14.4 Climate change and its influence on geomorphic systems

Climate change is reshaping Earth's surface in profound ways. Rising sea levels are eroding coastlines and altering coastal ecosystems. Changing precipitation patterns are triggering more landslides and debris flows on hillslopes, while retreating glaciers are transforming landscapes and river systems.

Permafrost thaw is destabilizing Arctic regions, releasing stored carbon. These geomorphic changes highlight how human-induced climate shifts are accelerating natural processes, creating new hazards and reshaping environments globally. Understanding these impacts is crucial for adapting to our changing planet.

Sea-level rise and coastal change

Causes and mechanisms of sea-level rise

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• Sea-level rise primarily driven by thermal expansion of oceans and melting of land-based ice due to global warming
  • Thermal expansion occurs as water molecules spread out when heated
  • Land-based ice includes glaciers, ice caps, and ice sheets (Greenland, Antarctica)
• Global mean sea level rising at accelerating rate
  • Current rate approximately 3.6 mm/year
  • Projections range from 0.3 to 2.5 meters by 2100 depending on emissions scenarios

Impacts on coastal geomorphology

• Coastal geomorphology shaped by interaction of marine and terrestrial processes
  • Waves, tides, currents shape coastlines through erosion and deposition
  • Sediment transport from land influences beach and barrier island formation
• Increased sea levels enhance coastal erosion, particularly in areas with soft sediments
  • Sandy beaches and barrier islands highly vulnerable
  • Cliff retreat accelerates in areas with erodible rock types
• Storm surge impacts amplified by higher sea levels
  • More severe coastal flooding and erosion events
  • Increased overwash on barrier islands leading to landward migration
• Saltwater intrusion into coastal aquifers and estuaries alters vegetation and sediment dynamics
  • Shifts in vegetation zones (salt marshes moving inland)
  • Changes in sediment cohesion and erosion resistance

Ecosystem responses and human interventions

• Coastal ecosystems struggle to keep pace with rapid sea-level rise
  • Salt marshes and mangroves may drown if vertical accretion rates insufficient
  • Loss of ecosystem services (storm protection, carbon sequestration)
• Human interventions have complex effects on coastal geomorphology
  • Seawalls can lead to beach narrowing and increased erosion downdrift
  • Beach nourishment temporarily mitigates erosion but alters natural sediment transport
  • Managed retreat allows for natural coastal processes but requires significant planning

Precipitation patterns and hillslope processes

Changes in precipitation patterns

• Climate change alters precipitation patterns globally
  • Changes in intensity, duration, and seasonality of rainfall events
  • Some regions experience increased precipitation while others face drought
• Increased rainfall intensity leads to higher rates of surface runoff and soil erosion
  • More energy available for sediment transport
  • Increased risk of gully formation and expansion
• Changes in soil moisture content affect soil cohesion and stability
  • Wetter conditions can reduce soil strength through increased pore water pressure
  • Drier conditions may lead to soil desiccation and cracking

Impacts on hillslope stability

• More frequent extreme precipitation events trigger increase in shallow landslides and debris flows
  • Rapid saturation of soil leads to loss of cohesion and reduced friction
  • Examples: increased landslide activity in Seattle, USA and Taiwan during intense storms
• Extended dry periods followed by intense rainfall enhance landslide risk
  • Reduced vegetation cover decreases soil reinforcement
  • Soil desiccation creates cracks that allow rapid water infiltration
• Alterations in freeze-thaw cycles in colder regions impact physical weathering and slope stability
  • More frequent freeze-thaw cycles can accelerate rock breakdown
  • Thawing of frozen ground reduces slope stability (especially in permafrost regions)

Compound effects and risk factors

• Coupling of changing precipitation patterns with land use changes exacerbates hillslope instability
  • Deforestation reduces root reinforcement and increases surface runoff
  • Urbanization alters drainage patterns and concentrates water flow
• Antecedent moisture conditions play crucial role in landslide initiation
  • Cumulative rainfall over weeks or months can predispose slopes to failure
  • Importance of monitoring long-term precipitation trends for hazard assessment
• Climate change impacts on vegetation can indirectly affect hillslope processes
  • Shifts in plant communities may alter root structures and soil moisture regimes
  • Increased wildfire frequency can lead to post-fire debris flows and erosion

Glacial retreat and fluvial systems

Glacial retreat and sediment dynamics

• Glacial retreat accelerating due to global warming
  • Exposing previously ice-covered landscapes
  • Altering sediment dynamics in glaciated regions
• Deglaciation leads to increased sediment availability
  • Moraines and other glacial deposits become exposed to erosion
  • Frost action and mass wasting processes mobilize sediment
• Proglacial lakes form in wake of retreating glaciers
  • Act as sediment traps, modifying downstream sediment flux
  • Can create outburst flood hazards if dams fail (jökulhlaups)

Paraglacial processes and sediment yield

• Paraglacial processes contribute to elevated sediment yields in recently deglaciated basins
  • Slope adjustments as glacial buttressing is removed
  • Increased mass wasting events (rockfalls, landslides)
• Temporal evolution of sediment yield follows a non-linear pattern
  • Initial pulse of high sediment yield followed by gradual decline
  • Timescales of adjustment can range from decades to millennia
• Changes in glacial meltwater discharge patterns affect stream power and sediment transport
  • Seasonal shifts in peak discharge timing
  • Long-term decline in meltwater contribution as glaciers shrink

Fluvial system adjustments

• Fluvial systems downstream of retreating glaciers often experience aggradation
  • Rivers adjust to increased sediment loads through channel widening and braiding
  • Floodplain deposition rates may increase
• Long-term fluvial system adjustments include:
  • Channel pattern changes (e.g., braided to meandering transitions)
  • Alterations in flood frequency and magnitude
  • Modifications to riparian ecosystems and habitats
• Cascading effects on downstream watersheds and coastal areas
  • Changes in sediment delivery to deltas and estuaries
  • Potential impacts on coastal geomorphology and ecosystems

Permafrost degradation and landscape stability

Permafrost characteristics and degradation processes

• Permafrost defined as perennially frozen ground highly sensitive to temperature increases
  • Covers approximately 24% of Northern Hemisphere land area
  • Depths range from a few meters to over 1000 meters (Siberia)
• Thawing permafrost leads to ground subsidence (thermokarst)
  • Formation of characteristic landforms (thermokarst lakes, alases)
  • Can trigger localized or regional-scale landscape instability
• Active layer thickening alters hydrological pathways
  • Increased groundwater contributions to surface water systems
  • Changes in soil moisture regimes and vegetation patterns

Geomorphic impacts of permafrost degradation

• Coastal erosion rates in Arctic regions amplified by combined effects of permafrost thaw and sea-level rise
  • Erosion rates exceeding 20 meters per year in some locations (Alaskan coast)
  • Loss of coastal infrastructure and cultural sites
• Thaw slumps and retrogressive thaw slumps characteristic mass wasting processes
  • Can mobilize large volumes of sediment and organic matter
  • Often initiated by thermal erosion along coastlines or river banks
• Changes in vegetation communities resulting from permafrost thaw modify geomorphic processes
  • Shifts from tundra to shrub or forest ecosystems
  • Alterations in surface energy balance and active layer dynamics

Biogeochemical and climate feedbacks

• Permafrost degradation releases stored carbon and nutrients
  • Potential creation of positive feedback loop in climate system
  • Estimates suggest up to 1700 gigatons of carbon stored in permafrost
• Increased methane emissions from thermokarst lakes and wetlands
  • Methane has higher global warming potential than CO2
  • Contributes to accelerated warming in Arctic regions
• Changes in surface hydrology affect carbon and nutrient cycling
  • Formation of new drainage networks and wetlands
  • Alterations in biogeochemical processes and ecosystem productivity