DP Biology HL D4.3 Climate Change

DP Biology HL D4.3 Climate Change - Full notes

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DP Biology HL | D4.3 Climate Change

DP BIOLOGY · THEME D: CONTINUITY AND CHANGE

D4.3

Climate Change

Full notes for HL · IB-style

Contents

1	The greenhouse effect
2	The enhanced greenhouse effect and rising CO₂
3	Evidence for climate change
4	Positive feedback and tipping points in the climate
5	Effects on ecosystems: polar, marine and terrestrial
6	Phenology, range shifts and evolution
7	Mitigation: reducing emissions and capturing carbon
8	The greenhouse effect
9	Sources of greenhouse gases
10	Evidence for climate change
11	Correlation and causation
12	Positive feedback in climate
13	Effects on ecosystems
14	Ocean acidification
15	Effects on coral reefs
16	Polar ecosystems
17	Carbon sequestration
18	Mitigation and adaptation
19	Analysing climate data
20	Mixed review: worked problems
21	Additional exam-style practice
22	Climate change and disease
23	Individual and global action
24	Common misconceptions and links
25	Exam-style question with markscheme
26	Check your understanding
27	Summary and topic map

These are full teaching notes. Read each section in order, try the Think about it question before you read on, and do every Quick check before moving to the next section.

Before you start

Rate yourself on each statement now, and again after you've worked through the notes. Shade R (not yet), A (getting there) or G (confident).

I can…	R	A	G
explain the natural greenhouse effect and the main greenhouse gases
explain how human activities have enhanced the greenhouse effect
evaluate evidence for climate change, including ice cores and the Keeling curve
explain positive feedback mechanisms such as melting ice and thawing permafrost
explain effects of climate change on polar habitats, coral reefs and terrestrial ecosystems
explain changes in phenology and species ranges, including mismatches
evaluate methods to reduce emissions and capture carbon

There is no additional HL content in this subtopic. SL and HL students study the same material.

Guiding questions

What are the drivers and consequences of climate change? How are ecosystems responding, and what can be done?

How this topic is examined

D4.3 is examined through data analysis (CO₂ and temperature records, ice core data, phenology graphs) and evaluation of mitigation strategies.

Distinguish the natural greenhouse effect (essential for life) from the enhanced greenhouse effect (human-caused warming), and correlation from causation.

Command term	What the examiner wants
State / Identify	a brief answer with no explanation needed
Outline	a brief account or summary
Describe	a detailed account, including trends or features, without reasons
Explain	a detailed account including causes, reasons or mechanisms
Distinguish / Compare and contrast	give the differences (and similarities), point by point
Calculate	a numerical answer with working and units
Deduce / Suggest	reach a conclusion from data; propose a reasonable explanation
Evaluate / Discuss	weigh strengths and limitations and reach a judgement

How to use these notes

Each section teaches one part of the syllabus in full. You'll see these boxes throughout; each has one job:

Box	What it's for
Think about it	a question to get you thinking before a section; the answer comes in the section
Key point	the one idea you must not forget
Common mistake	an error examiners see every year, and how to avoid it
In the real world	where the biology shows up outside the classroom
Try this	something quick you can do yourself to test the idea
Worked example	full solutions with IB-style marks (M1 method, A1 answer, R1 reasoning)
Quick check	short questions to test yourself at the end of each section

1 The greenhouse effect

Without it, Earth would be frozen.

Think about it

The Moon is the same distance from the Sun as Earth, yet its average surface temperature is about −20 °C. Why is Earth so much warmer?

Short-wave radiation (visible and ultraviolet light) from the Sun passes through the atmosphere and is absorbed by the Earth's surface, which warms. The warm surface emits long-wave infrared radiation. Greenhouse gases in the atmosphere absorb much of this infrared radiation and re-emit it in all directions, including back towards the surface, warming the lower atmosphere. This is the greenhouse effect.

The natural greenhouse effect keeps Earth's average surface temperature at about +15 °C, instead of about −18 °C without it. It is essential for life.

The main greenhouse gases are water vapour, carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O). Their impact depends on how strongly each molecule absorbs infrared and how long it stays in the atmosphere. Methane is about 80 times more powerful than CO₂ over 20 years but lasts only about 12 years; CO₂ persists for centuries.

Figure 1. The greenhouse effect.

Quick check

1. What type of radiation do greenhouse gases absorb?

2. Name three greenhouse gases.

Answers: 1. long-wave infrared 2. CO₂, methane, water vapour, nitrous oxide

2 The enhanced greenhouse effect and rising CO₂

Human activities are adding greenhouse gases.

Since the Industrial Revolution, human activities have increased the concentrations of greenhouse gases, enhancing the greenhouse effect and causing global warming. Burning fossil fuels (coal, oil, gas) releases CO₂ that was stored underground for millions of years. Deforestation releases carbon from trees and soils and reduces photosynthesis. Agriculture releases methane (from cattle and rice paddies) and nitrous oxide (from fertilizers). Landfill releases methane.

Atmospheric CO₂ has risen from about 280 ppm in 1800 to over 420 ppm today, higher than at any time in at least 800 000 years. Global average temperature has risen by about 1.2 °C since pre-industrial times. Climate change refers to the wider long-term changes in temperature, rainfall, extreme weather and sea level that result.

Figure 2. Global average temperature anomaly relative to the 1951-1980 average.

Quick check

1. Name three human activities that increase greenhouse gases.

2. How much has CO₂ risen since 1800?

Answers: 1. burning fossil fuels, deforestation, agriculture 2. from about 280 to over 420 ppm

3 Evidence for climate change

Ice, air and instruments tell the same story.

Direct measurements: thermometer records since the 1850s; the Keeling curve of atmospheric CO₂ measured at Mauna Loa since 1958 (C4.2); satellite data on sea level and ice cover.

Ice cores: as snow accumulates in Antarctica and Greenland, it traps tiny bubbles of air. Drilling ice cores and analysing the bubbles reveals past CO₂ and methane levels; the ratio of oxygen or hydrogen isotopes in the ice indicates past temperatures. Ice cores from Antarctica reach back about 800 000 years and show that CO₂ and temperature have risen and fallen together through ice ages, and that current CO₂ levels are far above anything in that record.

Other evidence: tree rings, coral growth bands, sediment layers, retreating glaciers, earlier spring events (phenology). The scientific consensus, summarized by the Intergovernmental Panel on Climate Change (IPCC), is that warming since the mid-twentieth century is mainly caused by human greenhouse gas emissions.

Correlation and causation: a correlation between CO₂ and temperature does not by itself prove that one causes the other. The causal link is established by the physics of how CO₂ absorbs infrared, by models, and by ruling out other causes such as changes in solar output.

Figure 3. Simplified ice core record: CO₂ and temperature have varied together.

Quick check

1. How do ice cores record past CO₂?

2. Why does correlation not prove causation?

Answers: 1. trapped air bubbles 2. another factor could cause both; need mechanism and evidence

4 Positive feedback and tipping points in the climate

Warming can cause more warming.

Several positive feedback mechanisms can accelerate warming. Melting ice: ice and snow reflect sunlight (high albedo); as they melt, darker ocean or land absorbs more heat, causing more melting. Thawing permafrost: frozen soils in the Arctic contain huge stores of organic carbon; as they thaw, decomposition releases CO₂ and methane, causing further warming. Forest dieback and fires release carbon and reduce uptake. Warmer oceans absorb less CO₂.

These feedbacks could push parts of the climate system past tipping points: thresholds beyond which change becomes self-sustaining and difficult to reverse, such as the collapse of ice sheets or the Amazon's shift to savanna (D4.2).

Figure 4. Positive feedback loops in climate change.

Quick check

1. Explain the ice albedo feedback.

2. Why is thawing permafrost a concern?

Answers: 1. less ice, less reflection, more absorption, more warming 2. releases stored carbon as CO₂ and methane

5 Effects on ecosystems: polar, marine and terrestrial

Habitats are changing faster than many species can adapt.

Polar habitats: Arctic sea ice is declining in extent and thickness. Polar bears depend on sea ice to hunt seals; less ice means less hunting time and lower body condition. Walruses and ringed seals also depend on ice.

Coral reefs: rising sea temperatures cause coral bleaching: stressed corals expel their symbiotic zooxanthellae (C4.1), losing colour and their main food source. If high temperatures persist, corals die. The Great Barrier Reef has suffered several mass bleaching events since 2016. Ocean acidification, as CO₂ dissolves in seawater forming carbonic acid, reduces carbonate ion availability and makes it harder for corals and shellfish to build calcium carbonate skeletons.

Terrestrial ecosystems: changes in rainfall and temperature shift biomes; droughts and wildfires increase; species in mountains move upslope and may run out of habitat.

Quick check

1. Why does warming cause coral bleaching?

2. How does ocean acidification affect shell-building organisms?

Answers: 1. stressed corals expel zooxanthellae 2. less carbonate available to build calcium carbonate

6 Phenology, range shifts and evolution

Timing and location of life cycles are changing.

Phenology is the study of the timing of seasonal biological events: flowering, leaf opening, migration, breeding. Many events are now occurring earlier in spring as temperatures rise.

Problems arise when interacting species respond differently. Phenological mismatch: in many forests, oak leaves now open earlier, so the peak of winter moth caterpillars comes earlier; great tits, which time breeding partly by day length, may hatch chicks after the caterpillar peak, reducing food for their young. Similarly, reindeer calving may no longer match the peak of plant growth in the Arctic.

Range shifts: many species are moving towards the poles or to higher altitudes, tracking suitable temperatures. Species unable to move or adapt fast enough face local extinction. Some species show evolutionary responses, such as changes in timing of breeding.

Figure 5. A growing mismatch between caterpillar abundance and great tit chicks.

Quick check

1. Define phenology.

2. Explain phenological mismatch with an example.

Answers: 1. timing of seasonal biological events 2. caterpillar peak earlier than great tit chicks; less food

7 Mitigation: reducing emissions and capturing carbon

Two approaches: cut emissions and remove CO₂.

Reducing emissions: switching from fossil fuels to renewable energy (solar, wind, hydroelectric) and nuclear power; energy efficiency in buildings and transport; electric vehicles; reducing deforestation; reducing meat consumption and food waste; better fertilizer management to reduce nitrous oxide.

Carbon sequestration (capture and storage): reforestation and afforestation (trees take up CO₂ as they grow); restoring peatlands and wetlands, which store large amounts of carbon; soil carbon from improved farming practices; and carbon capture and storage (CCS) technology, which captures CO₂ from power stations or directly from the air and stores it underground.

International agreements such as the Paris Agreement (2015) aim to limit warming to well below 2 °C, preferably 1.5 °C, which requires reaching net zero emissions around mid-century. Adaptation (such as flood defences and drought-resistant crops) is also needed for unavoidable changes.

Strategy	Strength	Limitation
Renewable energy	large emission cuts; falling costs	intermittency; storage needed
Reforestation	removes CO₂; habitat	slow; land competition; fire risk
Peatland restoration	large carbon stores protected	local; slow
CCS	removes CO₂ from emitters	expensive; unproven at scale

Key point

Mitigation reduces the causes of climate change; adaptation reduces its impacts. Both are needed.

8 The greenhouse effect

Natural and enhanced.

Short-wave radiation from the Sun passes through the atmosphere and warms the Earth's surface, which re-emits long-wave infrared. Greenhouse gases (CO2, CH4, water vapour, N2O) absorb and re-emit infrared, warming the lower atmosphere. The natural effect keeps Earth habitable; human emissions enhance it.

WORKED EXAMPLE 1 Mechanism (exam level)

Explain how greenhouse gases warm the Earth.

Solution

Earth's surface emits infrared radiationR1

Greenhouse gases absorb it and re-radiate some back to the surfaceR1

9 Sources of greenhouse gases

Human activities.

Gas	Main human sources
CO2	burning fossil fuels, deforestation, cement production
CH4	livestock (ruminants), rice paddies, landfill, gas leaks
N2O	fertilizers, industrial processes

WORKED EXAMPLE 2 Methane (standard)

Explain why cattle farming contributes to climate change.

Solution

Ruminant digestion produces methaneR1

Methane is a potent greenhouse gasR1

10 Evidence for climate change

Multiple lines.

Evidence includes rising global mean temperatures (about 1.1–1.2 °C since pre-industrial times), the Keeling curve, ice cores showing CO2 and temperature correlated over 800 000 years, shrinking glaciers and Arctic sea ice, rising sea levels, and earlier spring events.

WORKED EXAMPLE 3 Ice cores (standard)

How do ice cores provide evidence about past climates?

Solution

Air bubbles trapped in ice record past CO2 concentrationsR1

Isotope ratios indicate past temperaturesR1

11 Correlation and causation

Interpreting data.

CO2 concentration and global temperature are strongly correlated. Causation is supported by physical understanding of infrared absorption and by climate models; correlation alone does not prove causation.

Quick check

1. Why is correlation between CO2 and temperature not enough on its own to prove causation?

Answers: 1. other factors could be involved; a mechanism is needed

12 Positive feedback in climate

Amplifying warming.

Warming triggers processes that release more greenhouse gases or reduce reflection: melting permafrost releases methane; loss of ice reduces albedo (more absorption of sunlight); warmer oceans absorb less CO2; drought increases forest fires.

WORKED EXAMPLE 4 Albedo (exam level)

Explain how melting Arctic ice accelerates warming.

Solution

Ice reflects sunlight (high albedo)R1

Darker ocean absorbs more heat, causing further meltingR1

13 Effects on ecosystems

Shifting ranges and timing.

Species shift ranges towards poles and higher altitudes; phenology (timing of flowering, migration, breeding) changes, causing mismatches between species; coral bleaching increases; ocean acidification harms calcifying organisms.

WORKED EXAMPLE 5 Phenological mismatch (exam level)

Explain how earlier spring can reduce bird breeding success.

Solution

Caterpillars emerge earlier due to warmer temperaturesR1

Bird chicks hatch at the old time and miss peak food supplyR1

In the real world

Great tits in the Netherlands have struggled because caterpillar peaks now occur earlier, while bird egg-laying has not shifted as much.

14 Ocean acidification

CO2 dissolving.

About a quarter of emitted CO2 dissolves in the oceans, forming carbonic acid and lowering pH (by about 0.1 since pre-industrial times). Lower carbonate ion availability makes it harder for corals, molluscs and plankton to build calcium carbonate shells.

WORKED EXAMPLE 6 Chemistry (standard)

Explain why acidification harms corals.

Solution

Fewer carbonate ions availableR1

Harder to build calcium carbonate skeletonsR1

15 Effects on coral reefs

Bleaching.

High temperatures cause corals to expel their symbiotic zooxanthellae, losing colour and their main energy source. Repeated bleaching events can kill reefs, reducing biodiversity and coastal protection.

Quick check

1. Why do bleached corals often die?

Answers: 1. they lose the sugars supplied by zooxanthellae

16 Polar ecosystems

Rapid change.

The Arctic is warming about four times faster than the global average. Loss of sea ice threatens polar bears (hunting platform), walruses and ice algae at the base of food chains; tundra is being invaded by shrubs.

Quick check

1. Explain why polar bears are threatened by sea-ice loss.

Answers: 1. they hunt seals from the ice; less ice means less food

17 Carbon sequestration

Removing CO2.

Natural sequestration: forests, peatlands, mangroves and seagrass store carbon. Afforestation and restoration of wetlands increase storage. Technological methods include carbon capture and storage at power stations and direct air capture.

WORKED EXAMPLE 7 Evaluating afforestation (exam level)

Evaluate planting trees to reduce atmospheric CO2.

Solution

Trees absorb CO2 by photosynthesis and store carbonR1

Takes decades; trees may burn or die; land competitionR1

18 Mitigation and adaptation

Two responses.

Mitigation reduces emissions (renewable energy, efficiency, less meat, reducing deforestation). Adaptation reduces harm from changes that occur (flood defences, drought-resistant crops, assisted migration of species).

Quick check

1. Give one example of mitigation and one of adaptation.

Answers: 1. mitigation: solar power; adaptation: sea walls

19 Analysing climate data

Graphs and trends.

Climate data (temperature anomalies, CO2) are analysed for trends using moving averages and regression. Year-to-year variation (El Niño) is superimposed on the long-term trend.

Figure 6. Approximate global temperature anomaly relative to 1951–1980.

WORKED EXAMPLE 8 Rate (standard)

Estimate the rate of warming between 1980 and 2020 from the graph.

Solution

(1.01 − 0.26)/40M1

≈ 0.19 °C per decadeA1

20 Mixed review: worked problems

Bringing the ideas together.

WORKED EXAMPLE 9 Mixed problem (exam level)

Explain how increased CO2 affects both terrestrial and marine organisms.

Solution

Warming shifts ranges and timing on landR1

Ocean acidification harms calcifying organismsR1

Possible increased photosynthesis where CO2 limits growthR1

21 Additional exam-style practice

Paper 1 and Paper 2.

WORKED EXAMPLE 10 Paper 1 question (standard)

Which is a positive feedback in climate change? A. increased photosynthesis B. melting permafrost releasing methane C. cloud formation reflecting sunlight D. planting trees

Solution

BA1

WORKED EXAMPLE 11 Paper 2 question (exam level)

Outline two pieces of evidence for anthropogenic climate change.

Solution

Rising CO2 (Keeling curve) correlating with temperatureR1

Ice core records; isotopic signature of fossil-fuel carbonR1

22 Climate change and disease

Spreading vectors.

Warming expands the ranges of disease vectors such as mosquitoes (malaria, dengue) and ticks (Lyme disease) to higher latitudes and altitudes, increasing disease risk in new regions.

Quick check

1. Why might malaria spread to highland areas?

Answers: 1. warmer temperatures allow mosquitoes and the parasite to survive there

23 Individual and global action

From local to international.

International agreements (Paris Agreement, 2015) aim to limit warming to well below 2 °C, preferably 1.5 °C. Individuals contribute by reducing energy use, travel emissions, meat consumption and waste.

Quick check

1. State the main aim of the Paris Agreement.

Answers: 1. limit warming to well below 2 °C above pre-industrial levels

24 Common misconceptions and links

Students often think…	Actually…
the greenhouse effect is harmful	the natural effect is essential; the enhanced effect causes warming
the ozone hole causes global warming	ozone depletion and climate change are different problems
correlation proves CO₂ causes warming	causation is shown by physics, models and evidence

How D4.3 connects to the rest of the course

Topic	Connection
C4.2 Carbon cycle	sources and sinks
D4.2 Stability	tipping points
C4.1 Communities	coral mutualism
A4.2 Conservation	biodiversity loss
C1.3 Photosynthesis	CO₂ fixation and FACE

25 Exam-style question with markscheme

Try this under exam conditions (about 18 minutes) before reading the markscheme.

Figure 7. Timing of caterpillar peak and great tit chick hatching.

(a) Calculate the change in timing of the caterpillar peak from 1970 to 2020. [1]

(b) Suggest a reason for this change. [2]

(d) Explain how ice cores provide evidence of past climate. [3]

(e) Evaluate reforestation as a way to reduce atmospheric CO₂. [4]

Markscheme

Part	Answer	Marks
(a)	16 days earlier	A1
(b)	warmer springs	R1
	oak leaves and caterpillars develop earlier	R1
(c)	chicks hatch after peak food supply	R1
	less food for chicks	R1
	lower survival / breeding success	R1
(d)	trapped air bubbles show past CO₂	R1
	isotope ratios indicate temperature	R1
	dated by layers; 800 000-year record	R1
(e)	trees absorb CO₂ by photosynthesis; habitat benefits	R1 R1
	slow; land use conflict; carbon released if trees burn	R1 R1

Examiner's comments. In (c), the best answers linked the timing difference to food availability and breeding success.

26 Check your understanding

Section A: multiple choice

1. Greenhouse gases absorb

A. long-wave infrared radiation B. ultraviolet C. visible light D. radio waves

2. Atmospheric CO₂ before 1800 was about

A. 420 ppm B. 180 ppm C. 280 ppm D. 1 000 ppm

3. Coral bleaching is caused by

A. acid rain B. corals expelling zooxanthellae C. ozone loss D. plastic

4. Melting sea ice increases warming because

A. ice releases CO₂ B. water reflects more C. ice is a greenhouse gas D. less sunlight is reflected

5. Phenology is the study of

A. climate models B. timing of seasonal events C. ice cores D. greenhouse gases

6. Carbon sequestration includes

A. planting forests B. burning coal C. clearing peat D. flying

Section B: structured

7. Distinguish the greenhouse effect from the enhanced greenhouse effect. [3]

8. Explain two positive feedbacks in climate change. [4]

9. Explain how climate change affects polar ecosystems. [3]

Answers

1. A 2. C 3. B 4. D 5. B 6. A

7. Natural: keeps Earth habitable; enhanced: extra greenhouse gases from human activity cause additional warming.

8. Ice albedo; permafrost carbon release; forest dieback; reduced ocean uptake.

9. Sea ice loss; polar bears lose hunting platform; ice-dependent species decline; range shifts.

27 Summary and topic map

Idea	Remember
Greenhouse effect	infrared absorbed and re-emitted; natural and essential
Enhanced effect	fossil fuels, deforestation, agriculture; CO₂ 280 → 420 ppm
Evidence	Keeling curve, ice cores, temperature records
Feedbacks	albedo, permafrost, fires
Ecosystems	polar ice, coral bleaching, acidification, biome shifts
Phenology	earlier events; mismatches; range shifts
Mitigation	renewables, reforestation, peatlands, CCS; Paris Agreement

Before the exam, make sure you…

•distinguish natural and enhanced greenhouse effects
•use data with units and trends
•explain feedback loops step by step
•evaluate mitigation strategies

Climate Change on one page

Cover this page with a sheet of paper and try to rebuild it from memory. Whatever you can't recall is what to revise next.

Figure 8. Topic map for Climate Change.

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