11.2 Sea level rise and ocean acidification

Sea Level Rise

Causes of sea level rise

Three main factors drive sea level rise, and understanding each one helps you see why the problem is accelerating.

Thermal expansion accounts for roughly half of the global mean sea level rise observed from 1901 to 2018. As oceans absorb excess heat from the atmosphere, water molecules spread apart and take up more volume. No new water is added; the existing water simply expands.

Ice melt from land-based sources adds new water to the oceans. The Greenland and Antarctic ice sheets hold by far the largest potential for future sea level rise because of their massive size. Mountain glaciers in the Himalayas, Andes, and Alps also contribute, though to a lesser extent than the major ice sheets. Note that melting sea ice (like Arctic ice floating on the ocean) doesn't directly raise sea levels because that ice is already displacing water.

Land water storage changes play a smaller but real role. Human activities like pumping groundwater for irrigation move water from underground reservoirs into the ocean system. Dam construction can temporarily offset this by trapping water on land, but the net effect of groundwater depletion has been a contributor to rising seas.

Trends in sea level rise

Global mean sea level has risen about 21–24 cm (8–9 inches) since 1880, based on tide gauge records and, more recently, satellite measurements. The critical detail here is the acceleration: the rate of rise went from about 1.4 mm/year (1901–1990) to 3.6 mm/year (2006–2015). That's more than doubling in pace, which reflects the growing influence of both thermal expansion and ice sheet loss.

Sea level rise is not uniform around the world. Regional differences arise from ocean circulation patterns (like the Gulf Stream), wind patterns, and vertical land motion (some coastlines are sinking due to subsidence, while others are rising due to geological uplift). Parts of the western Pacific, for example, have experienced rates up to three times the global average. This regional variation matters enormously for coastal planning, because the sea level change a specific community faces can be quite different from the global number.

Ocean Acidification

Ocean acidification and CO₂

Ocean acidification is the decrease in ocean pH caused by the absorption of excess atmospheric $CO_2$. As $CO_2$ concentrations in the atmosphere rise, more $CO_2$ dissolves into seawater (following Henry's Law, which says gas solubility increases with higher atmospheric concentration of that gas).

Once dissolved, $CO_2$ reacts with seawater to form carbonic acid, which releases hydrogen ions and lowers pH:

$CO_2 + H_2O \leftrightarrow H_2CO_3 \leftrightarrow H^+ + HCO_3^-$

Since the start of the Industrial Revolution (around the 1750s), average ocean pH has dropped by about 0.1 units. That sounds small, but because the pH scale is logarithmic, a 0.1 drop represents a roughly 30% increase in hydrogen ion concentration (acidity).

Impacts of oceanic changes

Coastal communities face compounding threats from sea level rise:

• More frequent and severe coastal flooding and erosion, especially during storm surges and high tides. Cities like Miami and Venice already deal with regular flooding events.
• Saltwater intrusion into freshwater aquifers and agricultural land. In the Mekong Delta, for instance, this reduces both drinking water quality and crop yields.
• Damage to infrastructure (roads, buildings, ports) and displacement of populations in low-lying areas such as Bangladesh and Pacific Island nations, where millions of people are at risk.

Marine ecosystems are hit by both acidification and warming simultaneously:

• Ocean acidification reduces the availability of carbonate ions, which makes it harder for calcifying organisms to build and maintain their $CaCO_3$ shells and skeletons. The most affected groups include:
  1. Corals
  2. Mollusks (oysters, mussels)
  3. Certain plankton species (coccolithophores, foraminifera)
• Coral reefs are especially vulnerable because they face acidification, warming, and pollution all at once. This matters beyond the reefs themselves: coral ecosystems support roughly 25% of all marine species and provide services like coastal protection and fisheries.
• Shifting ocean temperatures and chemistry push species into new ranges and disrupt food web dynamics. When key species at the base of the food web (like calcifying plankton) decline, the effects cascade upward through the entire ecosystem.

The combination of acidification, warming, and deoxygenation creates compounding stress on marine life. These aren't isolated problems; they reinforce each other, and their combined effect on biodiversity and the communities that depend on ocean resources is projected to be far greater than any single factor alone.