More than 130 million more people could find themselves living in areas that will end up below sea level or in areas extremely vulnerable to storm surges by the end of the century due to sea level rise. According to new simulations, if the oceans rise about a meter-a climate scenario considered likely by 2100-coastal areas exposed to flooding could be much larger than expected.
This scenario emerges from research published in the scientific journal Nature, which challenges some of the bases used to date to estimate ocean heights and predict the impact of climate change on the planet’s coastlines.
A wrong starting point
The heart of the study concerns how science has calculated mean sea level. Many climate models use as a reference a theoretical surface related to Earth’s gravitational field, the so-called geoid, a kind of mathematical “zero level” of the oceans.
The problem is that this ideal surface does not always coincide with the actual water level. Ocean currents, prevailing winds, changes in temperature, water density, and tides can significantly change the actual height of the sea along coastlines. When these factors are incorporated into the calculations, the picture changes.
According to the study published in Nature, the ocean level used as a reference in many analyses may have been underestimated by about 30 centimeters on average. In some areas of the planet, the difference would be even wider. In the Indo-Pacific and Southeast Asian regions, for example, the gap between theoretical models and actual sea levels could be as much as a meter or more.
Greater risks to coastal areas
If the starting level is higher, future projections also change. With oceans rising about one meter by the end of the century-a threshold considered plausible by many climate scenarios-the area of land exposed to coastal flooding could be about 37 percent larger than estimated to date.
This means that the risk maps used to assess the impact of sea rise may be too optimistic. River deltas, tropical archipelagos, and large coastal plains appear to be among the most vulnerable areas, especially where population density is high. Many coastal defense plans, from storm surge barriers to flood insurance schemes, are based on models that may therefore not accurately represent the real situation: if sea levels are higher than some simulations indicate, the margin of safety on which much infrastructure has been designed may be smaller.
A phenomenon that is already accelerating
The problem is part of an established trend. Since the late nineteenth century, the global average sea level has risen about 16 to 21 centimeters, and nearly half of this increase has occurred in the past three decades. Indeed, warming oceans are causing thermal expansion of water, while melting glaciers continue to add new volumes to the ocean system.
In many coastal areas, the situation is also aggravated by subsidence, that is, the gradual lowering of the ground due to natural processes or human activities such as water or hydrocarbon extraction.
