PCE: Sea level rise in NZ & Devonport

Jan Wright gave a great presentation to the NZPI conference on Wednesday. A highlight. Sea level rise and its consequences so well and simply explained.

"We're used to the sea going up and down, but basically staying put..."

"It's all about the high tide. There's a natural variation in level. When there's a storm surge, when isobars show low pressure, the low pressure sucks the sea up to it. When the Wahine sunk the storm surge was measured by Ports of Tauranga as 88 centimetres...". (Research: Atmospheric air pressure at the time of the storm in 9 April 1968 (due to Ex-tropical Cyclone Giselle) dropped to 970 millibars (97.0 hPa) - a record low for New Zealand.) 

Picture from PCE's website promoting its 15 November 2015 report:
Preparing NZ for rising seas: Certainty and Uncertainty

Jan Wright built up the picture of what can cause flooding. River flooding will also be an issue. Every 1 degree warmer the sea becomes, there is 7% more moisture held in the atmosphere. Basic physics. So when it rains, more water comes down, and rivers will carry more water.

The PCE report about coastal flooding combines sea level rise, storm surge risk and river flooding to produce maps of low lying urban New Zealand that should prepare to be flooded in the next 20 to 30 years.

The most recent projections of global mean sea level rise by the IPCC relative to 1986–2005. The green band represents the range for the RCP2.6 scenario, and the overlapping purple band represents the range for the RCP8.5 scenario. The PCE has picked a conservative figure of 30cms for sea level rise for the present work.

Jan Wright says that PCE are being conservative choosing a sea level rise figure of 30 cms, combined with a storm surge figure of 20 cms, giving a total rise in coastal water level of 50 cms as a basis to produce maps. (30 cm background sealevel rise is conservative because all of the scenarios indicate that level of sea level rise is certain to occur - even with stringent mitigation.). PCE has produced maps showing the risks of flooding to urban areas throughout New Zealand. These very accurate maps - showing modelled inundation of land lying lower than 50 cms above sea level - have been enabled by the lidar accurate maps now being prepared by territorial authorities. Here is an example:

Here is the PCE map for a part of North Shore including Devonport. The purple areas are less than 50 cms above present natural sea level. These are the urban areas that will be prone to inundation under the conservative scenario described in the PCE report combining 30cm of climate change induced sea level rise + a 20cm storm surge.

This blog posting shows what happened around Devonport when there was a high tide and storm surge combined in January 2011. (Research: According to historic weather data, on that day in Auckland the atmospheric pressure dropped to 995 millibars. This was the main cause of the flooding that day).You just need to add 30cm in water height to the photos in this posting to get an idea of what PCE's forecasts suggest is certain to happen in the not too distant future. And that's without the sort of storm surge low pressure weather system that led to the sinking of the Wahine.