Climate change: Sea levels are rising faster than anticipated

Fuelled by more and more drainage of meltwater from Greenland and West Antarctica, seas are now rising 4.8 millimeters per year

Climate change: Sea levels are rising faster than anticipated

The United Nations’ Intergovernmental Panel on Climate Change (IPCC) last September raised a concern that the sea level is not just rising but accelerating at a rate of 3.2 millimeters per year. According to the report, the sea level rise was 1.4 mm per year from 1901-1990, which rose to 2.1 mm per year in 1970-2015. The rise accelerated to 3.2 mm per year in 1993-2015, and 3.6 mm per year from 2006-2015. The variation is so much over different periods that it was difficult to infer whether it was holding steady or accelerating. Two methods have been used to monitor sea-level rise - tide gauges which record the rise and fall of the water level at a fixed location over time, and satellites which use pulses of reflected radar to measure the ocean’s height (radar altimetry).

About 25 per cent sea-level rise is due to the addition of meltwater drained from glaciers in the Polar regions and about 75 per cent is attributed to the thermal expansion of water induced by global warming. Faster melting of West Antarctica and Greenland ice sheets has pushed the rate to 4.8 millimeters per year, according to NASA’s Jet Propulsion Laboratory. According to research published by Hamlington and colleagues in the Proceedings of the National Academy of Sciences, the first reported signs of the sea-level push up appeared in 2018. In a 2019 study in Nature Climate Change, a group led by Sönke Dangendorf, Old Dominion University, used tide gauge readings that predate satellite records to show seas rose 20 centimeters since 1900. The team’s data show that, after a period of global dam building in the 1950s that held back surface water and slowed sea level rise, it began to accelerate in the late 1960s. 

Fuelled by more and more drainage of meltwater from Greenland and West Antarctica, seas are now rising 4.8 millimeters per year and show few signs of slowing down. Oceanographers are about to get a sharper view of the trends by launching a new satellite this month, that will work much like its predecessors, using pulses of reflected radar to measure the ocean’s height. But its higher resolution measurements will allow it to gauge ocean height within 300 meters of the coastline, far closer than before.

The coasts are where sea level rise hits home—and where large local variations can mask the global average. In work published last month by Anny Cazenave, International Space Science Institute reanalyzed the satellite record and showed sea level rise at 20 per cent of the coastal sites they surveyed across Europe, Asia, and Africa was significantly different from that of the open ocean.  Some of the variations reflect the vertical motion of the land itself, due to the slow bobbing of continental plates that float on a viscous mantle. Coastal ocean currents, fresh water from nearby rivers, and weather patterns can also inject variability by causing water to pile up or retreat from the continents. But these effects are negligible.

The rising sea-level trends are worrisome. Aimée Slangen, a climate scientist at the Royal Netherlands Institute for Sea Research, and colleagues are putting together recent projections from climate models to predict when sea levels will rise 25 centimeters above 2000 levels, a point when 100-year floods on some coastlines could be a near-annual occurrence. In unpublished work, Slangen finds that the threshold will be reached sometime between 2040 and 2060. Efforts to slow climate change help to postpone it given the inertia of ocean warming and ice melt, though they could obviate much greater increases later in the century. 

The imminent consequences of sea-level rise will be coastal flooding. According to experts, small islands and low-lying cities might experience submergence by 2050. A study published in Scientific Reports, researchers predict that by 2100, the global population most likely exposed to periodic coastal flooding will increase from 128-171 million to 176-287 million. Is India with its 7,500-odd coastline and dense population in the coastal areas equipped to handle the impact of the phenomenon? 

Land-based reservoirs will be contaminated by saline water leading to scarcity of drinking water. As per some projections, climate change is expected to inundate significant sections of Mumbai by 2050, impacting millions of people. How will a city like Mumbai, which is home to an estimated 20 million people, handle this catastrophe? The memories of the 2005 and 2017 floods still linger, when Mumbai witnessed the worst inundation in its history. Incessant rain, coupled with storm surges and high tides killed over 1000 people and rendered many homeless. Other cities that regularly feature in the lists endangered by climate change include Guangzhou, Dhaka, Jakarta, Miami, and Manila.

The Indian coastline is up for rough weather on a different count. Compared to other seas, the Indian Ocean has been warming consistently over 50 years and the temperature trend is close to a degree Celsius for 1951-2015. Rising ocean temperatures are disturbingly altering the lives of fish. When the seas began to warm, the fish that prefer warmer waters expand their territory and explore new areas for breeding and feed. For example, Scientists from Central Marine Fisheries Institute found that sardines, a fish typically found in abundance along the Kerala and Karnataka coast, landed up on the east coast as well as Gujarat coast. 

Rising sea level is not a part of the national climate debate. There is very little discussion on this issue in the national action plan on climate change (2008). There is an urgent need for preventive planning, adaptive management techniques, and enforcement of coastal zone regulations, keeping in mind the probable inundation zones. The problem often tends to be region-specific and may need local responses. The city of Panaji and areas like Baina and Canacona are getting flooded every monsoon. The planning authorities need to act sooner to put in place a practical disaster-management plan to avert a major catastrophe. 

(The author is a scientist working on climate change topic for Polar regions)

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