11.2 Sea level rise and ocean acidification

Sea level rise is a growing concern, caused by thermal expansion of oceans and melting ice. Global sea levels have risen 21-24 cm since 1880, with the rate accelerating in recent decades. Regional variations exist due to factors like ocean currents and land movement.

Ocean acidification, caused by increased CO2 absorption, is lowering ocean pH. This threatens marine ecosystems, especially calcifying organisms like corals and mollusks. Combined with sea level rise, these changes pose significant risks to coastal communities and marine biodiversity.

Sea Level Rise

Causes of sea level rise

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• Thermal expansion
  • Oceans absorb excess heat from the atmosphere causing water molecules to expand and occupy more space resulting in rising sea levels
  • Responsible for approximately half of the observed global mean sea level rise (1901-2018)
• Ice melt
  • Melting of land-based ice including glaciers and ice sheets (Greenland, Antarctica) adds water to the oceans
  • Greenland and Antarctic ice sheets hold the largest potential for future sea level rise due to their massive size
  • Melting of mountain glaciers and ice caps (Himalayas, Andes, Alps) also contributes to sea level rise but to a lesser extent compared to the major ice sheets
• Land water storage changes
  • Human activities such as groundwater depletion for irrigation and dam construction can influence sea levels by altering the amount of water stored on land

Trends in sea level rise

• Global mean sea level rise
  • Increased by about 21-24 cm (8-9 inches) since 1880 based on tide gauge and satellite measurements
  • Rate of rise has accelerated from 1.4 mm/year (1901-1990) to 3.6 mm/year (2006-2015) indicating a growing influence of climate change
• Regional variations
  • Sea level rise varies geographically due to factors like ocean circulation patterns (Gulf Stream), wind patterns, and vertical land motion (subsidence, uplift)
  • Some regions such as the western Pacific have experienced sea level rise rates up to three times higher than the global average
  • Local sea level rise can deviate significantly from the global mean making it crucial for coastal planning and adaptation strategies

Ocean Acidification

Ocean acidification and CO2

• Ocean acidification refers to the decrease in ocean pH and increase in acidity caused by the absorption of excess atmospheric CO2
• As atmospheric CO2 concentrations rise more CO2 is absorbed by the oceans following Henry's Law
• Absorbed CO2 reacts with seawater forming carbonic acid ($H_2CO_3$) which lowers the ocean's pH through the following chemical reaction: $CO_2 + H_2O \leftrightarrow H_2CO_3 \leftrightarrow H^+ + HCO_3^-$
• Since the start of the industrial revolution (1750s) average ocean pH has decreased by 0.1 units equivalent to a 30% increase in acidity

Impacts of oceanic changes

• Coastal communities
  • Rising sea levels lead to more frequent and severe coastal flooding (Miami, Venice) and erosion especially during storm surges and high tides
  • Saltwater intrusion contaminates freshwater aquifers and agricultural land (Mekong Delta) reducing water quality and crop yields
  • Damage to coastal infrastructure (roads, buildings) and displacement of millions of people living in low-lying areas (Bangladesh, Pacific Islands)
• Marine ecosystems
  • Ocean acidification impairs the ability of calcifying organisms to build and maintain their calcium carbonate shells and skeletons by reducing carbonate ion availability
    1. Corals
    2. Mollusks (oysters, mussels)
    3. Some plankton species (coccolithophores, foraminifera)
  • Coral reefs which serve as biodiversity hotspots and provide critical ecosystem services are particularly vulnerable to the combined impacts of acidification, warming, and pollution
  • Changes in ocean chemistry and temperature alter species distributions (range shifts) and disrupt food web dynamics (trophic cascades)
  • The compounding effects of acidification, warming, and deoxygenation driven by climate change may lead to substantial biodiversity loss and degradation of marine ecosystems with far-reaching ecological and socioeconomic consequences