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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

Response: retreating from coasts and rivers.

“…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 wave heights 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 and subsidence can permanently lower or raise section of land relative to the ocean in minutes (Video 1) or over millennia. Harvesting water for irrigation and forcing Canterbury’s braided rivers into narrow channels to prevent floods, has cut off most of the sediment supply that once kept the coastline from growing outwards. Instead, it’s eroding.

“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 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

Eustatic (global) sea levels drop when Earth’s climate cools. Rain and snow that falls on the land builds up instead of being carried by rivers into the ocean. Glaciers form and eventually merge to become ice sheets several kilometres thick. At the same time, the ocean cools and contracts, so sea levels go down.

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?

“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

“It’s not a question of how much, but how fast and when

Most current estimates used by the Ministry for the Environment are based on the PCC 2013 Fifth Assessment Report (AR5), which didn’t predict the 2019 doubling of meltwater from Greenland.

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

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. But once those gasses are up there, the planet will continue to heat and sea-levels will continue to rise for thousands of years. The more we add, the more we ‘lock in’ the effects, and the more likely we are to reach dangerous tipping points such as the abrupt collapse of ice sheets, which will dramatically accelerate the pace of rising sea levels.

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, strongly indicate that if we don’t cut net carbon emissions we could see more than a 2-metre rise this century; much higher than most scientists expected a few years ago.

We’ve already warmed the planet 1.2°C, melting across Greenland and Antarctica is accelerating, and we’re on track to reach more than 3°C. The most recent estimates (2021) are:

“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

When ice caps were melting 13-14,000 years ago, sea levels jumped as much as 2 metres in 50 years following a Meltwater Pulse Event). Earth is heating up faster than at any since the comet took out the dinosaurs, it’s hard to know for sure what to expect (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.

Every coastline responds differently

“The conventional wisdom is that you harvest flood water in the winter and store it until it’s needed 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 a solution, but it’s also creating a problem.” – Dr Scott Lanard, NIWA

Around New Zealand, coasts have built up over millennia by rivers that carried sand and gravel to the coast (alluvium), ash and lava from volcanic eruptions, layers of peat and mud building up over time in ancient wetlands and lagoons, and tectonic uplift (earthquakes) that have lifted the land or ancient seabeds and coral reefs, or caused and continue to cause 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 allow waves to reach higher and further, undercutting the 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 around 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, so seawater can still reach the beach. However, the newly uplifted areas may help reduce erosion from waves.

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. This means the effects of rising sea levels including drainage, erosion and storm surges will occur much faster.

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. Nor is engineering to prevent inundation or drainage of low-lying areas factored in.

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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