2B.9C - Climate Change and Coastal Flood Risk

Climate change may increase coastal flood risk (frequency and magnitude of storms, sea level rise) but the pace and magnitude of this threat is uncertain. 

The UN's Intergovernmental Panel on Climate Change (IPCC) 5th Assessment Report in 2014 predicted that climate change warming of atmospheric and oceans would increase intensity of atmospheric circulation. 

High Confidence 

• Sea level will rise by 18-59 cm by 2100
  
  • however the pace and extent of sea level within the predicted range is uncertain due to population growth, economic development, natural positive and negative feedback, political commitment to restrict GHG emissions. 
  • it is also affected by adaption:
    • building sea walls, e.g. on the North Norfolk coast, 3 m sea wall on Malé. 
    • new artificial island, Hulhamalé created by the reclamation of sediment from the sea bed between 1997-2002, which is 4 m above sea level and cost $32 million to construct. 
    • building earth embankments, like the bunds in Bangladesh
    • storm surge barriers across river mouths - Thames Barrier, Eastern Scheldt Barrier in the Netherlands (part of the 2.5 billion euro project begun after the 1953 storm surge)
    • restoration of mangrove forest - protection belts, e.g. Sri Lanka replanting after the 2004 Indian Ocean tsunami killed 6,000 people in one coastal village where mangroves were cleared, but only 2 deaths in an adjacent village protected by a mangrove forest
    • mitigation - efforts to reduce magnitude of event. Reducing GHG emissions to limit level of global warming would mitigate sea level rise and cyclone intensity. 
• Delta flooding
  • The are area of the world's major deltas at risk from coastal flooding is likely to increase by 50%

Medium Confidence

• Wind and waves
  • Some evidence of increase wind speeds and large waves
• Coastal erosion
  • Erosion will generally increase because of the combined effects of changes to weather systems and sea level

Low Confidence - evidence weak and uncertainty high

• Tropical cyclones
  • The frequency is likely to remain unchanged, but there could be more larger storms
  • Predicted to increase in strength by 2-11% by 2100. Associated rainfall will increase by 20%.
  • Cyclone intensity would increase due to warmer ocean surface temperature and warmed atmosphere holding more moisture.
  • Number of tropical cyclones not predicted to increase - combination of factors form them, and a warm ocean temperature is only one of them. 
    • In the North Atlantic, the number of tropical storms becoming hurricanes has risen from 6 in the 1900s to 8 per year from 2000-2016
      
    • However, this is only low confidence - no observed increase in maximum intensity in the Pacific and Indian Oceans over the last 20 years of monitoring. 
    • The number and intensity of tropical cyclones is highly variable each year and decade - no statistically significant long term trend. 
• Storm surges
  • These are linked to depressions that are likely to become more common
  • More intense tropical cyclones will exhibit even lower surface air pressure producing larger temporary sea level rises as storm surges and increasing the risk of coastal flooding 
• Depressions
  • Polar front jet streams will accelerate, possibly increasing the number and intensity of depressions and storm surges in mid-latitudes

The magnitude and timing of all these changes is uncertain.