Climate change

Biology SL· D4.1· 25 min read

1. The Natural Greenhouse Effect Mechanism⏱ 5 min

Shortwave radiation from the sun penetrates the atmosphere and is absorbed by Earth’s surface, which re-emits energy as longer wavelength infrared radiation. Greenhouse gas molecules absorb this infrared radiation, re-radiating a portion back towards the surface rather than allowing it to escape directly to space.

📘 Definition

Greenhouse gas

GHG

Atmospheric gas molecule that absorbs infrared radiation, including carbon dioxide, methane, nitrous oxide, and water vapour

Example:

Water vapour is the most abundant natural GHG, but its concentration is controlled by global temperature rather than human activity.

📐 Worked Example

Calculate the approximate difference in average global temperature if there was no natural greenhouse effect, given the current average is 15°C and the greenhouse effect contributes 33°C of warming.

1
Identify the two values provided: current global average temperature = 15°C, total warming from natural GHGs = 33°C
2
Subtract the GHG warming contribution from the current average to find the hypothetical no-GHG temperature
3
$15 ^\circ C - 33 ^\circ C = -18 ^\circ C$
4
This confirms the natural greenhouse effect is non-negotiable for supporting liquid water and life on Earth.

2. Anthropogenic Drivers of Enhanced Warming⏱ 6 min

Since the industrial revolution, human activities have increased atmospheric CO₂ concentrations from 280 ppm pre-1750 to over 420 ppm in 2025. The largest single source is fossil fuel combustion for energy, transport and industry, followed by deforestation which removes natural carbon sinks.

Methane is released from livestock digestion, rice paddy agriculture, and leaky natural gas infrastructure
Nitrous oxide is emitted from synthetic nitrogen fertilizer use in industrial agriculture
CFCs are synthetic gases used historically as refrigerants that are extremely potent greenhouse gases

📐 Worked Example

A 1 hectare tropical forest absorbs 22 tonnes of CO₂ per year. Calculate how much additional CO₂ is released if that forest is cleared and burned, given the total above-ground biomass stores 180 tonnes of carbon.

1
Recall the conversion factor: 1 tonne of carbon burned produces 3.667 tonnes of CO₂
2
$180 \times 3.667 = 660.06 \text{ tonnes of } CO_2$
3
Add the lost annual sequestration value: 660.06 + 22 = 682.06 tonnes of net CO₂ released per hectare cleared.

✓ Quick check

Which of these is a purely anthropogenic greenhouse gas?
- Water vapour
- CFC
- Carbon dioxide
- Methane
Reveal answer
CFC —
CFCs do not occur naturally on Earth, all atmospheric concentrations are from human industrial production.

3. Empirical Evidence for Climate Change⏱ 5 min

Ice core data from Antarctica provides 800,000 years of historical atmospheric records: trapped air bubbles in compacted glacial ice allow scientists to directly measure past CO₂ concentrations, while oxygen isotope ratios act as a proxy for historical global temperature.

📐 Worked Example

Explain why ice core data is considered robust evidence that CO₂ and temperature are correlated over geological time.

1
Step 1: Drilled ice cores are dated layer by layer using annual dust deposition patterns, giving precise age control
2
Step 2: Air bubbles trapped at the time of ice formation are extracted and analysed for CO₂ concentration, no modern contamination
3
Step 3: Isotope ratio data from the ice itself is used to reconstruct local temperature at the time of deposition
4
Step 4: All 8 glacial-interglacial cycles in the 800k record show tight positive correlation between CO₂ and temperature.

4. Biological Impacts of Climate Change⏱ 6 min

Rising ocean temperatures trigger coral bleaching, where symbiotic zooxanthellae algae are expelled from coral tissues, leading to coral death if stress persists. Arctic sea ice loss reduces habitat for polar bears and seals, disrupting entire polar food webs. Shifting rainfall patterns reduce global staple crop yields, threatening food security.

📐 Worked Example

Outline the causal chain linking rising atmospheric CO₂ to reduced calcification in marine coral species.

1
Step 1: Excess atmospheric CO₂ dissolves in seawater, forming carbonic acid
2
Step 2: Carbonic acid dissociates, lowering ocean pH and reducing the concentration of free carbonate ions
3
Step 3: Corals require dissolved carbonate ions to build their calcium carbonate skeletons, so reduced availability slows growth rates.

5. Common Pitfalls

Wrong move:

Stating the greenhouse effect is a harmful, human-caused problem

Why:

This confuses the natural and enhanced versions of the effect, which is a very common mark deduction in IB exams

Correct move:

Explicitly distinguish the two, noting the natural effect is essential for life while the enhanced effect is anthropogenic and damaging

Wrong move:

Claiming water vapour is the main driver of modern climate change

Why:

Water vapour concentration is a feedback effect, not a forcing: its levels are controlled by global temperature, not direct human emissions

Correct move:

Name CO₂ as the primary anthropogenic forcing driving modern climate change

Wrong move:

Stating ice core data proves CO₂ rises cause temperature rises

Why:

During deglaciation events, initial temperature rise releases CO₂ from oceans, which then amplifies warming, creating a positive feedback

Correct move:

Note that ice core data shows correlation, and modern measured CO₂ rises precede modern temperature rises to confirm causation

Wrong move:

Confusing ozone layer depletion with climate change

Why:

These are separate environmental issues with different causes and impacts, even though CFCs contribute to both

Correct move:

Never mix the two topics in exam answers, keep their mechanisms distinct

Wrong move:

Claiming all species will adapt to rising temperatures over time

Why:

Current warming rate is ~10x faster than post-glacial natural warming rates, so most species cannot evolve or shift range fast enough

Correct move:

Note that rapid warming is the key factor that makes modern climate change a major extinction threat

6. Quick Reference Cheatsheet

Greenhouse Gas	Main Anthropogenic Source	Relative 100-year GWP	Atmospheric Lifetime
Carbon dioxide	Fossil fuel combustion, deforestation	1	~300 years
Methane	Livestock, rice agriculture	28	12 years
Nitrous oxide	Synthetic fertilizers	265	120 years
CFC-12	Historic refrigerants	10200	100 years

7. Frequently Asked

Do I need to memorize exact global warming potential values for the exam?

You only need to compare relative values, e.g. methane is far more potent per molecule than CO₂, rather than recalling exact numerical scores.

Is climate change an examinable practical topic for IB SL?

Yes, you may be asked to analyse secondary data sets of temperature or CO₂ trends for Paper 3 section A questions.

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2024 · Paper 2
Coral bleaching and climate change impact
2022 · Paper 2
Ice core data analysis question
2021 · Paper 3
Arctic ecosystem climate impacts

What's Next

Mastering climate change gives you a foundational framework to analyse ecosystem resilience, human impacts on biogeochemical cycles, and conservation strategies for threatened species, all high-weight topics in IB Biology SL Paper 2. You will next explore how rising temperatures interact with other stressors like habitat loss to amplify biodiversity loss, and evaluate real-world mitigation policies such as carbon pricing and reforestation. This content also directly prepares you for the data analysis question in Paper 3, where you will be asked to interpret trends in long-term environmental datasets.