Intergovernmental Panel on Climate Change:
Fifth Assesment Report; Working Group One:
Chapter 13: Sea Level Change
Chapter Summary and Implications
13.7: Projections of 21st Century Sea Level Extremes and Waves
Currently, sea level extremes and waves are driven by two main factors. First, extratropical and tropical storms are the main drivers of sea level extremes and waves. Therefore, future changes in frequency, intensity, and duration of storms will have a significant impact on sea level extremes and waves. Secondly, GMSLR increases sea level extremes and wave magnitude regardless of the storm modulated component. Observational tide-gauge records from the past few decades have shown an increasing trend in the height of sea level extremes. Although many factors are at work, the strength and variability of climate modes, such as El Niño Southern Oscillation (ENSO) and North Atlantic Oscillation (NAO), are known to influence sea level extremes.
Sea Level Extremes
CMIP5 climate-model forcings were applied to storm surge models in order to predict changes in future sea level change. Using hypothetical low-pressure systems (storms) to propagate waves, the model projections found that it is very likely that sea level extremes and storm surges will be increase in some regions by 2100 (Figure 13.25). The models reaffirm that future changes will be due to the combined effects of sea level rise and changes in storminess. While there is high confidence that extremes will increase in the future, there is low confidence for region-specific predictions due to variability at smaller scales.
Waves
Waves are mainly driven by low-level atmospheric winds, including long-standing circulation streams and more transient low-pressure storm systems. Scientists look to changes in these two processes in the future to determine the effect they will have on wave direction and magnitude. Although recent satellite observations indicate a (likely) positive trend in significant wave height, it is also likely that this trend is due to natural variations in wind forcing. Intercomparison of region specific models show common features, but there is low confidence in wave model projection due to large uncertainties regarding future wind patterns and strength (Figure 13.26).
Figure 13.25: The projected multiplication factor by which the frequency of extreme sea level events will occur for a sea level rise of (a) 0.5 m and (b) under RCP scenario 4.5. The warmer colors indicate an elevated frequency of extreme sea level events.
Figure 13.26: Projected changes in global wave conditions for 2075-2100 relative to 1980-2009. (a) Annual mean, (b) Jan.-Mar., (c) July-Sept. Hashed regions indicate where the difference is greater than the model ensemble standard deviation.