Understanding Global Warming and Its Biological Impact Revision Notes | GCSE AQA Higher Biology

1. The Greenhouse Effect and Rising Gas Levels

Picture Earth wrapped in an invisible blanket. This blanket - our atmosphere - keeps us warm enough for life to exist. But we're making this blanket thicker every year, and that's causing problems.

Here's the key point: both gases are increasing rapidly due to human activities. CO₂ levels have risen from 280 parts per million in 1880 to over 420 ppm today. Methane concentrations have more than doubled in the same period.

These gases trap heat that would normally escape to space. More greenhouse gases mean more trapped heat, leading to global warming. This sounds simple, but the biological consequences are complex and far-reaching.

2. Temperature Changes and Their Immediate Effects

Global average temperatures have risen by approximately 1.1°C since pre-industrial times. This might sound small, but it's huge in climate terms. During the last ice age, global temperatures were only 6°C cooler than today.

This warming isn't uniform across the planet. The Arctic is warming twice as fast as the global average - a phenomenon called Arctic amplification. Polar ice reflects sunlight back to space, but as ice melts, darker ocean water absorbs more heat, accelerating warming further.

Rising temperatures cause immediate physical changes: glaciers retreat, ice caps shrink, and sea levels rise due to thermal expansion of seawater and melting ice. These changes set the stage for biological impacts.

3. Biological Consequences - Species and Habitats

Now we reach the heart of the biological impact. Global warming affects living organisms in several key ways, and these effects cascade through entire ecosystems.

Habitat Loss and Fragmentation: As temperatures rise, suitable habitats shift towards the poles and up mountains. Arctic species like polar bears lose sea ice hunting grounds. Mountain species run out of cooler areas to migrate to - they literally reach the top of the mountain with nowhere else to go.

Timing Mismatches: Many species rely on precise timing for survival. Birds migrate based on day length, but their food sources (insects, plants) respond to temperature. Warmer springs mean insects emerge earlier, but birds arrive at the usual time to find their food source has already peaked.

Coral Bleaching: Corals have a narrow temperature tolerance. When water temperatures rise by just 1-2°C above normal, corals expel their symbiotic algae, turning white (bleaching). Without these algae, corals starve and die, destroying entire reef ecosystems.

4. Food Chains and Ecosystem Disruption

Food chains don't just shift - they can collapse entirely. Here's how global warming creates a domino effect through ecosystems.

Worked example: Analysing Food Chain Disruption in the Arctic

Explain how global warming affects the Arctic food chain: phytoplankton → krill → fish → seals → polar bears

1. Phytoplankton (primary producers) live under sea ice — They use nutrients from ice and sunlight filtering through
2. Less sea ice = less habitat for phytoplankton — Reduced primary production affects the entire chain
3. Fewer phytoplankton → less krill → fewer fish → fewer seals — Each level depends on the one below
4. Polar bears lose both hunting platform (ice) and prey (seals) — Double impact: habitat loss AND food shortage

Answer: Global warming disrupts Arctic food chains by removing sea ice habitat for primary producers and hunting grounds for top predators, causing population crashes at multiple trophic levels.

Ocean Acidification: As CO₂ levels rise, oceans absorb more CO₂, forming carbonic acid. This makes seawater more acidic, dissolving the shells of marine organisms like molluscs and affecting the entire marine food web.

Phenological Shifts: This refers to changes in the timing of biological events. Spring arrives earlier, affecting when plants flower, insects emerge, and birds breed. These timing changes can break the synchronisation between predators and prey, or between pollinators and plants.

5. Species Migration and Adaptation

Species respond to global warming in three ways: migrate, adapt, or face extinction. Each response has different success rates and timescales.

Migration Patterns: Many species are shifting their ranges poleward and upward in elevation. In the UK, butterfly species are moving northward at an average rate of 6km per year. However, migration isn't always possible - islands, urban areas, and mountain tops create barriers.

Evolutionary Adaptation: Some species can adapt through natural selection, but this takes many generations. Fast-reproducing species like insects have better chances than slow-reproducing mammals. However, the rate of climate change often exceeds the rate of evolutionary adaptation.

Extinction Risk: Species with small populations, limited ranges, or specific habitat requirements face the highest extinction risk. Polar bears, mountain gorillas, and coral reef fish are particularly vulnerable because they cannot easily migrate to suitable new habitats.

6. Exam Technique and Common Mistakes

Global warming questions often appear in the ecology section of your exam. Here's how to maximise your marks and avoid common pitfalls.

Command Word Focus: 'Describe' questions want you to state what happens without explanation. 'Explain' questions require you to give reasons using scientific principles. For example: 'Describe: Coral bleaching occurs when corals turn white.' 'Explain: Coral bleaching occurs because increased water temperatures stress corals, causing them to expel their symbiotic algae.'

Mark-winning phrases: Use precise scientific language like 'symbiotic relationship', 'trophic levels', 'phenological mismatch', and 'thermal tolerance'. These show you understand the underlying biology, not just the surface effects.

Remember: global warming questions test your understanding of ecological relationships, not just your ability to list consequences. Always explain the biological mechanisms behind the effects you describe.