D4.3 Climate change
The Earth’s climate has always changed, but the warming now under way is happening far faster than past natural shifts, and it is driven by human activity. At the heart of D4.3 is a piece of straightforward physics: certain gases in the atmosphere trap heat, and by adding more of them we are turning up that trapping. The consequences ripple through every level of biology — from the timing of when a plant flowers to the survival of entire species — which is why climate change is the final, integrating topic of the course. Your task is to understand the mechanism precisely and to reason from it to biological effects.
The greenhouse effect
The greenhouse effect is a natural and essential process; without it the planet would be far too cold for life. It works in stages: short-wavelength solar radiation passes through the atmosphere and is absorbed by the Earth’s surface, warming it. The warmed surface then re-emits energy as longer-wavelength infrared (heat) radiation. Greenhouse gases in the atmosphere absorb this outgoing infrared radiation and re-radiate some of it back towards the surface, keeping the lower atmosphere warm.
The crucial distinction is between the natural greenhouse effect and the enhanced greenhouse effect. By raising the concentration of greenhouse gases, human activity increases the amount of infrared radiation absorbed, so more heat is retained and global temperatures rise. The mechanism itself is unchanged; we have simply intensified it.
Greenhouse gases and their sources
The two greenhouse gases emphasised by the syllabus are:
- Carbon dioxide (CO2), released mainly by the combustion of fossil fuels and by deforestation. It is the largest single contributor to the enhanced effect because of the huge quantities emitted.
- Methane (CH4), released from livestock digestion, rice paddies, landfill and melting permafrost. Per molecule it traps far more heat than carbon dioxide, though it is present in much smaller amounts.
The impact of a gas depends on both its concentration in the atmosphere and its ability to absorb infrared radiation. Water vapour is the most abundant greenhouse gas, but its concentration is controlled by temperature rather than directly by human emissions, so attention focuses on carbon dioxide and methane, whose levels we control. The famous Keeling curve, the long-term record of rising atmospheric carbon dioxide measured at Mauna Loa, provides clear evidence of the increase.
Biological consequences
Rising temperatures and shifting climate patterns affect organisms in many ways. The syllabus highlights several:
- Changes in distribution. Species shift their ranges towards the poles or to higher altitudes as conditions warm; those unable to move or with nowhere to go face local extinction.
- Changes in the timing of life cycles (phenology). Events such as flowering, breeding and migration shift earlier, which can break the synchrony between species — for example, a bird hatching after the insects it feeds its chicks have already peaked.
- Loss of ice habitat. Melting sea ice threatens species such as polar bears that depend on it for hunting.
- Ocean effects. Warming and the absorption of carbon dioxide (causing ocean acidification) damage coral reefs through bleaching and impair shell-forming organisms.
Because species interact, these effects are not isolated: a change to one species ripples through food webs, so the overall result is a loss of biodiversity and reduced ecosystem stability.
Responses: mitigation and the carbon cycle
Limiting climate change means reducing the net release of greenhouse gases. Strategies (mitigation) include cutting fossil-fuel use by switching to renewable energy, improving energy efficiency, halting deforestation and reforestation, since growing trees remove carbon dioxide by photosynthesis and act as a carbon sink.
Understanding the response requires the carbon cycle: photosynthesis removes carbon dioxide from the atmosphere while respiration, decomposition and combustion return it. For most of history these flows were roughly balanced. Burning fossil fuels releases carbon that had been locked away for millions of years, adding it to the atmosphere faster than sinks can absorb it, which is why concentrations are rising. Because the climate system responds slowly and some changes are effectively irreversible, the precautionary principle argues for acting now rather than waiting for complete certainty.
Key terms
- Greenhouse effect
- The warming of the lower atmosphere caused by gases absorbing and re-radiating infrared radiation emitted by the Earth’s surface.
- Enhanced greenhouse effect
- The intensified warming caused by human activities raising the concentration of greenhouse gases.
- Greenhouse gas
- A gas, such as carbon dioxide or methane, that absorbs outgoing infrared radiation and contributes to warming.
- Infrared radiation
- Long-wavelength (heat) radiation re-emitted by the warmed Earth’s surface and absorbed by greenhouse gases.
- Carbon sink
- A reservoir, such as a forest or the ocean, that absorbs more carbon dioxide than it releases.
- Ocean acidification
- The lowering of ocean pH as it absorbs carbon dioxide, harming corals and shell-forming organisms.
- Phenology
- The timing of seasonal biological events such as flowering, breeding and migration, increasingly shifted by warming.
- Carbon cycle
- The movement of carbon between the atmosphere, organisms, oceans and rocks through processes such as photosynthesis, respiration and combustion.
- Mitigation
- Action taken to reduce the causes of climate change, such as cutting emissions and reforestation.
Exam technique
- Describe the mechanism in order: solar radiation absorbed → surface re-emits infrared → greenhouse gases absorb and re-radiate it — do not skip the change in wavelength.
- Distinguish the natural greenhouse effect (essential for life) from the enhanced effect (caused by human emissions); confusing them loses marks.
- Name specific gases and their sources — CO2 from combustion and deforestation, CH4 from livestock and landfill — not just pollution.
- When asked about effects on organisms, reason from the mechanism to a named consequence such as range shifts or disrupted phenology.
- Use the carbon cycle to explain why burning fossil fuels raises CO2: it adds long-stored carbon faster than sinks can remove it.
- Greenhouse gases emit solar radiation that the surface absorbs as heat
- Solar radiation is absorbed by the surface, which re-emits infrared radiation that greenhouse gases absorb and re-radiate
- Greenhouse gases block incoming solar radiation, trapping it above the atmosphere
- Infrared radiation from the Sun is absorbed directly by carbon dioxide
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