Global Warming

Overview

Global Warming is a critical component of the 'Global Challenges' topic in your OCR GCSE Biology specification. This guide will dissect the science behind the headlines, focusing on the precise details examiners look for. We will distinguish between the essential natural greenhouse effect and the problematic enhanced greenhouse effect, detailing how specific types of radiation interact with atmospheric gases. A significant portion of marks in this area are awarded for applying knowledge, particularly in interpreting data that links human activity to climate change. You will learn to analyse graphs, explain the dual impact of deforestation, and describe the consequences of rising global temperatures with the scientific precision required for higher-tier marks. This topic frequently links to others, such as photosynthesis and decay, so a firm grasp here will strengthen your understanding across the syllabus.

Key Concepts

Concept 1: The Natural Greenhouse Effect - Earth's Central Heating

The greenhouse effect is a naturally occurring process that is essential for life on Earth. Without it, the planet's average temperature would be a frigid -18°C. It works by regulating the Earth's temperature, much like a planet-sized central heating system.

1. Incoming Solar Radiation: The Sun emits energy that travels to Earth in the form of short-wavelength radiation, including visible light and ultraviolet (UV) radiation.
2. Atmospheric Transparency: This short-wavelength radiation passes largely unobstructed through the gases in the Earth's atmosphere.
3. Surface Absorption: The Earth's surface (land and oceans) absorbs this energy, causing it to warm up.
4. Outgoing Infrared Radiation: The warmed Earth re-emits this energy as long-wavelength infrared (IR) radiation, which we feel as heat.
5. Absorption by Greenhouse Gases: Unlike the incoming short-wave radiation, this outgoing long-wave IR radiation is absorbed by greenhouse gas molecules (like CO2 and CH4) in the atmosphere.
6. Re-radiation: After absorbing the energy, the greenhouse gas molecules re-radiate it in all directions. Some escapes into space, but a significant amount is directed back towards the Earth's surface, further warming it.

This process of absorption and re-radiation is what maintains a habitable average global temperature. It is crucial for candidates to use the terms 'absorb and re-radiate' rather than 'reflect' or 'trap', as the latter are imprecise and will not be credited in an exam.

Concept 2: The Enhanced Greenhouse Effect & Global Warming

This is the concept that is often, and incorrectly, used interchangeably with the greenhouse effect. The enhanced greenhouse effect refers to the amplification of the natural effect due to human activities increasing the concentration of greenhouse gases in the atmosphere. This enhancement leads to global warming—the observed century-scale rise in the average temperature of the Earth's climate system.

Key Greenhouse Gases:

• Carbon Dioxide (CO₂): The primary driver of the enhanced effect. Its concentration has risen dramatically since the Industrial Revolution.
• Methane (CH₄): A more potent greenhouse gas than CO₂ over a shorter timescale, but present in lower concentrations.

An increase in the concentration of these gases means that more of the outgoing long-wavelength infrared radiation is absorbed and re-radiated back to Earth, leading to a net increase in global average temperatures.

Concept 3: Human Activities and Their Impact

Examiners require candidates to link specific human activities to the release of specific greenhouse gases. Vague terms like 'pollution' will not earn marks.

• Burning Fossil Fuels (for electricity, transport, industry): This is the largest source of anthropogenic CO₂. The combustion of coal, oil, and natural gas releases vast quantities of carbon that were previously locked away underground.
• Deforestation: This has a critical two-fold impact that is often tested. Firstly, the act of clearing forests, often by burning, releases large amounts of stored carbon as CO₂ (combustion). Secondly, and just as importantly, the removal of trees reduces the planet's capacity to absorb atmospheric CO₂ through photosynthesis.
• Agriculture: Modern farming practices are a major source of methane. Cattle farming produces methane through the digestive processes of livestock (enteric fermentation). Rice paddies, where flooded fields prevent oxygen from penetrating the soil, lead to anaerobic decomposition of organic matter, which releases methane.
• Waste Decomposition in Landfills: Similar to rice paddies, organic waste decomposing in landfills under anaerobic conditions produces methane.

Concept 4: Consequences of Global Warming

Candidates must describe specific biological and environmental consequences, not just state that the planet is getting warmer.

• Rising Sea Levels: Higher temperatures cause thermal expansion of seawater and the melting of polar ice caps and glaciers, leading to a rise in global sea levels. This threatens low-lying coastal areas with flooding and habitat loss.
• Loss of Biodiversity: As climates change, habitats are altered faster than many species can adapt. This can lead to extinction. For example, the loss of sea ice affects polar bears that rely on it for hunting.
• Changes in Migration and Distribution: Species may move to different areas to find suitable climates. Birds may migrate earlier or shift their ranges northwards. This can disrupt food webs and introduce new diseases to areas.
• Ocean Acidification & Coral Bleaching: Increased atmospheric CO₂ dissolves in seawater, forming carbonic acid and lowering the ocean's pH. This acidification damages marine ecosystems, particularly coral reefs, which undergo 'bleaching' (expelling their symbiotic algae) and die in response to both warmer temperatures and lower pH.