Arctic Ice Watch

Post by **kenneal - lagger** » 25 Jul 2016, 13:45

I think that the important thing on the rate of ice loss is the exponential function. At a linear rate of increase the time to complete loss of the Greenland ice cap might be 9000 years but at an exponential rate of loss, which is much more likely, the time could shorten drastically and could well be much less than 500 to 1000 years.

There is commonly quoted to be over 350 years of coal left on the earth at current rates of consumption but at the exponential increase which is seen with resource use there is less than 50 years worth. The actual length of time obviously depends on the rate of increase.

johnhemming2 · Post by **johnhemming2** » 25 Jul 2016, 13:52

You can sensibly predict it is no linear, but not sensibly predict exponential. That is quite unlikely in fact. It may accelerate, but not an exponential basis (using the mathematical definition of exponential).

Post by **kenneal - lagger** » 25 Jul 2016, 14:02

johnhemming2 wrote:You can sensibly predict it is no linear, but not sensibly predict exponential. That is quite unlikely in fact. It may accelerate, but not an exponential basis (using the mathematical definition of exponential).

I understand exponential to mean "at a constant annual rate of increase", ie at, say, 7% per year every year which would give a doubling, or halving, time of 10 years. What is likely to happen is that because of feed back mechanisms the percentage rate will increase every year which is a lot worse than exponential.

Post by **clv101** » 25 Jul 2016, 21:31

Ice sheets are complex. There are lots of different processes happening at the same time at different speeds, different rates of changing speed. There are also tipping points and brake points.

For example, currently about half of Greenland mass loss is surface melting and runoff, the other half is solid ice discharge from marine terminated glaciers. However, it won't always be 50/50. At the moment both runoff and discharge are increasing - however, as glaciers retreat they will eventually lift themselves clean out of the water. There won't be any discharge anymore, just surface melting. This will be a major regiem change from what we have now.

Post by **clv101** » 25 Jul 2016, 21:37

UndercoverElephant wrote:What about "rebound" when the weight of ice has gone? Or is that too slow a process to be relevant?

There are two different types of "rebound", elastic and isostatic. The elastic rebound actually happens instantly - you can see (with GPS) the rock sinking in the winter as the mass increases with snowfall and rises in the summer with melt. The isostatic response takes thousands of years - there's still a significant rebound signal from the last ice age (which has to be compensated from in radar and laser satellite altimetry measurement of surface elevation change).

The rebound has little impact on mean global sea level but has a major impact on the local, regional sea level around the Greenland coast. The other major local impact is changes to the Earth's gravitation field. The mass of the Greenland ice sheet attracts the ocean to it, increasing local sea level. Melt all the ice means there's more water in the ocean - BUT there's less gravitational attraction meaning that locally, sea level actually goes down.

UndercoverElephant · Post by **UndercoverElephant** » 25 Jul 2016, 22:02

clv101 wrote:
UndercoverElephant wrote:What about "rebound" when the weight of ice has gone? Or is that too slow a process to be relevant?
There are two different types of "rebound", elastic and isostatic. The elastic rebound actually happens instantly - you can see (with GPS) the rock sinking in the winter as the mass increases with snowfall and rises in the summer with melt. The isostatic response takes thousands of years - there's still a significant rebound signal from the last ice age (which has to be compensated from in radar and laser satellite altimetry measurement of surface elevation change).

The rebound has little impact on mean global sea level but has a major impact on the local, regional sea level around the Greenland coast. The other major local impact is changes to the Earth's gravitation field. The mass of the Greenland ice sheet attracts the ocean to it, increasing local sea level. Melt all the ice means there's more water in the ocean - BUT there's less gravitational attraction meaning that locally, sea level actually goes down.

OK cheers for explaining that.

As for local sea levels...I have been trying to get my head round the way tides work recently, as part of research for a book on plant foraging (which will include a section on seaweeds, the best of which are only available at low spring tides). This has involved trying to understand tides in different places - I never realised how much they are influenced by local conditions (and you've just added another thing to the list of what is influencing them). Places near Lulworth Cove have a double low tide - the sea goes out, then comes in a bit and goes out again before it comes back in properly. Just round a headland or three in Poole Harbour, there is a double high tide instead. And in the gulf of Mexico there is only one low tide and one high tide each day. Maybe people are already aware of this diversity of tidal action, but I certainly wasn't! For something so obviously the result of the predictable laws of physics, the details of the tides remain incredibly mysterious and hard to understand...

johnhemming2 · Post by **johnhemming2** » 25 Jul 2016, 22:20

Musicians are used to resonance and harmonics.

UndercoverElephant · Post by **UndercoverElephant** » 25 Jul 2016, 22:44

johnhemming2 wrote:Musicians are used to resonance and harmonics.

I am also a musician, but I can't say it has helped much in understanding tides!

Post by **kenneal - lagger** » 26 Jul 2016, 03:03

Portsmouth and Southampton have two high and two low tides a day as one comes up the Solent from the west and the other comes round the Isle of Wight from the east.

johnhemming2 · Post by **johnhemming2** » 26 Jul 2016, 08:56

UndercoverElephant wrote:
johnhemming2 wrote:Musicians are used to resonance and harmonics.
I am also a musician, but I can't say it has helped much in understanding tides!

Try listening to a guitar with a single open string when harmonic tones are played and then stopped. Or indeed playing harmonics on the guitar. The trumpet etc is rather obvious, but harmonics on the guitar demonstrates how resonance can happen in practice.

UndercoverElephant · Post by **UndercoverElephant** » 26 Jul 2016, 09:48

johnhemming2 wrote:
UndercoverElephant wrote:
johnhemming2 wrote:Musicians are used to resonance and harmonics.
I am also a musician, but I can't say it has helped much in understanding tides!
Try listening to a guitar with a single open string when harmonic tones are played and then stopped. Or indeed playing harmonics on the guitar. The trumpet etc is rather obvious, but harmonics on the guitar demonstrates how resonance can happen in practice.

What hast that got to do with the tides, John?

UndercoverElephant · Post by **UndercoverElephant** » 26 Jul 2016, 09:54

kenneal - lagger wrote:Portsmouth and Southampton have two high and two low tides a day as one comes up the Solent from the west and the other comes round the Isle of Wight from the east.

Most places have two low and two high tides a day. According to the charts, Portsmouth and Southampton are typical in this respect!

http://www.tides4fishing.com/uk/england/portsmouth

Compare with Lulworth's four lows:

http://www.tides4fishing.com/uk/england/lulworth

Or Poole's four highs:

http://www.metoffice.gov.uk/public/weat ... /gcn89bnfb

woodburner · Post by **woodburner** » 26 Jul 2016, 11:18

Southampton is slightly different from most places. It gets two peaks at each high tide which effectively extends the high tide state to about two hours.

https://www.tidetimes.org.uk/southampton-tide-times

UndercoverElephant · Post by **UndercoverElephant** » 26 Jul 2016, 11:52

woodburner wrote:Southampton is slightly different from most places. It gets two peaks at each high tide which effectively extends the high tide state to about two hours.

https://www.tidetimes.org.uk/southampton-tide-times

So Southampton is like Poole.

Different tide tables not agreeing with each other is another problem!

Post by **kenneal - lagger** » 26 Jul 2016, 15:19

Most places get two highs and two lows a day as the moon goes around the earth once every 24 hrs. There is a bulge on the moon side of the earth where the moon pulls the water up and a bulge the opposite side where the centrifugal force pushes the water up. This was explained on Prof Brian Cox's program recently.

Blackpool is typical of this with a high tide at 0303 and 1503 today and lows at 0943 and 2204 today.

The South Coast, from my geography lessons of years ago, is different in that it gets the Isle of Wight effect at Portsmouth and Southampton which is compounded by a reflected wave of water from Cherbourg and the north coast of France at Bournemouth.

If you go further along the coast to Eastbourne, for instance, the double tides effect is lost and you're back to the normal two tides per day.

Sorry for sowing confusion!

(Edited twice)