IB Biology HL D4.3 Climate change Notes

Human activities increase atmospheric carbon dioxide, methane, and other greenhouse gases. The main sources are fossil-fuel combustion, cement production, agriculture, deforestation, and land-use change. The exam chain is source, gas increase, enhanced greenhouse effect, and climate impact.

Human sources increase greenhouse gases, which strengthen heat retention in the atmosphere.

Positive feedback amplifies an initial climate change. Warming can reduce ice cover, lowering albedo and causing more heat absorption. It can thaw permafrost and release methane, reduce ocean CO2 storage, or increase wildfire, each adding further warming pressure.

A positive feedback loop does not just respond to warming; it amplifies it.

Boreal forests can store carbon, but warming can push them toward becoming carbon sources. Warming, drought, reduced snowfall, browning, insect outbreaks, and fire can reduce growth and increase carbon release. A tipping point risk appears when feedbacks make the forest less able to remain a carbon sink.

A strong answer shows why a sink can become a source, not just that forests are affected.

Melting landfast ice and sea ice changes breeding, feeding, and resting habitat. Emperor penguins depend on ice timing and extent for breeding success. Walruses use sea ice for resting and access to feeding areas. A strong answer connects ice change to a specific life-process failure.

A strong answer shows why a sink can become a source, not just that forests are affected.

Ocean warming can strengthen stratification, meaning warm surface water mixes less with deeper water. Reduced mixing can reduce nutrient upwelling. With fewer nutrients at the surface, phytoplankton productivity drops, reducing food supply for higher trophic levels.

Reduced upwelling links a physical climate change to lower biological productivity.

As climate zones move, species ranges can shift poleward, move upslope, or contract. Montane birds and North American tree species show that distribution limits can change when temperature and moisture conditions shift. A range shift is evidence that climate affects where a species can persist.

A range shift is evidence that climate can move the suitable habitat, not that every individual adapts instantly.

Coral reefs face two linked climate stresses. Warming disrupts the coral-zooxanthellae mutualism, causing bleaching when corals lose photosynthetic symbionts. Ocean acidification lowers carbonate availability and suppresses calcification. Together these threaten reef biodiversity and increase collapse risk.

Do not merge bleaching and acidification: they are two different routes to reef collapse.

Carbon sequestration captures and stores atmospheric carbon dioxide. Afforestation, agroforestry, forest regeneration, and peatland rewetting increase carbon stores by increasing plant biomass or protecting carbon-rich soils. The strongest answers say where carbon is stored and why the method reduces atmospheric CO2.

Phenology studies the timing of seasonal biological events. These events include flowering, budburst, migration, nesting, breeding, and insect emergence. Climate change matters because warming can shift timing, not just location or population size.

Phenology is about timing, cues and whether interacting events still line up.

Climate change can desynchronize species that depend on matched timing. If plants grow earlier but reindeer migration does not shift enough, food availability and arrival timing mismatch. If caterpillar peaks shift away from great tit chick feeding needs, breeding success can fall.

Climate disruption often means the relationship between events breaks, not that one event disappears.

Warmer temperatures can shorten insect development time. If development finishes sooner, some insects can add an extra generation per year. Spruce bark beetles can shift from one to two annual attack periods, increasing pressure on forests.

Climate change can alter selection pressures. If milder winters reduce snow cover, darker tawny owls may have higher survival than when snow cover was common. Over generations, phenotype frequencies can shift because the environment changes which variants survive or reproduce best.

Evolution from climate change means selection changes population frequencies over generations.

Core D4.3 is secure when every climate impact is explained as a chain: human greenhouse-gas sources or feedbacks change climate conditions, which alter habitats, oceans, carbon stores, or species distributions. Carbon sequestration is the mitigation chain that stores atmospheric CO2.

HL D4.3 adds timing and evolution. Phenology tracks when seasonal events happen; warming can desynchronize interacting species, shorten insect development, add generations, and change selection pressures so phenotype frequencies shift.

Core climate-change transfer answers should not list endangered examples. They should identify the climate driver, explain the physical or chemical mechanism, then state the biological consequence. Use this for greenhouse gases, feedbacks, boreal forests, ice-dependent species, upwelling, range shifts, reefs, and carbon sequestration.

Explain how climate change or a mitigation strategy affects an ecosystem or species.

HL climate questions require reasoning from timing to ecological or evolutionary consequence. The answer starts with a seasonal event or life-cycle stage, explains how warming shifts timing or selection pressure, and states the effect on interaction success, number of generations, attack periods, survival, or phenotype frequency.

Explain how climate change can alter seasonal timing, species interactions, or phenotype frequencies.