Deforestation and Global Warming

Deforestation is the large-scale removal of trees, usually to use the land for something else. It has happened on a huge scale in tropical areas — the warm, wet regions near the equator, such as the Amazon and parts of Southeast Asia.

Humans clear tropical forest for several reasons. AQA 8461 names clearing land for agriculture (cattle and rice fields) and for biofuel crops; selling timber is a further common driver:

Reason for deforestation	What the land is used for
To provide land for agriculture	Grazing cattle and growing rice in fields
To grow crops for biofuels	Crops such as oil palm, burned or processed for fuel
To harvest timber (extra context — not named in AQA 8461)	Selling wood for building and paper

The land underneath tropical rainforest is valuable for farming and fuel crops, and the trees themselves can be sold. These economic pressures drive the clearing.

Key term — biofuel: a fuel made from recently grown plant or animal material, rather than from fossil fuels. Crops grown for biofuels still take up land that was once forest.

The two big environmental problems this causes are more carbon dioxide in the atmosphere and reduced biodiversity — the next slides work through each.

This is the most commonly examined idea, and it has two separate mechanisms. To get full marks you must describe both — many students only give one.

Mechanism 1 — deforestation releases more CO₂ into the air.

• When forest is cleared, the trees are often burned. Combustion of the wood releases the carbon stored in it as carbon dioxide.
• Trees and other plant material that are not burned are broken down by decomposers (microorganisms). As these decomposers respire, they also release carbon dioxide.

Mechanism 2 — deforestation removes less CO₂ from the air.

• Living trees carry out photosynthesis, which absorbs carbon dioxide from the atmosphere.
• With fewer trees, less photosynthesis happens, so less carbon dioxide is taken out of the air.

The net effect is a rise in atmospheric CO₂ from both ends at once:

	Before deforestation	After deforestation
CO₂ released	Normal (respiration)	High (burning + decomposers)
CO₂ removed	High (many trees photosynthesising)	Low (few trees photosynthesising)
Net atmosphere	Roughly balanced	CO₂ rises

Exam rule: a top answer states that deforestation increases the release of CO₂ and reduces its removal. Giving only one mechanism is the single most common reason answers lose marks here.

Exam questions often give a scenario and ask you to explain the effect on carbon dioxide. Here is how to build a full-mark answer step by step.

Question: "A large area of rainforest is cut down and burned to make a cattle farm. Explain how this affects the level of carbon dioxide in the atmosphere."

Step 1 — name the released-CO₂ mechanism. Burning the felled trees is combustion, which releases the carbon stored in the wood as carbon dioxide.

Step 2 — add the decomposer point. Any remaining dead plant material is broken down by decomposing microorganisms; their respiration releases more carbon dioxide.

Step 3 — name the reduced-removal mechanism. The trees that have been removed can no longer photosynthesise, so less carbon dioxide is absorbed from the atmosphere.

Step 4 — state the net result. Both effects push the same way, so the level of carbon dioxide in the atmosphere increases.

A model answer reads: "Burning the trees releases carbon dioxide, and decomposers respiring on dead material release more. At the same time, fewer trees means less photosynthesis, so less carbon dioxide is removed. Overall, atmospheric carbon dioxide increases."

Notice the structure: two release routes + one reduced-removal route + a net conclusion. Reuse this skeleton for any deforestation CO₂ question.

Biodiversity is the variety of all the different species of organisms in an ecosystem. Tropical rainforests hold an enormous share of the planet's species, so clearing them does serious damage.

When forest is removed:

• Habitats are destroyed. Species that can only live in the forest lose the conditions, food and shelter they depend on.
• Populations shrink, and some species may become extinct locally or entirely.
• The number of different species in the area falls, so biodiversity is reduced.

Cause	Direct effect	Consequence
Trees burned / felled	Carbon released; carbon store lost	Atmospheric CO₂ rises
Fewer trees remain	Less photosynthesis	Less CO₂ removed from air
Forest habitat removed	Species lose food and shelter	Biodiversity reduced

Lower biodiversity matters because it makes ecosystems less stable: a community with many interdependent species copes better with change than one with only a few.

Watch the word "biodiversity": it means the range of species, not just the number of organisms. Cutting forest reduces the variety of species, not simply the total count of living things.

Carbon dioxide is one of several greenhouse gases. For AQA Biology the two you must name are carbon dioxide (CO₂) and methane (CH₄), and the specification states that their levels in the atmosphere are increasing.

Methane is added to the atmosphere by, for example, cattle (livestock farming) and decay in waterlogged conditions such as rice fields — the same agricultural changes that drive deforestation also raise methane.

How the greenhouse effect works:

• The Sun's energy reaches the Earth and warms the surface.
• The warmed Earth radiates energy back out as heat.
• Greenhouse gases in the atmosphere absorb and trap some of this heat instead of letting it escape into space.
• More greenhouse gas means more heat trapped, so the average global temperature rises. This rise is global warming.

Don't confuse two things: the greenhouse effect is the natural trapping of heat that keeps Earth warm enough for life. Global warming is the extra warming caused by rising levels of greenhouse gases.

(Extra context — not required by AQA 8461.) Atmospheric CO₂ passed about 420 parts per million by the mid-2020s, up from roughly 280 ppm before industrialisation. You do not need figures for the exam; the spec only requires that you know the levels are increasing.

The specification asks you to describe biological consequences of global warming. Each consequence has a mechanism — knowing why it happens earns the marks.

Consequence	Mechanism (why it happens)
Loss of habitat	Rising temperatures melt ice, so sea levels rise and flood low-lying land, destroying the habitats there
Changes in species distribution	As temperature or rainfall changes in a region, species move to areas that now suit them and disappear from areas that no longer do
Changes in migration patterns	Some animals migrate in response to temperature or seasonal cues; as these shift, the timing or route of migration changes

Worked walk-through — explaining one consequence:

Question: "Explain how global warming can cause a loss of habitat."

• Step 1: Global warming raises average global temperatures.
• Step 2: Higher temperatures melt ice and cause sea water to expand, so sea levels rise.
• Step 3: Rising seas flood low-lying coastal areas.
• Step 4: The organisms living there lose their habitat — a biological consequence of global warming.

A "consequence" answer is only complete when it links warming → physical change → biological effect. Stating "habitats are lost" with no mechanism rarely gets full marks.

The specification asks you to evaluate the evidence linking human activity to rising greenhouse gases, and to understand why scientists handle this evidence carefully.

Why the data is treated as strong:

• The scientific consensus is based on systematic reviews of thousands of peer-reviewed publications, not on one study.
• Peer review means other independent scientists check a study's methods and conclusions before it is published, which filters out flawed work.
• Evidence that is valid (it measures what it claims to) and reproducible (other scientists get the same result when they repeat it) is trusted far more than a one-off finding.

Why some uncertainty remains:

• Climate is a complex system with many interacting factors, so it is hard to be certain about the size of every future effect.
• Early or unrepeated data, and data reported in the media without peer review, may be unreliable.

Evaluate means weigh both sides. State that the overwhelming peer-reviewed evidence links human activity to rising CO₂ and methane, then note where uncertainty remains — rather than treating the question as a simple yes/no.

This is why you should be cautious about a single dramatic statistic or a claim that has not been peer reviewed: good science depends on evidence that other researchers can check and reproduce.

1. Giving only one CO₂ mechanism

Deforestation raises atmospheric CO₂ in two ways: it increases release (burning and decomposers) and reduces removal (less photosynthesis). An answer with only one of these is incomplete.

2. Saying trees "make oxygen" instead of removing CO₂

The carbon point that scores marks is that trees absorb carbon dioxide during photosynthesis. Fewer trees means less CO₂ removed. Oxygen production is not the focus of this topic.

3. Forgetting methane

The greenhouse gases named by the spec are carbon dioxide and methane. Many students name only CO₂. Methane levels are rising too and also contribute to global warming.

4. Confusing the greenhouse effect with global warming

The greenhouse effect is the natural trapping of heat. Global warming is the extra warming from rising greenhouse gas levels. They are not synonyms.

5. Describing a consequence with no mechanism

"Habitats are lost" or "animals migrate differently" alone is weak. Link it: warming → sea-level rise / changed temperature or rainfall → the biological effect.

6. Treating biodiversity as the number of organisms

Biodiversity is the variety of species. Deforestation reduces the range of species present, not just the headcount of living things.