Global Warming and Deforestation

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 4.15 — The enhanced greenhouse effect, global warming and its consequences
• 4.18 — The effects of deforestation: leaching, soil erosion, evapotranspiration, the carbon cycle and atmospheric gases (bold B statement — Paper 2 only)

Specification section 4(d) Human influences on the environment — also taught in The Carbon and Nitrogen Cycles, Air Pollution and the Greenhouse Gases, Water Pollution and Eutrophication.

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

The greenhouse effect is natural and necessary. What has changed is its strength: rising concentrations of carbon dioxide, methane and the other greenhouse gases mean more of the radiation the Earth emits is absorbed and re-radiated back, so less energy escapes to space. That is the ENHANCED greenhouse effect, and the resulting rise in mean global temperature is GLOBAL WARMING.

The temperature rise itself is small in everyday terms — a degree or two — and that is exactly why the consequences need explaining rather than asserting. A mean global temperature is an average across the whole planet and the whole year, so a shift of one degree in it represents an enormous change in the total energy the Earth is holding. The biological consequences follow from that energy, not from anyone feeling warmer.

The consequences of global warming, each with its mechanism

1. MELTING ICE AND RISING SEA LEVELS. Ice sheets and glaciers melt, adding water to the oceans, and the water already there expands as it warms. Low-lying land is flooded, destroying habitats and displacing the organisms and people living there.
2. CHANGED DISTRIBUTION OF SPECIES. Each species has a range of conditions it can tolerate. As regions warm, species that can move do so — towards the poles or up mountains — while those that cannot, or that reach the top of a mountain with nowhere further to go, decline or become extinct.
3. CHANGED SEASONAL BEHAVIOUR. Warming shifts the timing of flowering, migration and breeding. This matters because it does not shift everything by the same amount: an insect emerging earlier than the plant it feeds on, or a bird arriving after the caterpillars it feeds its chicks have peaked, breaks a relationship that both depended on.
4. MORE EXTREME WEATHER. A warmer atmosphere holds more water vapour and more energy, producing more intense storms, floods and droughts, which destroy habitats and crops.
5. REDUCED BIODIVERSITY. Species unable to move or adapt fast enough become extinct, and the variety of species in affected communities falls.

Notice that the third of those is about TIMING rather than temperature tolerance, and it is the one most people leave out. Ecological relationships are built on things happening in the right order, and warming pulls the pieces apart at different rates. A plant flowering two weeks earlier is not harmed by the warmth — it is harmed if its pollinator has not shifted with it.

Think of it like an orchestra drifting out of time

Every player is still playing the right notes, and no individual is playing badly. But the sections have crept apart, so the strings arrive slightly before the brass and the timing that made the piece work has gone. Nothing has been destroyed — the whole ensemble has simply lost its coordination, and the result is unusable. That is what warming does to a community whose species have kept their tolerances but lost their synchrony.

Deforestation — five consequences, and why each follows from removing the trees

1. LESS CARBON DIOXIDE ABSORBED. The trees are no longer photosynthesising, so the carbon dioxide they would have taken from the atmosphere remains in it.
2. MORE CARBON DIOXIDE RELEASED. Burning the felled trees releases their stored carbon immediately as carbon dioxide, and decomposers respiring on the remains release more. Both add to the enhanced greenhouse effect.
3. SOIL EROSION. Tree roots bound the soil together and the canopy sheltered it from the direct impact of rain. Without them the topsoil is washed away by rain and blown away by wind, and it is the topsoil that holds the nutrients.
4. LEACHING. Rain now passes straight through the soil instead of being taken up by the trees, and dissolved mineral ions such as nitrates are carried down beyond the reach of roots — so the soil that remains is also less fertile.
5. REDUCED EVAPOTRANSPIRATION. Trees draw up enormous volumes of water and release it as vapour from their leaves. Remove them and less water enters the atmosphere locally, so cloud formation and rainfall fall in the region.
6. LOSS OF HABITAT AND BIODIVERSITY. The species that lived there lose their habitat and food, and many become extinct — tropical forest holds a very large share of the world's species.

Deforestation therefore appears twice in this topic, and it is worth being explicit about that when answering. It is a CAUSE of the enhanced greenhouse effect, through both routes above — less absorbed and more released. And it is a set of LOCAL consequences in its own right, on the soil and the rainfall, which would matter even if the atmosphere were unaffected. A question asking about its effects wants both, and the strongest answers say which is which.

One thing worth being careful about with soil. It is tempting to think a cleared forest leaves rich ground behind, since the forest grew so well on it — but tropical forest soils are often thin and poor, because almost all the nutrients are held in the living trees rather than in the ground. Decomposers return them and the trees immediately reabsorb them, so the cycle runs almost entirely above the surface. Remove the trees and there is very little left, which is why cleared land is often exhausted within a few years.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

Learn this definition · Enhanced greenhouse effect

The increase in the natural greenhouse effect caused by rising concentrations of greenhouse gases, so that more of the radiation emitted by the Earth is absorbed and re-radiated back to the surface.

Learn this definition · Global warming

The increase in the mean surface temperature of the Earth caused by the enhanced greenhouse effect.

Statement 4.15 — the consequences of global warming, each with its mechanism

Rising sea levels

Ice sheets and glaciers melt and seawater expands as it warms, flooding low-lying land and destroying habitats.

Changed species distribution

Species move towards the poles or up mountains as their range shifts; those that cannot move, or have nowhere further to go, decline or become extinct.

Changed seasonal timing

Flowering, migration and breeding shift — but not all by the same amount, so relationships such as a plant and its pollinator can fall out of step.

More extreme weather

A warmer atmosphere holds more water vapour and more energy, giving more intense storms, floods and droughts that destroy habitats and crops.

Reduced biodiversity

The population of any species unable to move or adapt quickly enough falls until it becomes extinct, so the variety of species in the community is reduced.

Statement 4.18 — the effects of deforestation, each with the reason

Less carbon dioxide absorbed

The trees no longer photosynthesise, so that absorption stops and the carbon dioxide stays in the atmosphere.

Why mature trees matter most

A mature tree holds far more biomass, and therefore far more stored carbon, than a young one.

More carbon dioxide released

Burning the felled trees releases their stored carbon at once; decomposers respiring on the remains release more.

Soil erosion

Roots no longer bind the soil and the canopy no longer shelters it, so topsoil — which holds the nutrients — is washed and blown away.

Leaching

Rain passes through the soil instead of being taken up by trees, carrying dissolved nitrates and other mineral ions below the reach of roots.

Reduced evapotranspiration

Trees drew up water and released it as vapour from their leaves; without them less water enters the atmosphere locally, so cloud formation and rainfall fall.

Loss of habitat and biodiversity

Species lose their habitat and food supply, and many become extinct — tropical forest holds a large share of the world's species.

Deforestation appears twice — say which you are answering about

A. AS A CAUSE of the enhanced greenhouse effect: less carbon dioxide absorbed by photosynthesis, and more released by burning and decomposition.

B. AS A LOCAL EFFECT in its own right: soil erosion, leaching of mineral ions, reduced rainfall through lower evapotranspiration, and loss of habitat — all of which would matter even if the atmosphere were unaffected.

Why the timing consequence matters more than it sounds

• Warming shifts flowering, migration and breeding EARLIER
• It does not shift them all by the SAME amount, because different species respond to different cues — some to temperature, some to day length
• A plant flowering before its pollinator emerges may not be pollinated, and the pollinator may find no food
• A migrating bird arriving after the caterpillars it feeds its chicks have peaked may raise fewer young
• Neither species has been harmed by the temperature itself — what has been lost is the SYNCHRONY between them

Not this: A cleared tropical forest leaves rich soil behind, since the forest grew so well there.

This: Tropical forest soils are often thin and poor. Almost all the nutrients are held in the living trees and recycled rapidly between them and the decomposers, so removing the trees removes the nutrients.

Model answer [4 marks]

Explain how deforestation contributes to the enhanced greenhouse effect. [4]

The trees that are removed are no longer photosynthesising, so the carbon dioxide they would have absorbed from the atmosphere remains in it. If the felled trees are burned, the carbon they had stored is released immediately as carbon dioxide. Decomposers feeding on the remains respire, releasing further carbon dioxide. The concentration of carbon dioxide in the atmosphere therefore rises, so more of the radiation emitted by the Earth is absorbed and re-radiated back, enhancing the greenhouse effect.

Model answer [4 marks]

Explain why the soil in an area that has been deforested rapidly becomes less fertile. [4]

The roots of the trees held the soil together and the canopy sheltered it from heavy rain, so once they are removed the topsoil is washed away by rain and blown away by wind. It is the topsoil that contains most of the nutrients. Rainwater now passes down through the soil instead of being absorbed by the trees, leaching dissolved mineral ions such as nitrates below the reach of roots. In tropical forests most of the nutrients were held in the trees themselves rather than in the soil, so removing the trees removes most of the nutrients from the area.

Model answer [3 marks]

Explain why rainfall may decrease in a region after large-scale deforestation. [3]

Trees absorb large volumes of water through their roots and release it as water vapour from their leaves by transpiration, a process known as evapotranspiration. When the trees are removed, far less water vapour enters the atmosphere in that region. With less water vapour there is less cloud formation, so rainfall in the area falls.

Mark-losing trap. The greenhouse effect is natural. It is the ENHANCED effect that causes global warming — use the word.

Mark-losing trap. Give the MECHANISM for every consequence. 'Habitats are destroyed' scores nothing without saying how.

Mark-losing trap. Deforestation raises carbon dioxide in TWO ways: less absorbed, and more released. Say both.

Mark-losing trap. Sea level rises from melting ice AND from seawater expanding as it warms. The second is usually forgotten.

Mark-losing trap. Leaching removes dissolved mineral ions downwards; erosion removes the soil itself. They are different processes.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · State [1 mark] — Give one way in which deforestation increases the concentration of carbon dioxide in the atmosphere.
2. Grade 7 · Explain [2 marks] — Explain why the topsoil is often washed away after a forest is cleared.
3. Grade 8 · Explain [5 marks] — Explain how global warming may cause a species of alpine plant, which grows only near mountain tops, to become extinct. Select every statement that belongs in a full-mark explanation.
4. Grade 9 · Calculate [4 marks] — A hectare of tropical forest absorbs 6.4 tonnes of carbon dioxide per year. An area of 12 500 hectares is cleared, and burning the felled trees releases 480 tonnes of carbon dioxide per hectare. Calculate the total increase in atmospheric carbon dioxide in the first year after clearance, counting both the carbon dioxide released and the absorption that no longer takes place.
5. 9+ · Analyse [5 marks] — A government proposes to offset the carbon dioxide released by clearing 10 000 hectares of mature tropical forest by planting the same area with fast-growing saplings elsewhere. Select every statement that belongs in a full-mark evaluation of this proposal.

The people behind this science

Two ways into the same idea — the one who wrote about the sea before she wrote about pesticides, and the one who established what a forest does to the air. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Rachel Carson — the one who wrote about the sea before she wrote about pesticides

Carson's first three books were about the ocean, and The Sea Around Us describes it as a system in slow motion — currents, sediments and sea levels changing over timescales no individual can perceive. That is exactly the difficulty this page presents: consequences that arrive over decades are easy to dismiss because nothing appears to happen from one year to the next. She then spent her final years demonstrating that a harm invisible in any single place can be established by assembling evidence across many, which is how the case on this page is made too.

• “Why is a slow change so much harder to persuade people about?”
• “How do you build a case from evidence spread across a continent?”
• “What did studying the sea teach you about how systems change?”
• “How should we decide to act on a harm we cannot yet see?”

Jan Ingenhousz — the one who established what a forest does to the air

The first consequence of deforestation on this page is that the trees stop removing carbon dioxide from the air, and Ingenhousz is the person who established that green plants do that at all — and only in sunlight. His 1779 experiments showed that leaves purify the air by day and spoil it at night, and that only the green parts do it. Every claim on this page about what a forest was doing before it was cleared rests on the distinction he drew.

• “What does a forest do to the air around it during the day?”
• “And what does it do at night?”
• “How did you measure something happening in a jar of water?”
• “What happens to the balance if the green parts are removed?”

Carry on through the course

• Previous lesson: Air Pollution and the Greenhouse Gases
• Next lesson: Water Pollution and Eutrophication
• Populations, Ecosystems and Sampling with Quadrats — Four nested words for where organisms live — and how to count a population you could never count in full.
• Biodiversity, Distribution and Limiting Factors — Why organisms are where they are and not somewhere else — and how to measure the variety of life in a place rather than just its quantity.
• Food Chains, Webs and Pyramids — Who eats whom, drawn three different ways — and why only one of the three can come out upside down.
• Energy Transfer Along a Food Chain — About nine tenths of the energy disappears at every step. Here is exactly where it goes — and what that means for how the world is fed.
• The Carbon and Nitrogen Cycles — The same atoms, used again and again — and the four bacteria that do the work nothing else can.
• All of Biology · Biology key terms · required practicals · Incandio Science · The 4BI1 specification