# Climate change

> Biology SL · IB Diploma Programme Biology SL
> Source: https://www.owlsprep.com/study/ib-biology-sl-u4-climate-change/

This module explains the greenhouse effect mechanism, human drivers of enhanced warming, empirical evidence for rising temperatures, and biological impacts on marine and terrestrial ecosystems.

**Prerequisites:** [Carbon cycling and energy flow in ecosystems](https://www.owlsprep.com/study/ib-biology-sl-u4-carbon-cycling/)

## Learning objectives

- Identify key greenhouse gases and their natural and human-caused sources
- Distinguish between the natural greenhouse effect and anthropogenic enhanced climate change
- Interpret empirical evidence for climate change including ice core and atmospheric CO₂ datasets
- Analyse documented biological impacts of climate change on coral reefs, arctic ecosystems and food security

## The Natural Greenhouse Effect Mechanism

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.

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

*Notation:* GHG

*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.

> **tip**
>
> Never state the greenhouse effect is 'bad' in exam answers: the natural version is essential for life, only the enhanced anthropogenic version is problematic.

## Anthropogenic Drivers of Enhanced Warming

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.

**Check your understanding**

1. Which of these is a purely anthropogenic greenhouse gas?

   - Water vapour
   - CFC
   - Carbon dioxide
   - Methane

   *Why:* CFCs do not occur naturally on Earth, all atmospheric concentrations are from human industrial production.

## Empirical Evidence for Climate Change

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.

**Exam command terms**

- **Evaluate climate change evidence** — You must include at least one limitation of proxy data, e.g. ice core temperature lags CO₂ during deglaciation events

## Biological Impacts of Climate Change

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.

## Common pitfalls

- **Wrong:** Stating the greenhouse effect is a harmful, human-caused problem
  - Why it fails: This confuses the natural and enhanced versions of the effect, which is a very common mark deduction in IB exams
  - Correct: Explicitly distinguish the two, noting the natural effect is essential for life while the enhanced effect is anthropogenic and damaging
- **Wrong:** Claiming water vapour is the main driver of modern climate change
  - Why it fails: Water vapour concentration is a feedback effect, not a forcing: its levels are controlled by global temperature, not direct human emissions
  - Correct: Name CO₂ as the primary anthropogenic forcing driving modern climate change
- **Wrong:** Stating ice core data proves CO₂ rises cause temperature rises
  - Why it fails: During deglaciation events, initial temperature rise releases CO₂ from oceans, which then amplifies warming, creating a positive feedback
  - Correct: Note that ice core data shows correlation, and modern measured CO₂ rises precede modern temperature rises to confirm causation
- **Wrong:** Confusing ozone layer depletion with climate change
  - Why it fails: These are separate environmental issues with different causes and impacts, even though CFCs contribute to both
  - Correct: Never mix the two topics in exam answers, keep their mechanisms distinct
- **Wrong:** Claiming all species will adapt to rising temperatures over time
  - Why it fails: Current warming rate is ~10x faster than post-glacial natural warming rates, so most species cannot evolve or shift range fast enough
  - Correct: Note that rapid warming is the key factor that makes modern climate change a major extinction threat

## 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 |

## 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.

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