The threat of rising sea levels

Context: The World Meteorological Organisation (WMO) has found in a new report, entitled ‘State of the Global Climate 2022’, that the world’s sea level is rising at an unprecedented rate.

What is average sea level?

  • It is the position of the air-sea interface, to which all terrestrial elevations and submarine depths are referred. 
  • The sea level constantly changes at every locality with the changes in tides, atmospheric pressure, and wind conditions. 
  • Longer-term changes in sea level are influenced by Earth’s changing climates.
  • Consequently, the level is better defined as mean sea level, the height of the sea surface averaged over all stages of the tide over a long period of time.

How sea levels are changing now?

  • According to the WMO report, the sea level has been rising in the three decades for which satellite altimeter data is available (1993-2022). But, while the rate of sealevel rise was 2.27 mm/year in 1993-2002, it shot up to 4.62 mm/year in 2013-2022.
  • Long-term changes in global mean sea level (GMSL) are predominantly driven by three processes:
    • Ice Melt: Due to the warming atmosphere and ocean, ice sheets and mountain glaciers are melting, resulting in the addition of fresh water into the ocean.
    • Thermal Expansion: Ocean water expands as it absorbs trapped heat, causing sea levels to rise. As increasing concentrations of carbon dioxide and other greenhouse gases drive global warming, 90% of the ‘extra’ heat is stored in the oceans. This leads to ocean warming. And as the ocean heats up, it undergoes thermal expansion, which in turn leads to a rise in the GSML
    • Land Water Storage: Water that is either removed from land (through groundwater pumping, for example) or impounded on land (through dam building, for example) can cause a net change in the total water found in the ocean.
  • According to the report, in 2005-2019, loss of glaciers and ice sheets contributed 36% to the GSML rise. Ocean warming — the phenomenon of rising mean ocean temperatures — contributed 55%, and changes in the storage of land water contributed less than 10%.
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State of Global Climate report 2022:

The WMO State of the Global Climate report 2022 focuses on key climate indicators – greenhouse gases, temperatures, sea level rise, ocean heat and acidification, sea ice and glaciers. It also highlights the impacts of climate change and extreme weather.

Drought, floods and heatwaves affect large parts of the world and the costs are rising

Global mean temperatures for the past 8 years have been the highest on record

Sea level and ocean heat are at record levels – and this trend will continue for many centuries

Antarctic sea ice falls to lowest extent on recordEurope shatters records for glacier melt
General factors behind sea-level changes:

Short-term Effects:
Variations of sea level on periods ranging from minutes to weeks that arise from processes like storm surge, wave runup, wave setup, astronomical tides and freshwater input.

Variability in Ocean circulation: This includes large-scale climate signals like the seasonal cycle, El Niño-Southern Oscillation, North Atlantic Oscillation, and Pacific Decadal Oscillation. This also includes longer-term changes in ocean circulation that may occur in the future and global sea level rise associated with long-term warming of the ocean and associated expansion.

Glaciers: Glaciers outside of the ice sheets account for about 1% of total ice trapped on land. These changes are expressed regionally through gravitational, rotational and deformational (GRD) changes that have a characteristic pattern, or fingerprint.

Land Water Storage: Changes associated with the transfer of water between land and ocean. This includes variability in the global water cycle, groundwater withdrawal, and water impoundment.

Ice Sheet: The Greenland and Antarctic ice sheets contribute to global sea level rise.

Subsidence: Movement of the land at the coast (could also be uplift) in response to a range of physical processes including groundwater and hydrocarbon withdrawal, tectonics, and glacial isostatic adjustment. These changes can vary widely in both time and space and can be associated with human activities.

What can be the possible impacts of Sea level rise?

  • Changes in land – ocean configuration which in turn can alter the heat absorption rates.
  • Reduced land for human activities – as rising seas swallow more of the land cover, particularly in coastal areas, coastal communities will face an “acute shortage of land for human use”
  • Increase in cyclones – As the GSML continues to rise, along with a rise in ocean temperatures, the chances of cyclones could increase, affecting coastal communities and leading to large economic liabilities for tropical countries such as India and South Africa, which have high population densities.
  • Increase in salinity – more sea water could seep into the ground, leading to the groundwater — which is usually freshwater — turning more and more saline. 
  • All these factors will affect the agriculture, infrastructure and Settlements.

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