Global Sea Level Rise: WMO Report – Explained, pointwise

Introduction

The World Meteorological Organization (WMO) has released a Fact Sheet, ‘Global Sea Level Rise and Implications’. The fact sheet has made some alarming observations. It notes that the sea level has risen at the rate of 4.5mm per year during 2013-22. The rise has been linked to climate change. Even in the low greenhouse gas emission scenario, the sea level rise of 0.6 metre is expected by 2100. A rise of this magnitude can have disastrous consequences for Island States as well as coastal cities. WMO has termed the sea-level rise a ‘major economic, social and humanitarian challenge‘.

What are the key highlights of WMO Report on Sea Level Rise?

First, Sea-level rise threatens several low-lying small islands and high-population coastal cities.

Second, The impacts of average sea-level rise is further exacerbated by storm surges and tidal variations.

Third, Human influence was very likely the main driver of these Sea-level increases since at least 1971.

Fourth, The global ocean has warmed faster over the past century than since the end of the last deglacial transition (around 11,000 years ago).

Fifth, Thermal expansion explained 50% of sea-level rise during 1971–2018. Among other factors, Ice loss from glaciers contributed 22%, ice sheets 20% and changes in land-water storage 8%. The rate of ice-sheet loss increased by a factor of four between 1992–1999 and 2010–2019.

Sixth, Sea-level rise is not globally uniform and varies regionally.

Seventh, Over the next 2000 years, global mean sea-level will rise by about 2-3 m if warming is limited to 1.5°C, 2-6 m if limited to 2°C and 19-22 m with 5°C of warming.

Eighth, Continued sea-level rise will increase risks to food security in vulnerable regions between 1.5°C and 2°C Global warming level. Sea-level rise poses a distinctive and severe adaptation challenge as it implies dealing with slow onset changes. Increased frequency and magnitude of extreme sea-level events which will escalate in the coming decades. There are significant impacts and challenges to those populations faced with sea-level rise living in coastal urban areas in least developed and low-middle income countries.

Source: Sea Level Rise, WMO Fact Sheet. The trend indicates that the rate of sea-level rise is increasing. The rate was 2.1mm/year between 1993-2002, 2.9mm/year between 2003-2012, and 4.5mm/year between 2013-2023. The sea level has risen by ~100mm since 1993.

What are the reasons for Sea Level Rise?

Ocean Thermal Expansion: Instrumental records reveal that the world’s oceans have warmed since 1955, accounting for more than 80% changes in the energy content of the Earth’s climate system during this period. During the period 1961 to 2003, the 0-3000 m ocean layer has absorbed up to 14.1 × 10^22 Joules, equivalent to an average heating rate of 0.2 Watts/m2 (per unit area of the Earth’s surface). Warming of ocean water leads to expansion contributing to rising oceans. WMO estimates that thermal expansion contributed ~50% of the observed rise in water levels (i.e., contributing ~2.3mm/year rise between 2013-2023. It contributed to a rise of ~1.6mm/year between 1993-2002).

Glacial Melt from Greenland and Antarctica: According to the IPCC AR4 (Assessment Report), it is very likely (> 90% probability) that the Greenland Ice Sheet (GIS) shrunk from 1993 to 2003. An assessment of the data suggests shrinking of Greenland Ice Sheet (~50-100 Gigatons/year) contributing to rising global sea levels of 0.14 to 0.28 mm/yr from 1993 to 2003. There is a risk of a much higher sea-level rise due to potential intrusion of sea water under the Antarctic glaciers, as NASA has demonstrated in its recent published scientific studies.

Loss of Snow on Land: Snow cover has decreased in most regions, especially in spring as confirmed by Satellite observations. This means less water trapped in snow and more water in the oceans, leading to rise in water levels.

Permafrost: Permafrost and seasonally frozen ground in most regions have displayed large changes in recent decades. Temperature increases at the top of the permafrost layer of up to 3°C since the 1980s have been reported. Permafrost warming has also been observed with variable magnitudes in the Canadian Arctic, Siberia, the Tibetan Plateau and Europe.

All the above reasons are attributable to global warming caused by accumulation of Greenhouse Gases generated by anthropogenic activities.

Relative Sea Level Changes Due to Vertical Land Motions: At the local scale, vertical land motions such as uplift or subsidence of the ground due to tectonic and volcanic activity, sediment loading, groundwater pumping, and oil and gas extraction can produce sea level variations relative to the seafloor.

Source: Indian Express

What are the harmful impacts of Sea Level Rise?

Impact on Island States and Coastal Cities: (a) Large coastal urban centers located on low-lying coastal areas will become prone to flooding. Initially, the coast may suffer episodic inundation, but later this may become permanent; (b) High and low tide lines will advance landward, part of the present intertidal zone will become permanently submerged and, consequently, significant land loss is likely to occur; (c) It will cause extensive submergence of low-lying deltaic plains. Large coastal urban centres like Mumbai, Chennai, Kolkata, New York, London, Shanghai, Dhaka, Bangkok, Jakarta, Lagos, Cairo, Copenhagen, Los Angeles, Buenos Aires and Santiago etc. are vulnerable. This may lead to large scale displacement of population; (d) Storm Surges can become more destructive as was evident during landfall of hurricane Sandy in New York and Cyclone Idai in Mozambique; (e) Small Island States with low elevation like Kirbati, Maldives, Solomon Islands, Micronesia, Tuvalu, Palau etc. face threat of complete submergence with rise in sea levels. This will lead to large scale climate-induced migration.

Impact on Freshwater: Freshwater in coastal delta and estuaries will get contaminated by salt sea-water. Water and soil salinity along the coast will increase with the rise in sea levels, destroying normal characteristics of coastal soil and water. This is already happening in Sunderbans delta in West Bengal and Bangladesh.

Landward movement of Water Salinity Boundary in Sunderbans, Bangladesh

Impact on Coastal Ecosystems: The Sundarbans are the largest mangrove forest in the world, covering 6,500 sq. km. The Sundarbans will be completely lost with 1m rise in ocean levels (World Bank, 2000). This will be a great loss of heritage, biodiversity, fishery resources, life and livelihoods. Salinity intrusion has led to ‘top-dying’ disease in Sundari trees. A 2018 report has pointed out that in the last 30 years, 1.44 million cubic meters of Sundari trees,  have been lost to ‘top-dying’ disease.

Impact on Fisheries and Aquaculture: The rise in sea level will have a significant impact on fish habitat and their breeding ground. Rise in water levels will change the location of river estuaries. This will have significant impact on fisheries, aquaculture and consequently on the livelihoods of coastal population.

Coastal Erosion: Sea level rise will cause increased coastal erosion as water will wash out top soil of the coast. In addition to this, the backwater effect is accelerated by sea level rise that will also cause erosion. The forecasted land erosion will lead to displacement of coastal population.

Impact on Agriculture: The landward shift of water salinity boundary will cause salinity intrusion in land which will decrease agricultural production in coastal areas. It will also cause soil degradation. Salinity also diminishes the germination rate of some plants. A World Bank (2000) study suggested that increased salinity alone from a 0.3 metre sea level rise will cause a net reduction of 0.5 million metric tonnes of rice production. Salinity intrusion degrades soil quality which in turn inhibits rice production.

Impact on Health: Reduction of freshwater in coastal regions can cause water-related diseases like diarrhoea. Decrease in food production can contribute to malnutrition among coastal population. Flooding in coastal areas can increase outbreaks of water-borne diseases like cholera.

Extreme Events: Coastal countries will face extreme events. Cyclones are already intensifying rapidly due to more moisture and heat from warming of oceans. Cyclones are bringing more rain than earlier e.g., Super Cyclone Amphan (2020) caused large-scale flooding and inundated tens of kms inland with saline water.

Cascading and Compounding Impacts: Sea-level rise will bring cascading and compounding impacts. Losses of coastal ecosystems and ecosystem services, groundwater salinization, flooding and damage to coastal infrastructure will cascade into risks to livelihoods, settlements (causing displacement), health, well-being, food security, water security, and cultural values in the near to long-term.

What should be done going ahead?

Reduce Carbon Footprint: Greenhouse gases are a key cause of sea-level rise. It will be beneficial to minimise sea level rise by reducing the amount of greenhouse gases emitted each year and developing containment mechanisms. The transition to green energy must be expedited.

Climate Action Plan: Many cities and Nations do not have plans to address climate change. Hence, preparing a climate action plan from individual cities to international level will synchronize the efforts to tackle the sea level rise. There is also a need for an international alliance and agreement, similar to the Paris Climate Agreement, that is explicitly dedicated to looking into the issue of sea level rise. Developed countries must step-up climate finance and technology transfer to the developing counties to enhance their capacity for adaptation and mitigation.

Ecosystem-based Solutions: Coastal wetlands, marshes, and mangrove swamps can hold sediments and expand vertically at rates equal to or greater than the mean rate of sea level rise. These habitats can absorb carbon 40 times faster per hectare than tropical forests, making them extremely beneficial for reducing climate change. Wetlands provide natural buffers for coastal communities during rainstorms and hurricanes. They soak up rain and storm surge water.

Conclusion

WMO Fact sheet on Sea Level Rise has indicated that the rate of water-level rise in oceans is increasing rapidly. Major urban centres and island Nations are vulnerable to the rising water levels. This will have widespread economic, social and humanitarian impacts. There is a need for coordinated action at the global level to address the challenge. Else the consequences can be disastrous.

Syllabus: GS I, Changes in critical geographical features; GS III, Conservation, Environmental pollution and degradation.

Source: WMO, WMO, Indian Express, World Bank Environment Department, Institute for Social and Economic Change