Sea level variability and trends: Wellington Region

All sea level estimates for this century may be conservative, and while MfE is staying with 0.8 metres, Rob Bell who wrote the MfE reports, thinks that 1.5 metres, relative to baseline of 1990, should now be used (phone conversation).

Screenshot 2015-07-14 16.38.35James Hansen predicted up to 4 metres quite some time ago.  Hansen was legendary during his long career as NASA’s chief climatologist for being ahead of the curve on seeing the threat of catastrophic climate change. What happens depends on whether we have already passed some tipping points, and the end game may not become clear until some time after global emissions have peaked.

Screenshot 2015-07-14 16.37.17

A survey of 90 sea-level experts from 18 countries reveals what amount of sea-level rise the wider expert community expects. With unmitigated warming, the likely range was 70-120 cm by 2100 and two to three meters by the year 2300. However, note the long tail of about a dozen “pessimists” who are worried about a much larger sea-level rise.
See more at: www.realclimate.org/index.php/archives/2013/11/sea-level-rise-what-the-experts-expect/#sthash.299zFhEH.dpuf

The bottom line, as Slate’s Eric Holthaus writes, is that “glaciers in Greenland and Antarctica will melt 10 times faster than previous consensus estimates, resulting in sea-level rise of at least 10 feet in as little as 50 years.” A sea-level rise of 10 feet would inundate parts of major cities from New York to Shanghai.

Jonathan Musther, has just published a series of highly detailed maps projecting future sea level rise scenarios onto the New Zealand coastline…
“Study after study shows that we have underestimated ice-sheet instability, and it is almost universally accepted that large sea-level rise will be a consequence. Unfortunately, most studies place this sea-level rise at some unspecified time in the future – when, we’re not sure, but it’s far enough away that we needn’t worry…”

There is a lag effect of 20 to 30 years of GHG already in the atmosphere and excess heat now stored in the oceans. The heat stored in oceans tends to partially release back to the atmosphere in El Nino years.  On that basis this year should break some more records.

NZ is a slow mover in the adoption of low carbon technologies, and consequently our emission trajectory is now out of sync with many European countries. If low-carbon sectors of our economies were encouraged to expand and create jobs, and high-carbon sectors encouraged to contract, we might be winning. The call is for wealthy countries to cut their emissions by somewhere in the neighbourhood of 8-10% a year –
http://kevinanderson.info/blog/eu-2030-decarbonisation-targets-and-uk-carbon-budgets-why-so-little-science/

Sea level spectrum

Screenshot 2015-07-14 16.39.59
Variations are subject to
– Tidal fluctuations determined by Moon and Earth movements around sun
– Storm surge related to barometric pressure and winds
– Seasonal annual cycle
– climate variability including inter annual and inter-decadal oscillations
– climate change trends
– vertical land movement – tectonic and ice-age crustal readjustment
– tsunami

Ocean thermal expansion and glacier melting have been the dominant contributors to 20th century global mean sea level rise, explaining 75% of the observed rise. The contribution of the Greenland and Antarctic ice sheets has increased since the early 1990s, partly from increased outflow induced by warming of the immediately adjacent ocean.

http://www.climatechange2013.org/images/report/WG1AR5_Chapter13_FINAL.pdf

Screenshot 2015-07-14 16.41.14

Globally, sea level rise is 3mm per annum. Wellington Harbour has experienced an average sea level rise of 2mm per annum over the last 100 years, there has been some acceleration over 10 years, with 4 or 5mm per year in NZ region, but need longer period to verify that. This is especially so in Wellington region over last 10 to 20 years, possibly due to subsidence contribution. Some indication that we are slightly higher than IPCC recommendations, but 5 to 10cm already added in MfE guidance.

Due to convergence of the Australian and Pacific crustal plates some 20-40km beneath surface, our region is also subsiding tectonically with Wellington city subject to slow slip events that have produced an average subsidence of the land mass of 1.7mm/year since 2000. Records over 6 years up to 2012 show subsidence varies across the region from around 1mm per year on the Kapiti coast up to between 2 to 3mm per year along the Wairarapa coast.

Screenshot 2015-07-14 16.43.01

There are other cycles that relate higher sea levels with La Nina episodes and the 20-30 year positive phase of the Inter-decadal Pacific Oscillation. The relative change in sea level is currently tracking towards 0.8m rise by the 2090s, though it is suggested by Rob Bell that 1.5m relative to baseline of 1990 should be used.

Catastrophic contributions to sea-level rise from collapse of the West Antarctic Ice Sheet or the rapid loss of the Greenland Ice Sheet are not considered likely to occur in the 21st century. However, the occurrence of such changes becomes increasingly likely as greenhouse gas concentrations continue to rise – see below “Ice melt from West and East Antarctica grounded ice”..

It is recommended that predictions are updated every 5 years, taking into account recent tectonic land movements, and IPCC updates.

Consecutive IPCC reports are coming out with higher predictions for sea level rise. Latest research has indicates that the Western Antarctica ice sheet is melting at a much faster rate in basins below sea level with warming from below and above when air temperatures rise above zero degrees. Using paleo history of the Pliocene, it was found that rapid ice melt occurred in similar conditions to today, giving sea level rises of 20 metres or so, and this was found to match fairly closely the ice volume available in the deep basins in WAIS and East Antarctica ice sheet.. Predictions for sea level rise this century vary between 80 cm about 4 metres, but it seems that the outliers are becoming increasingly plausible.

A new review analyzing three decades of research on the historic effects of melting polar ice sheets found that global sea levels have risen at least six meters, or about 20 feet, above present levels on multiple occasions over the past three million years. What is most concerning is that amount of melting was caused by an increase of only 1-2 degrees (Celsius) in global mean temperatures, and modern atmospheric carbon dioxide levels equivalent to those today.  According to Anders Carlson, an Oregon State University glacial geologist and paleoclimatologist, and co-author on the study, the ominous aspect to this is that CO2 levels are continuing to rise.

Storm surges
In the short term, storm surges are the biggest contributor to waves spilling over and eroding coastlines. Storm surges are generated by rapidly lowering pressures and strengthening winds close to a deepening depression, and are expected to become more regular and increase in severity with climate change. The worst impact occurs when they coincide with king tides.

Greater Wellington has commissioned a report on the level of storm surges around the Wellington region. Storm surges historically reach 1 metre above normal tidal levels on the Kapiti Coast, to half a metre within Wellington Harbour. Coastal flooding occurred in Lambton Quay in the February 1936 storm, with a tide height of 1.7 m above normal.

This level also varies around the harbour, with the largest surge in the area of longest direct fetch likely from west of Petone to Nguaranga Gorge.

At the shoreline, the maximum vertical elevation reached by the sea is a combination of the wave set-up that is induced landward of the wave breaking zone and wave run-up (or swash).  These act on top of the storm-tide level.  Wave run-up is highly variable even over a short length of coast, varying according to the type of beach, the beach slope, the backshore features and presence of any coastal defence structure.

Wellington storm 20 June 2014
The damage was unprecedented according to Kiwirail.The impact of the outage was felt throughout the Wellington transport system as people who usually caught the train needed to look to other methods of transport. It took almost a week to get the rail line restored with services resuming on the morning of 27 June.

The storm severely affected Wellington’s transport network, in particular damage to the Hutt Valley rail line, and consequent disruption to passenger rail services for the 6 days following the storm. Estimated economic impacts of transport disruption resulting from storm was between $12 and $43 million. Some of those costs – like the $5.3 million in repair works – are unique to the outage however the same amount again is simply due to the travel delays caused by the mode shift and ensuring congestion. This might not sound like much but consider that it is just for four working days so equates to about $1.3 million per day. That helps to give us an idea as to just how much impact the rail network in Wellington is having on congestion relief (http://transportblog.co.nz/2014/02/11/the-impact-of-rail-disruption-in-wellington/)

Storm tide 2 February 1936
Scott Stephens, Glen Reeve, Rob Bell NIWA

The storm tide was modelled for the 2090s, taking into account climate change impacts.  Normal spring tides of 0.83m relative to datum are considered low. However, this means that storm surges are a relatively important component of storm tide, and this is confirmed by examining all extreme tides, such as the event of 1936 in Lambton Quay in 1936 with a tide height of 1.7 m above normal.

Ice melt from West and East Antarctica grounded ice
Twin papers show that the rate of ice loss from West Antarctica is increasing — with the acceleration particularly pronounced in the past decade — and also why this is happening: Warmer ocean waters are pushing up from below and bathing the base of the ice sheet.
The findings add to a growing body of evidence suggesting that the effects of climate change are outpacing scientific predictions.
The Antarctic Ice Sheet contains enough water to raise global sea levels more than 50 metres. It poses the single greatest threat to the world’s shorelines and coastal cities. Emerging geological records imply a surprising sensitivity of the ice sheet, with serious implications for our future response to climate and ocean warming. Recent observations show an accelerating retreat of some major outlet glaciers, especially in West Antarctica, where the bed of the ice sheet lies hundreds of metres below sea level – hinting that a massive runaway ice retreat is already underway. Transitions between glacial, intermediate and collapsed states are relatively rapid, taking one to several thousand years.

Totten Glacier, the size of California in East Antarctica, is in danger of melting away, which could lead to an extreme thaw increases sea levels by about 11.5 feet (3.5 meters) worldwide if the glacier vanishes, a new study finds http://www.livescience.com/50174-east-antarctica-glacier-melt.html

Researchers have found two seafloor channels underneath the floating ice shelf of Totten Glacier in East Antarctica. The channels may let the warmest waters near the glacier to enter beneath the floating ice shelf, causing the rapid thinning of the ice shelf observed to date, the scientists said.

As the ice shelf thins, the point where the glacier starts to float will retreat, raising the sea level, and exposing more ice to the ocean, said the study’s lead author, Jamin Greenbaum, a doctoral candidate at the University of Texas at Austin’s Institute for Geophysics.
[Vanishing Glaciers: See Stunning Images of Earth’s Melting Ice]

Professor Robert DeConto
Climate System Research Center, University of Massachusetts

While estimates of sea level in the geological past remain difficult to constrain, Antarctica is now thought to have contributed 15 to 20m of sea level
rise in the Pliocene (about 3 million years ago) and 4-6m during the geologically recent Last Interglacial.

Screenshot 2015-07-14 16.44.54

The magnitude of ice sheet retreat required to produce these high sea levels (during past climates only slightly warmer than today) has been difficult to reconcile in most ice sheet models. A continental ice sheet-shelf model is enhanced with new physics accounting for 1) the influence of surface meltwater on ice-shelf calving, and 2) structural failure of large tidewater ice cliffs in places where ice shelves retreat back to the grounding line during episodes of climate warming. Coupled with high-resolution atmosphere and ocean components, the ice sheet model is used to simulate the Antarctic Ice Sheet during the Pliocene and through the LIG. For the first time, the model simulates an Antarctic contribution to sea-level rise of ~17m during the Pliocene, and 4m during the LIG, in approximate agreement with (albeit uncertain) geological sea-level indicators.

When applied to long-term future simulations assuming extended RCP greenhouse gas emission scenarios, the same model shows a dramatic retreat of Antarctic marine-based ice over the next 500 years, beginning within a few decades in the Pine Island Bay sector of West Antarctica. In the most extreme RCP scenarios, subsequent breakup of the Ross and Filchner-Ronne ice shelves triggers the near-total collapse of the West Antarctic Ice Sheet (WAIS) within a few centuries, followed by eventual retreat into the deep sub-glacial basins underlying the East Antarctic Ice Sheet (EAIS).

NZ govt dumps national environmental standard for sea level rise
The New Zealand government has ordered officials at the Ministry of Environment to stop work on the development of a national environmental
standard (NES) on sea level rise, enquiries by the Science Media Centre have revealed. Lack of an NES for future sea level increases will force
each local authority to make up its own […] [Get the full story at Hot Topic…]

References

Rob Bell’s Rob.Bell@niwa.co.nz Coastal hazards – page 23
www.mfe.govt.nz/publications/climate/coastal-hazards-climate-change-guidance-manual/

Professor Robert DeConto- Department of Geosciences, University of Massachusetts
The Fate of the Antarctic Ice Sheet: Lessons from the Geological Past and How they are Informing Future Predictions
2014 S.T. Lee Lecture

Totten glacier melting
http://www.livescience.com/50174-east-antarctica-glacier-melt.html

IPCC Chapter 13 http://www.ipcc.ch/report/ar5/wg1/

A strategy to guide the Wellington Regional Council’s climate resilience activities
http://www.gw.govt.nz/assets/council-reports/Report_PDFs/2014.650a1.pdf

J Hannah (Vision NZ Ltd)

Adaptive Urbanism : Sea Level Rise, Resilience & Urban Development
Stephenson & Turner

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