Excerpts from an interaction with Ancha Srinivasan.

The recent floods in Chennai have been unprecedented. What do you attribute this to?

This could be directly attributed to four main causes, in the following order of importance.

Poor urban land use planning without paying attention to floodplains and river flows, outdated stormwater drainage systems, and excessive dumping of waste, including plastic, in water bodies; silting of rivers and reservoirs and low carrying capacity of water bodies; urban heat island effect leading to cloud-burst phenomenon, primarily due to climate change; and land subsidence due to over-extraction of groundwater beyond levels of recharge.

We should note, however, that storm surges due to rising sea levels induced by climate change will increasingly pose a threat to life and infrastructure.

You had hinted in August this year during your visit to Hyderabad that Mumbai and Kolkata are among the cities vulnerable to such flooding.

All our coastal cities are prone to storm surge and flooding. A recent World Bank-OECD study reported that average global flood losses will multiply from $6 billion in 2005 to $52 billion a year by 2050. The report notes that global flood damage for large coastal cities could cost $1 trillion a year if cities don’t take steps to adapt.

More inland flooding of coastal cities is expected in future because of sea level rise and storm surges, and stronger tropical cyclones. Adaptation must become the key feature of urban planning in the short to medium term. Along with better structural defences such as dykes, cities will need better crisis management and contingency planning, including early warning systems and evacuation plans.

Infrastructure losses from flooding will be significant. Most of the current infrastructure in coastal cities cannot withstand the impacts of future floods and storm surges. Cities should be prepared to allocate more budget for flood protection infrastructure and their operations and maintenance. Infrastructure resilience should be integrated into urban planning.

Human suffering, especially for the most vulnerable sections of society, will increase. The poor are most at risk as rapid urbanisation has pushed them into the most vulnerable neighbourhoods, often in low-lying areas and along waterways prone to flooding

When the sea surface temperature rises by one degree Celsius, the strength of tropical cyclones increases considerably. There are six main requirements for genesis of tropical cyclones – sufficiently warm sea surface temperatures, atmospheric instability, high humidity in the lower to middle levels of the troposphere, enough Coriolis force to develop a low pressure centre, a pre-existing low level focus or disturbance, and low vertical wind shear. All these factors will increase in the backdrop of climate change.

The once-in-a-century events are likely to happen more frequently than before. It means the interval between extreme climate events will decrease, thereby making our preparedness inadequate. Disaster preparedness in a changing climate thus becomes significant.

Apart from El Nino, what factors contributed to the recent heavy rains in Tamil Nadu and Andhra Pradesh?

El Nino is not necessarily a direct reason for heavy rains but can be a major contributing factor. Usually La Nina brings high precipitation.

We should note that for every one degree Celsius rise in temperature, the moisture holding capacity of clouds increases by 7 per cent. Prolonged drought associated with El Nino can also increase the moisture holding capacity of clouds. When the right weather system develops, cloudburst occurs, dumping large volumes of water in a short time.

How long do you think this erratic behaviour of climate will continue?

The erratic behaviour in climate will continue for generations, as there is already enough greenhouse gases accumulated in the atmosphere to create weather abnormalities. It is important to note that flash floods will become more frequent. Therefore, we need to be prepared in all forms.

Urbanisation increases the possibility of such calamities two to six times over what would occur on natural terrain. During periods of urban flooding, streets can become swift moving rivers, while basements and viaducts can become death traps as they fill with water. This is why future urban planning should allow for more absorption of rain water.

What is the role of global warming in it?

An Australian study showed that a rise in mean temperature by 3 degrees C will heighten flood peaks in urban areas by as much as 20 per cent.

The most intense downpours are getting more extreme at warmer temperatures, dumping larger volumes of water over less time, while the least intense periods of precipitation are getting weaker. If this trend continues, the risk of flooding due to short-term extreme bursts of rainfall could increase even if the overall volume of rain during storms remains the same.

A British study has recently shown that global warming will make downpours a more frequent event. While it is difficult to pinpoint that each extreme event is directly linked to climate change, given the overall trend observed so far in various cities and countries, one could confidently say that climate change is a major factor behind increasing the frequency and intensity of such events.