Ocean acidification Study Guide

📖 Core Concepts Ocean Acidification (OA) – the ongoing decline in seawater pH caused by uptake of atmospheric CO₂. pH – $p\!H = -\log{10}[H^{+}]$; a 0.1‑unit drop ≈ 26 % more hydrogen ions because the scale is logarithmic. Carbonate Chemistry – CO₂ dissolves, forms carbonic acid (H₂CO₃), which dissociates: $${\rm CO2 (aq)} + {\rm H2O} \rightleftharpoons {\rm H2CO3} \rightleftharpoons {\rm HCO3^-} + {\rm H^+} \rightleftharpoons {\rm CO3^{2-}} + 2{\rm H^+}.$$ Total Alkalinity (TA) – the seawater capacity to neutralize added acid; not altered by CO₂ uptake. Saturation State (Ω) – $$\Omega = \frac{[Ca^{2+}][CO3^{2-}]}{K{sp}}$$ where $K{sp}$ is the solubility product of a calcium‑carbonate mineral. Ω > 1 → precipitation possible; Ω < 1 → dissolution. Polymorphs – Aragonite (more soluble) vs Calcite (less soluble). Organisms that build aragonite shells are more vulnerable. --- 📌 Must Remember Surface ocean pH fell from ≈ 8.15 (1950) → 8.05 (2020). 0.1 pH‑unit = 26 % rise in \([H^{+}]\). Current decline rate: 0.017–0.027 pH units per decade (late‑1980s to present). Atmospheric CO₂ > 422 ppm (2024) vs pre‑industrial ≈ 280 ppm. Oceans absorb ¼ of anthropogenic CO₂ → ≈ 175 ± 35 Gt C sequestered. Ω < 1 → existing calcium‑carbonate structures dissolve. Aragonite Ω reaches critical values sooner than calcite because of higher solubility. SSP5‑8.5 projection: pH down ≈ 0.44 units by 2100 → pH ≈ 7.7, a 2–4× increase in \([H^{+}]\) relative to today. --- 🔄 Key Processes CO₂ uptake – colder, denser water absorbs more CO₂; warming reduces solubility locally. Carbonic‑acid dissociation – sequential release of two H⁺ ions (see equation above). Shift in carbonate system – ↑ \([H^{+}]\) → ↓ \([CO3^{2-}]\) → lower Ω. Calcification – organisms combine Ca²⁺ + CO₃²⁻ → CaCO₃; lower \([CO3^{2-}]\) slows or reverses this reaction. Feedback to climate – weaker calcification weakens the biological pump, reducing CO₂ drawdown. --- 🔍 Key Comparisons Aragonite vs Calcite – Aragonite is more soluble → dissolves at higher Ω than calcite. Ocean Acidification vs Ocean Warming – Acidification lowers Ω; warming lowers CO₂ solubility (temp‑driven moderation of pH) but raises metabolic stress. Anthropogenic CO₂ vs Natural Variability – Current pH drop is unprecedented in the last 26 000 yr; natural fluctuations are ±0.02 pH units only. Total Alkalinity vs pH – TA remains constant when CO₂ is added; pH changes because the carbonate speciation shifts. --- ⚠️ Common Misunderstandings “The ocean is now acidic (pH < 7)” – false; seawater stays alkaline (pH > 8) despite the decline. “Adding CO₂ raises alkalinity” – incorrect; TA is unchanged by CO₂ dissolution. “Warmer water always means less acidification” – warming does reduce CO₂ solubility locally, but the concurrent metabolic and deoxygenation stresses outweigh any modest pH buffering. “All calcium‑carbonate minerals dissolve at the same rate” – aragonite dissolves 2–3 × faster than calcite. --- 🧠 Mental Models / Intuition Log‑scale shortcut – each 0.3‑unit drop ≈ doubling of \([H^{+}]\). Balance‑beam analogy – think of the carbonate system as a seesaw: adding CO₂ pushes the pivot toward H⁺ (left side) and away from CO₃²⁻ (right side). Ω as a safety margin – Ω = 1 is the “red line”; the farther above 1, the safer the mineral; the farther below, the faster dissolution. --- 🚩 Exceptions & Edge Cases High‑latitude waters absorb more CO₂ → faster pH decline despite cooler temperatures. Upwelling zones bring CO₂‑rich deep water to the surface → localized spikes in acidity. Freshwater influx (Arctic meltwater) adds acidity and lowers TA locally, accelerating OA. Short‑term cooling events can temporarily increase CO₂ uptake, deepening the pH dip. --- 📍 When to Use Which (Mitigation Choices) | Situation | Preferred Action | |-----------|------------------| | Root‑cause control | Immediate CO₂ emissions reductions (energy transition, policy). | | Need for rapid pH buffering in a hotspot | Ocean alkalinity enhancement (add alkaline minerals) – works locally, high cost, side‑effects. | | Large‑scale, long‑term CO₂ removal | Direct Air Capture + storage or enhanced weathering on land; indirect ocean benefit. | | Limited budget, low tech readiness | Prioritize behavioral / policy measures over experimental CDR. | | Synergy with other stressors | Combine emission cuts with protected areas to reduce cumulative “deadly trio” impact. | --- 👀 Patterns to Recognize Steady pH decline across decades → look for 0.017–0.027 pH/decade in data tables. Ω crossing below 1 precedes observed shell dissolution in field studies. Aragonite Ω reaches critical thresholds earlier than calcite in the same region. Regions with strong upwelling (e.g., eastern boundary currents) show higher acidity spikes. Concurrent warming + OA → amplified mortality in polar pteropods and coral bleaching events. --- 🗂️ Exam Traps Distractor: “Ocean pH will fall below 7 in the next 50 years.” – Wrong; pH stays above 8 even under high‑emission scenarios. Distractor: “Total alkalinity rises as CO₂ is absorbed.” – Incorrect; TA is largely unchanged by CO₂ uptake. Distractor: “Aragonite is less vulnerable because it is a stronger mineral.” – Opposite; aragonite is more soluble. Distractor: “Warming always offsets acidification by reducing CO₂ solubility.” – Only a modest local effect; overall OA continues. Distractor: “All calcifiers are equally affected.” – Species‑specific; aragonite builders suffer first, non‑calcifiers may be less directly impacted. ---