A rising tide lifts all boats? Not always...

The geographical distribution of sea level rise
Once you've read a fair amount about a certain topic, anything new you read generally sounds familiar, and repeats a lot of things you've heard before. In fact, it is a good sign that you are becoming acquainted with a field of knowledge when you don't run across many new ideas anymore.

But when you do, the surprise can be all the more powerful.

I thought I knew that gravity pulls us down towards the centre of the earth and that a rising ride lifts all boats. It turns out that the story is a little more complex.

I was aware that predictions of sea level rise (current best estimates are between 0.75-2 metres by 2100) would not be equally distributed around the globe, due to slow movements in the height of various landmasses, some of which are sinking or rising at a rate of a few millimetres each year from subsidence or glacial rebound after the last ice age. I was also aware that currents and local topography affect tidal amplitude. I was aware that sea levels are rising due to both thermal expansion and the melting of land-based ice (though not sea-ice).* But what I didn't realise was how significant the local gravitational effects of large ice-masses can be.
*Actually, the melting of floating ice can have a tiny effect due to differing densities of fresh water (stored as ice) and salt water in the ocean.

Of gravity and glaciers
Everything in the universe attracts every other thing in the universe; that is gravity. So my quip above about gravity only pulling down is too simplistic. We are also pulled up by the moon when it is overhead and sideways by local mountain ranges. Of course, these effects are so slight compared to the downward tug of the bulk of the earth that we don't feel them (though weighing yourself when the moon is overhead can make you almost half a gram lighter). But we do notice the tides, which are caused by the moon's gravitational pull.

In a similar, but even smaller way, land masses, mountain ranges, and large ice sheets exert a gravitational tug. Above submarine mountain ranges, the water is slightly higher than above valleys. Why this matters is that as ice sheets in Greenland and Antarctica melt, changes in their mass have gravitational effects on the distribution of sea level rises, increasing the rate of rise further from the poles and decreasing it closer to the ice-mass. This effect is so significant that if Greenland were to entirely melt (as most predictions say it will within the next few hundred or few thousand years), an average of about 7 meters of sea level rise would result, but in Hawaii this would be closer to 10m, while in Iceland, levels would actually drop slightly.

Counterintuitive? Yes. Significant? Yes, not only because it helps to solve a longstanding puzzle concerning the variable distribution of observed sea level rises, but also because it helps predict future sea level rises a little more precisely. You can read more about this here or here.

I love learning new things. Some days I think I should have pursued a career in the geosciences.
Significant changes in ice-mass can also effect changes in the earth's rotational axis, with further knock-on effects on ocean distribution.