Climate change—causes and impacts

Direct evidence comes from modern instrument measurements (e.g., thermometers, satellites). Proxy evidence comes from indirect natural records (e.g., ice cores, tree rings) that reconstruct past climate.

Examples include ice cores, tree rings, coral bands, pollen in sediments, and ocean/lake sediments.

Global average temperature has increased over the long term, with the warmest years concentrated in the most recent decade.

It means the rate of sea level rise is increasing over time (the slope becomes steeper), not just that sea level is rising.

Proxy data is indirect evidence of past climate preserved in natural archives such as ice cores, tree rings, corals, and sediments.

Multiple independent datasets reduce uncertainty and make the conclusion more robust (e.g., temperature records, CO2, sea level, ice extent all point to warming).

Describe = state what the data shows using numbers and trends. Explain = give reasons/mechanisms for the pattern shown.

Examples include: (1) long-term temperature records from weather stations, (2) measured atmospheric CO2 concentrations (e.g., observatory records), (3) sea-level measurements from tide gauges/satellites, (4) satellite observations of ice extent.

Examples include atmospheric CO2 concentration, global mean temperature, sea level, Arctic sea ice extent, and glacier mass/length.

Include: (1) overall trend (increase/decrease), (2) specific data values with units and time period, (3) any change in rate or notable anomalies.

Direct measurements are taken with calibrated instruments and have higher precision and less interpretation. Proxy data extends further back in time but requires inference (e.g., linking ring width to climate).

Proxy data extends climate records back beyond the instrumental period (before modern measurements), allowing reconstruction over thousands to hundreds of thousands of years.

A correlation means CO2 and temperature tend to change together over time (both rise/fall in related patterns). It does not, by itself, prove causation.

Ice cores trap ancient air bubbles and preserve isotopic signals. Air bubbles show past greenhouse gas concentrations, and isotopes help infer past temperatures, allowing comparison of CO2 and temperature over long time periods.

Proxy data requires interpretation (calibration) because the climate signal is inferred from biological/chemical indicators, which can be influenced by multiple factors.

Global warming potential (GWP) is a measure of how much heat a greenhouse gas traps compared with CO2 over a specified time period.

Anthropogenic means caused by human activities (e.g., burning fossil fuels, deforestation, agriculture).

Examples include Milankovitch cycles, volcanic eruptions, solar output variations, and changes in ocean circulation (El Niño/La Niña).

CO2 has the largest overall impact because it is emitted in far greater quantities and persists for a long time, so its cumulative effect is very large.

Large eruptions release aerosols/ash that reflect incoming solar radiation, reducing the energy reaching Earth’s surface for months to a few years.

CO2: fossil fuel combustion/deforestation. CH4: livestock/rice paddies/landfills. N2O: fertiliser use/combustion/industry.

Fossil fuel combustion → CO2. Livestock/rice paddies/landfills → CH4. Fertiliser use → N2O. Refrigerants → fluorinated gases.

Deforestation removes a carbon sink (less CO2 absorbed by photosynthesis) and often releases stored carbon as CO2 when biomass is burned or decomposes.

A carbon source releases CO2 (e.g., fossil fuel combustion). A carbon sink absorbs CO2 (e.g., forests via photosynthesis or oceans dissolving CO2).

Natural factors (solar output, volcanic activity) do not show changes large enough to match observed warming, while greenhouse gas concentrations from human activity rise sharply and align with temperature increases.

A range shift is when a species’ geographic distribution moves (often poleward or to higher altitude) to track suitable temperatures and conditions as climate warms.

Examples include rising global temperatures, melting glaciers/ice sheets, sea level rise, permafrost thaw, and increased frequency/intensity of extreme weather events.

Examples include sea level rise, melting glaciers/ice sheets, more extreme weather, species range shifts, ocean acidification, and coral bleaching.

Thermal expansion of seawater, and melting of land-based ice (glaciers/ice sheets).

Phenology is the timing of seasonal biological events. Example: earlier flowering, earlier insect emergence, or earlier bird migration due to warmer springs.

Thermal expansion of seawater as it warms, and melting of land-based ice (glaciers and ice sheets).

Use cause → effect chains. State the climate driver (warming, drought, sea level, acidification), then the biological/physical change, then the consequence for populations and biodiversity.

Initially, increased melting adds extra runoff. Over time, glacier volume shrinks so there is less ice left to melt, reducing dry-season flow.

Thermal expansion is the increase in volume of seawater as it warms, which raises sea level even without adding extra water.

CO2 dissolves forming carbonic acid, lowering pH and reducing carbonate ions needed to form calcium carbonate. Shell formation becomes harder and shells can be thinner.

Sustained high sea temperatures stress corals, causing them to expel symbiotic algae (zooxanthellae). Corals lose colour and a major energy source, increasing mortality risk.

Ocean warming is a temperature increase that stresses organisms (e.g., coral bleaching). Ocean acidification is a pH decrease from dissolved CO2 that reduces carbonate availability for shells/skeletons.

Thaw allows decomposition of previously frozen organic matter, releasing CH4 and CO2, increasing the greenhouse effect and causing more warming and further thaw.

Example: polar bears. Reduced sea ice decreases access to hunting platforms for seals, reducing feeding success and affecting reproduction/survival.

Ocean acidification is the decrease in ocean pH as CO2 dissolves forming carbonic acid. It reduces carbonate ions, making it harder for organisms to build calcium carbonate shells/skeletons.

No. Sea ice is already floating, so when it melts it largely displaces the same volume of water. Melting land ice raises sea level because it adds water to the ocean.

Not correct. Melting sea ice does not significantly raise sea level because it already floats. Sea level rises mainly from thermal expansion and melting land-based ice.

Warming shifts plankton bloom timing (cause) → mismatch with fish larvae feeding period (effect) → lower fish survival → fewer prey for seabirds/marine mammals.

Rising sea level increases coastal flooding and erosion, pushes saltwater into wetlands and aquifers, and reduces habitat area for coastal ecosystems (e.g., mangroves and salt marshes).

Thawing permafrost allows organic matter to decompose, releasing CO2 and methane (CH4). These greenhouse gases increase warming, causing more thaw.

Food security is when all people have reliable access to sufficient, safe, and nutritious food.

Food security, water security, human health, infrastructure damage, economic costs, and displacement are major impact areas (any four).

Examples include heatstroke, dehydration, and increased cardiovascular stress during heatwaves.

Warmer temperatures and changed rainfall can expand the range and season of vectors (e.g., mosquitoes), increasing diseases such as malaria or dengue in new areas.

Water security is reliable access to adequate quantities of acceptable quality water for health, livelihoods, ecosystems, and production.

Organise by sectors (food, water, health, infrastructure, economy). For each: describe impact, explain mechanism, add an example, then include equity/climate justice.

Examples include coastal flooding damaging roads/ports, stronger storms destroying buildings, and permafrost thaw destabilising foundations and pipelines.

Examples include more frequent drought/heatwaves causing water stress, increased flooding/storm damage, and expansion of pests/diseases into new areas.

Malnutrition from reduced crop yields, mental health stress after disasters, or increased disease spread are indirect health impacts.

Saltwater intrusion is seawater moving into coastal aquifers. Sea level rise increases pressure and allows seawater to push further inland, contaminating freshwater.

They often have greater exposure (e.g., agriculture dependence), fewer resources for adaptation, weaker infrastructure, and limited healthcare and insurance coverage.

People forced to move because climate impacts (e.g., sea level rise, drought, extreme storms) make their home unsafe or livelihoods impossible.

Those who contributed least to greenhouse gas emissions (often LEDCs and small island states) tend to face the greatest impacts and have fewer resources to adapt.

Warmer water can increase algal blooms; decomposition/respiration can reduce dissolved oxygen, increasing hypoxia risk.

Heat reduces labour productivity and increases cooling costs; disasters damage assets; insurance costs rise and supply chains are disrupted.