# 13.2 Climate Change Impacts on Ocean Ecosystems The primary impacts of climate change on ocean ecosystems are a **rise in ocean temperature**, a **decrease in ocean pH (ocean acidification)**, and subsequent negative **effects on marine biodiversity**. ## Rise in Ocean Temperature Oceans absorb about 90% of the excess heat trapped by greenhouse gases, leading to a significant increase in their temperature. ### Causes of Ocean Warming * **Global Warming and Greenhouse Gases**: Human activities (burning fossil fuels, deforestation) release greenhouse gases ($\mathrm{CO_2}$, methane) that trap heat in the atmosphere. The ocean absorbs the majority of this excess heat. * **High Heat Capacity of Water**: Oceans can absorb and store immense amounts of heat without a drastic immediate temperature increase, but this stored heat accumulates over time. * **Thermal Expansion**: As water warms, its molecules move further apart, causing it to expand. This process is a major contributor to global sea-level rise. * **Altered Ocean Circulation**: Climate change disrupts the ocean currents that distribute heat around the globe. * **Atlantic Meridional Overturning Circulation (AMOC)**: A major ocean current that transports warm water from the tropics northward. Melting polar ice introduces freshwater, reducing the salinity and density of the surface water. This slows the AMOC, causing heat to build up in tropical and subtropical regions. * **El Niño and La Niña**: These are natural climate patterns in the Pacific Ocean that affect global weather. Climate change can intensify El Niño events, which are characterized by warmer-than-average ocean surface temperatures, leading to spikes in global ocean heat. * **Melting Sea Ice (Albedo Effect)**: Sea ice has a high albedo, meaning it reflects sunlight back into space. As it melts, it exposes the darker ocean water below, which has a low albedo and absorbs more solar radiation, creating a positive feedback loop that accelerates warming. ## Ocean Acidification (Decrease in pH) This is the ongoing decrease in the pH of the Earth's oceans, caused by the uptake of atmospheric carbon dioxide ($\mathrm{CO_2}$). * **Ocean as a Carbon Sink**: The ocean has absorbed approximately 30% of the excess $\mathrm{CO_2}$ released into the atmosphere by human activities. * **Chemical Process**: 1. Carbon dioxide ($\mathrm{CO_2}$) dissolves in seawater ($\mathrm{H_2O}$) to form carbonic acid ($\mathrm{H_2CO_3}$). $$\mathrm{CO_2 + H_2O \rightarrow H_2CO_3}$$ 2. Carbonic acid is weak and dissociates into bicarbonate ions ($\mathrm{HCO_3^-}$) and hydrogen ions ($\mathrm{H^+}$). $$\mathrm{H_2CO_3 \rightarrow HCO_3^- + H^+}$$ 3. The increase in hydrogen ions ($\mathrm{H^+}$) concentration is what lowers the ocean's pH, making it more acidic. * **Regional Variations**: Acidification is not uniform. Polar regions are more vulnerable because colder water can absorb more $\mathrm{CO_2}$, leading to faster acidification rates. * **Feedback Loop**: Warmer water holds less dissolved gas. As the ocean warms, its capacity to absorb $\mathrm{CO_2}$ decreases, leaving more $\mathrm{CO_2}$ in the atmosphere and accelerating global warming. ## Harmful Effects on Marine Biology The combined effects of warming and acidification have severe consequences for marine life. ### Effects of High Ocean Temperature | Impact | Description | | :--- | :--- | | **Coral Bleaching** | Stressed by heat, corals expel their symbiotic algae (*zooxanthellae*), causing them to turn white. This leaves them vulnerable to disease and death. | | **Ecosystem Disruption** | Temperature-sensitive species (e.g., certain fish, plankton) are forced to migrate to cooler waters, altering food webs. | | **Reduced Fish Stocks** | Warmer water can alter fish metabolism, leading to smaller body sizes and reduced reproductive success. | | **Habitat Loss** | Melting polar sea ice destroys grounds for species like polar bears and seals. It also affects krill populations. | | **Ocean Deoxygenation** | Warmer water holds less dissolved oxygen, creating "hypoxic zones" where most marine life cannot survive. | ### Effects of Ocean Acidification * **Impact on Calcifying Organisms**: * Acidification reduces the availability of carbonate ions ($\mathrm{CO_3^{2-}}$), which organisms like corals, shellfish (oysters, clams), and some plankton (pteropods) need to build their calcium carbonate ($\mathrm{CaCO_3}$) shells and skeletons. * This leads to weaker, thinner shells and slower growth, making them more vulnerable to predation and erosion. * **Food Web Disruption**: * Planktonic organisms with carbonate shells are a fundamental food source. A decline in their populations can cause a cascading effect on larger animals like fish and whales. * **Behavioral Changes**: * Elevated $\mathrm{CO_2}$ levels and lower pH can impair the sensory abilities of fish, affecting their ability to detect predators and navigate. ### Biomass and Trophic Levels In marine ecosystems, as in terrestrial ones, the biomass decreases at higher trophic levels. This is due to the energy loss (approx. 90%) at each transfer between levels.