APES Topics 9.6-9.7 notes-Ocean Warming and Acidification

Primary Cause: Ocean warming is directly caused by the increase in greenhouse gases (GHGs) in the Earth's atmosphere.
Thermodynamic Relationship: - There is a constant flux between atmospheric warming and oceanic warming. - The ocean serves as a massive heat sink, absorbing heat that is radiated back to Earth by greenhouse gases. - Oceans absorb a significant portion of Earth's heat—up to $90\%$ —due to the high specific heat of water.
Heat Distribution Mechanisms: - Thermohaline Circulation: This global system of surface and deep-water currents distributes the heat absorbed at the ocean surface to greater depths and different geographic areas of the Earth. - Atmospheric Feedback: Heat absorbed by the ocean can be transferred back to the atmosphere for decades, prolonging the effects of warming even if atmospheric concentrations stabilize.

Solubility of Oxygen: - Warmer water holds less dissolved oxygen. - Consequences: This leads to respiratory stress or suffocation in marine organisms.
Disruption of Life Cycles and Behavior: - Migratory Routes: Modified as species seek cooler waters. - Mating Seasons: Altered due to temperature cues. - Reproductive Timing: Disrupted, potentially leading to a mismatch where food sources are not present when larvae or offspring are expected.
Toxic Algal Blooms: - Warming facilitates the growth of toxic blue-green algae (cyanobacteria). - Environmental Impacts: These blooms release toxins that can kill marine species and/or block sunlight, leading to hypoxia (low oxygen conditions). - Human and Animal Health Risks: Cyanobacteria can produce toxins causing gastrointestinal issues, skin irritation, and severe damage to the liver or nervous system. It is crucial to avoid visibly discolored water, especially if it exhibits scum or a foul odor.

Mutualistic Symbiotic Relationship: Coral reefs rely on a partnership between coral polyps and photosynthetic algae known as zooxanthellae. - Algae's Contribution: Provide sugars (carbohydrates) via photosynthesis. - Coral's Contribution: Provide carbon dioxide ( $CO_2$ ) and detritus (nutrient-rich organic matter) to the algae.
The Mechanism of Bleaching: - Algae have a narrow range of tolerance for temperature. - Response to Stress: When temperatures increase, the algae leave the reef. - Appearance: Without the algae, the coral loses its color and turns white (bleaching). - Vulnerability: Bleached coral is stressed, loses its food source (sugar), and becomes more vulnerable to disease. While some corals recover, many eventually die.
Additional Stressors on Coral Reefs: - Sea Level Rise: Causes thermal stress and increased sedimentation. - Storm Patterns: Stronger, more frequent storms cause direct destruction of reef structures. - Precipitation Changes: Increased freshwater runoff carries sediment and land-based pollutants, which can smother coral. - Runoff Pollutants: Sediments, pesticides, and sunscreens can force algae to leave the reef. - Altered Currents: Change the connectivity and temperature regimes, impacting the dispersal of larvae and food availability.

Definition: Ocean acidification is the ongoing decrease in the $pH$ of the Earth's oceans.
Primary Driver: Increased concentration of carbon dioxide ( $CO_2$ ) in the atmosphere.
Anthropogenic Contributions: Activities that increase atmospheric $[CO_2]$ include: - Burning of fossil fuels. - Vehicle emissions. - Deforestation.
The Chemical Process of Acidification: 1. Direct Exchange: Atmospheric $CO_2$ is absorbed by the ocean. 2. Formation of Carbonic Acid: $CO_2$ combines with ocean water ( $H_2O$ ) to form carbonic acid. - $CO_2 + H_2O \rightarrow H_2CO_3$ 3. Dissociation: Carbonic acid dissociates, releasing hydrogen ions ( $H^+$ ) and bicarbonate ions ( $HCO_3^-$ ). - $H_2CO_3 \rightarrow H^+ + HCO_3^-$ 4. Carbonate Interference: The free $H^+$ ions bond with carbonate ions ( $CO_3^{2-}$ ) already in the water to form more bicarbonate ( $HCO_3^-$ ). - $H^+ + CO_3^{2-} \rightarrow HCO_3^-$ 5. Result: This reaction consumes the carbonate ions that marine organisms need for calcification.

Calcification Process: Shellfish and corals use calcium ions ( $Ca^{2+}+$ ) and carbonate ions ( $CO_3^{2-}$ ) to build their shells and skeletons out of calcium carbonate ( $CaCO_3$ ).
Structural Damage: As acidification reduces the availability of carbonate ions, it becomes difficult for organisms to form shells.
Affected Species: This negatively impacts corals, mollusks, and urchins, leading to brittle or broken shells and decreased growth rates.
Ecosystem Integrity: Acidification decreases the structural integrity of reefs, making them more susceptible to erosion and destruction.

The Global Carbon Cycle Shift: - Pre-Industrial: $CO_2$ absorption and release were balanced over centuries/millennia through land plants, rivers, and the ocean. - Modern Era: Humans release fossil carbon (coal, oil, natural gas). The ocean absorbs approximately one-third ( $\frac{1}{3}$ ) of this anthropogenic $CO_2$ . - Source vs. Sink: The ocean has shifted from being a net source of $CO_2$ to the air to a net $CO_2$ sink.
pH Scale and Acidity Measurements: - Historical Change: Ocean $pH$ has decreased from $8.2$ to $8.1$ over the past $150$ years. - Future Projections: Estimates suggest $pH$ could drop to $7.8$ by the year $2100$ . - Logarithmic Scale Significance: Because the $pH$ scale is logarithmic, a decrease of only $0.1$ represents a $30\%$ increase in acidity.