Local sea level rise: A question of gravity
Imagine that sea level is more or less stable around the Earth (which it was, more or less, before the start of the twentieth century). Just like the moon causes tides because its gravity exerts a pull on the oceans, the ice sheets are large masses and their gravity also attracts ocean water, so the average sea level is higher closer to Greenland and to Antarctica. But there is only a finite volume of water in the oceans, so a higher sea level close to the ice sheets means lower sea levels further away in the tropics for example.
As the ice sheet melts and gets smaller, its gravitational pull becomes smaller too, so the average height of the sea around Greenland and Antarctica is lower than it was before, but the water gets redistributed around the Earth until it is in equilibrium with the gravitational pull of the ice sheets again. The sea level in other places is therefore much higher than it would have been without that gravitational effect.
And in general, the further away from an ice mass you are, the more these gravitational processes affect your local sea level change. In Northern Europe, it often surprises people (also here in Denmark) to learn that while Greenland has a small influence on our local sea level, it’s not very much because we live relatively close to it. However the loss of ice from Antarctica is much more important in affecting our local sea level rise.
Currently, most of the ice contributing to sea level is from the small glaciers around the world, and is we can notice a graviational effect here as well. The melt of Alaska and the Andes are more important to our sea level than the Alps or Norwegian glaciers because we are far from the American glaciers but close to the European ones.
This figure below illustrates the processes:
This is why it is important to not only study the total ice mass loss, but to also understand where it is happening. PRECISE is making a crucial contribution to this understanding by improving the state-of-the-art ice sheet models, to improve simulations of the ice sheets in the future and reduce uncertainties of the ice mass loss in Greenland and Antarctica. Other projects like PROTECT focuses on the science to policymakers pipeline. The PROTECT project is described in this Frontiers paper, which includes a graphic explaining what affects your local sea level.
As you can see, it very much depends on what time and spatial scale you’re looking at, with the two ice sheets affecting sea level on the longest time scales.
A good summary about local sea level rise is provided by this policy briefing. The most important points are worth highlighting here:
We expect that 2 m of global mean sea level rise is more or less baked in, it will be very difficult to avoid this, even with dramatic reductions in greenhouse gas emissions. But the timescale, as in when that figure will be reached, could be anything from the next hundred years to the next thousand.
What the map shows is that the timing at which any individual place on Earth reaches 2 m is strongly dependent on where on Earth it is. In general lower latitudes close to the equator will get to 2 m before higher latitudes, and while there are ocean circulation and other processes that are important here – to a large extent your local sea level is controlled by how close to the ice sheets you are and how quickly those ice sheets will lose their ice.
There are other processes that are important – especially locally, including how much the land you are on is rising or sinking, as well as changes in ocean and atmosphere circulation.
This article has been modified from a blog post by Ruth Mottram on https://sternaparadisaea.net/.