This is a complex problem. Miami, is chosen because it is the centre of a lot of arguments, not because it is in Australia. I assume everyone knows the place in Florida USA.
If the rate of ocean rise remains linear over the next ten years, or longer, then Miami may get some increased surge effects, but it should probably be ok – although it is likely the limestone ‘bedrock’ will suffer before then, and salt water will permeate the water table. Many people claim that the average rise per year has been millimeters a year, or less, over the last 100 years, and thus it is impossible for the rise to be a problem in a short term.
However, the sea level rise may not be linear and stay at the same rates as it has done in the past. The real problem occurs if the rate of melting of land ice accelerates, or accelerates wildly in a feedback loop: less ice, so the temperature is higher, less light is reflected, and so more ice melts etc. etc. The high temperatures we are reaching in normally cold regions, suggests the melt rate could stop being linear soon, if it hasn’t already. This is the worry. Likewise if Siberia starts releasing stored methane as its permafrost melts, which it seems to be doing, then this will speed temperature rises and lead to more ice melting elsewhere.
On the other hand if the Gulf stream collapses, which some think is likely, and the UK does not get its warming currents, then the UK and northern Europe might have more severe winters, and lead to more ice formation. My guess is that this will not be permafrost, so waters will rise.
The earth climate system is a complex system, predicting behaviours in complex systems is very difficult. You certainly cannot give an exact date for when Miami will likely become unlivable because of recurrent flooding. I read that floods there seem to be getting worse, and that “sunny day flooding” (ie no rain) is now relatively common when it was not before.
It may take 30 of more years as opposed to 10 for Miami to become uninhabitable but its very likely to happen – there are no absolute certainties. If you want to bet against climate change then buy coastal properties, or properties in recent flood zones, to help those who are concerned move out. That way you may make a killing, and be useful to people.
One of the main problems we seem to face is that many people seem uncomfortable with non-linear thinking. They seem to think that if something has been going along at a particular rate then that rate will not change, or that everything will continue to proceed calmly, rather than that there can be tipping points and feed back loops which produce acceleration and can lead us rapidly into the unknown. It also seems difficult to recognise that small changes can have big effects, or that they can combine with other small changes to produce big effects. Thus even an increase in ocean levels of .5 cm or less can changes the patters we observe, and lead to much greater, and more common, storm surges which can lead to significant flooding and significant effects on the land…. There are already places in the world where people have had to leave, despite the smallness of the changes. Changes effectively multiply.
Despite all this, it is, however, probably sensible to plan for the worst, do what we can to slow rates of climate change down, and then relax a bit if less than the maximum tragedy occurs.