🌍 Understanding Climate Change and Our Oceans (Syllabus 6.8)
Welcome to one of the most important topics in Marine Science! Climate change isn't just about hot weather; it's fundamentally changing the ocean environment that marine life depends on. Don't worry if this chapter seems big—we will break down the science into simple, clear steps focusing only on what the IGCSE syllabus requires!
1. Climate History: A Look at the Past
Climate change is often in the news, but the Earth's climate has always changed over its history. What is concerning scientists now is the speed of change caused by human activity.
Key Historical Context (Syllabus 6.8.1)
- Scientists compare ancient climates to the present day, using fossil evidence and ice cores.
- We know that the climate of different regions has changed significantly between 335 million years ago and today, linked to natural variations in carbon dioxide level and temperature.
- The current rise in global temperature and CO₂ levels is happening much faster than in natural cycles, which gives marine organisms less time to adapt.
Quick Review: Climate naturally changes, but current human-caused changes are happening at a dangerous speed.
2. The Enhanced Greenhouse Effect
The Earth stays warm thanks to the natural greenhouse effect. However, human pollution is making this effect much stronger.
How the Greenhouse Effect Works (Syllabus 6.8.2)
Think of the Earth's atmosphere like a cozy blanket. This blanket contains gases called greenhouse gases (GHGs).
Step-by-step Process:
- Sunlight (energy) enters the atmosphere and warms the Earth’s surface.
- The Earth radiates this heat energy back towards space.
- Greenhouse gases, like carbon dioxide (CO₂) and methane (CH₄), trap some of this outgoing heat, preventing it from escaping. This keeps the planet warm enough for life.
Present-Day Pollution and Effects
When we burn fossil fuels (like coal, oil, and gas) and release methane (from farming and industry), we dump extra greenhouse gases into the atmosphere.
- This results in an enhanced greenhouse effect (a thicker blanket).
- The atmosphere traps too much heat, leading to climate change, which includes a rise in global average temperatures.
Key Takeaway: Pollution by methane and CO₂ intensifies the greenhouse effect, leading to climate change.
3. The Ocean's Critical Role in Heat Absorption
The ocean is a massive heat sponge and carbon sink. It is helping to slow down global warming, but this comes at a huge cost to the marine environment.
Ocean Heat Absorption (Syllabus 6.8.4)
The ocean plays a vital role in absorbing heat and maintaining the stability of global temperature. Water has a very high specific heat capacity, meaning it takes a lot of energy to raise its temperature.
- The ocean has absorbed over 90% of the excess heat trapped by greenhouse gases since the mid-20th century.
- Without this absorption, temperatures on land would be far higher than they are now.
Consequences of Heat Absorption
Absorbing this heat has two major consequences for the ocean itself:
- Ocean Warming: The temperature of the seawater increases (especially near the surface).
- Thermal Expansion (Syllabus 6.8.4a): When water heats up, its molecules spread out, causing the total volume of the water to increase. This is a primary contributor to sea level rise.
Did you know? A warm cup of water takes up slightly more space than the same mass of cold water! This small effect, applied to the entire ocean, causes significant sea level rise.
4. Impacts on Seawater Chemistry: The Double Threat
Increasing atmospheric CO₂ concentrations have two key effects on the seawater itself: warming and acidification (Syllabus 6.8.3).
Increasing Temperature (Syllabus 6.8.3a)
As the ocean warms, the most direct effect is that increasing temperature reduces the solubility of gases in water. This means:
- Less dissolved oxygen can be held in warmer water, creating "dead zones" where large organisms struggle to breathe. (We will see this again in ecosystem impacts).
Decreasing pH: Ocean Acidification (Syllabus 6.8.3b)
The ocean absorbs about 30% of the CO₂ released by humans. When CO₂ dissolves in sea water, it reacts to form carbonic acid.
- CO₂ + H₂O $\rightarrow$ Carbonic Acid
- This acid increases the concentration of hydrogen ions, causing the pH of the seawater to decrease (become more acidic).
Analogy for pH: The pH scale measures how acidic or alkaline (basic) something is. Pure water is pH 7 (neutral). Seawater is naturally slightly alkaline (around pH 8.1). When it drops, even slightly, it is called ocean acidification. A small drop in pH is like a huge change to marine life.
Key Takeaway: Increased CO₂ means warmer water (less oxygen) and lower pH (acidification).
5. Sea Level Rise: Thermal Expansion vs. Melting Ice
Sea level is rising globally, threatening coastal ecosystems and human communities. The syllabus requires us to compare the two main causes (Syllabus 6.8.5).
1. Thermal Expansion of Sea Water (Syllabus 6.8.5)
- As discussed above, warmer water takes up more volume.
- This effect accounts for a significant portion of current sea level rise.
2. Melting of Land-Based Ice (Syllabus 6.8.5)
- This involves the melting of glaciers and ice sheets located on land (e.g., Greenland and Antarctic ice sheets).
- When this frozen water melts, it runs off the land and adds new liquid water volume to the ocean, causing sea levels to rise.
Common Mistake to Avoid: Melting sea ice (like the ice floating in the Arctic) does NOT significantly contribute to sea level rise because it is already displacing water (like ice cubes melting in your drink). We are concerned with *land-based* ice.
Key Takeaway: Sea level rises due to water expanding when heated (thermal expansion) AND new water added from melting land ice.
6. Impacts on Marine Ecosystems
The increase in greenhouse gases leads to severe consequences across all marine habitats (Syllabus 6.8.6).
Consequences of Climate Change on Ecosystems
1. Increase in Sea Level and Loss of Habitat and Land (Syllabus 6.8.6a)
- Rising water permanently floods low-lying coastal areas like mangrove forests, salt marshes, and estuaries.
- This leads to the destruction of critical nursery grounds and coastal protection barriers.
2. Coral Bleaching (Syllabus 6.8.6b)
- Corals live in a mutualistic relationship with tiny algae called zooxanthellae.
- When water temperatures become too high (even for a short time), the coral polyps expel the zooxanthellae.
- The coral turns white (bleaching) and starves, often leading to death if the temperature does not quickly return to normal.
3. Decreased Dissolved Oxygen in the Oceans (Syllabus 6.8.6c)
- Warmer surface water holds less dissolved oxygen (remember solubility decreases with temperature).
- This can lead to wider areas of low oxygen, stressing or killing marine life, especially those trapped in deep water.
4. Changing Distribution and Migration Patterns (Syllabus 6.8.6d)
- Many marine species, especially fish and plankton, are highly sensitive to temperature.
- As local waters warm, species are forced to move towards the poles or into deeper, cooler waters to survive. This changes where fisheries operate and disrupts existing food webs.
5. Loss of Species (Syllabus 6.8.6e)
- Organisms sensitive to pH changes (like those that build shells or skeletons from calcium carbonate, such as *oysters, pteropods*, and *corals*) suffer from ocean acidification.
- If organisms cannot migrate or adapt fast enough to temperature or pH changes, they face extinction or local disappearance (loss of species).
6. Increase in Extreme Weather (Syllabus 6.8.6f)
- Warmer sea surface temperatures provide more energy for tropical storms (hurricanes and cyclones).
- These more frequent and intense storms cause devastating physical damage to coastal habitats, including massive erosion and destruction of coral reefs and mangrove forests.
Key Takeaway: Climate change impacts ecosystems through higher sea levels, warmer water (bleaching, low oxygen), chemical changes (acidification), and disruptive weather.
Summary Checklist for 6.8
A successful student should be able to:
- Describe the historical relationship between CO₂ and temperature.
- Explain the enhanced greenhouse effect caused by CO₂ and methane.
- Explain how ocean heat absorption leads to thermal expansion and stabilizes global temperature.
- Explain why increasing CO₂ causes ocean warming and acidification (decreasing pH).
- Compare sea level rise caused by thermal expansion versus melting land ice.
- List and explain at least three major impacts of climate change on marine ecosystems (e.g., coral bleaching, habitat loss, species migration).