Q&A: What we don't know about sea-level rise

Rising sea levels and extreme weather are threatening effects of climate change. Photo / File

The damage that climate change could bring to New Zealand's coastal communities has become a pre-election talking point, with a 284-page draft Government report being leaked to media by the Green Party. But how much certainty can scientists place around what to expect in coming decades - and what can't they say yet? Science reporter Jamie Morton put some of the key questions to a lead author of the leaked report, Niwa scientist Dr Rob Bell.

In general terms, what range of sea-level rise can be expected by the end of the century in New Zealand, and what models can scientists and policymakers use to estimate this?

In the next few decades the rise in sea level is fairly certain in the narrow range of 20cm to 30cm by 2050.

But by the end of the century the expected rise will widen out to between 30cm and 1m, depending on how global emissions track.

Larger increases above 1m can't be ruled out if polar ice sheets become more unstable later this century.

These figures come from the 5th Assessment Report of the IPCC (Intergovernmental Panel on Climate Change), gleaned from many different climate-ocean model simulations internationally and verified using past observations including sea-level rise over last century.

What degree of uncertainty comes with projecting sea level rise - and its implications for damage to infrastructure and buildings - decades into the future? What challenges do scientists face here?

Possible global sea-level rise projections become increasingly uncertain with time as it depends on global greenhouse gas emissions and we don't know how social and economic change will occur in our global society.

There are other uncertainties around modelling, such as how, and over what timeframes, polar ice sheets will respond to ongoing warming.

Sea-level rise will continue for several centuries - but what is uncertain is the rate at which the rise occurs, as it strongly depends on the world's future greenhouse gas emissions.

This uncertainty presents a major challenge for coastal communities, their assets and infrastructure - and well beyond this century.

The challenges are around how much change will occur, what is the impact of more frequent hazard events, at what point do existing communities adapt, and how can foreseeable climate-change impacts be avoided for new coastal developments?

These are all challenging questions that scientists, planners and engineers are researching to underpin more informed decision making.

Why are our estimates becoming better informed and what new science is under-pinning this?

Scientists in New Zealand are also developing projections of sea-level rise specifically for New Zealand that include vertical land movement and more refined estimates on the contribution from the polar ice sheets in Antarctica.

A new MBIE Endeavour Fund project over the next five years involving Victoria University, Niwa and GNS Science on this work has just been announced.

Research is also underway in the National Science Challenges on developing more adaptive planning tools for coastal areas, approaches to planned retreat that could work in the New Zealand context and improved ways of engaging with coastal communities and iwi and hapu to develop an adaptation plan.

Relatively new science is also helping refine a "dynamic adaptive pathways" approach, where sequences of different pathways or response options - beach nourishment, groynes, rerouting a road or planned retreat - are mapped out with communities.

Switching between these pathways is tied to agreed decision points or triggers when an alternative pathway needs to be implemented.

Triggers, such as when a certain number of coastal flooding events have occurred, reaching a specific sea-level rise, or erosion reaching within X metres of houses, are then regularly monitored and reviewed to inform progress towards the next decision point.

This adaptive approach eases the burden on decision makers trying to second-guess a specific sea-level rise value within the planning timeframe and reduces the chances of creating future inflexibilities - especially if the seas rise much faster or slower than anticipated.

We have seen damage figures of $52 billion, and $19 billion, covering only building replacement costs - and not all regions. From what we understand about sea-level rise so far, are these figures likely to change a lot, and are they useful or meaningful?

Both these values come from a 2015 Niwa report for the Parliamentary Commissioner for the Environment, which was the first time a national assessment had been done of the risk exposure we face in our low-lying coastal areas, especially in our urban centres built around harbours.

The 2015 assessment used land elevation up to 3m above mean high water spring as the basis for risk exposure, noting that this is not just from sea-level rise, but also from storm-tide and wave flooding that can presently reach up one to two metres in exposed places.

Coastal land that is below three metres has nearly 167,000 buildings which have a replacement value of around $52b (2011 building costs), while land less than 1 metre high in New Zealand has around 68,000 buildings with a replacement value of $19b (2011).

No other asset or infrastructure was costed - other than identifying the kilometres of road and rail exposed and number of airports.

A research project in the Deep South National Science Challenge is following up this initial assessment by developing specific coastal flooding and sea-level rise scenarios for each region and extending to regions not previously covered in the 2015 report.

This type of risk exposure information is useful for focusing the effort on adapting to climate change, as it shows what is at risk, where the hotspots are likely to occur and establishes a profile of future impacts tied to different rises in sea level.

Why is it critical that communities and policymakers be informed?

Already some low-lying or eroding coastal communities are facing the challenge of "where to from here?".

With the rise in sea level, which won't stop at 2100, communities and councils or agencies who provide the services and infrastructure need to be fully aware of the rising risk they face and decide what is the threshold for the risk becoming intolerable before implementing the next adaptive pathway or response option.

Science can inform policy, community engagement processes and council decision making by refining local sea-level projections, the coastal hazard exposure to buildings and services and providing more adaptive planning approaches that work around the future uncertainties faced by coastal communities.