Photo: Abhijith P/Unsplash.
Bengaluru: Imagine living in Mumbai, and waking up to the view of the Arabian Sea every morning. Now imagine having to abandon your home because sea level on Mumbai’s coast has risen, and every time there’s a storm or a cyclone, a wall of high waves rides on that water, breaching the embankment and submerging everything in its path. This scenario hasn’t played out yet – but it is likely to happen in the future.
Scientists have predicted this because there is considerable evidence that the sea level is rising everywhere on Earth. In fact, measurements show that the mean sea level has been rising faster in recent decades than in the entire 20th century. Researchers are also quite sure, through meticulous measurements taken across the globe, that much of the sea-level rise is due to human-induced global warming that’s melting ice sheets and glaciers, and heating up the oceans.
In the 20th century, the global mean sea level rose at 1.4 mm per year, according to the 2019 IPCC Special Report on Ocean and Cryosphere in a Changing Climate (SROCC). Between 2006 and 2015, the rate increased to 3.6 mm per year. In fact, mean sea level is projected to rise beyond 2100 no matter how much we change our greenhouse gas emissions from this point on, according to the report.
Another report released by India’s Ministry of Earth Sciences (MoES) and the Indian Institute of Tropical Meteorology (IITM-MoES), Pune, in June this year suggests India is following similar trends. Between 1874 and 2004, the mean sea level in the northern Indian Ocean rose by 1.06-1.75 mm per year – but at about 3.3 mm per year from 1993 to 2017.
The rising mean sea level is evidence that climate change is definitely happening, said Steven Nerem, a professor of aerospace engineering sciences at the University of Colorado, Boulder. It’s like popping a thermometer and taking your temperature when you feel sick, he told The Wire Science. “Measuring sea level changes is another way of diagnosing what’s happening with Earth.”
So the numbers tell us that the mean sea level around India is rising. The term ‘mean sea level’ – or the average height of the ocean – also sounds simple.
As it turns out, tracking it reliably is not simple at all.
Various levels of the sea
Unless you live on the coast and keep a close watch on the ocean’s comings and goings, perhaps the one time you do think about the mean sea level is when talking about the elevation or depth of a place. Official figures put Bengaluru at an average height of around 900 m above mean sea level, for example. Kanchenjunga in Sikkim has a height of around 8,586 m above mean sea level. Some parts of Kerala like Kuttanad are estimated to be a metre or two below mean sea level. Chennai is nearly at sea level.
But the height of the ocean – from any reference surface or point – isn’t constant. There are waves on the ocean’s surface. The combined effects of the gravitational pull of the Sun and the Moon produce high tides and low tides. Then there are weather patterns like storms or the El Niño that constantly move the water around.
The ocean is also not continuous. It is interrupted by lumpy masses in the form of continents and ice sheets, whose different density and therefore gravity pulls the sea slightly toward them. Any change in these masses, say, due to ice sheets melting can affect the gravitational field and change regional sea levels.
Land masses are also not stationary. They’re subtly, but constantly, moving up or down. Tectonic activity, for instance, can cause land to subside or rise. So can the gradual build-up or removal of sediments deposited by rivers in delta regions like the Sundarbans, or the excessive withdrawal of groundwater.
There were also massive ice sheets covering much of North America, and Northern Europe in the last glacial period, about 20,000 years ago. As the planet’s surface warmed, the ice melted and retreated to the sea, and land – no longer saddled by the weight – began to rebound, and continues to do so very slowly. Think of how your mattress resumes its original shape after you get up.
To understand how the sea level is changing in relation to your city, town or village, we need to account for both the land’s and the ocean’s movements. “If land subsides, or goes down, then you feel that sea level is rising. If there’s no subsidence, then we get to see the actual sea level,” M. Ravichandran, director of the National Centre for Polar and Ocean Research in Vasco da Gama, said.
So how do scientists determine if sea level is rising or not?
To get a sense of changing sea levels in the 20th century, scientists have banked principally on instruments called tide gauges. These devices, installed within observatories along the Indian coast and on some islands, measure how the sea level changes relative to the point on land where the observatory is built – also called the relative sea level.
India’s tide gauges are mostly mechanical: any change in water level displaces a float, whose movement is conveyed to a needle that records the ups and downs of the water on a graph chart wound around a rotating drum. The charts are then sent to the Survey of India office in Dehradun, the central agency responsible for managing tide gauges in the country, where researchers extract the data and record it digitally for further analysis, according to S.K. Singh, former director of the geodetic and research branch of the Survey of India.
In many places around the world, float-activated tide gauges have been replaced by electronic or other kinds of gauges. But in India, these old gauge types continue to be in use. They’re also typically more reliable.
There are 36 tide-gauge observatories on India’s mainland coastline and islands, Singh added, although he wasn’t sure how many were functional at the moment.
In fact, not all tide gauges have been functional all the time in the past. But to be able to see if climate change is indeed causing the sea level to rise, you need not a month, not a year, but several decades’ worth of continuous data – at least. This is so natural variations in the sea level aren’t mistaken for a rise or fall. “If you use a shorter period of data, there will be other factors contributing to sea level,” A.S. Unnikrishnan, a retired chief scientist of CSIR-National Institute of Oceanography, Goa, said. “In some years, there will be El Niño, for example, that will reflect on the sea level. If you take longer periods, those signals will get averaged.”
So how many decades of data do you need? Some researchers have suggested we need more than 50 to 60 years of continuous measurements, with minimal gaps. But tide gauges don’t often collect data over such a long period, and researchers have had to make do. Unnikrishnan, for example, has used data from tide gauges with more than 40 years of continuous records to determine long-term sea level trends for India in the last century.
There aren’t many of those in India. The tide gauge in Mumbai, installed by the British East India Company in the late 1800s, is the only observatory in the country that has been offering more than a century’s worth of sea level measurements. A few others, in places like Kochi, Diamond Harbour in Kolkata, Visakhapatnam and Chennai, have more than 40 years of recordings. But data from some of these tide gauges have their limitations too.
In a paper published in 2006, for example, Unnikrishnan and his colleagues studied data from tide gauges in Mumbai, Kochi, Chennai and Visakhapatnam to understand mean sea level change along the Indian coast up to 1994. In that analysis, they found that while Mumbai, Kochi and Visakhapatnam showed a sea-level rise of 0.78, 1.14 and 0.75 mm per year respectively, the estimate for Chennai showed a decrease, by about 0.65 mm per year.
In a subsequent study, published the next year, the team analysed tide gauge data from four observatories in the Arabian Sea – Aden, Karachi, Mumbai and Kochi – to estimate sea-level trends across the north Indian ocean. For the Bay of Bengal, they used one tide gauge in Visakhapatnam. This time, the team excluded data from Chennai. “We later on realised that that [trend] cannot be taken seriously because there are gaps in the data,” says Unnikrishnan.
The 2007 paper estimated that the mean sea level had increased at 1.06-1.75 mm per year in the north Indian Ocean up to 2004. These results aligned with the global mean sea level increase rate of 1 to 2 mm per year in the same time period. And India’s National Action Plan for Climate Change, released in 2008, uses this estimate.
However, a national mean sea level rise is only an average indication of what’s happening across the north Indian Ocean. For a country like India, which has a long coastline plus includes numerous islands, the average hides many local differences. These differences could affect how plans for combating climate change are designed.
For example, tide gauge data from Diamond Harbour in Kolkata wasn’t included in the 2006 or the 2007 analyses of final average trends (although the 2007 paper did report Diamond Harbour’s data individually). This is because making sea level measurements in the region is tricky in general. There are frequent ‘signals of storm surges’ – that is, a temporary increase in sea level due to storms pushing more water towards the coast – reflected in the sea level data, Unnikrishnan and his colleagues wrote in the 2006 paper. And there’s land subsidence.
But subsequent studies (this and this) that did analyse data from Diamond Harbour tide gauge found that the net mean sea level rise there was nearly 5 mm per year between 1948 and 2010. The Ministry of Earth Sciences first noted this trend only in 2019, in the Lok Sabha – that Diamond Harbour’s sea level rise measurements were much higher than the national average during a similar period.
Some researchers say that the delta region has been sinking over time, although the data to pinpoint the exact mechanisms influencing the subsidence isn’t very clear. Land subsidence combined with an actual rise in the mean sea level could be exacerbating the impact of sea-level rise in the Bay of Bengal delta, experts said. Imagine standing in the middle of a swimming pool that’s both being filled with water, and whose bottom is sinking.
“The Sundarbans delta is subsiding quite fast,” said Sugata Hazra, of the school of oceanographic studies, Jadavpur University, Kolkata. “It is not the mathematical sea level but what is actual or the relative sea level that is creating the hazard for people.”
But to get to a ‘true’ relative sea level, we need information on whether, and at what rate, the land is moving up or down. There is a global model to account for some of the glacial isostatic adjustment of land – the very slow rebound of land due to ice sheets melting. On the flip side, there isn’t much data in India on how tectonics, sediment compaction, water extraction and other local processes are changing the land’s vertical movement.
Ideally, you would have this vertical land-movement information from the same place as the tide gauge observatory. This is because tide gauge observatories are built on land, and any vertical motion of Earth’s crust affects the observatory’s height relative to the sea.
After the 2004 Indian Ocean tsunami, the Survey of India installed GPS instruments to measure land movement at around 20 observatories along the Indian coast. But the measurements have been discontinued, according to Singh. “Due to maintenance, administrative and other problems, the project could not go [on for] very long. We have some data, but it’s not [for] long enough, and there were problems with the instruments also.”
There is another big limitation of using tide gauge data. While some observatories do offer a long-term look at sea level changes in India, going back several decades or even a century in the case of Mumbai, there are not enough of them. This makes understanding local variations across the Indian coastline a challenge.
“Tide gauges sample the ocean sparsely and non-uniformly,” said Swapna Panickal, a scientist with IITM-MoES. “They provide point observations mostly confined along the coastlines and ocean island stations, and therefore [are] often not representative of offshore conditions.”
Fortunately, scientists have another tool to measure sea level.
Since 1992, satellite altimeters have been the primary source of information on sea level. But these satellites measure a different kind of sea level compared to tide gauges. Data from tide gauges allows us to determine the relative sea level. Satellite altimeters on the other hand measure the absolute sea level – which are changes in the height of the ocean relative to a fixed centre of the planet. This way, it doesn’t matter if land nearby is rising or sinking.
Satellite altimeters have a simple mechanism. The satellites have radars onboard that bounce signals off the ocean’s surface. The time it takes for a pulse to go all the way down and come back up is used to calculate each satellite’s distance from the ocean. Subtracting this distance from a satellite’s altitude relative to Earth’s centre yields the ocean surface’s height from Earth’s centre.
Satellite altimeters cover the entire globe, unlike tide gauges that record changes in sea level at distinct, well-separated locations. But while tide gauges monitor sea level every day, altimeters cover the planet in about 10 days, which means there’s a sea level recording of a particular place every 10 days. “We’ve been doing that for 27 years now, so we have 27 years of 10-day maps of sea level that are accurate to a centimetre or two,” Nerem said.
By analysing satellite altimetry data for the north Indian Ocean, a recent study found that the mean sea level across this region had risen at a rate of about 3.2 mm per year between 1993 and 2012 – much faster relative to the entire 20th century. For the northern and eastern coasts of the Bay of Bengal, where West Bengal is located, satellite altimetry-derived trends are even more alarming: about 5 mm per year.
So it seems the sea level around India isn’t just rising – it’s accelerating. But the study’s authors caution that a period of two decades is “not long enough to resolve the natural sea level variability at interannual and decadal timescales”.
Even globally, the rates of mean sea level rise have been found to be picking up pace in the last two decades. What’s causing the acceleration?
For the north Indian Ocean, the exact mechanisms aren’t very clear yet, says Unnikrishnan.
But speaking of the planet as a whole, the acceleration is likely because of two reasons, according to the 2019 IPCC SROCC. First, Greenland and Antarctic ice sheets as well as glaciers seem to be melting faster this century than they did in the previous one. Second, since the ocean stores more than 90% of the excess heat trapped by greenhouse gases, studies indicate that the ocean is becoming hotter than before – and hot water expands.
The Indian Ocean, in fact, seems to be heating up more than others. Between 1951 and 2015, its surface temperature increased by 1º C on average, reports suggest, compared to the global average rise of 0.7º C in the same time period.
Scientists have been able to figure out the contribution of ocean heating to sea-level rise using devices called argo floats. A network of about 3,000 of these robotic instruments have been lowered into the ocean around the world. The floats move with currents, measure temperature and salinity of the ocean at various depths, and transmit the data to satellites. “India provides about 150 floats of data at any point of time,” Ravichandran said.
To estimate the contribution of melting ice, researchers have turned to NASA’s twin Gravity Recovery and Climate Experiment (GRACE) satellites. These satellites help measure how the mass of ice sheets or glaciers is changing on Earth by studying how the force of gravity varies over Earth.
“GRACE provides not only how much total mass is being added to the ocean, it also tells you at which parts it is being added to, which parts of Antarctica, which parts of Greenland or Alaska,” said Srinivas Bettadpur, director of the Centre for Space Research at the University of Texas at Austin. “An oceanographer can then decide whether the entire addition of mass will make the sea level rise uniformly or if it will make it rise non-uniformly in different parts of the oceans.”
In general, satellite measurements have been key to figuring out possible causes of sea level rise, according to Nerem.”If all we had was tide gauge data, I think we would all be arguing about … what’s causing sea level change, how much is changing. So satellite data has really helped us understand what is happening.”
On the flip side, satellite altimeters can’t take a closer look at the land because of competing signals from shallow water and terrestrial sources. For many climate studies, getting closer to the coast isn’t very necessary, according to Nerem. But if people want to know what’s happening or going to happen on the coast where they live, say in Mumbai, then data from the coast is essential. “If you have to design a plan for coastal resilience, you have to depend on the local observations and assessments,” Hazra said.
This data comes mainly from tide gauges.
Panickal said we need continuous monitoring of the Indian coast line with an extensive network of tide gauges with co-located GPS systems.
There are also gaps in our understanding of how sedimentation and groundwater extraction affect land movements in India, particularly in places like the Sundarbans delta region. It’s surprising we don’t have this information even after three Sundarbans islands have completely disappeared and several others, including tiger habitats, have shrunk, Hazra added.
Finally, there aren’t enough regional projections that can shed light on which parts of the country are most at risk from sea level rise. That is, there aren’t enough studies for us to know which parts of the country will be inundated and to what extent, or which places will suffer frequent floods, twenty years from now.
“All these IPCC models are global level models – global models are okay for certain purposes, like studying the acceleration of mean sea level,” Unnikrishnan said. “But if you want to know what is happening where, regional projections aren’t very good now. There are many things to be done. And some people have already started in this direction.”
Note: This article was edited at 9:30 pm on August 30, 2020, to clarify Sugata Hazra’s institutional affiliation.
Shreya Dasgupta (@ShreyaDasgupta) is an independent science writer based in Bengaluru, India. Her work has appeared in Mongabay, Nature, BBC Earth, Smithsonian.com, New Scientist, Ensia, and other publications.