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The threat of rising sea levels

Context: The World Meteorological Organisation (WMO) has found in a new report that the world’s sea level is rising at an exceptional rate, portending potentially disastrous consequences for the weather, agriculture, the extant groundwater crisis, and social disparities.

The report

• The report, entitled ‘State of the Global Climate 2022’, was published last week.
• Along with accelerating sea-level rise, it focused on a consistent rise in global temperatures, record-breaking increases in the concentration of greenhouse gases as well as glacier loss, sustained drought-like conditions in East Africa, record rainfall in Pakistan, and unprecedented heat waves that struck Europe and China in 2022.

The rising sea?

• The rate of global mean sea-level((GSML) rise has doubled between the first decade of the satellite record and the last.
• Since the 1990s, scientists have been measuring sea-level rise using satellite altimeters.
• These instruments send radar pulses to the sea surface and measure the time they take to get back and the change in their intensity. The higher the sea level, the faster and stronger the return signal.
• Researchers are able to determine GSML by collecting this data from different points on earth and calculating the average.
• To calculate the rate of change in the GSML — i.e. how fast or slow the sea level is changing — we can calculate the difference in the GSML across a few years, usually a decade, and then divide the difference by the number of years. This provides an estimate of the rate of sea-level change.
• 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 sea-level rise was 2.27 mm/year in 1993-2002, it shot up to 4.62 mm/year in 2013-2022.

What causes accelerated sea-level rise?

• The WMO report points to the following factors as being responsible for a rising GSML:
  • ocean warming
  • ice loss from glaciers and ice sheets, and changes in land water storage.

• 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%.
• 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. One measure of ocean warming is the ocean heat content (OHC). As per the report, OHC measures in 2022 touched a new record.
• The report also says that the earth’s ice cover, known as the cryosphere, has thinned.
  • The cryosphere includes the Arctic and Antarctic regions (called “sea ice”), glaciers, the ice sheets of Greenland and Antarctica (area of ice on land covering more than 50,000 km2), seasonal snow cover, and permafrost (mass of land that remains below 0 degree Celsius for at least two straight years).

What problems will sea-level rise cause?

• One, the accelerated pace will cause changes in land cover, i.e., “what will be land and what will be sea”, in the future.
  • Rising seas swallow more of the land cover, particularly in coastal areas, coastal communities will face an “acute shortage of land for human use”.
  • This land crunch, will mean that those who are better off will be able to cope better than marginalised groups, leading to an increase in social disparities between people living in coastal areas.

• Second, weather formations such as cyclones are known to typically originate in the open seas.
• 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.
• Third, as the GSML continues to rise, more seawater could seep into the ground, leading to the groundwater — which is usually freshwater — turning more and more saline.
  • This, in turn, can exacerbate water crises in coastal areas as well as agriculture in adjacent regions.