West Bengal's climate change conundrum Part II: Changing rainfall patterns have left Kolkata vulnerable to flooding; here's why
One of the reasons for Kolkata’s increasingly vulnerability to flooding is the rapid decline in the east Kolkata wetlands, the natural drainage system of the city.
Rapid decline in the east Kolkata wetlands has left the city increasingly vulnerable
Kolkata has been found to be one of the most flood-prone cities in the world
Extreme events and monsoon variability are fundamentally higher in a warmer world
Editor's note: Kolkata and the Sundarbans face a deadly melange of climate change impacts: Intensifying heat waves and rainfall extremes, an exceptionally rapid rise in sea levels and intensifying cyclones. Chirag Dhara, a climate physicist, visited Kolkata and the Sundarbans in November 2018. He interviewed a wide cross-section of people – college students and professionals, taxi drivers and street dwellers – on their experience of changes in their city’s climate.
He also spoke to experts and activists working in health, science and environment. This five-part series integrates public perception with expert opinion. It contextualises local climate trends within country-wide and global trends, using photographs, videos, satellite imagery, infographics, concept schematics and the latest developments in climate research. Important scientific concepts have been simplified to better explain the causes and consequences of these changes. This is the second part of the series.
“[Earlier] it would rain only during the monsoon season. Now, rain can fall any time. It rained in November. In summer… floods occurred,” said Prannoy, a student of mass communications at St Xavier’s College in Kolkata. “In my youth, it rained for two or three days continuously,” said 96-year-old Samir Chatterjee, a former table tennis and bridge champion. “I don't find that happening now.”
From teens to nonagenarians, everyone in Kolkata is complaining about the monsoon season and how rainfall is increasingly sporadic. These observations are remarkable in how they mirror globally observed changes in rainfall patterns. Yet, perception is fickle and makes caution imperative in interpreting them.
That being the case, it is essential that we ask ourselves a few fundamental questions:
- Do rain records in Kolkata bear out public perception?
- Why are these changes happening?
- How does it matter?
- What does the future hold?
Capriciousness of rainfall
Temperatures remain reasonably consistent among different parts of a city although large green areas may be cooler and business districts warmer (the “urban heat island effect”). In contrast, parts of a city could see a deluge while others remain absolutely dry. Rainfall is inherently variable. Whether between seasons, years, or from one place to another. At the most basic level is change in total rainfall. However, what arguably has a greater impact on human societies and ecosystems are changes in patterns of rainfall and its extremes.
How are each of these changing in Kolkata and how is it related to changes seen around the world today?
Changes in annual rainfall in Kolkata, the Gangetic plains, and the rest of India
The fertile Gangetic plains of West Bengal, that include Kolkata and the Sundarbans, receive around 1500 mm rainfall during the year. These include what are known locally as the Kal Baisakhi, violent pre-monsoon thundershowers specific to Odisha and West Bengal that bring short spells of intense rain, and relief from the summer heat.
Q&A: Why is rainfall measured in millimeters (mm)?
Imagine a bath tub and a coffee mug left out in the rain. How much will each fill?
At what may seem counter-intuitive at first, both would fill to the same level, assuming that they are of “regular” shape (cylindrical, for instance). This is because the coffee mug, though much smaller, has a proportionally smaller rainfall interception area than the bath tub.
This independence of the volume of the interception device is why rainfall is measured in millimeters: the height to which the device is filled. The interpretation of Kolkata’s annual rainfall is that it would blanket the city (where ever it rains) with water up to a height of 1500 mm (approx 5 feet) in one year.
The map below shows trends in the average annual rainfall in India between 1950 and 2015. These data show that Kolkata and the Indian Sundarbans have seen a reduction in rainfall (of approximately 10 percent) over the past six decades.
Significant as this decrease is, it pales in comparison to the decreases seen in vast swaths of central India and the northern Gangetic plains, that host some of the highest population densities in the world.
While some parts of the country have seen moderate to large increases in rainfall, monsoon in India as a whole has substantially weakened.
Q&A: Why is the Indian monsoon weakening?
The South West Indian Monsoon bringing rain to the subcontinent from June to September is not just one of the most distinctive rainfall patterns in the world, but also one of the most complex.
A primary mechanism that drives the South West Monsoon is that the land mass of the Indian subcontinent warms up much faster than the Indian Ocean during summer. This creates areas of low pressure over land and prompts the flow of moist air from the ocean into the subcontinent. Precipitation of this moisture delivers monsoon rain.
Research led by Roxy Mathew Koll of the Indian Institute of Tropical Meteorology in Pune (IITM), published in the journal Nature in 2017 found that the temperature difference between land and ocean driving the South West Monsoon is decreasing “due to rapid warming in the Indian Ocean and a relatively subdued warming over the subcontinent.”
One reason for this subdued warming is suspected to be the large aerosol haze over India called the Atmospheric Brown Cloud that shades the country and partially counteracts greenhouse warming. Yet another is the increasing use of irrigation that also suppresses temperature rise, much like the evaporation of sweat moderates body temperature.
The South West Monsoon system, however, has many layers of complexity and is notoriously difficult to predict using even the most sophisticated climate models.
How will the amount of precipitation change globally?
Climate models project that global warming will cause rainfall to increase in wetter parts (regions of high rainfall) of the world and decrease in drier parts, popularly called “wet becomes wetter, dry becomes drier”.
Large as the variations between regions may be, warming is expected to increase rainfall in the global average. As temperature rises and air becomes warmer, moisture evaporates faster from the land and oceans into the atmosphere. More moisture in the air generally means that we can expect greater rainfall.
Yet, observations have found little change in (global average) rainfall so far despite robust warming around the world. It is strongly believed that this is the result of the effect of aerosol pollution — byproducts of fossil fuel combustion — that are suppressing rainfall, counteracting the increase due to greenhouse warming.
Rainfall becoming increasingly erratic in most parts of country
On 24 June, 2018, the Santacruz Observatory in Mumbai recorded 231 mm or rainfall in a single day, amounting to over 10 percent of the annual average of the city. On 27 August 2018, Palam Observatory in Delhi recorded 101 mm in a single day, nearly 13 percent of the annual average. Heavy and extreme rainfall of short durations can contribute a significant fraction of the annual amount of rainfall.
A study by Haider Ali and co-workers from IIT, Gandhinagar, in 2014 analysing changes in rainfall at urban centers in India from 1900 to 2010 found that Kolkata is among the cities that has seen the largest increases in episodes of extreme rainfall. Intense rainfall events are contributing an increasing fraction to the total yearly rainfall in the city, which means that monsoon dry spells are lengthening. This is precisely what Samir described, as did most others who were interviewed.
Kolkata is far from an isolated case. Increasing monsoon variability is a pan-Indian, pan-global phenomenon.
A study by Deepti Singh and co-workers at Stanford University in 2014 analysed changes in extreme wet and dry spells from 1951 to 2011 in central India that hosts very high population densities. They found an increase in the intensity of extreme wet spells and incidences of extremely dry spells despite the total monsoon rainfall having decreased.
Roxy Mathew Koll of IITM, Pune, along with co-workers worldwide also showed that extreme rain events have risen threefold from 1950 to 2015 in central India.
A substantial and growing body of research has established that longer spells of moderate rainfall of yesteryears are increasingly giving way to longer dry spells interspersed by short periods of heavy rainfall.
In short, monsoon variability is inexorably rising.
Global changes in rainfall patterns
An extensive review of scientific literature by Mohammed Dore published way back in 2005 analysing rainfall patterns around the world found an “increased variance of precipitation everywhere”.
Rainfall is, of course, extremely variable by its very nature, affected by several natural phenomena that affect circulation patterns. The question that arises is: Are the changes being observed worldwide down merely to natural variations or has human intervention had any discernible influence as well? This is one of the more challenging questions to answer in climate science.
A study published in 2013 in the journal PNAS studied climate model projections together with observational records and found that, indeed, many of the observed changes would have been extremely unlikely to occur in the absence of the rapid rise in greenhouse gases and knock-on effects. In other words, we humans are indeed responsible, at least in part, for the manner in which rainfall patterns are changing.
In addition, there are fundamental physical reasons to anticipate that spottier rainfall must be a primary manifestation of climate change due to greenhouse warming:
Q&A: Why does global warming increase rainfall variability?
Climate models have been predicting and warning for over 25 years that global warming will increase rainfall extremes.
Kevin Trenberth, lead author of several Intergovernmental Panel on Climate Change (IPCC) reports and one of the world’s leading climate scientists, argued two decades ago that an increase in heavy precipitation events should be a primary manifestation of climate change due to increasing greenhouse gases in the atmosphere.
At a basic level, it is easy to understand why. As a rough analogy, we can visualise the atmosphere as a water tank. In this framework, moderate rainfall and dry spells correspond to the time it takes for the tank to fill up and a heavy rainfall episode occurs when the tank bursts.
The size of the atmospheric tank increases with temperature since a warmer atmosphere contains more moisture. Logically, the larger the tank that bursts, the greater the intensity of the deluge. This analogy illustrates why we expect more intense extreme rainfall events in a warmer world. Additionally, a larger tank takes longer to subsequently replenish with water, increasing the length of dry spells.
More technically: The total moisture content of the atmosphere increases by about 6 to 7 percent for each degree rise in temperature because of which the frequency of high intensity rainfall events increases by about the same amount.
Increase in rainfall is, however, much more muted at about 2 percent per degree.
This causes the frequency of extreme events to rise much faster than total rainfall and necessarily means that the contribution of rainfall extremes to the total amount increases with warming. Consequently, there must be a reduction in moderate intensity rainfall and an increase in dry spells i.e. rainfall variability increases.
Flood-prone Kolkata: yet another grave concern for the city
River discharge and land subsidence in river deltas combined with storm surges and rising sea levels make coastal and riverine cities extremely vulnerable to flooding.
Kolkata has been found to be one of the most flood-prone cities in the world because of its exposure to cyclones that form over the Bay of Bengal, and an exceptionally rapid rise in relative sea levels in these parts.
Kolkata has a population of 14 million and growing, a third of whom are homeless or live in slums. That places millions of the poorest and most vulnerable at grave risk. The Delta Alliance, an international organisation aiming to improve resilience of the world’s river deltas, published a report projecting that Kolkata (and Dhaka) will see a staggering 4 to 11 times increase in flood related risk between 2017 and 2030.
An Organisation for Economic Co-operation and Development (OECD) report has projected Kolkata to be among the top three cities in the world seeing the greatest economic losses due to flooding until 2050 and to have the most people exposed to coastal flooding by 2070 of any city in the world.
One of the reasons for Kolkata’s increasingly vulnerability to flooding is the rapid decline in the east Kolkata wetlands, the natural drainage system of the city. Wetlands are defined as areas that are covered in water for at least one season. They act as a natural sponge, soaking in water and holding it until it can slowly infiltrate into the ground.
Increasing population and rapid real estate development over these wetlands is robbing the city of its natural defense mechanism against floods.
Fortunately, there has been a promising recent development. Kolkata acquired India’s first city-level Flood Forecasting and Early Warning System in 2018. Four hundred sensors placed at hot-spots around the city feed real-time data on inundation and other related parameters to a cloud-based dashboard where they are automatically analysed. Risk information is expected to be shared with citizens and community groups to help mitigate flood risk.
Conserving and restoring the wetlands, construction of dikes, floodgates, drainage ditches, canals and pumping stations are some of the measures the city may have to invest in to stave off the worst ravages of urban flooding.
What does the future portend?
As discussed above, extreme events and monsoon variability are fundamentally higher in a warmer world.
Since the marriage of modern life to greenhouse gas emissions is going strong and the Earth continues to warm at an accelerating rate, it is natural to expect that recent trends in rainfall patterns will amplify.
Pendergrass and co-workers analysed climate model projections finding that both variability and extreme events will increase in almost every part of the world by the year 2100, with India projected to experience some of the largest increases over inhabited land areas.
As Pendergrass succinctly summarises what is required: “Society needs to take measures to deal with little change most of the time and a handful of events with much more rain.”
To make it even more pertinent to India, one might add, “… and a declining summer monsoon”.
The author is a climate physicist currently freelancing for Firstpost. You can get in touch with him on email@example.com
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