Precipitation
There is greater variability in the direction of projections of percentage change in precipitation for these cities. Some cities are expected to see increases in precipitation while others are projected to experience sharp declines in precipitation by the 2080s. For cities in higher latitudes projected to see increased precipitation, such as Toronto and New York, most of the precipitation increase will be in the form of rainfall, as snowfall is likely to decline given the warmer temperatures.
Precipitation changes will differ dramatically by region. In general, mid-to-high latitude cities such as Toronto, New York, and Tokyo are expected to experience precipitation increases. However, some mid-latitude cities, such as London, are expected to experience significant summer-time drying, which could lead to net precipitation decreases. Other cities at the boundaries between the mid-latitudes and subtropics are expected to experience drying, including Harare and Melbourne. Some tropical cities are expected to experience more precipitation, while others are expected to experience less. In those cities where precipitation is strongly correlated with inter-annual variability, changes in the modes with climate change will be critical, and introduce an added element of uncertainty. These cities include Harare, Melbourne, Delhi, and Sao Paulo for ENSO, and Athens and London for the NAO.
It is important to note that in some cases the GCMs have trouble accurately simulating the baseline/observed precipitation in particular regions. This is especially true in cities that have strong seasonal precipitation cycles. With a poor handle on the baseline, the projections for the city are therefore skewed. The projections for Delhi, which has a monsoonal climate, provide an example of this, as extreme values for increased precipitation may overestimate how wet the climate may become. It is therefore important to focus on the direction of change, not so much the actual values. Because it was not possible to include all cities across the globe, the projections presented here can be used as a proxy for other cities with similar climate conditions.
Precipitation projections for Athens, Greece
Athens is one city where annual precipitation is projected to decrease over the next century. In the observed climate record for annual precipitation, there is no statistically significant trend. Slight decreasing trends in other precipitation variables, such as wet days, have been observed. What appears to be occurring in Greece, as well as many other parts of the globe, is an increase in extreme daily precipitation (Nastos and Zerefos, 2007). While total rainfall and the number of wet days show no change or a decline, when there is precipitation, it is more intense. This could potentially be most damaging, as these rain events can cause short-duration, flash flooding.
As shown in Table 3.4 Athens is projected to see drops in precipitation between 10 and 25 percent by the 2080s. Although seasonal projections are less certain than annual results, the climate models project much of the drying to occur during the summer months. Figure 3.17 shows the full range of GCM projections for annual total precipitation in Athens. Even using 10-year smoothing, there remains large historical variability in the observed trend. For the projections, only from approximately the 2040s onward does the B1 scenario produce smaller precipitation decreases compared to the other two scenarios.
Sea level rise
As the oceans warm and expand and land-based ice continues to melt, all the coastal cities analyzed here are expected to experience sea level rise this century. However, the rate will differ by city, for two primary reasons. First, the local height of the adjacent ocean can differ by city, due to the influence of ocean currents, water temperature, and salinity, and the influence of wind and air pressure. Second, local land height change can differ by city. Some cities such as Shanghai are sinking due to the effects of groundwater extraction and compaction of soils by the expanding built environment.
Projections for sea level rise are presented in a case study format for New York City. Described are the methods used to make these projections and the projections themselves.
Sea level rise methods
The GCM-based methods used to project sea level rise for the New York City region include both global (global thermal expansion and meltwater from glaciers, ice caps, and ice sheets) and local (local land subsidence and local water surface elevation) components.
Within the scientific community, there has been extensive discussion of the possibility that the GCM approach to sea level rise may underestimate the range of possible increases, in large part because it does not fully consider the potential for land-based ice sheets to melt due to dynamical (motion-related) processes (Horton et al., 2008). For this reason, the NPCC developed an alternative "rapid ice-melt" approach for regional sea level rise projection based on observed trends in melting of the West Antarctic (Velicogna and Wahr, 2006) and Greenland ice sheets (Rignot and Kanagaratnam, 2006) and paleoclimate studies of ice-melt rates during the most recent postglacial period (Fairbanks, 1989). Starting around 20,000 years ago, global sea level rose 120 meters and reached nearly present-day levels around 8,000-7,000 years ago. The average rate of sea level rise during this ~10,000-12,000 year period was 9.9 to 11.9 cm per decade. This information is incorporated into the rapid ice-melt scenario projections.
The GCM-based sea level rise projections indicate that sea level in New York City may rise by 5 to 13 centimeters in the 2020s, 18 to 30 centimeters in the 2050s, and 30 to 58 centimeters in the 2080s.
Figure 3.18: New York City sea level rise. Combined observed (black line) and projected sea level rise is shown. Projected model changes through time are applied to the observed historical data. The three thick lines (green, red, and blue) show the average for each emissions scenario across the seven GCMs used for sea level rise. Shading shows the central range. The bottom and top lines, respectively, show each year’s minimum and maximum projections across the suite of simulations. A 10-year filter has been applied to the observed data and model output. The dotted area between 2002 and 2015 represents the period that is not covered due to the smoothing procedure.
Sea level projections for the three emissions scenarios agree through the 2040s. Figure 3.18 shows that the B1 scenario produces smaller increases in sea level than the A1B and A2 scenarios beginning in the 2050s, and only around 2080 does the A2 scenario produce larger values than A1B. The divergence of A2 from A1B occurs approximately 10 years earlier for temperature than for sea level rise, in part reflecting the large inertia of the ocean and ice sheets relative to the atmosphere.
Sea level rise projections for the New York City region are higher than global sea level rise projections (generally by approximately 15 cm for twenty-first century projections) (IPCC, 2007). One reason is that the New York metropolitan region is subsiding by approximately 8 to 10 cm per century. The climate models also have a tendency to produce accelerated sea level rise along the northeast US coast, associated in large part with a projected weakening of the Gulf Stream (Yin et al., 2009).
The model-based sea level rise projections shown in Table 3.5 are characterized by greater uncertainty than the temperature projections, due largely to the possibility that dynamic processes in polar ice sheets not captured by the GCMs may accelerate melting beyond currently projected levels. This uncertainty is weighted towards the upper bound: that is, the probability of sea level rise lower than that described in the GCM-based projections in Table 3.5 is very low, and the probability of sea level rise exceeding the GCM projections is relatively high.
Table 3.5: Sea level rise projections including the rapid ice-met scenario for New York City.
|
New York City |
2020s |
2050s |
2080s |
|
Sea level risea |
+5 to 13 cm |
+18 to 30 cm |
+30 to 58 cm |
|
Rapid ice-melt sea level riseb |
~13 to 25 cm |
~48 to 74 cm |
~104 to 140 cm |
a Based on seven GCMs and three emissions scenarios.
b "Rapid ice-melt scenario" is based on acceleration of recent rates of ice-melt in the Greenland and West Antarctic ice sheets and paleoclimate studies.
The rapid ice-melt sea level rise scenario addresses this possibility. It is based on extrapolation of recent accelerating rates of ice-melt from the Greenland and West Antarctic ice sheets and on paleoclimate studies that suggest sea level rise on the order of 9.9 to 11.9 cm per decade may be possible. Sea level rise projections for New York City in the rapid ice-melt scenario are shown in the bottom row of Table 3.5. The potential for rapid ice-melt is included in the regional projections for New York City because of the great socio-economic consequences should it occur. To assess the risk of accelerated sea level rise and climate change for the New York City region over the coming years, climate experts need to monitor rates of polar ice-melt, as well as other key indicators of global and regional climate change.
Extreme events
Some of the largest climate change effects on cities are associated with extreme events, such as heat waves, intense precipitation events, and coastal storms. The frequency, intensity, and duration of many extreme events are expected to increase with climate change. The following quantitative New York City example describes the types of extreme event threats faced by many cities, although each city will face slightly different extreme event and natural disaster risks.
Following the New York City case, we present an example of qualitative projections for tropical cyclones, an extreme event that impacts many cities included in this topic. It should be noted that extreme climate events and natural disasters are intertwined, partly because climate hazards represent a major portion of all natural disasters. Furthermore, climate extremes (such as intense precipitation or drought-induced forest fires) may cause secondary natural disasters (such as landslides).
Extreme events projections for New York City
Extremes of temperature and precipitation (with the exception of drought) tend to have their largest impacts at daily rather than monthly timescales. Because monthly output from climate models is considered more reliable than daily output, simulated changes in monthly temperature and precipitation were calculated; monthly changes through time from each of the 16 GCMs and three emissions scenarios described earlier in this section were then applied to the observed daily Central Park record from 1971 to 2000 to generate 48 time series of daily data.3 This is a simplified approach to projections of extreme events, since it does not allow for possible changes in the patterns of climate variability through time. However, because changes in variability for most climate hazards are considered highly uncertain, the approach described provides an initial evaluation of how extreme events may change in the future. This level of information with appropriate caveats can assist long-term planners as they begin to prepare adaptation strategies to cope with future extreme events.
Adaptation to sea level rise in Wellington, New Zealand
INTRODUCTION
Wellington is a coastal city, with an inner harbor and exposed southern coast. A pilot study focusing on the impacts of sea level rise on a low-lying city suburb has commenced to inform the adaptation approach across the city.
Part of the area included in this pilot study (Box Figure 3.3) has been identified as a key growth node for urban intensification. The area also contains a range of key infrastructure including a significant highway, an international airport, utilities, businesses, housing, and community facilities such as schools, pools, libraries, a marina, and a surfing beach.
METHODOLOGY
Sea level rise was viewed as one of the most critical climate change impacts on the study area, because it lies between only 1 m and 3 m elevation.
Box Figure 3.3: The study area.
The latest New Zealand guidance on coastal hazards2 associated with climate change recommends that councils consider the impacts of a 0.8 m increase in sea level by 2090. However, given the considerable uncertainty in projections and the possibility of catastrophic events, an approach was taken based on testing infrastructure resilience and response via a range of scenarios. This will allow the development of strategies to manage the expected risk.
Three core scenarios were examined (0.5 m, 1 m, and 2 m) with each having an additional 0.5 m storm surge component within the harbour and a 1 m component on the exposed southern coast. These scenarios reflect the most recent scientific probabilities in the short term (50-100 years), while allowing for possible higher levels in the longer term.
Evaluation of the scenarios was carried out in an interdisciplinary cross-council workshop including water, drainage, roading, hazards, transport, coastal and recreational, and urban planning experts. For each asset the following information was gathered: description, ownership, criticality, condition, relocatability, economic value, proposed upgrades. Each asset was then tested against each sea level rise scenario to determine potential risks and impacts. Feasible response options were then proposed.
Mapping the scenarios
Sea level rise scenarios were mapped based on ground elevation data from LiDAR (Light Detection And Ranging). The LiDAR data were captured at 1 cm vertical intervals with +/-10 cm accuracy and were verified by field survey. The data were then used to create a digital terrain model. A local vertical datum was developed based on mean high water springs. This allowed for assessment of the highest likely sea level that includes mean tidal elements.
Infrastructure and key existing community facilities were mapped against the sea level rise scenarios so the impacts could be evaluated holistically.
RESULTS
Through qualitative assessment of likely impacts and appropriate responses to the sea level rise scenarios a number of issues were identified:
• Degradation of the level of service from the storm-water system
• Rising groundwater levels
• Need to evaluate response options for at-risk coastal areas across the city
• Need for early decision-making for response planning
• Interactions and interdependency between assets
• Need to prioritise adaptation responses across the city
The workshop highlighted that rising sea levels are likely to have some impacts on the storm-water system in the short term (next 20-30 years). Some solutions were proffered that could be developed as part of normal asset management.
Low-lying parts of the study area may be susceptible to increased flooding due to a rise in the water-table. This was regarded as more urgent than "over-topping." Several likely options for responding were identified and will be consulted on with Councillors and the community. Further detailed work is required to examine the impact, behavior, and response of groundwater in the study area, together with the likely costs and benefits of each response option.
Different responses may be appropriate for the natural dune environment of the southern coast compared to the structurally modified northern coast. Maintaining a dune environment on the south coast would help the area retain its high aesthetic and amenity values. Moreover, this could be a more successful adaptation response, given the adverse effects that "hard engineered" structures can have on a beach.
The importance of taking into consideration linkages between infrastructure elements was recognised. For example, pumping stations require power and telecommunications, which must therefore be maintained through the area at all times.
Similar impacts may occur in other parts of the city, and an overall cost-benefit analysis cannot be completed in isolation within a limited area.
These findings will inform the proposed intensification plans and other ongoing development, maintenance, and asset management plans for the area. Findings will also be used for discussions within council and the community around prioritizing, costs, and residual risks.
CONCLUSIONS
This study gathered and evaluated key information needed to make an initial assessment of climate change impacts in a localized urban area. It has indicated where further detailed work could be undertaken to derive a more accurate assessment of costs and benefits. A modified approach based on this pilot study will be used across other coastal areas within Wellington City.
