Government projections for sea level rise for 2100 used by councils for planning and adaptation may need to be doubled to 2 m according to new research from the world’s leading experts on Greenland and Antarctic ice sheets.
This new research says there is a one in twenty chance that seas could rise by more than 2 metres by 2100 if unchecked carbon emissions and “business as usual” leads to average global warming of 5°C.
The current government guidance has a worst case scenario of sea level rise of 1m by 2100. This is based on the 2013 Intergovernmental Panel on Climate Change report and a few 2014 and 2015 research papers. The red line on this graph from the guidance is the RCP 8.5 H+ scenario. The guidance requires councils to use this scenario H+ and avoid hazard risk in respect of coastal subdivision, greenfield developments and major new infrastructure.
The H+ scenario in the current guidance does take some account of possible instabilities in polar ice sheets and is based on the RCP8.5 (83rd percentile) projections from Kopp et al (2014). Councils are therefore already required to plan for low probability sea level rise projections.
But the current guidance does not take account of the most recent new satellite measurements which are showing ice mass loss happening faster than models expected, or other studies which have shown ice cliffs in Antarctica could collapse
2 m of sea level rise by 2100 is double the current projection in the government guidance. The authors of this recent study acknowledge that the chances of hitting the high end of this range are small, around 5 per cent or 1 in 20, but they are plausible and should not be discounted, according to the lead author.
“If I said to you that there was a one in 20 chance that if you crossed the road you would be squashed or that your plane would crash you wouldn’t go near it,”
“Even a 1 per cent probability means that a one in a hundred-year flood is something that could happen in your lifetime. I think that a 5 per cent probability, crikey – I think that’s a serious risk.” said lead author Prof Bamber.
To put that in perspective the January 2018 storm surge in Thames was a 1 in 100-year event. The probability of sea level reaching 2 m by 2100 is, therefore, five times greater than the probability of that Thames event, which we have just experienced.
This new research reveals how hopelessly out of date and conservative the current guidance Sealevel projections really are. We were promised by the Ministry for the Environment that these projections would be reviewed annually. It is now 18 months since the current guidance was published.
Any revised government projections are going to be higher. And any revised low probability estimates are going to have to be followed by councils for major projects like subdivisions.
The review needs to happen urgently so that councils have the most up-to-date science and data to guide them with coastal planning and adaptation.
In the meantime, the message to councils which are planning major infrastructure or subdivision is – don’t rely on the current guidance because very soon it’s going to be reviewed with significantly higher sea level rise projections.
Some councils such as Greater Wellington Regional Council are already taking account of these 2m + projections for 2100 –
Here is what is left of Thames and Whitianga/Cooks Beach with 2m of sea level rise and a mid-range storm surge (maps generated from Waikato Regional Council’s coastal flood simulator). Serious question for a Council – if there was a 1 in 20 chance of this level of catastrophic of flooding of these townships within 80 years would you allow major new subdivision or infrastructure?