Climate change affects ecosystems through greenhouse-gas forcing, feedback cycles, habitat shifts, coral stress, phenology changes, and evolutionary responses across many environments.
Anthropogenic climate change occurs when human activities alter atmospheric composition and energy flows, increasing warming or other climate drivers.
Fossil-fuel combustion, land-use change and agriculture add greenhouse gases or reduce sinks. The gases change radiative balance, which shifts temperature and circulation.
Trace source; atmospheric change; energy effect; observed climate response.; separate driver, mechanism, response and timescale
Burning coal adds CO2 that persists in the atmosphere and increases outgoing-heat absorption, contributing to warming. This gives a concrete prediction from the stated climate condition.
One weather event cannot prove the cause of climate change; attribution uses long-term data and models. Interpret the result within the stated evidence and scenario limits.
This objective is assessed through structured response, commonly using Explain / Outline / Discuss.
Explain / Outline / Discuss / Describe / State
Build the answer around this relationship: The greenhouse effect is natural, but human activity enhances it by increasing greenhouse gas concentrations.
Confusing the greenhouse effect with ozone-layer depletion or ultraviolet radiation reaching Earth.
Representative question
Explain the impact of anthropogenic activity on climate change.
a. greenhouse effect describes that greenhouse gases trap long-wave radiation from the Earth's surface and absorbs it keeping the Earth warm
OR
increase in greenhouse gases trapped in atmosphere increases Earth's temperature;
b. burning of fossil fuels/peat/coal/natural gas by humans has released more CO2 into the atmosphere causing temperature rise on Earth;
c. raising livestock for feed has added methane/greenhouse gas to the atmosphere causing more heat to be trapped on Earth;
d. industrial processes / fertilizer production has increased the concentration of nitrous oxides/greenhouse gases in the atmosphere;
e. deforestation has increased the amount of CO2 in the atmosphere/reduced an important sink for CO2;
f. increased temperatures have caused melting of land ice leading to changes to polar habitats/decreasing species ability to survive
OR
melting of ice/snow causes reduction in albedo, so there is more insolation absorbed at Earth's surface;
g. warmer surface water prevents upwelling of nutrients/decreases ocean productivity;
h. coral reef habitat destruction due to changes in water pH / temperature;
i. warmer temperatures/decreased snowfall has led to droughts and loss of life/habitats;
j. rates of decomposition of peat/organic matter increases greenhouse gases;
k. ocean/sea level rise from melting ice/thermal expansion of oceans has impacted coastal regions/loss of land;
I. changing weather patterns due to global warming has caused loss of life and infrastructure;
m. poleward migrations/migrations to more suitable places due to climate changes at lower latitudes;
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Marking guidance:
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A positive climate reinforcing loop reinforces an initial change, making the final response larger than the direct forcing alone.
Warming can reduce ice, lower albedo and increase absorbed sunlight; it can also alter water vapour or carbon release. The sign is amplification, not value judgement.
Identify the initial change, the reinforcing loop variable and whether it pushes in the same direction.; separate driver, mechanism, response and timescale
Warming melts snow, darker ground absorbs more radiation, and additional warming follows. This gives a concrete prediction from the stated climate condition.
Positive reinforcing loop does not mean warming is inevitable at one fixed rate; strength and thresholds vary. Interpret the result within the stated evidence and scenario limits.
This objective is assessed through structured response, commonly using Identify / Explain.
Identify / Explain
Build the answer around this relationship: Positive feedback reinforces the original warming instead of opposing it.
Representative question
Explain how positive feedback cycles could increase the rate of warming of the Earth.
a. increased (named) greenhouse gases increases global warming / vice versa;
b. increased deforestation increases atmospheric CO2;
c. warmer ocean water releases carbon dioxide (because it is less soluble);
d. melting of snow/ice so less reflection/more radiation absorbed;
e. increased (peat) decomposition releases methane/ CO2;
f. melting of permafrost releases methane/ CO2;
g. increased forest fires/burnt fossil fuels release CO2;
h. increasing cellular respiration (in cellular organisms) increases CO2 production OR
increased livestock increases release of methane;
Marking guidance:
Accept greenhouse gases as CO2.
a. Do not accept increased CO2 decreases ozone layer.
a. Do not accept an increase in temperature as an increase in global warming.
b. Accept deforestation (decreases carbon sink so) more CO2 remains in the atmosphere.
b. Accept other examples of loss of algae/plants as 'deforestation'.
d. Do not accept melting icebergs increase ocean levels. Accept albedo as a measure of reflection.
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Boreal forests can shift toward a different state when warming, drought, fire and pests reduce regeneration faster than recovery.
Tree mortality lowers canopy and carbon storage; altered albedo, fuel and soil moisture can reinforce further loss. A threshold is a risk of abrupt or persistent change, not a guaranteed date.
Separate stressor, ecological response, reinforcing loop and evidence of persistence.; separate driver, mechanism, response and timescale
Repeated hot fires remove seed sources and dry the soil, so shrubs or grass replace mature conifers over a large area. This gives a concrete prediction from the stated climate condition.
A regional tipping risk is not a precise global tipping point; uncertainty and management matter. Interpret the result within the stated evidence and scenario limits.
This objective is assessed through structured response, commonly using Explain.
Explain
Build the answer around this relationship: Boreal warming can increase drought, fires, pests, disease, and tree mortality.
Representative question
An increase in global temperatures poses a critical threat to boreal forests. Explain the consequences of climate change to this northern ecosystem.
a. higher temperatures so more transpiration/droughts/dehydration/water shortage
b. more forest fires
c. more/new pests/diseases because of the changed conditions
d. competition from trees/plants «that colonize/spread to boreal forests»
e. trees/«named» organisms «of boreal forests» not adapted to warmer conditions OR
trees/«named» organisms migrate/change their distribution due to warmer conditions
f. trees die so loss of habitat for animals
g. faster decomposition/nutrient cycling «so conditions in the ecosystem change»
h. standing water/floods due to more snow/permafrost melting
2 max
Warming reduces sea ice and changes snow, permafrost and seasonal timing, altering habitat, transport and food-web conditions in polar systems.
Ice loss removes platforms and algae habitat; earlier melt or thaw changes access to prey and releases stored carbon. Species responses differ with mobility and dependence on ice.
Link physical change to habitat function before predicting population outcome.; separate driver, mechanism, response and timescale
Earlier sea-ice breakup shortens a seal’s platform season and changes when polar bears can hunt effectively. This gives a concrete prediction from the stated climate condition.
Not every polar species declines in the same way; local food supply and adaptation capacity differ. Interpret the result within the stated evidence and scenario limits.
This objective is assessed through essay response, data analysis, commonly using Outline / State / Distinguish.
Outline / State / Distinguish / Describe / Analyse / Discuss
Build the answer around this relationship: Loss of ice habitat can reduce survival and reproduction of ice-dependent species.
Assuming all polar sea ice changes have the same direction in Arctic and Antarctic data.
Representative question
Discuss the use of Adélie penguins in studying the effects of global warming.
a. global warming leads to climate / environmental change; eg temperature change / ice melting
b. stable ice associated with stable population / no climate change;
c. ice changes associated with population changes;
d. changes in penguin population size can indicate climate change / global warming;
e. example of how climate change can alter population; eg prey availability / habitat loss;
f. not all species will be affected in the same way (so care needed in applying conclusions more widely)
g. there is information on changes of population over the past 35000 years;
Climate change can alter ocean circulation by changing temperature, salinity, stratification and wind forcing, with consequences for heat and nutrient transport.
Freshwater input lowers density; warming increases stratification; together they can weaken or redirect mixing. Biological effects follow altered oxygen, nutrients and temperature.
Distinguish the physical driver, circulation change and ecological consequence.; separate driver, mechanism, response and timescale
Freshwater from ice melt lowers surface salinity, increasing stratification and reducing nutrient mixing into the sunlit layer. This gives a concrete prediction from the stated climate condition.
A circulation change is not inferred from one seasonal observation; attribution requires sustained measurements and models. Interpret the result within the stated evidence and scenario limits.
This objective is assessed through multiple choice.
Build the answer around this relationship: Warmer surface water can strengthen stratification and reduce vertical mixing.
Representative question
What is a consequence of ocean water having a very high temperature?
Decreased bleaching of coral reefs
Increased production of oxygen
Increase in energy flow through food chains
Reduced nutrient upwelling to the surface
D
Range shifts occur when changing temperature, moisture or seasonality makes former areas less suitable and new areas more suitable, subject to dispersal and barriers.
A species can move poleward, uphill or into deeper water, but fragmented habitat, competition and slow reproduction limit the response.
Compare climatic suitability with the species’ dispersal ability and life history.; separate driver, mechanism, response and timescale
A butterfly’s suitable temperature band moves uphill; populations expand upslope only where habitat corridors exist. This gives a concrete prediction from the stated climate condition.
A range edge is not a direct proof of climate causation; land use and monitoring effort can also change records. Interpret the result within the stated evidence and scenario limits.
This objective is assessed through multiple choice, structured response, commonly using Explain / Compare / Discuss.
Explain / Compare / Discuss / Suggest
Build the answer around this relationship: Warming can move suitable climate zones and food supplies northward or upslope.
Representative question
The data shows how the hardiness zones in part of North America are predicted to change over the next 25 years. A hardiness zone is an area that has a certain average annual minimum temperature, a factor relevant to the survival of many plants. The lower the number, the more cold
resistant the plants must be.
What is a likely consequence of this change for tree species?
Tree species will spread northwards as climate changes.
Tree species that are not cold-resistant will decline.
There will be no change in the distribution of the tree species.
Tree species that currently live in the north will outcompete other tree species.
A
Coral reefs are threatened when warming causes bleaching and altered seawater chemistry reduces calcification, while pollution and overfishing weaken recovery.
Heat disrupts coral–algal symbiosis; acidification lowers carbonate ion availability; local stressors reduce resilience and recruitment.
Separate direct heat stress, chemistry effects and local pressures before evaluating a reef outcome.; separate driver, mechanism, response and timescale
A marine heatwave expels symbiotic algae, bleaching coral; repeated heat before recovery raises mortality risk. This gives a concrete prediction from the stated climate condition.
Bleaching is a stress response, not immediate death; outcome depends on duration, species and recovery conditions. Interpret the result within the stated evidence and scenario limits.
This objective is assessed through structured response, commonly using Describe / Suggest / Outline.
Describe / Suggest / Outline / Discuss / Deduce / Explain
Build the answer around this relationship: Coral bleaching occurs when heat stress disrupts the coral-zooxanthellae symbiosis.
Treating bleaching as colour loss only, without explaining zooxanthellae expulsion and reduced nutrient supply.
Representative question
Outline the reasons that climate change is a threat to coral reefs.
a. increased carbon dioxide/greenhouse gases increase greenhouse effect;
b. increase in temperature in the oceans with global warming;
c. coral bleaching/loss of symbiotic Zooxanthellae/algae (due to temperature increase);
d. reduced supply of nutrients from photosynthesis (for the coral);
e. bleaching reduces coral growth/can kill corals;
f. increased carbon dioxide dissolves in the oceans;
g. lowers pH / increases acidity of the ocean / reacts with water to form carbonic acid;
h. coral skeletons dissolve/corals cannot make skeletons/corals cannot absorb carbonate;
5 max
Carbon sequestration lowers atmospheric CO2 when carbon is transferred into a store and remains there for a meaningful period.
Forests, soils, wetlands, oceans or engineered stores take up carbon, but fire, decay, disturbance, leakage and saturation can return it. Net effect depends on additionality and permanence.
Check source, sink, duration, leakage and what would happen without the project.; separate driver, mechanism, response and timescale
Restoring a wetland can bury carbon in waterlogged soils, but drainage later can reverse the storage and release CO2. This gives a concrete prediction from the stated climate condition.
Sequestration complements emissions cuts; temporary uptake does not cancel ongoing fossil emissions one-for-one. Interpret the result within the stated evidence and scenario limits.
This objective is assessed through multiple choice, structured response, commonly using Explain.
Explain
Build the answer around this relationship: Photosynthesis removes carbon dioxide from the atmosphere and stores carbon in organic matter.
Representative question
Which action will decrease carbon sequestration?
Afforestation
Primary production
Deforestation
Rewetting peatlands
C
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.
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.
Phenology is the timing of recurring biological events such as flowering, migration, breeding or leaf-out.
Temperature, day length and resource cues control schedules. Climate warming can advance or delay events, changing overlap among interacting species.
Record the event, cue, date shift and interacting species before inferring ecological impact.; separate driver, mechanism, response and timescale
Warmer springs advance caterpillar emergence; a bird that migrates on a fixed cue may miss the food peak. This gives a concrete prediction from the stated climate condition.
A date shift alone is not a population decline; consequences depend on synchrony and alternative food. Interpret the result within the stated evidence and scenario limits.
This objective is assessed through multiple choice, data analysis, commonly using Identify / Analyse.
Identify / Analyse
Build the answer around this relationship: Phenology is the study of seasonal timing in biological events.
Representative question
Which of the following could be a subject of phenological research?
Timing of budburst in a tree species each year
Changes in allele frequencies in a population over time
The relationship between changes in ocean acidification and carbon dioxide concentration
The emergence of a new species of migratory bird by divergent evolution
A
Phenological mismatch occurs when interacting species shift seasonal timing by different amounts, reducing overlap in a resource or life-cycle window.
If plants flower earlier but pollinators do not, or prey peak before predators arrive, energy transfer and reproduction can fall. The mechanism is timing, not simply warming.
Compare both event dates and the overlap needed for the interaction.; separate driver, mechanism, response and timescale
Earlier plankton blooms can occur before a fish larva hatches, leaving less food during its critical growth window. This gives a concrete prediction from the stated climate condition.
Mismatch can be partial or temporary; species may adjust behaviour, migrate or adapt. Interpret the result within the stated evidence and scenario limits.
Many insects develop faster in warmer conditions within their tolerance range, potentially increasing the number of life cycles per year.
Development rate, survival and winter conditions determine voltinism. More generations can raise herbivory or disease transmission, but heat beyond tolerance can reduce success.
Check temperature range, development threshold, survival and host availability before predicting generation count.; separate driver, mechanism, response and timescale
A pest that formerly produced two generations may produce three in a warmer season, increasing crop exposure. This gives a concrete prediction from the stated climate condition.
Warming does not always help insects; drought, extreme heat and host decline can reverse the effect. Interpret the result within the stated evidence and scenario limits.
Climate change can alter allele frequencies when new conditions change survival or reproduction and the relevant variation is heritable.
Selection pressure changes; individuals with suitable traits leave more offspring; frequencies shift over generations. Plasticity and migration can produce short-term change without evolution.
Separate immediate acclimation, range movement and inherited frequency change.; separate driver, mechanism, response and timescale
If heat-tolerant coral genotypes consistently survive warm years and reproduce, heat-tolerance alleles may become more common. This gives a concrete prediction from the stated climate condition.
Observed trait change over one generation does not prove evolution; genetic evidence and repeated selection are needed. Interpret the result within the stated evidence and scenario limits.
This objective is assessed through structured response, commonly using Predict / Suggest.
Predict / Suggest
Build the answer around this relationship: Climate change can create new selection pressures on survival and reproduction.
Representative question
Suggest how climate change can influence the natural selection of organisms that live in the Arctic oceans.
a. individuals in a population will show a variation of adaptations to climate change
b. organisms that resist temperature changes
OR current changes of the ocean/melting ice/more acidity/changes in food chains will survive better
c. reproduce more and pass on their characteristics
d. organisms with less adaptation will disappear with time
e. example «eg polar bears have less ice to be able to catch prey/seals and are starving the ones that manage to find other food sources will survive» OWTTE
f. changes will occur within species OR
new species may appear «over time»
Marking guidance:
Accept any valid example of an Arctic ocean organism.
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.
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.