Global Warming and Its Impact on Marine Ecosystems (9693 A Level)

Welcome to one of the most critical topics in Marine Science! This chapter explores Global Warming—how our planet is heating up—and, crucially, how this affects the vast, interconnected marine environment. Understanding these impacts is essential, as the oceans regulate our global climate.

Quick Review: What are the Key Ideas?

  • The difference between the natural and enhanced greenhouse effect.
  • The scientific evidence supporting global warming and the role of humans.
  • Major physical and biological changes in the ocean: sea level rise, coral bleaching, and circulation changes.

Section 1: The Greenhouse Effect (NGE and Enhanced EGE)

The Natural Greenhouse Effect (NGE) - Keeping Earth Habitable (9.2.1)

The Earth’s temperature is maintained at a level suitable for life thanks to a natural process called the Natural Greenhouse Effect (NGE).

Think of the Earth's atmosphere as a warm blanket.

Certain gases, known as Greenhouse Gases (GHGs), trap heat (infrared radiation) that the Earth radiates after being warmed by the Sun.

Key Greenhouse Gases (GHGs) include:

  • Carbon dioxide (\(CO_2\))
  • Methane (\(CH_4\))
  • Water vapour (\(H_2O\))

Without this natural insulating blanket, Earth would be extremely cold (around -18 °C), freezing water and making life as we know it impossible.

The Enhanced Greenhouse Effect (EGE) and Global Warming (9.2.2)

Global warming occurs because human activities have added extra GHGs to the atmosphere, making the "blanket" thicker. This is called the Enhanced Greenhouse Effect (EGE).

The primary way humans increase GHGs, especially \(CO_2\), is through the combustion of fossil fuels (like coal, oil, and gas) for energy, transport, and industry.

Step-by-Step: Enhanced Warming

  1. Humans burn fossil fuels, releasing massive amounts of stored carbon (as \(CO_2\)) into the atmosphere.
  2. The concentration of \(CO_2\) and other GHGs increases significantly above natural levels.
  3. More heat radiation is trapped close to the Earth's surface (both land and ocean).
  4. This trapped heat causes the average global temperature to rise, resulting in Global Warming.

Key Takeaway: The natural process is necessary for life; the enhanced process (caused by excess human CO2) leads to problematic warming.

Section 2: Evidence and Attribution (9.2.3 & 9.2.4)

Evidence for Global Warming (9.2.3)

Scientists have amassed compelling evidence that the planet is warming:

  • Rising Global Mean Temperature: Direct measurements show a steady increase in land and sea surface temperatures over the past century.
  • Melting Ice: Retreat of glaciers, reduction in Arctic sea ice extent, and melting of ice sheets in Greenland and Antarctica.
  • Ocean Heat Content: The oceans absorb most of the excess heat, and deep ocean monitoring confirms rising water temperatures.
  • Atmospheric Composition Data: Data from ice cores show that current atmospheric \(CO_2\) concentrations are far higher than they have been for hundreds of thousands of years. (Analogy: Ice cores are like time capsules; bubbles trapped in the ice reveal the atmosphere's ancient composition.)

The Human Link: Evaluating the Evidence (9.2.4)

While some natural cycles (like solar variability) affect climate, the current rapid warming trend is strongly linked to human activity.

Evidence FOR Human Contribution:

  • Isotope Signatures: The carbon being added to the atmosphere has a specific isotopic signature (less Carbon-13), which is characteristic of burning ancient plant matter (fossil fuels), not natural volcanic emissions.
  • Rate of Change: The speed at which GHGs are increasing and temperatures are rising is unprecedented in geological history, suggesting an unnatural, external influence.
  • Climate Models: Computer models that incorporate *only* natural factors (like solar and volcanic activity) cannot reproduce the observed warming trend. Models that include human GHG emissions match the observations perfectly.

Evidence AGAINST/Alternative Hypotheses (and why they are often refuted):

  • Solar Activity: Some argue solar output changes cause warming. Refutation: Solar irradiance has actually slightly decreased or remained stable since the 1970s, while global temperatures continued to soar.
  • Natural Cycles: Critics point to natural climate cycles. Refutation: While cycles exist, the current warming goes far beyond the expected range and rate of natural variations.

Evaluation Summary: The vast majority of scientific evidence (data showing rising CO2 matching fossil fuel emissions, and computer modeling) strongly supports the hypothesis that human activity significantly contributes to global warming.

Key Takeaway: Evidence includes rising temperatures and historic CO2 levels from ice cores. The unique signature of the carbon released confirms that humans (fossil fuels) are the primary cause of the accelerated warming.

Section 3: Possible Impacts of Global Warming on the Marine Environment (9.2.5)

The rise in global temperature affects the ocean's physical, chemical, and biological systems in several interconnected ways.

3.1 Sea Level Rise

Global warming leads to sea level rise through two main mechanisms:

  1. Thermal Expansion: As water heats up, its density decreases, and its volume increases. This is the single largest contributor to current sea level rise.
  2. Melting of Land Ice: Melting glaciers and ice sheets (like Greenland and Antarctica) add huge volumes of water to the ocean. (Note: Melting sea ice, like Arctic ice, does not directly raise the sea level, just as ice melting in a glass of water doesn't overflow.)

Impacts of Sea Level Rise:

  • Habitat Loss: Low-lying coastal ecosystems, such as mangrove forests and salt marshes, are flooded, disrupting their ability to survive in the littoral zone.
  • Coastal Erosion: Increased wave action and higher baseline water levels accelerate the erosion of sandy shores and coastlines.

3.2 Coral Bleaching

Coral reefs are particularly sensitive to small changes in temperature.

Coral relies on a mutualistic symbiotic relationship with photosynthetic algae called zooxanthellae. These algae live within the coral tissue and provide the coral polyp with essential organic compounds (like glucose) they produce via photosynthesis.

The Process of Bleaching

When Sea Surface Temperature (SST) rises above the coral’s normal range (even by just 1–2 °C for a prolonged period):

  1. The coral polyps become stressed.
  2. The zooxanthellae start producing harmful reactive oxygen species instead of useful nutrients.
  3. The coral expels the zooxanthellae from its tissue.
  4. The coral loses its colour (becoming white, or "bleached") and its main food source.

If the water temperature quickly returns to normal, the coral may recover and reacquire algae. If the high temperature persists, the coral will starve and die.

Did you know? Major global bleaching events, like those in 2016 and 2017, were directly linked to unusually high ocean temperatures caused by climate change and El Niño events.

3.3 Changes in Species Distribution

Marine organisms, from plankton to large fish, have specific ranges of temperatures they can tolerate (their thermal tolerance).

As sea water warms, species must move to find waters that match their preferred temperature range.

  • Poleward Migration: Many species (including commercial fish stocks) are shifting their distribution towards the poles where the water is cooler.
  • Ecosystem Disruption: This movement disrupts established food webs and predator-prey dynamics in their original habitats, leading to ecosystem instability.
  • Fishing Impacts: Fisheries reliant on specific species (like cod or anchovy) may find their stocks moving out of traditional fishing grounds, causing economic and social impacts.

3.4 Potential Changes to Global Circulation of Sea Water

Ocean currents, particularly the deep-water currents known as the Global Ocean Conveyor Belt (Thermohaline Circulation), are crucial for distributing heat and nutrients globally.

This circulation is driven by differences in water density (due to temperature and salinity).

The Threat to the Conveyor Belt

Intense warming and the subsequent melting of vast quantities of freshwater ice (from Greenland) pose a threat to this system:

  1. Fresh water is less dense than salt water.
  2. This large influx of fresh, cold water at high latitudes (e.g., in the North Atlantic) makes the surface water less dense.
  3. Since the surface water is less dense, it is less likely to sink, which is the driving force for the deep-ocean circulation (the 'pump' slows down).
  4. If the Conveyor Belt slows or stops, it could lead to extreme and unpredictable climate changes, especially impacting temperature regulation in the North Atlantic region, and drastically altering nutrient distribution across the globe.

Key Takeaway: Global warming drives sea level rise (thermal expansion and melting land ice), destroys sensitive habitats (coral bleaching), forces species to move, and risks disrupting the massive ocean circulation systems.

Quick Review Box

NGE: Necessary for life, uses natural GHGs.
EGE/Global Warming: Caused by excess human-released CO2.
Impacts:
1. Sea level rise (thermal expansion).
2. Coral bleaching (high SST expels zooxanthellae).
3. Species shift (migrate poleward due to thermal tolerance).
4. Circulation change (fresh meltwater disrupts sinking, potentially slowing the Global Ocean Conveyor Belt).