Climate Change Leadership Forum report number: 7

Planning for sea level rise in New Zealand – considering prudence and pragmatism

Briefing for the Climate Change Leadership Forum

Prepared by NIWA and Ministry for the Environment
Date: 19 June 2008

Background

Councils and engineers need expert advice to help in their planning of infrastructure and other decision-making activities that are sensitive to sea level rise. Such planning and decisions often have consequences that extend for periods in excess of 50-100 years. Within this timeframe, the sea level may rise significantly.

The Ministry for the Environment has commissioned an update of the “Coastal Hazards and Climate Change” guidance manual designed to help councils with their decision-making. That document features advice on the height of sea level rise recommended for planning.

This paper explains how the sea level rise advice was developed for inclusion in the guidance manual. It details the current draft advice and provides detail of the outcomes of discussion with key stakeholders.  

Observed climate change

In 2007, the Intergovernmental Panel on Climate Change (IPCC) completed its Fourth Assessment of climate change science. It found that global average temperatures have warmed by over 0.7ºC over the last 100 years.  There has also been widespread melting of snow and ice. The ocean is absorbing 80% of the heat added to the climate system and average ocean temperatures have increased down to depths of 3000 m. This warming causes expansion of ocean water. This, along with the additional water from melting of land-based ice, causes a rise in sea level. Consequently, sea levels around New Zealand have risen on average 1.8 mm/year over the last 40 years with the total sea level rise over the last century of 0.17 m. The rate of global-average sea level appears to have increased recently, with an average of 3.1 mm/yr being observed between 1993-2003. However, it is still not completely clear if this a temporary increase in the rate of sea level rise or if it reflects a change to a significantly higher trends than have been observed in the past.

The future to 2100

Future sea level rise can be estimated from numerical models of the physical processes involved in the atmosphere/ocean system. Current model results suggest that sea levels could rise by between 0.18 and 0.59 m by 2100 (see Table SPM 3 WG1 below, taken from the IPCC Fourth Assessment Report). However, these models do not include some processes that may contribute to further sea level rise, such as carbon-cycle feedbacks, nor do they include the full effects of changes in ice sheet flow because these changes are still uncertain. Because of the processes not accounted for in the available models, the IPCC noted that the upper values of the ranges given in the table are not to be considered upper bounds for sea level rise.

Table SPM-3. Projected globally averaged surface warming and sea level rise at the end of the 21st century. {10.5, 10.6, Table 10.7}

Table SPM-3. Projected globally averaged surface warming and sea level rise at the end of the 21st century.

The future beyond 2100

Very large sea-level rises would result if widespread deglaciation of Greenland and West Antarctic ice sheets occurred. There is medium confidence that at least partial deglaciation of the Greenland ice sheet, and possibly the West Antarctic ice sheet, may occur over a period of time ranging from centuries to millennia for a global average temperature increase of 1-4°C (relative to 1990-2000), causing a contribution to sea level rise of 4-6 m or more. The complete melting of the Greenland ice sheet and the West Antarctic ice sheet would lead to a sea-level rise of up to 7 m and about 5 m, respectively. However, the IPCC expressed high confidence that such widespread deglaciation will not occur significantly in this century (i.e., before 2100).

Regional changes

The level of the sea varies around the globe in response to localised changes in wind, ocean temperature, currents and gravity variations. For New Zealand, initial estimates from models suggest that, by 2100, we may experience sea levels in addition to the global average of up to 0.1 m.  However, the research in this area is ongoing, and we expect these numbers to change as we learn more. 

Expert Opinion

In producing its Fourth Assessment Report, the IPCC drew on the best expert advice on sea level rise that is available internationally. The IPCC concluded that it could “not assess the likelihood, nor provide a best estimate or an upper bound for sea level rise”.

Despite this, councils and engineers still need to make planning decisions that are sensitive to sea level rise. In preparing the “Coastal Hazards and Climate Change” guidance manual, expert opinion was sought by the Ministry for the Environment from NIWA as to the level of sea level rise that would be considered “diligent” to plan for. NIWA considered the global climate modelling results, the IPCC documents and subsequent research papers published in scientific journals.  In building the advice, NIWA consulted several experienced coastal planning consultants, Environment Waikato and Environment Bay of Plenty, as well as drawing on its own experience in advising councils on coastal planning. This advice was reviewed by Ministry for the Environment officials and discussed with climate scientists from universities. Feedback was also sought from other stakeholders such as Lifelines engineers, emergency managers, council planners and asset managers. The feedback received has suggested that the current draft advice is well formulated and well sought after.

Setting the numbers

The IPCC reported that computer modelling has suggested a range of sea level rise from 0.18-0.59 m by the end of the century. The IPCC assessed that a further 0.1 to 0.2 m sea level rise might occur if the contributions from Greenland and Antarctic ice sheets were to grow linearly with global temperature change.

In formulating the advice (see Box 1 below), a ‘base’ value of 0.5 m has been chosen. This can be seen as representing a mid-high value from the modelling or as a mid-range value from the modelling together with a further contribution from an increase in ice sheet loss.

The draft sea level rise advice for planning

In its Fourth Assessment Report, the IPCC has found that "Because understanding of some important effects driving sea-level rise is too limited, this [Fourth] report does not assess the likelihood, nor provide a best estimate or an upper bound for sea-level rise." While there are uncertainties associated with the science around sea-level changes, national and local governments and individuals must continue to make decisions which either implicitly or explicitly make assumptions about what this rise will be. This report therefore recommends that in such cases a planning allowance for sea-level rise is used which is based on the best models available, as assessed by the IPCC, with an additional allowance for the factors which these models have not included1. The recommended additional allowance depends on the assessed risks, i.e. on the lifetime, cost, and potential impact as follows:

  1. For planning and decision timeframes out to the 2090s (2090–2099)2 a base value sea-level rise of 0.5 m relative to the 1980–1999 average should be used to assess vulnerability.
  2. A further assessment needs to be made of the greater risk resulting from a possible higher sea level rise of 0.9 m by the 2090s relative to the 1980–1999 average. This could have significant implications for decisions around important or vulnerable infrastructure such as hospitals, schools, key sub-stations etc.
  3. For planning and decision timeframes beyond 2100 where, as a result of the particular decision, future adaptation options will be limited, an allowance for sea-level rise of 10 mm/year beyond 2100 is recommended (in addition to the above recommendation). 

1 The factors not included in the models are a possible high emission scenario and uncertainties associated with increased contribution from Greenland and Antarctica Ice sheets, carbon cycle feedbacks and possible differences from the global average in the New Zealand region.

2 Note: assuming an average rate of mean annual sea-level rise of 1.6 mm/yr, sea levels will have risen on average by about 24 mm between the midpoint of this timeframe (1990) and 2006. This should be accounted for when using currently observed sea levels. 

  

The advice also suggests that an assessment is made of the impact of 0.9 m sea level rise. This value represents the sum of the ice sheet contribution and regional changes plus the upper bound of the combined projections from the modelling. Note that the IPCC explicitly does not exclude higher values of sea level rise, but states that the “understanding of these effects [not included in the models] is too limited to assess their likelihood or provide a best estimate or an upper bound.”

Risk - Small chance of bigger rises

There is high confidence that sea level rise will not be less than the lower values (i.e., 0.25 m). This is because the lower values are largely a result of the warming that has already occurred or is inevitable because of past emissions. There is less confidence in the upper estimates. For example, some recent research (Proudman Oceanographic Laboratory, Stefan Rahmstorf) suggests that higher sea level rises of between 0.5 m and 1.4 m by 2100 are possible. While the science community has yet to reach a consensus on this, the advice lies within the lower bounds of these higher estimates. The Ministry for the Enviroment guidance advice advocates the level for which we think it is prudent to plan for based on currently reviewed science - future sea level rises of an even higher amount have not been excluded. Ministry advice will need to be reviewed and revised in the future as the information and knowledge base develop further.

How the advice is used

The sea level rise advice is presented in the Coastal Hazards and Climate Change guidance manual. This manual outlines the use of a risk management framework for assessing the implications of climate change on council activities.

Risk management is a consideration of the hazard in its context. If the consequences of inundation or erosion are likely to be unacceptable, the adaptation response needs to be early and to remove all risk. Consequences could include loss of life, injuries, displacement of people from their homes, temporary loss of use of hospital facilities, loss of utilities like water and power, etc.

An example of unacceptable risk may be a (new or existing) subdivision at frequent risk of inundation.  If the consequences of inundation are less severe, then the response may allow more flexibility. For example, the situation could include ‘inert’ infrastructure such as road, rail and public spaces where occasional inundation can be coped with.  

Note that it is the consequence of the hazard that is important. Although 0.9 m of sea level rise is less likely to eventuate, the consequences of inundation at this level are likely to be greater. If the consequences are particularly serious at the 0.9 m level, then it is this level of sea level rise (rather than 0.5 m) that should be planned for.

Risk assessment of the consequences involved for each situation will be required to determine whether adaptation to sea level rise should involve retreat, ‘holding the line’, or an intermediate step between these two.

Whatever decision is made there will be some residual risk resulting from factors like the occurrence of an infrequent but higher impact event. The assessment of the consequences of this residual risk is encouraged as part of the risk management framework. This will often require an emergency management response, and often presents itself as part of the risk being covered by insurance.

Why not plan for higher values?

It can be costly to plan for sea level rise, particularly when the response is to build or enhance physical defences.  However, the precautionary principle implies that we should plan for what is likely to realistically occur. Planning is therefore a balance between what is prudent and what is pragmatic. The risk management guidance in the manual helps in assessing the impacts of climate change, and hence in evaluating the risks being faced.

In practice, if the costs of reducing risk are too expensive, the level of risk deemed “acceptable” is often increased. For example, adaptation options for existing infrastructure will often be more expensive than that for green-field developments due to limitations on adaptation options. This leads to the acceptance of higher risk on existing infrastructure and tighter constraints on new developments. Accepting

higher levels of risk creates greater residual risk, placing additional pressure on the emergency management and insurance sectors.

Examples include the level of protection afforded to an existing subdivision in a coastal area prone to inundation, in comparison to the development of a green-fields subdivision in a similar place.  Although both cases would face similar risks, the planning response would be quite different.

For existing subdivisions, the risk may be sufficient to require an adaptation response, e.g., increasing coastal protection works. There may also be additional restrictions placed in the district plan on any further development in this area. Note that changes in planning criteria will only affect existing structures when changes requiring consent are to be made. Changes to existing buildings (e.g., extensions, retrofitting or post-event reconstruction) may require measures to make these buildings more resilient, e.g., raising existing floor levels.

For the new development example, the risk faced may well be deemed unacceptable, and planning permission declined. For the new subdivision development to go ahead, planning permission could impose significant restrictions, e.g., on floor heights or enhanced coastal protection. These may lead to the project being deemed financially unviable.

The result of using the sea level rise advice in this risk management context is that pressure will be brought to bear on new proposals to avoid areas likely to be inundated in the future, and for developments in existing subdivisions to be more
“future-proof”.

Feedback from key sectors

A meeting was held with Local Government New Zealand to discuss the advice, and feedback was sought from key council practitioners from around the country. Comments received indicated that the advice was well founded, and no significant negative feedback was made. They considered the advice would fit well with approaches currently being used by councils. However, comments were received as to issues such as to where the liability lay if the council used (or failed to use) the advice, as well as about issues such as existing land-use rights and the planning timeframes that should be considered. There was quite some discussion over the use of regulatory tools (e.g., National Environmental Standards) to enforce planning considerations of sea level rise.   

Insurance representatives, while positive about the general values of sea level rise set in the advice, they were keen to make the wording around the use of the 0.9 m stronger. This has been reflected in the new draft above.  The insurance representatives also discussed the likely increase in the pool of risk to housing and infrastructure that may result from sea level rise. This has implications not only for green-field developments but also for current infrastructure. Where provisions have become more restrictive since the original planning permission, insurance payouts for damage from severe events may be required to refit houses to standards better than they were before the event. This may require further consideration into the future by the insurance industry. Comments also suggested that the insurance industry is likely to look to increase premiums and restrict where cover is offered when sea level rise results in more frequent coastal inundation threatening property and infrastructure. This could occur at sea level rise levels of as little as 0.15 m, or sooner if storm frequency or intensity changes.  Note that the pool of insurance risk will increase due to sea level rise impacts on existing infrastructure. The use of the sea level rise advice in planning is intended to avoid the risk resulting from new developments.

Feedback from engineers also looked to strengthen the wording around the 0.9 m sentence. However, the general approach was considered to fit well with current engineering practice. Discussions also scanned other issues, including the current proficiency with which storm surge is incorporated into planning.  They also recognised that sea level rise may well be of great concern in the future, particularly where adaptive capacity is limited, e.g., at ports and for some airports.