Monday, May 17, 2010

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.


Anthony Douglas said...

I'm slow. Only just noticed the slight revamp to the site. It's a lovely photo, well chosen, even in composition.

But I liked the old tagline (the first part, at least - that old guy just liked big words!)

Oh, and on topic: yes, it is interesting. I have to confess, though, that I'm still not sold on the moon having enough pull to do the tides. I think it's a conspiracy...

byron smith said...

Ah, yes, the moon-gravitational effect, always championed by lunatics...

As for the re-vamp, it was actually an experiment that I forgot to reverse. I've been meaning to do something about the site's visual appearance for about two years, but busyness, forgetfulness and especially my own visual and virtual incompetence have consistently thwarted me.

Anthony Douglas said... I simply can't see how it should be able to control how long a month is.

Well, count me as one vote for keeping the pic. I love how it leaves implicit room for new things!

jessica smith said...

I like the picture, but I also liked the tag line.... about thinking about the future in a not doomy gloomy kind of way.... especially considering that when you wrote the tag line you didn't know what the blog was actually going to be about and how right you were!

stef said...

is it too late to switch careers? Surely there is some cross-over from and arts/philosophy/theology training to the geosciences - though I suspect all the maths and physics would be a bit of a catch up session!