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Sea Level Rise and Increasing Coastal Flooding

How high and how fast sea level will rise over the coming decades isn’t clear, but it will continue to rise. It is estimated that globally there are currently millions of people living on land that will be below sea level or exposed to regular coastal flooding by the end of the century, but this figure could also be far higher.

BACKGROUND

Sea levels and coastlines have never been static. For millions of years sea level has drastically risen and fallen. The current sea level is just that; what the level happens to be right now. However, this will change over time as there is no correct or fixed point.

Earth has always experienced ‘ice ages’, periods of colder weather lasting for tens of millions of years, during which time large areas of the planet are covered by ice sheets and glaciers. It is estimated that there have been at least 5 major ice ages throughout history, within which are glacial periods (when ice coverage increases) and interglacial periods (warmer periods when ice retreats). There have also been times in history when there has been no polar ice at all, and the sea level was hundreds of feet higher than it is today; this last happened around 40 million years ago. Conversely, at the end of the most recent glacial maximum (greatest coverage of ice) around 20,000 years ago, sea level was approximately 120m lower than its current level.

Contributing factors

Many factors influence sea level at different locations around the planet, such as the isostatic rebound of land from the compressive forces of glaciers which have crushed down for millions of years. Other factors include the thermal expansion of seawater as it heats up, regional ocean currents, wind, and high/low pressure weather systems.

We are currently experiencing an interglacial period which began approximately 11,000 years ago and has seen sea level slowly rise as glaciers and ice sheets have receded.

Sea level hasn’t changed a great deal in the last 2,000 years, which in the grand scheme of time is no time at all. Since sea level began to be documented around 1880, the global average rose steadily at about 1.5mm per year, but over the last two decades this has increased to 3.2mm per year.

So, while on the one hand the level of the sea relative to the land has always fluctuated, this more recent, accelerated rise in level is being attributed to human activity. This is because the growing concentration of anthropogenic greenhouse gases (GHGs) (particularly CO²) in Earth’s atmosphere trap solar energy, preventing it from passing through our atmosphere and bouncing back out into out into space.

Industrialisation, burning fossil fuels, the cultivation of rain forests, and melting permafrost’s and glaciers keep pushing up the concentration of GHGs. CO² levels in the atmosphere are currently around 415 parts per million, which is the highest they have been for 800,000 years.

When you consider the data in the table below, and the interglacial periods where CO² levels are high, we are in unchartered territory. The CO² levels are off the scale!

If the main driver behind sea level rise is the warming of our oceans and atmosphere as a result of increasing levels of CO² and other GHGs, then what might the future bring?

Projections

The Intergovernmental Panel on Climate Change (IPCC) have run models and produced a special report on ocean and cryosphere in a changing climate, which projects a sea level rise of between 0.3m and 0.6m if future global CO² emissions are on the low scale. If CO² emissions are on the high end of the scale, sea level rise could increase to between 0.6 and 1.1m, but substantially higher estimates should not be ruled out. Beyond this, sea level will continue to rise, and remain at these higher levels for many thousands of years more.

The image below is a table which was put together by renowned oceanographer and sea level rise expert John Englander, and illustrates the historic relationship between sea level, global temperature and the concentration of atmospheric CO². These 3 things appear to have moved up and down in sync over the last 400,000 years.

Looking ahead?

Globally, CO² outputs are still on the rise, which doesn’t bode well for keeping to the Paris Agreement target limit of +1.5°c above pre industrial levels.

The rate of sea level rise has doubled over the last 30 years, up from 1.7mm per year to 3.2mm per year, and it can’t be accurately estimated how much more seal level rise is already ‘locked in’ or what this will look like over next 30, 100, or 500 years.

Are we now just waiting for global temperature and average sea level to follow the same steep upward curve as CO² levels have, and if this does happen, what would it mean?

There is growing opinion and evidence that sea level rise will not only continue but will rise faster and higher than the current forecasts suggest. So, is it realistic to continue building larger coastal flood risk management schemes indefinitely, or should we now be planning for the inevitable?

Sources: NOAA, Climate Central, IPCC

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