The Mediterranean Sea is an area particularly vulnerable to climate change. Its high density of coastal population (34% compared to 10% globally) and its economy based largely on sun and beach tourism have encouraged the urbanization of the coast. Once anthropized, these coasts are more vulnerable to sea level rise and increasingly face flooding and erosion phenomena that put the population and infrastructure at risk.
The researchers combined tide gauge and satellite data with measurements of ice melt to model sea level change in the Mediterranean basin since 1960. To their surprise, they found that the sea level fell by about 9 mm between 1960 and 1989, due to the increase in atmospheric pressure over the basin.
But since 1989, warming oceans and melting land ice have led to rapid sea level rise, reaching an average rate of 3.6 mm per year in the Mediterranean basin over the past two decades. However, the increase has not been evenly distributed.
Their findings, which are published in JGR Oceans, show that the Adriatic, Aegean and Levantine seas have increased by 8 cm over two decades, while the passage of Crete in the eastern Mediterranean has increased by half this amount.
The causes of sea level rise
20,000 years ago, when Homo sapiens was still a hunter-gatherer species, the surface of the oceans was about 120 m below its present level. The reason is that much of the water that is part of the oceans today was ice then. These large polar caps slowly shrank due to natural processes to their present extent and, like climatic conditions, have been stable for about 10,000 years.
During this period, called the Holocene, human civilization developed, largely favoured by stable climatic conditions. It is not surprising, therefore, that we have considered our coasts as static lines, where the sea level has remained practically unchanged for millennia, to the point that today the regions near the sea concentrate, in relative terms, most of the human population.
With global warming derived from the emission of greenhouse gases into the atmosphere, the planet is undergoing changes in its climatic conditions of a magnitude and at a speed unprecedented in hundreds of thousands of years.
One of these tangible, and potentially most damaging, changes is sea level rise. Globally, the sea level rises for two reasons: firstly, due to the warming of the oceans, which expand by accumulating the heat that results from absorbing more than 90% of the extra energy from global warming. Secondly, due to the loss of continental ice, both from mountain glaciers and from the polar caps, which when melting adds more water to the oceans.
Sea level has risen by about 20 cm
On average, the sea level has risen by about 20 cm since the beginning of the 20th century, of which 8 cm has been during the last 30 years. That is, it is not only increasing, but the rate at which it is increasing is accelerating.
When the sea level rises, the configuration of the coasts changes, but not in the same way everywhere. To begin with, sea level changes are not homogeneous across the planet, mainly due to regional differences in heat absorption and ocean circulation.
In some regions, the sea level can rise up to 30% more than the global average. Furthermore, not all coastlines respond in the same way to a change in the position of the ocean level. For example, low-lying coastal areas, such as river deltas, can be permanently flooded; instead, sedimentary shorelines respond dynamically, while corals grow vertically when the water column that covers them does.
How much has it increased since there were records?
Our cores are some of the oldest environmental records, obtained from tide gauge observations. A tide gauge is an instrument anchored to the coast that continuously measures changes in sea level with respect to a fixed reference.
In the Mediterranean, the sea level has risen 16 cm since there are records and half of them only in the last 30 years. As a consequence of these changes, the Mediterranean coasts have eroded and have receded 0.5 m/year since the 1980s.
Sea levels will continue to rise in the coming decades and centuries. Climate models that represent possible future scenarios of the planet’s climate indicate that, assuming a drastic reduction in emissions (compatible with a global temperature increase of 1.5 ℃ compared to the pre-industrial era – let’s not forget that we have already arrived at 1.2 ℃), the sea level at the end of this century will be almost 40 cm above what we had at the beginning of the 21st century.
In a more negative scenario, with temperature increases of up to 5℃, this value would probably be at least 80 cm. And what is more striking is that, due to possible instabilities of the Antarctic ice sheet, with such an increase in global temperature, values of 1.5 m at the end of the century, 2 m at the middle of the next century and down to 10 m by the year 2300.
The implications of a change of this magnitude would be enormous in all coastal areas of the world, including the complete submergence of large urban areas in the long term.
Disappearance of the Mediterranean beaches
In an environment as sensitive to sea level rise as the Mediterranean, the consequences will already be noticeable in the coming decades. The coasts are increasingly vulnerable to the impacts of extreme events that cause flooding.
With higher sea levels, the magnitude and frequency of extreme events will increase and what is now a rare event that occurs once in 100 years will be observed every 5 years by mid-century and by the year 2100 it will be something common that will happen several times a year. Coastal erosion will also worsen, with average setbacks of between 50 and 80 m, depending on the climate scenario.
This effect will result in the disappearance of a large part of the Mediterranean beaches, especially the urban and highly anthropized ones that do not have enough space to move the beach inland as the sea level rises, with the impact that this entails for economies based on recreational coastal tourism.
Some of the consequences of sea level rise are now unavoidable since a part of future changes is compromised due to past and current emissions. However, the full magnitude of the changes and the speed at which they occur depend on actions and emissions now and over the next few decades. Our actions can mean the difference between the ability or not adapt to a new configuration of our coasts and the irreversible loss of currently densely populated regions.
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