Coastal assets and ocean changes: how can Asset Managers prepare?

Humans are changing the Earth’s climate, which changes the oceans.

Coastal asset managers ought to account for:

• Increasing temperatures,
• Intensifying storms,
• Worsening storm surges,
• Rising sea levels,
• Acidifying waters.

Warming oceans and intensifying storms

As humans increase the concentration of greenhouse gases in the atmosphere, heat gets trapped and warms the air.

Much of this heat is absorbed by the oceans, warming the water temperature, mainly near the surface with some reaching the depths. From 1971-2010, the ocean surface down to about 700 metres took up an estimated 63% of the excess heat added to the planet. Water can absorb a lot of heat without a large temperature increase, but we still see the oceans getting hotter.

Since the industrial revolution, sea surface temperature has risen on the order of 1.0°C with a possible rise over 3.0°C by the end of the century, depending on how we act to stop climate change.

Warmer waters might not directly affect coastal land assets. However, they play a significant role in increasing storm intensity, which can indirectly impact these areas.

Tropical cyclones (hurricanes, cyclones, and typhoons) are expected to decrease in number due to human-caused climate change. However, when one forms, its strength is partly driven by heat from the ocean while warmer air holds more moisture. So, although reducing in frequency, the average wind speed and rainfall are expected to increase.

Moreover, tropical cyclones may be slowing down as they travel, leaving more time to dump rain over a single location. Houston’s terrible river flooding and surface flooding during Hurricane Harvey in 2017 showed these signals of increased intensity and has been partly attributed to human-induced climate change.

Whether or not tropical cyclones are expanding their ranges under climate change is unclear, as comprehensive records of storms have only been maintained since the 1960s. Numerous multi-decade climate variabilities affect and continue to affect where tropical cyclones form and where they travel. Extracting the climate change signal from all these complications is ongoing work.

Nevertheless, asset managers along any low-lying coast must consider the possibility of a storm and flooding that may follow – most notably river flooding, surface flooding, and coastal flooding – irrespective of human-caused climate change.

Plus, tropical cyclones impacts might not be confined to the coastline but extend inland.

In 2021, following parts of the paths of at least three previous storm tracks in living memory, Hurricane Ida made landfall along the Gulf of Mexico coastline and then swung northeast. It brought intense rainfall and killed over three dozen people across four east coast states, by moving overland toward the Atlantic Ocean.

Storm surge

As part of intensifying storms, significant physical risks to coastal assets can come from storm surges.

Storm winds blowing across the ocean toward land push water into the coast, making the tide seem to be higher. The longer the water distance over which the wind blows, called fetch, the more the water piles up - at times up to several metres.

This water level rises even more as the low atmospheric pressure at the storm’s centre sucks water up. The latter increases the water’s height by about one centimetre for every millibar that atmospheric pressure drops, with tropical cyclones being, on average, around 60-70 millibars below average atmospheric pressure at sea level.

Storm surges can propagate up estuaries and rivers, producing brackish river flooding. It also further inhibits rainfall drainage, contributing to surface flooding.

Coastal assets will generally be designed and placed to be out of range of the yearly maximum tide, perhaps with some extra room to account for decadal variations in tidal level, storms, or waves.

An extreme storm surge is rarely considered, especially to a height greater than the entire tidal variation.

Waves riding atop the storm surge can surpass sea walls, applying tremendous force that weakens their structure. In addition, these waves can circulate around flooded properties, breaking through windows and doors, and eroding foundations.

Rising Sea Levels

As the world’s air temperature rises, temperature rises, snow, ice, and permafrost are melting.

This water runs off into the oceans, adding mass and volume. The oceans’ increased mass does not really push the ocean floor downwards, nor push aside the continents. The main way to account for the increased volume is to go up, witnessed as sea-level rise, with this melt accounting for around half of sea-level rise seen at the moment.

The other half of today’s sea-level rise is from ‘thermal expansion’.

Water’s density changes with temperature. As the oceans absorb heat from the atmosphere, the water’s temperature rises, its density decreases, and so its volume increases. We witness this volume increase as sea-level rise, which compounds coastal flooding.

Sea-level rise has averaged about 1.3 millimetres per year from 1901-1971, accelerating to close to 4.0 millimetres per year over the past decade. This rate continues to accelerate. Without any changes to our actions causing climate change, by 2100 sea level could rise to over 1.0 metre above the 1980’s level, at the high end. One complication in all these calculations is that sea level is not the same everywhere all the time. The average level - not considering waves, tides, or wind effects - varies by tens of centimetres globally. Also, ocean mixing is incomplete leading to large regional variations in sea level.

Tens of centimetres of variation is small compared to potential maximum sea-level rise. If the huge ice sheets covering Kalaallit Nunaat (Greenland) and Antarctica melt significantly, then they could add more than a dozen metres to sea level—higher than a four-storey building—over coming centuries. If all this ice melts, then sea level would reach the top of high-rise buildings. The US east coast exemplifies, with rich areas from Miami Beach to Manhattan far underwater.

Asset managers have centuries to prepare for these full impacts which would reconfigure the world’s shores and require major ports to be rebuilt – or perhaps would encourage underwater and amphibious living along the old coastlines!

Other physical risks

Before sea levels reach these extreme values, asset managers must deal with ocean acidification.

The ocean absorbs carbon dioxide from the atmosphere, which combines with water to form carbonic acid.

As a consequence, the pH lowers, acidifying the oceans.

Impacts on different building materials and designs is not well-investigated. Buildings close to the ocean, especially those made of limestone that touch the water, would deteriorate faster than they currently do. Impacts of sea spray on exposed structures need to be considered, but the consequences compared to the salt spray and other factors have not yet been quantified in detail.

Other coastal changes offer little immediate impact. Freshwater added to the saline seas reduces the salinity, but rarely enough to be noticeable while incomplete mixing of the oceans around the world can make these changes highly localised. Much bigger changes are hard to determine. The oceans are connected like a conveyer belt moving water around the world, called the thermohaline circulation. The most famous segment is the Gulf Stream from the Caribbean area to northern Europe, technically called the Atlantic Meridional Overturning Circulation (AMOC). If these circulations change, then the impacts on the oceans and the weather could be swift and severe.

Data for climate adaptation

These multiple, intersecting, physical risks from oceans impacted by climate change should be analysed in tandem to determine what could happen to assets and how to adapt to the changing seas.

Data on past and present local-to-global oceans can input into and be combined with future projections of different ocean scenarios.

Although the satellite era does not always give sufficient data to remove all uncertainties, it provides a wealth of continuous data that providers and modellers can use, improve, add to other data, and apply for risk analysis and asset management.

The storm surge height of Hurricane Sandy in 2012 can be checked with the maximum surface flooding depth of Hurricane Ida in 2023, both of which badly affected New York City.

The extent of coastal flooding and river flooding in New Orleans during and after Hurricane Katrina in 2005 can be compared with what would have happened if the storm had been the strongest possible.

Extensive data on wind and water from Hurricane Andrew in 1992, which just missed Miami, could be mapped onto that city if a similar tropical cyclone made a direct hit today.

All such scenarios can be run for sea levels of a hundred years ago, today, and a hundred years from now to check the differences in damages, costs, and loss of assets.

Then, add in data on and expected changes in water temperature and ocean acidity to understand the full, wide-ranging impacts on assets of oceans under climate change. What adaptation interventions, from sea walls to building-by-building retrofitting, would offer the best benefit-cost ratio over two, ten, and fifty years?

Then again, not linked to climate change, a tsunami could happen at any time with limited notice, as seen around the Indian Ocean on 26 December 2004 and along different Japanese coasts on 11 March 2011 and 1 January 2024.

Ultimately, for ocean changes, coastal asset managers must be flexible, adaptable, and ready for anything.