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This lesson covers the enhanced greenhouse effect, evidence for climate change, effects on biodiversity, and mitigation strategies, as required by the Edexcel A-Level Biology specification (9BI0), Topic 10 -- Ecosystems.
The natural greenhouse effect is essential for life on Earth:
Key Point: The greenhouse effect itself is not harmful -- it is essential for life. The problem is the enhanced greenhouse effect caused by increased concentrations of greenhouse gases from human activities.
Human activities have increased the concentration of greenhouse gases in the atmosphere, intensifying the natural greenhouse effect and causing global warming -- a sustained increase in the Earth's average surface temperature.
| Greenhouse Gas | Pre-industrial Level | Current Level | Global Warming Potential (GWP, 100yr) | Main Human Source |
|---|---|---|---|---|
| Carbon dioxide (CO2) | ~280 ppm | ~420 ppm | 1 (reference) | Burning fossil fuels, deforestation, cement production |
| Methane (CH4) | ~700 ppb | ~1,900 ppb | 28 | Rice paddies, livestock, landfill, natural gas |
| Nitrous oxide (N2O) | ~270 ppb | ~335 ppb | 265 | Fertiliser use, combustion, industrial processes |
| CFCs / HFCs | 0 (synthetic) | Various (ppt) | 1,000--23,000 | Refrigerants, aerosols, industrial processes |
GWP (Global Warming Potential) measures how effective each gas is at trapping heat relative to CO2 over 100 years. Methane is 28 times more effective per molecule than CO2, but CO2 is present in much larger quantities, making it the dominant contributor overall (~75% of warming).
| Type of Evidence | Description | Strength of Evidence |
|---|---|---|
| Ice core data | Air bubbles trapped in Antarctic/Greenland ice sheets contain ancient atmosphere; CO2 and temperature data for the last 800,000+ years show a strong correlation between CO2 and temperature | Very strong: 800,000-year record; Vostok and EPICA cores |
| Temperature records | Direct measurements since ~1850 show a clear warming trend, especially in the last 50 years | Strong: multiple independent datasets agree |
| Sea level rise | Thermal expansion of water and melting of ice sheets/glaciers; ~20 cm rise since 1900 | Measurable with satellite altimetry |
| Glacier retreat | Photographic and satellite evidence shows widespread glacier shrinkage globally | Dramatic visual evidence; >90% of glaciers retreating |
| Arctic sea ice decline | Satellite data shows a significant decrease in summer Arctic sea ice extent since 1979 | ~40% decrease in minimum extent since 1979 |
| Phenological changes | Earlier spring flowering, earlier bird migration, later autumn leaf fall | UK data: oak leafing 2--3 weeks earlier than 1950 |
| Ocean acidification | CO2 dissolves in seawater, forming carbonic acid; ocean pH has decreased by ~0.1 since pre-industrial times | pH 8.2 to 8.1 = 26% increase in H+ concentration |
| Extreme weather events | Increased frequency and intensity of heatwaves, droughts, and heavy rainfall events | Statistical analysis of weather records |
Exam Tip: When discussing evidence for climate change, distinguish between evidence for temperature change (ice cores, temperature records) and evidence for the consequences of temperature change (sea level rise, glacier retreat, species range shifts). Also note the difference between correlation (CO2 and temperature rise together) and causation (the mechanism by which greenhouse gases trap heat).
Phenology is the timing of seasonal biological events:
This phenological mismatch between trophic levels can reduce reproductive success and threaten populations.
flowchart LR
A["Increased\nCO2 emissions"] --> B["Enhanced\ngreenhouse effect"]
B --> C["Global\nwarming"]
C --> D["Permafrost\nthaws"]
D --> E["Methane (CH4)\nreleased"]
E --> B
C --> F["Ice melts"]
F --> G["Reduced albedo\n(less sunlight\nreflected)"]
G --> C
This diagram shows two positive feedback loops: the permafrost-methane loop and the ice-albedo loop. Positive feedback amplifies the original change, making climate change harder to reverse.
| Strategy | Mechanism | Scale of Impact |
|---|---|---|
| Reducing fossil fuel use | Transitioning to renewable energy (solar, wind, hydro, nuclear) reduces CO2 emissions | Very high: fossil fuels are the largest source |
| Carbon capture and storage (CCS) | Capturing CO2 from power stations and storing it underground in geological formations | Moderate: technology still developing at scale |
| Reforestation / afforestation | Planting trees to absorb CO2 through photosynthesis; increasing carbon sinks | Moderate: significant but cannot offset all emissions |
| Reducing deforestation | Maintaining existing forests as carbon sinks; programmes like REDD+ | High: deforestation accounts for ~10% of emissions |
| Improving energy efficiency | Better insulation, more efficient engines and appliances reduce energy demand | Moderate: cost-effective but incremental |
| International agreements | Paris Agreement (2015): aim to limit warming to 1.5 degrees C above pre-industrial levels | Variable: depends on compliance |
| Dietary changes | Reducing meat consumption (especially beef) to decrease methane from livestock | Moderate: agriculture accounts for ~14% of emissions |
| Carbon offsetting | Investing in projects that reduce emissions elsewhere to compensate for your own | Controversial: risk of "greenwashing" |
| Peatland restoration | Rewetting drained peatlands to prevent CO2 release and restore carbon sink function | Moderate: UK peatlands store ~3.2 billion tonnes C |
Question: Atmospheric CO2 concentration was 315 ppm in 1958 and 420 ppm in 2023. Calculate the percentage increase.
Answer:
Percentage increase=315420−315×100=315105×100=33.3%
Question: Data shows that in a UK woodland, the average date of first oak leaf emergence has advanced from April 25th in 1950 to April 8th in 2020. The average date of peak caterpillar abundance has advanced from May 15th to May 5th. The average date of peak great tit chick demand has advanced from May 20th to May 17th. Explain the ecological significance.
Answer:
All three events have advanced due to warmer spring temperatures, but they have not advanced at the same rate. The gap between peak caterpillar abundance and peak chick demand has changed:
The phenological mismatch has increased. Great tit chicks now hatch further past the peak caterpillar abundance, meaning less food is available during the critical growth period. This could lead to reduced chick survival, lower reproductive success, and a potential decline in the great tit population over time. This is an example of how climate change can disrupt trophic interactions even without directly killing organisms.
"The greenhouse effect is bad." The natural greenhouse effect is essential for life -- without it, Earth would be -18 degrees C. The problem is the enhanced greenhouse effect from human-generated greenhouse gases.
"CO2 is the only greenhouse gas." Methane, nitrous oxide, and fluorinated gases are also important. Methane is 28 times more effective per molecule at trapping heat than CO2.
"Climate change only affects polar regions." Every biome on Earth is affected, from tropical coral reefs (bleaching) to temperate woodlands (phenological mismatches) to deserts (expansion).
Climate change is the integrative end-point of Topic 5 — the lesson where the photosynthetic light reactions of lesson on photosynthesis, the respiratory CO₂ flux of every aerobic cell, the carbon and nitrogen biogeochemical cycles of lessons 3–4, the succession of lesson 5, the density-dependent regulators of lesson 6 and the field-methodology of lesson 7 collide with twenty-first-century human industrial output. The Edexcel 9BI0 treatment requires candidates to reason at three linked scales: (i) the molecular scale of greenhouse-gas absorption — why H₂O, CO₂, CH₄ and N₂O absorb long-wave infrared while N₂ and O₂ do not; (ii) the planetary scale of radiative balance — incoming short-wave solar versus outgoing long-wave infrared, and how altering the absorbing-gas inventory perturbs that balance; and (iii) the ecological scale of consequence — phenological mismatch, range shifts, ocean acidification, succession derailment and feedback loops that amplify or damp the original perturbation. This deep dive works a paper-format radiative-balance question, lays out the synoptic web that ties climate change to every prior lesson, and trains the mark-scheme literacy required for the long-answer "evaluate the mitigation strategy" questions that recur in Section B.
The Edexcel 9BI0 specification embeds climate change in Topic 5: On the Wild Side — Photosynthesis, Energy and Ecosystems, on Paper 2 (Energy, Exercise and Coordination). Specification statements concern: the greenhouse effect as a natural radiative phenomenon and the enhanced greenhouse effect as its anthropogenic intensification; the principal greenhouse gases — water vapour, carbon dioxide, methane, nitrous oxide and the synthetic halocarbons — and their relative global warming potentials; evidence for past and contemporary climate change drawn from ice-core records, instrumental temperature series, sea-level data, glacier and Arctic-sea-ice records and phenological monitoring; biological consequences including range shifts, phenological mismatch, coral bleaching, ocean acidification and habitat loss; feedback loops — positive (permafrost methane, ice-albedo) and negative (CO₂ fertilisation, ocean uptake) — and the mitigation strategies of fossil-fuel substitution, reforestation, biofuels, carbon capture, and dietary shift toward lower trophic levels; refer to the official Pearson Edexcel 9BI0 specification document for exact wording. Synoptic links radiate to lesson 1 — Ecosystems and Communities (the niche framework that range shifts disturb), lesson 2 — Energy Transfer in Ecosystems (lower-trophic eating as a mitigation lever), lesson 3 — The Carbon Cycle (the perturbed compartment), lesson 4 — The Nitrogen Cycle (fertiliser-derived N₂O), lesson 5 — Ecological Succession (climax communities shifting under changing climate), lesson 6 — Population Dynamics (extinction risk under environmental change) and lesson 7 — Investigating Ecosystems (long-term monitoring underpins all evidence). The earlier Topic 5 lesson on photosynthesis (light and dark reactions) is the molecular partner: photosynthesis is both the pre-industrial CO₂ sink that buffered the carbon cycle for millennia and the process candidates must reason about when evaluating reforestation as a mitigation lever.
Question (8 marks):
The atmosphere transmits the bulk of incoming short-wave solar radiation but absorbs a large fraction of the long-wave infrared radiation re-emitted by the Earth's surface.
(a) Explain, at the molecular level, why carbon dioxide and methane absorb infrared radiation while diatomic nitrogen (N₂) and oxygen (O₂) — the dominant atmospheric gases — do not. (3)
(b) The pre-industrial atmospheric CO₂ concentration was approximately 280 ppm; current concentrations exceed 420 ppm. Calculate the percentage rise from the pre-industrial baseline and explain why a percentage rise of this magnitude in a trace gas can alter Earth's surface temperature. (3)
(c) State and justify the direction of the surface-temperature response to a permafrost-methane positive feedback loop. (2)
Solution with mark scheme:
(a) M1 (AO1.1) — infrared absorption requires a change in the molecular dipole moment during a vibrational mode. M1 (AO1.2) — CO₂ is linear and symmetric at rest but its asymmetric stretching and bending modes produce a transient dipole that couples to infrared photons; CH₄ has analogous asymmetric stretching and bending modes; H₂O is permanently polar. A1 (AO2.1) — N₂ and O₂ are homonuclear diatomic molecules: their only vibration is a symmetric stretch that does not change the dipole moment, so they are essentially transparent to infrared and contribute negligibly to the greenhouse effect even though they constitute ~99% of the atmosphere by volume.
(b) M1 (AO2.1) — percentage rise = (420 − 280) / 280 × 100 = 50% (accept "approximately 50%"). M1 (AO1.2) — although CO₂ is a trace constituent (parts per million), it absorbs in spectral windows where Earth's outgoing long-wave emission is strong; absorption in these windows controls the rate at which the surface loses heat to space. A1 (AO3.1a) — increasing the column abundance of CO₂ raises the effective emission altitude (where the atmosphere becomes optically thin) into colder air, reducing outgoing infrared at the top of the atmosphere; the surface must warm to restore radiative balance, so a fractional change in trace-gas concentration produces a measurable surface-temperature response.
(c) M1 (AO2.1) — warming thaws permafrost, releasing methane (and CO₂) sequestered in frozen organic matter. A1 (AO3.2a) — added methane absorbs further infrared, reinforcing warming; the loop is positive (self-amplifying), and the direction of the response is to push surface temperature higher than the original anthropogenic forcing alone would predict, with no automatic equilibrium until the permafrost reservoir is exhausted or another negative feedback dominates.
Total: 8 marks.
Question (6 marks): A government advisory committee is evaluating reforestation as a national-scale climate-change mitigation strategy. Discuss the biological mechanisms by which reforestation would reduce atmospheric CO₂, evaluate two limitations of relying on this strategy alone, and identify one synoptic link to another Topic 5 process.
Mark scheme decomposition by AO:
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