Also, Baz - can you crash course me on global warming and its effect on Sydney if sea levels rise?
I can try...while I'm interested in the science, it's not really my forte and I haven't actually studied/worked in the field for a long time.
I presume you understand the premise...warming earth, theoretically related to carbon based greenhouse gases in the atmosphere, melts ice/warms the ocean thus changing the level of the sea....?
The first major issue when talking about effects is that no one actually knows. It's not as simple as a linear progression up the coastline, because water depth and coastal topography affect the actual level as much as anything else. So there are models (
http://www.ozcoasts.gov.au/climate/Map_images/Sydney/mapLevel2.jsp) but it's very difficult to say. Theoretically, even with a 2.2m rise as shown on the severe mapping models there, Sydney isn't going to be affected too badly, if at all depending on whether preventative measures can be taken. Sydney Harbour is deep and the land on the shoreline is actually a lot steeper than it looks. It would take a really significant rise in levels to inundate Sydney, somewhere in the 4m+ region if I remember rightly. Most models don't predict anything like that, but again, these are models based on data that we don't truly understand in all cases, and based on the assumption that A) there is a trend and B) that trend will hold stable or even increase. Remember that geologically speaking the time we've measured is a fraction of a fraction of a heartbeat...and much of that is guesswork from before quality measuring, too. What looks like a severe, obvious trend to a species with a lifespan of 80 years might simply be a passing storm for a planet 4 billion years old.
The second major issue with predicting sea level rise is that we have absolutely no idea how much water it takes to raise the water level, how much water is really locked in ice, how much of that will melt and how much of that will make it to the sea. Modelling is practically guess work. For example, many models equate melting sea ice with a sea level rise, which is mostly fallacious. In the same way that ice melting in a glass of water doesn't change the level, sea ice shouldn't really change the level of the sea. Theoretically, anyway. Its volume is already accounted for, if you like. Antarctic land based ice is unlikely to melt without severe changes, to the extent that if it does the sea level will be the least of our worries. Some glaciers are melting, some aren't, and we don't understand that in the slightest. Not all that water will go into the ocean, either, and it's tough to predict. Many of these glaciers are free standing, so to speak, or are actually set on glacial lakes with no (or very little) through flow to the ocean. Of course, all this water will enter the rain cycle in one way or another and that's a whole other kettle of complexity.
I guess the other big thing is salinity. Salt water is denser than fresh, obviously. So mixing all this fresh water from ice into the sea will dilute the ocean, and therefore lower its density. While a warmer climate may affect the surface temperature of the sea very slightly, on the whole it won't make any difference so if the ice does melt, you have to factor in the drop in density of the water in the oceans. That might seem minute, but when you're talking about such vast quantities it does make a measurable difference and could allow the ocean to absorb a lot more water than we think before the levels change.
In short, I think the major answer is no one really knows. We base all of our proposals on models that are derived from hypotheses, essentially. Different sets of data can give wildly different results and with so much conflicting data it's almost impossible to give a definitive answer. Some data, for example, shows an increasing upward trend...some data shows an upward trend that has hit a plateau and a few sets even show a downward trend, and as yet I don't think anyone can explain the discrepancies. Which is basically science to a tee. The theory of evolution is one of the most widely accepted, yet there's still evidence that doesn't fit....
