Sea Level Rise and Ocean Acidification

Course Assignment: BI 306, April 16, 2026.
Observed Sea Level Change (1993–2023): * Data provided by NOAA Climate.gov and UHSLC indicate a clear upward trend in global sea levels over the three-decade period from 1993 to 2023. * During this interval, the cumulative change in sea level has reached approximately $10\,cm$ to $15\,cm$ based on satellite altimetry, though the scale of the graph provided accommodates values from $-20\,cm$ to $20\,cm$ .

The elevation of sea levels is primarily driven by two physical processes: the melting of land-based ice and the thermal expansion of ocean water.

Ice Melting and Mass Loss: * Sea Ice: Arctic winter sea ice has reached record-low maximum extents for two consecutive years (as of 2026) since satellite monitoring began in 1979. In addition to reduced surface area, the remaining ice is significantly thinner than in previous decades. * Glaciers: Since 1970, there has been a consistent decline in global glacier mass. The cumulative glacier mass balance has dropped from $0$ in 1970 to approximately $-30\,m$ of water equivalent by 2020. * Polar Ice Sheets (Greenland and Antarctica): These represent the largest potential contributors to sea level rise.
Thermal Expansion: * As seawater absorbs heat and warms, the molecules move more vigorously, causing the water to become less dense and occupy a larger volume. * Impact: Thermal expansion is responsible for nearly half ( $50\%$ ) of the total observed sea-level rise to date.

Glacial Retreat Examples: * Pedersen Glacier, Alaska: Comparative photography from 1917 and 2005 shows a complete transformation from a massive ice front to a lush, vegetated landscape with a small lake. * Muir Glacier, Alaska: Photos from August 1941 compared to August 2004 show the total disappearance of the glacier from the visible fjord frame, replaced by open water.
Ice Sheet Mass Anomalies: * Greenland Ice Sheet: Losing mass at a rate of $-281\,Gt\,yr^{-1}$ . If fully melted, Greenland contains enough water to raise global sea levels by approximately $6\,m$ . * Antarctic Ice Sheet: Losing mass at a rate of $-118\,Gt\,yr^{-1}$ . If fully melted, Antarctica contains enough water to raise global sea levels by approximately $60\,m$ . * Monitoring: The GRACE-FO (Gravity Recovery and Climate Experiment Follow-On) mission tracks these changes by measuring gravitational anomalies from space.

It is critical to distinguish between different types of ice, as they do not affect sea level equally:

Grounded Ice vs. Floating Ice: * Grounded Ice (Glaciers and Ice Sheets): Melting of ice currently resting on land adds new water volume to the ocean, directly causing sea level rise. * Sea Ice and Ice Shelves: Because they are already floating, their melting does not directly increase sea level (following Archimedes' Principle). However, they play a vital structural role.
Ice Shelf Collapse Mechanism: 1. Stable State: The glacier flow is driven by gravity, while the buoyant (hydrostatic) force at the ice shelf front supports the ice mass. The grounding line marks the point where ice leaves the bedrock and begins to float. 2. Warmer Temperatures: Melt water percolates through the glacier, acting as a lubricant that causes the glacier to speed up (primarily in summer). Simultaneously, water-filled fractures carve through the ice shelf, leading to disintegration. 3. Unstable Front: Once the shelf retreats past the grounding line, buoyant support decreases. The glacier front begins to calve rapidly. 4. Acceleration: The lower part of the glacier steepens and accelerates, losing mass into the ocean and causing sea level rise.

Projections for 2100: * Projections vary based on emissions scenarios (Low to High). * The "High" scenario anticipates sea level rise of over $7\,feet$ above 2000 levels by the year 2100. * The "Intermediate" scenario projects approximately $3\,feet$ to $4\,feet$ of rise.
Ecosystem Transformation (Ghost Forests): * Rising sea levels and saltier groundwater facilitate the transition of coastal forests into salt marshes. * 2020: Freshwater forested wetlands exist with a defined vadose zone and high water table. * 2060: The water table rises; high tides and storm surges push salt water further inland. * 2100: The original forest is killed by salinity and flooding, becoming a transition zone or salt marsh. These "ghost forests" represent a loss of vital ecosystem services.

Definition: Often referred to as "The other $CO_2$ problem," ocean acidification is the ongoing decrease in the $pH$ of the Earth's oceans caused by the uptake of carbon dioxide ( $CO_2$ ) from the atmosphere.
General Statistics: * The ocean absorbs approximately one-third ( $33.3\%$ ) of the anthropogenic $CO_2$ released into the atmosphere. * Dissolved $CO_2$ reacts with seawater to form carbonic acid ( $H_2CO_3$ ), which lowers the $pH$ .

Chemical Process: 1. Atmospheric $CO_2$ dissolves in the ocean. 2. Reaction: $CO_2 + H_2O \rightarrow H_2CO_3$ (Carbonic acid). 3. Dissociation: Carbonic acid releases Hydrogen ions ( $H^+$ ) and Bicarbonate ions ( $HCO_3^-$ ). 4. The increase in $H^+$ ions results in a lower $pH$ (increased acidity).
Current Trends: * Hawaii (Station ALOHA/Mauna Loa): Atmospheric $CO_2$ has risen from approximately $315\,ppm$ in 1958 to over $400\,ppm$ by 2018. During this time, ocean $pH$ at Station ALOHA has dropped from roughly $8.12$ to approximately $8.05$ . * While the ocean remains slightly basic (alkaline), it is moving toward a more neutral $pH$ ; it is becoming "less alkaline."

Carbonate Ion Availability: Acidification reduces the concentration of carbonate ions ( $CO_3^{2-}$ ).
Calcification Issues: * Many marine organisms (corals, oysters, clams, mussels, and various plankton) require carbonate ions to build their calcium carbonate ( $CaCO_3$ ) shells and skeletons. * Effects: Slowed calcification rates, weakened skeletal structures, deformed shells (resembling "osteoporosis of the sea"), and increased biological stress.
Multiple Stressors: Ocean acidification does not act in isolation; it occurs simultaneously with ocean warming and deoxygenation, creating a synergistic "multiple stressor" environment for marine life.
Socioeconomic Impact: These changes ripple through food webs, potentially damaging commercial fisheries and coastal protection provided by coral reefs.