Impacts, scale and risk of greenhouse gases
Chemistry of the atmosphere • Carbon dioxide and methane
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Key concepts
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Definition and primary drivers
Global climate change refers to long-term changes in average temperature, precipitation and weather patterns caused by altered energy balance in the Earth system. Increased atmospheric concentrations of CO2 and CH4 raise the greenhouse effect because both gases absorb outgoing infrared radiation. Human activities, especially burning fossil fuels, agriculture and land-use change, increase emissions of these gases. Natural factors such as volcanic eruptions and solar variability produce shorter-term variations but do not explain the current sustained warming trend. Climate forcing measures the change in energy balance (watts per square metre) produced by a driver such as increased CO2. Climate sensitivity quantifies the expected temperature change for a given forcing. Both parameters limit precision in regional projections and contribute to uncertainty in impact timing and magnitude.
Sea-level rise and coastal impacts
Cause → Effect: Increased global temperature causes thermal expansion of seawater and melting of land ice (glaciers and ice sheets). The combined processes raise global mean sea level and increase coastal erosion and flooding. Low-lying coastal communities and small island states experience higher flood frequency, permanent loss of land and saltwater intrusion into freshwater supplies. Scale and risk: Sea-level rise is global but local effects vary with land subsidence, ocean currents and coastal geomorphology. Risk depends on exposure (population and infrastructure in coastal zones) and adaptive capacity (coastal defences and planning). Long-term implications include migration, loss of coastal habitats (mangroves, saltmarshes) and damage to economic assets.
Ecosystems, biodiversity and species distribution
Cause → Effect: Rising temperatures and changing precipitation alter habitats, species ranges and life-cycle timing. Species adapted to narrow temperature ranges face population declines or local extinction. Shifts in phenology (timing of flowering, migration) disrupt ecological interactions such as pollination and food webs. Scale and risk: Impacts appear first at local and regional scales but accumulate to cause global biodiversity loss. Risk increases for endemic species and ecosystems with limited ability to migrate, such as alpine and island biotas. Environmental implications include reduced ecosystem services (pollination, water purification) and weakened ecosystem resilience.
Agriculture, food security and water resources
Cause → Effect: Altered rainfall patterns, increased drought frequency, and heat stress reduce crop yields and pasture quality in many regions. Changes in pest and disease ranges increase crop losses. Water availability becomes more variable, with some regions experiencing reduced freshwater supply and others increased flood risk. Scale and risk: Food security risk varies by region, crop type and socioeconomic factors. Lower-income regions with limited irrigation and infrastructure face higher vulnerability. Environmental implications include increased competition for water, pressure for land conversion, and potential increases in food price volatility.
Extreme weather, human health and infrastructure
Cause → Effect: A warmer atmosphere holds more moisture and alters atmospheric circulation, increasing the frequency and intensity of heatwaves, heavy rainfall, storms and droughts. Extreme events cause direct damage to infrastructure, increase mortality and morbidity, and disrupt services such as power and transport. Scale and risk: Extreme weather impacts are episodic but can cause disproportionate damage when they hit densely populated areas or critical infrastructure. Risk assessment uses likelihood, exposure and consequence to prioritise responses. Environmental implications include lost habitats, contaminated water supplies and long-term economic costs.
Feedbacks, tipping points and long-term implications
Cause → Effect: Warming can trigger feedbacks that amplify climate change. Examples include reduced albedo from ice melt (more solar absorption) and methane release from thawing permafrost. Positive feedbacks accelerate warming, while negative feedbacks can offset some changes but rarely fully compensate. Scale and risk: Tipping points are thresholds beyond which systemic changes become rapid and irreversible on human timescales. Risk increases near thresholds such as major ice-sheet collapse. Long-term environmental implications include persistent changes to the carbon cycle, altered ocean chemistry and lasting biodiversity loss.
Scale, risk assessment and limiting factors
Scale refers to spatial (local to global) and temporal (seasonal to centuries) dimensions of impacts. Risk assessment combines hazard (magnitude and frequency of climate-driven events), exposure (people, ecosystems, assets at risk) and vulnerability (sensitivity and adaptive capacity). Limiting factors include uncertainty in climate sensitivity, variability in regional projections, socioeconomic development pathways, and effectiveness of mitigation and adaptation measures. Clear definitions of exposure and vulnerability improve the accuracy of risk rankings and policy responses.
Key notes
Important points to keep in mind