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Rising sea levels

Summary

“…72,000 New Zealanders [are] currently exposed to present-day extreme coastal flooding, along with about 50,000 buildings worth $12.5 billion. The risk exposure increases markedly with sea-level rise, particularly during the first metre of rise, which means long-term planning to address the risk is urgent. There is near certainty that the sea will rise 20-30 cm by 2040.”NIWA

“A common response to increasing climate risk is to “harden the coasts” to defend property from inundation. However, engineering solutions like seawalls, stopbanks, and levees only delay damage at best and might even be counterproductive, as it encourages intensification in hazardous locations. Responses to sea level rise insurance retreat should attempt to eliminate the underlying risk by moving homes out of harm’s way. – Storey et al, 2020

  • Sea levels rise relative to the land for many reasons (Fig. 6) including land sinking or rising (top image).
  • Sea level rise due to climate change is accelerating because the ocean is warming and the Greenland and Antarctic icecaps are melting at an unprecedented pace.
  • There is a lag time between rising temperatures and sea levels. Currently, we are already committed to a rise in sea-levels of 2-3m but the time frames are uncertain due to uncertainty around the tipping points of melting ice sheets.

Effects

“Sea level rise does not look like the ocean coming at us… It looks like the groundwater coming up.” – Tada, 2020

Home > Climate wiki > Impacts > Rising sea levels

Summary

“…72,000 New Zealanders [are] currently exposed to present-day extreme coastal flooding, along with about 50,000 buildings worth $12.5 billion. The risk exposure increases markedly with sea-level rise, particularly during the first metre of rise, which means long-term planning to address the risk is urgent. There is near certainty that the sea will rise 20-30 cm by 2040.”NIWA

“A common response to increasing climate risk is to “harden the coasts” to defend property from inundation. However, engineering solutions like seawalls, stopbanks, and levees only delay damage at best and might even be counterproductive, as it encourages intensification in hazardous locations. Responses to sea level rise insurance retreat should attempt to eliminate the underlying risk by moving homes out of harm’s way. – Storey et al, Dec. 2020

  • Sea levels rise relative to the land for many reasons (Fig. 6) including land sinking or rising (top image).
  • Sea level rise due to climate change is accelerating because the ocean is warming and the Greenland and Antarctic icecaps are melting at an unprecedented pace.
  • There is a lag time between rising temperatures and sea levels. Currently, we are already committed to a rise in sea-levels of 2-3m, but the time frames are uncertain due to uncertainty around the tipping points of melting ice sheets.

Effects

“Sea level rise does not look like the ocean coming at us… It looks like the groundwater coming up.” – Tada, 2020

Sea levels are constantly changing

Sea level is generally referred to as ‘mean sea level’ or ‘MSL’ because the height of the ocean relative to the land is constantly changing for a multitude of reasons (Figs. 1 & 5). Some changes, like waves and tides, are episodic and temporary. Others such as El Niño /La Niña and the episodic wobble of the Moon leading to higher than normal tides over longer periods, can last for months, while earthquakes can change this in minutes (Video 1) or over millennia. Taking water for irrigation and forcing Canterbury’s braided rivers into narrow channels to prevent floods, has also prevented the rivers from delivering sediment to the coast, replacing what was taken by waves.

“The conventional wisdom is that you harvest flood water in the winter and store it until it’s needed (for agriculture) in the summer. However, floods are required to carry gravels to the coastal zone and if there’s not enough gravel, the waves get hungry and start eroding the land. Sure, it’s [storing water] a solution, but it’s also creating a problem.” Dr Scott Lanard, NIWA

Video 1: In less than 2 minutes the Papatea Fault, part of the 2016 Kaikoura earthquake sequence, raised the seabed up to 3m in places. However, rising sea levels will still affect areas of the coast as only parts of the shoreline were uplifted and the sea can still reach inland behind the raised reefs. (Geonet NZ)

Sea levels driven by climate change

When the climate cools, eustatic (global) sea levels drop because rain and snow that falls on the land builds up into glaciers instead of being carried by rivers into the ocean. Glaciers eventually merge to become ice sheets several kilometres thick. At the same time, the ocean cools and contracts. Both if these result in sea levels dropping.

When the climate warms, glaciers and ice sheets on the land melt and drain into the ocean. At the same time, the ocean warms and expands, so sea levels rise.

Today, eustatic sea levels are rising because Earth’s climate is getting warmer. While this is a global change, sea levels are not the same height everywhere because of local and regional factors including salinity, temperature, currents, gravity, and sinking coastlines (Figs. 1, 4 & 6). This means rising sea levels affect different coastlines in different ways.

How high have global sea levels risen?

Instructions for interactive graphs (Credit: The 2°Institute.)

  • Mouse over anywhere on the graphs to see the changes in global sea levels over the last thousand years.
  • To see time periods of your choice, hold your mouse button down on one section then drag the mouse across a few years, then release it.
  • To see how this compares to the past 800,000 years, click on the ‘time’ icon on the top left.
  • To return the graphs to their original position, double-click the time icon.

Instructions for interactive graphs (Credit: The 2°Institute.)

  • Mouse over anywhere on the graphs to see the changes in sea levels over the last thousand years.
  • To see time periods of your choice, hold your mouse button down on one section then drag the mouse across a few years, then release it.
  • To see how this compares to the past 800,000 years, click on the ‘time’ icon on the top left.
  • To return the graphs to their original position, double-click the time icon.

How much higher will they rise?

The models used by the Intergovernmental Panel on Climate Change predict a sea level rise contribution from Greenland of around 10 centimeters by 2100, with a worst-case scenario of 15 centimeters. But that prediction is at odds with what field scientists are witnessing from the ice sheet itself. According to our findings, Greenland will lose at least 3.3% of its ice, over 100 trillion metric tons. This loss is already committed – ice that must melt and calve icebergs to reestablish Greenland’s balance with prevailing climate.” – Alun Hubbard, Prof. of Glaciology, University of Tromsø, August 2022

“A sobering thought is that even if we somehow managed to turn global warming off right now, the atmosphere would keep warming for some years to come because of the heat that’s stored in the ocean.” Dr Craig Stevens, NIWA

Sea levels don’t instantly respond to warming, just as the climate doesn’t instantly respond to adding too many greenhouse gases into the atmosphere. Earth is large, and there’s a long lag time, so we know that sea-levels will continue to rise for thousands of years. We are now reaching dangerous tipping points, where ice sheets may collapse, which will dramatically accelerate rising sea levels.

Most estimates used by the Ministry for the Environment are based on the IPCC 2013 Fifth Assessment Report (AR5), which didn’t predict the extraordinary rate of of melting fthat’s been happening these past few years, from the Greenland ice sheet. These outdated estimates are:

  • 20cm – 30cm by 2040
  • 50cm – 1.1m by 2100

Research and observations of melting ice sheets in Greenland and Antarctica since the 2013 IPCC Report, including the 2019 IPCC Report on the State of the Cryosphere, make it clear that if we don’t cut net carbon emissions we could see more than a 2-metre rise this century.

We know sea-levels can rise this quickly because 13-14,000 years ago, they rose as much as 2 metres in 50 years following a Meltwater Pulse Event. Today, Earth is warming far faster (see ‘How hot could it get?‘).

“Around 13,000 year ago, for several centuries, sea level was rising about 4 metres per century.” – Professor Eric Rignot University of California and Senior Research Scientist for NASA’s Jet Propulsion Laboratory (Video 2: 19.22 – 20.20).

Video 2: Professor Eric Rignot presentation at the National Academies of Sciences and Engineering.

“The last time the world was 4°C warmer, the Ross Ice Shelf was gone, the West Antarctic Ice Sheet was gone. Sea level was about 20 metres higher than it is today.” – Professor Tim Naish, Victoria University of Wellington

Every coastline responds differently

Around New Zealand, coasts have built up over millennia by rivers that carried sand and gravel (alluvium) to the coast, ash and lava from volcanoes, peat and mud in wetlands and lagoons building up over time, and earthquakes that have lifted the land or ancient seabeds and coral reefs, or caused the land to drop.

In the future, as temperatures rise, storms will become stronger and waves are likely to become larger, so ‘soft shore’ coastlinessand, gravel and rocks, mud, ash etcwill erode faster than hard rocky coastlines. In places where these ‘soft’ coasts are cliffs, such as South Taranaki and South Canterbury, rising sea levels means the waves will reach higher and further inland, undercutting the soft cliffs and causing them to collapse. Some of this eroded material may be carried along the shore by currents and washed up on nearby beaches, but the land above the cliffs will be lost (see Canterbury case studies).

Overall, erosion will happen faster along beaches that have bigger waves. While rocky volcanic cliffs such as those around Banks Peninsular and low lying rocky beaches won’t erode much in our lifetimes, low-lying areas will eventually be inundated (drowned) by rising seas. The impacts will be compounded in areas where the land is sinking, such as Christchurch and North Canterbury (Fig. 3)

Unless an earthquake lifts an entire coastline evenly, parts of the coastline will still be affected by rising sea levels. The Papatea Fault (Video 1) for example, lifted a section of the sea bed at an angle to the coast. That means rising seas will still reach the beach. However, the newly uplifted areas may help reduce the impact of wave erosion.

NIWA’s ‘coastal sensitivity index’ takes multiple factors into consideration to map the vulnerability of coastlines to erosion (Fig. 2).

Fig. 2: (Image: NIWA)
Fig. 3: Click on this image to be taken to the SeaRise map. Here, you can zoom in to any area, to see how much land is sinking along coastal areas. In these areas, the effects of rising sea levels will be felt sooner. These effects include: erosion, made worse by storm surges; poor drainage, especially after heavy rains or flooding; and salt water entering what were freshwater aquifers, affecting wells and coastal wetlands.

Flooding: a risk multiplier

“By the end of the century, depending on whether global greenhouse gas emissions are reduced, it could rise by between 0.5 to 1.1 m, which could add an additional 116,000 people exposed to extreme coastal storm flooding.”NIWA

Low-lying coasts near rivers are particularly vulnerable during storms. Low-pressure systems raise sea levels, storm waves are bigger and reach further inland, and the water from rain-filled rivers and rain-drenched land can’t drain away. Together, this can result in widespread flooding inland as well as along coastlines, and coastal erosion.

Small sea-level rise increments of 10–20cm predicted to happen around the NZ coast in the next 20–30 years may not seem like much. But the number of times coastal areas are likely to flood is increased. According to NIWA the current exposure to coastal flooding across New Zealand is several billion dollars (Fig. 5).

Inundation maps: ‘bathtub’ estimates

Inundation maps (eg: Fig. 4) are based on topography. They’re useful where a coastline is a hard rocky shore (like the edge of a bathtub), but they do not factor in how dynamic coastal processes will change the shape of estuaries or hapua, how river mouths might migrates inland, or how much sand and gravel that high storm waves will erode and carry into water that’s too deep for smaller gentler waves or currents to bring back onshore. It doesn’t include engineering works, such as sea-walls.

The way these maps are made, a 2m sea level rise ‘inundation map’ of, for example, South Canterbury or South Taranaki, would show little or no change to the coast. In reality, considerable chunks of both areas will wash away long before sea levels reach the tops of the cliffs. For example, see this map of Pegasus Bay showing the coastline as it was 9,500 years ago.

If you live in Christchurch or the Banks Peninsula, see the city Councils’ risk hazard map here.

Fig. 4: This is a snapshot of an inundation map around the Ashley River estuary north of Christchurch. The coastline here is ‘soft’, so coastal processes including increasing storm waves will rearrange the configuration of the landscape as sea levels rise. Click on the image to use the interactive online tool to view other areas around New Zealand and the world, based on different temperatures and sea level heights.

Current coastal flooding exposure by region: this risk will increase as sea levels continue to rise

Fig. 5 (image: NIWA)

Reasons why sea levels change and are not the same everywhere

Fig. 6. Sea levels rise and fall relative to the land for many reasons. Every strip of coastal land responds differently to the interplay between complex and ever-changing dynamic forces, from waves and storms to melting ice caps, all of which need to be considered when making decisions about managing and living on or close to coastal environments (Table: Whitelaw).

New Zealand stories

Several well-research stories that cover the impacts on New Zealanders are published by Stuff, some using a storytelling multi-media platform. These stories breath life into what is now an everyday reality, not a distant problem, for many Kiwis:

Fig. 7: After debris flow from the valley behind hit homes in 2005, Matatā residents were asked to shift due to the risk. (Image: Dominico Zapata/ Stuff)

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