The biology of a warming planet. The data, the mechanisms, the projections, the choices.
Every sub-topic below feeds at least one of these questions.
What are the drivers of climate change?
What are the impacts of climate change on ecosystems?
The required syllabus content for D4.3, in order. Each card is one lesson-sized checkpoint.
Nature of Science: Students should be able to distinguish between positive and negative correlation and should also distinguish between correlation and causation.
Positive feedback cycles in global warming
Change from net carbon accumulation to net loss in boreal forests as an example of a tipping point
Melting of landfast ice and sea ice as examples of polar habitat change
Changes in ocean currents altering the timing and extent of nutrient upwelling
Poleward and upslope range shifts of temperate species
Threats to coral reefs as an example of potential ecosystem collapse
Afforestation, forest regeneration and restoration of peat-forming wetlands as approaches to carbon sequestration
The natural greenhouse effect raises Earth's average surface temperature by about 33 °C above what it would be without an atmosphere. Without it, oceans would freeze.
Solar radiation passes through the atmosphere and warms the surface. The warm surface emits infrared radiation back outward. Greenhouse gases — H₂O vapour, CO₂, CH₄, N₂O — absorb some of that outgoing IR and re-emit it in all directions, including back down. The result: warmer surface than the planet would otherwise have.
The effect is essential to life — but adding more greenhouse gases (burning fossil fuels) increases it, making Earth warmer than it has been for at least the past million years.
Atmospheric CO₂ has risen from ~280 ppm pre-industrial to over 420 ppm today — entirely from human activity.
Multiple independent lines of evidence converge on the same conclusion: Earth is warming, and humans are the cause.
Global mean temperature has risen ~1.2 °C since 1900. Arctic sea ice has lost ~40% summer extent since 1979. Sea level rising at ~3.4 mm/yr and accelerating. All measured directly.
Ice cores capture atmospheric composition for ~800,000 years — current CO₂ levels are higher than at any point in that record. Phenological records (timing of bird migration, flowering) shift consistently with warming.
Climate models reproduce observed warming only when human emissions are included. With natural factors alone (solar, volcanic), models predict slight cooling. The pattern of warming (lower atmosphere warming, upper atmosphere cooling) matches the greenhouse gas signature, not the solar signature.
Every ecosystem on Earth is responding to climate change. Some can adapt; many cannot keep pace.
Species ranges are shifting poleward (to higher latitudes) and upward (to higher elevations) as climate zones move. Coral reefs cannot move and are being lost where waters warm.
Plants flower earlier in warmer springs; some pollinators don't shift their emergence to match. Birds arrive at breeding grounds before their food peaks. Tight ecological partnerships break.
~30% of CO₂ from emissions dissolves in oceans. pH has dropped ~0.1 — small but biologically major. Threatens corals, molluscs, plankton that build calcium carbonate shells.
Sustained high water temperature stresses corals → they expel their photosynthetic zooxanthellae → they starve. Mass bleaching events have occurred globally since the 1980s and are intensifying.
Mitigation: reduce emissions to slow further change. Adaptation: prepare for change that's already locked in.
Switching from fossil fuels to renewables (solar, wind, hydro). Improving energy efficiency. Protecting and expanding forests (carbon sinks). Carbon capture from industrial sources. Plant-based food systems to reduce livestock emissions.
Sea defences against rising sea levels. Drought- and heat-resistant crop varieties. Improved infrastructure for extreme weather. Health systems for heat-related illness and shifting disease vectors. Conservation corridors for shifting species ranges.
Greenhouse gases persist in the atmosphere for decades to centuries. The climate change happening today reflects emissions from years ago. Every additional ton of CO₂ extends the warming further into the future. The case for fast mitigation is overwhelming — even partial success buys time for adaptation.
An extra 4 sub-topics for HL — same syllabus, deeper mechanism.
Phenology as research into the timing of biological events
Disruption to the synchrony of phenological events by climate change
Increases to the number of insect life cycles within a year due to climate change
Evolution as a consequence of climate change
If you can't define one of these in a sentence, that's where to revise next.
“What are the impacts of climate change at each level of biological organization?”
“What processes determine the distribution of organisms on Earth?”
“What is the distinction between artificial and natural processes?”