Sea Level Rise, Coral Reefs and Ocean Governance

Why sea level rises — and one thing that does not raise it. Global mean sea level is rising by roughly 3–4 mm a year and the rate is accelerating. Two mechanisms dominate. Thermal expansion: warmer water occupies more volume, and because the ocean stores over 90% of the extra heat trapped by greenhouse gases, this alone accounts for about a third to a half of the rise. Melting land ice: glaciers and the Greenland and Antarctic ice sheets add water that was previously stored on land. The trap: melting sea ice does not raise sea level, because floating ice already displaces its own weight of water. Local sea level change also depends on the land: coasts that are subsiding (deltas compacting, groundwater extraction) experience faster relative rise, while some high-latitude coasts are still rebounding after the last ice age and see little change.

Consequences are uneven. Low-lying deltas such as the Ganges–Brahmaputra and the Mekong face increased flooding, saline intrusion into farmland and aquifers, and loss of mangroves. Small island states may lose habitable land and freshwater lenses long before they are submerged. Wealthy cities can build barriers; poor communities usually cannot, so the burden falls hardest on those who contributed least to the cause — an equity point that scores in evaluation questions.

Coral reefs: why they matter and how they bleach. Reefs cover under 1% of the ocean floor but support about a quarter of marine species, protect coasts by absorbing wave energy, and underpin fisheries and tourism worth billions. Coral polyps host symbiotic algae (zooxanthellae) which supply most of their food and their colour. When water is 1–2 °C above the summer maximum for several weeks, the coral expels the algae and turns white: bleaching. Bleached coral is not dead — if temperatures fall quickly it can recover — but it is starving, and if the stress persists it dies. The real threat is that bleaching events now recur faster than reefs can recover. Reefs also face ocean acidification (dissolved CO₂ lowers pH and makes it harder to build carbonate skeletons), sediment and nutrient runoff, destructive fishing and tourism damage.

Governance: who controls the sea? Under the UN Convention on the Law of the Sea, a coastal state has full sovereignty over its territorial sea (12 nautical miles) and sovereign rights over resources in its exclusive economic zone (200 nautical miles). Beyond that lie the high seas — about two thirds of the ocean — where no state has jurisdiction and management depends on regional fisheries organisations and treaties. This is the classic open-access problem: no owner, so no one has an incentive to conserve. Tools include marine protected areas (which work only if enforced and large enough to cover the range of the species), catch quotas, gear restrictions, and cutting the fuel subsidies that make distant-water fishing profitable. A 2023 treaty on biodiversity beyond national jurisdiction created a legal basis for high-seas protected areas — a step, not a solution.