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Impacts: rising sea levels

Rising sea levels

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(Image: click to read ‘Beach Road’ story by Charlie Mitchell/Stuff)

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

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 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.

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, and even gravity (Figs. 1 & 5). This means rising sea levels affect different coastlines in different ways.

How high have global sea levels risen?

Eustatic sea levels have been rising at an accelerating rate for over 120 years.

  • Between 1901 and 2010 sea levels rose ~19cm at an average rate of 1.7mm/year.
  • Between 1992 and 2014 sea levels rose ~5cm at an average rate of 3.4mm/year (Fig. 1).
  • In 2019, melting just from the Greenland icecap alone (ie, no other sources) was 2.2mm in two months; twice as much as predicted in the PCC 2013 Fifth Assessment Report (AR5) 6 years earlier.

“Within our four largest cities, at least 10,000 houses currently sit within a 1-in-100-year coastal flood zone. Nationally, around 450,000 houses are within 1km of the coast. These homes are likely to be affected by more frequent and intense storms and by sea level rise. Worsening coastal hazards are not yet fully reflected in homeowners’ decisions to purchase, develop or renovate coastal property. New Zealand is also still building new residential developments in climate-risky locations. What does this mean for our house insurance?“  – Deep South Challenge

Fig. 1. Sea level change 1992 - 2014, based on data collected from the TOPEX/Poseidon, Jason-1, and Jason-2 satellites. Blue regions are where sea level has gone down, and orange/red regions are where sea level has gone up. Since 1992, seas around the world have risen an average of nearly 5cm, however this is just an average. The color range for this visualization shows regional variations -7cm to +7cm, though measured data extends above and below 7cm. This particular range was chosen to highlight variations in sea level change, not absolute changes, as some extreme changes are temporary (see Fig. 5).
Fig. 1. Sea level change 1992 – 2014, based on data collected from the TOPEX/Poseidon, Jason-1, and Jason-2 satellites. Blue regions are where sea level has gone down, and orange/red regions are where sea level has gone up. Since 1992, seas around the world have risen an average of nearly 5cm, however this is just an average. The color range for this visualization shows regional variations -7cm to +7cm, though measured data extends above and below 7cm. This particular range was chosen to highlight variations in sea level change, not absolute changes, as some extreme changes are temporary (see Fig. 5).

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

  • Mouse over anywhere on the graph to see the changes in global sea levels over the last thousand years.
  • To see details for time periods of your choice, hold your mouse button down on one section then drag the mouse across a few years, and release it.
  • To see how this compares to the past 771,000 years, click on the ‘time’ icon on the top left.
  • Compare this to rising global temperatures by clicking the planet/thermometer icon at the top left corner.
  • To return the graph to its original position, double-click the time icon to the left of the thermometer/planet 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

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 gasses 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.

Then again, we’ve already warmed the planet more than 1°C, melting across Greenland and Antarctica is accelerating, and in spite of the 2015 Paris Accord, we’re on track to reach more than 3°C.

“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. Today, Earth is heating up faster than it has in tens of millions of years, so 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

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 the land to drop.

In the future, as temperatures rise, storms will become stronger and waves are likely to become larger, so ‘soft shore’ coastlines—sand, gravel and rocks, mud, ash etc—will 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.

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. 2: (Image: NIWA)

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. 4).

Inundation maps: ‘bathtub’ estimates

Inundation maps (eg: Fig. 3) 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.

Fig. 3: 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.
Fig. 3: 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. 4 (image: NIWA)
Fig. 4 (image: NIWA)

Reasons why sea levels change and are not the same everywhere

Fig. 5. 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).
Fig. 5. 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. 6: After debris flow from the valley behind hit homes in 2005, Matatā residents were asked to shift due to the risk in 2016. (Image: Dominico Zapata/ Stuff)
Fig. 6: After debris flow from the valley behind hit homes in 2005, Matatā residents were asked to shift due to the risk in 2016. (Image: Dominico Zapata/ Stuff)

Explainers

Why does a warming ocean raise sea levels?

The oceans cover 70% of Earth and the deepest areas are almost 4,000m; that’s a huge area to store heat (Fig. 7). Additionally the ocean is dark, so it has a very low albedo meaning that it can absorb much more heat than the land. As water gets warmer, it expands. As it can only expand upwards, relative the to land, it rises.

However, the ocean doesn’t absorb heat evenly. Local factors such as ocean currents and weather systems like El Niño /La Niña also play a role in the temperature of water, as do the changing seasons.

Recent research reveals that the oceans are heating 40% faster than predicted in the IPCC 2013 Fifth Assessment Report. Water around New Zealand has also been warming faster (Fig. 8).

Fig. 7: Based on IPCC AR4 5.2.2.3 for the period 1993-2013. (Image: Skeptical Science).
Fig. 7: Based on IPCC AR4 5.2.2.3 for the period 1993-2013. (Image: Skeptical Science).
Fig. 8: Changes in ocean temperatures around New Zealand 2010 - 2019 (Image: NIWA).
Fig. 8: Changes in ocean temperatures around New Zealand 2010 – 2019 (Image: NIWA).

Gravity and sea levels:

It may seem perverse but as the Greenland and Antarctic ice caps melt into the ocean,they lose mass. This means the pull of gravity is reduced, so not as much ocean water is pulled towards them. This causes a relative drop in sea levels around them. See Fig. 5 (16).

Gravitational changes are also one of several ways that help ‘fingerprint’ where Meltwater Pulses (see below) originated.

The 2019 IPCC report on the State of the Cryosphere

This report acknowledged that Greenland was most likely responsible for much of the 4 metre rise in sea levels ~125,000 years ago during the Eemian Epoch. Back then, temperatures were 1-2°C warmer that pre-Industrial levels and CO2 in the atmosphere was 280ppm. Today, temperatures have passed 1.1°C and atmospheric CO2 is 410ppm and climbing.

The IPCC didn’t include melting ice caps in their models for rising sea levels until 2018 and 2019. By then, the Paris Accord, which is based on the earlier IPCC 2013 report, had been signed. In effect, many countries including New Zealand are working to keep warming under 1.5°C based on information that’s out of date, and models that don’t include the single largest concern around rising sea levels: the rapidly melting Greenland and Antarctica.

Net carbon emissions:

This means gross greenhouse gas emissions (all industrial activities + burning fossil fuels for energy + agriculture) minus (carbon sinks from forestry + changing agricultural to improve soils + regenerating natural ecosystems). Covid-19 has meant a temporary respite in carbon dioxide due to reduced transport, however methane emissions from agriculture, gas production, and also manufacturing in China have increased. Moreover, dangerous tipping points are being breached, with natural carbon sinks now becoming sources of methane and carbon dioxide.

Abrupt sea level rise (Meltwater Pulse events):

At the end of the last glacial (not the end of the ice age: we still are in the Pleistocene Ice Age) the ice sheets that covered much of the planet began to melt. While some meltwater flowed into the ocean, gradually raising sea levels, huge volumes of water also pooled into vast meltwater lakes dammed by the natural shape of the land, (still melting) glaciers, and/or ice shelves along coasts. As the Earth continued to warm, the lakes kept filling until they eventually burst (Video 3) and the meltwater rushed into the ocean.

Video 3: Abrupt collapse of ice dam

In other areas, coastal glacier (ice shelves) collapsed allowed glaciers to disgorge into the ocean. These ‘Meltwater Pulses’ (MWP)(Fig. 9) raised sea levels rapidly. Some events match stories of great floods from the cultures known to have inhabited the affected regions at that time.

There is still some uncertainty about how many of these meltwater pulse events occurred, as sea levels continued to rise at different rates 4,500 years ago, after which they, along with the global climate, remained relatively stable.

MWP-1Ao: ~19,600 – 18,800 years ago sea levels rose at least 10 metres.

MWP-1A: ~14,700 – 13,500 year ago sea levels rose 16 – 25 metres, much of which occurred over a 400–500 year period. The average rate was 40–60 mm/year, however there were periods of extremely rapid rise of up to 4 metres/century for several centuries.

Fig. 9: Meltwater 'pulse' events as ice sheets collapsed at the end of the last Glacial Maximum (Pleistocene Epoch). It's not clear how many MWP events occurred. Some researchers suggest there may have been 15 or 16, however the fastest appears to have been MWP-1A (Image: NASA).
Fig. 9: Meltwater ‘pulse’ events as ice sheets collapsed at the end of the last Glacial Maximum (Pleistocene Epoch). It’s not clear how many MWP events occurred. Some researchers suggest there may have been 15 or 16, however the fastest appears to have been MWP-1A (Image: NASA).

Younger Dryas: ~12,800 years ago thousands of cubic kilometres of icy water and icebergs from Lake Agassiz abruptly poured into the North Atlantic and Arctic Oceans (Fig. 10). This effectively switched off the North Atlantic’s circulation system, chilling the Northern Hemisphere. Winter temperatures in northern Europe plummeted by as much as 22°C until ~11,500 years ago, after which the global warming trend continued.

The volume of water released does not seem to have had a significant impact on the overall rate of sea-level rise, probably because the abrupt cooling meant glaciers started to expand again, locking away the excess water.

This highlights some of the complex feedback effects of abrupt warming. 

Fig. 10: Lake Agassiz in North America abruptly released freshwater into the Arctic and North Atlantic Oceans (Image:Sheffield University).
Fig. 10: Lake Agassiz in North America abruptly released freshwater into the Arctic and North Atlantic Oceans (Image:Sheffield University).

References and further reading

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