Ice, oceans and sea level: the mechanisms

Prelims and MainsCurrent affairs on this: Cryosphere, oceans and sea level

The cryosphere is the frozen part of the planet, sea ice, ice sheets, glaciers, snow and permafrost, and the questions on it are about mechanism: what melts, where the water goes, and what the ocean does with the heat. The ocean has taken more than 90 per cent of the extra heat trapped since 1970, and sea level rises because warm water expands and because land ice melts into it; sea ice, already afloat, raises nothing when it melts but exposes dark water that absorbs more sun. Global mean sea level rose about 20 cm between 1901 and 2018, and the rate has nearly tripled, from 1.3 mm a year in the early twentieth century to 3.7 mm a year in 2006 to 2018. The same carbon dioxide dissolving in seawater has lowered its pH by about 0.1 since industry began, ocean acidification, which is chemistry rather than temperature.

• Where sea level rise comes from, 1971 to 2018: thermal expansion of warming water about half, glaciers about a fifth, the Greenland and Antarctic ice sheets about a fifth, and water moved from land by groundwater pumping the rest, as in the figure. The ice sheets are the wild card: they hold enough water for tens of metres, and their loss has accelerated.
• Sea ice against land ice: Arctic sea ice reaches its yearly minimum each September and has shrunk by about 12 per cent a decade since satellites began watching in 1979; Antarctica's sea ice sits around a continent of ice sheet, and it is the sheet, not the sea ice, that matters for sea level. Snow and ice reflect sunlight, so their loss is a feedback, the ice albedo effect.
• Acidification: carbon dioxide and water make carbonic acid, which releases hydrogen ions and uses up carbonate ions, so corals, shellfish and plankton find it harder to build shells of calcium carbonate; a 0.1 fall in pH is about a 30 per cent rise in acidity, since the scale is logarithmic. A marine heatwave is a spell of at least five days with sea surface temperature in the hottest tenth for the season, and the Indian Ocean is among the fastest warming.
• The Himalaya, the Third Pole, holds the largest ice outside the poles: about 10,000 glaciers in the Indian Himalaya feed the Indus, Ganga and Brahmaputra, and the assessments of the International Centre for Integrated Mountain Development find the loss accelerating decade on decade, with a third to a half of the ice gone by 2100 even at 1.5°C to 2°C. A glacier retreating leaves a lake behind a wall of loose moraine, and a glacial lake outburst flood is what follows when the wall gives way; the glacial lake risk subject has India's atlas and criteria.
• Permafrost, ground frozen for at least two years, holds about twice the carbon now in the air; its thaw releases carbon dioxide and methane and loosens mountain slopes, which is the mechanism the Himalayan glacier retreat subject describes.

Mains: India's exposure is at both ends of the same water: a coast of 7,500 km and cities like Mumbai and Kolkata at sea level, and mountain valleys below lakes that did not exist a generation ago, so adaptation means coastal planning and glacial lake monitoring at once, paid for from the same budget.

See also: Himalayan glacier retreat · Glacial lake risk in India · The Nepal flood of August 2026 · INCOIS El Niño bulletins for the seas · Coral reefs of India · Disasters: the notified calamities and the warning chain · The IPCC and the numbers a climate question uses