Sea Levels
The present, increasingly heated, debate seems to be drawing all sorts of red herrings across the path.
For example sea level rise is being promoted as a serious issue and reason for action. But long before this has any real impact there will be a wide range of much worse outcomes of global warming; including crop failures and weather events.
Al Gore's Inconvenient Truth is one such overstatement, wildly quoted by sceptics, as it suggests that Manhattan will be swamped within the lifetime of current buildings (<20 years). Other prominent 'green activists' have claimed that we are facing a five or six meter sea level rise this century. We already have politicians in Western Australia and some Pacific Island nations proclaiming that current coastal inundation is due to sea level rise.
In a runaway feedback loop temperatures could climb higher than expected but if anthropogenic warming is fast enough to cause a rise of 6 metres (or more) this century we will have a lot more to worry about than declining coastal property values; we will have nothing to eat.
Land based ice has been melting for around the past 22,000 years and sea levels have risen about 130 metres over that period as a result. But less than two metres of this rise occurred during the past 5,000 years.
Sea levels have been fairly accurately measured since the 1920's and there have been more accurate satellite data since the late 1990's. Prior to that tidal statistics are less reliable. Back in the 1800's there were some places with good measurement but a whole world view (necessary to measure global sea level) was not available.
Before that, local rises and falls in the earth's crust begin to be significant (where do you measure from). But we can say with some confidence that mean sea levels have risen by around 20 to 30 cm (around a foot) in the past 100 years. More accurate satellite records put this at 6 cm in the past 20 years (3 mm per year) on a linear trend. This is due to land based ice continuing to melt and to ocean expansion due to warming.
Source: Wikipedia, the free encyclopaedia
This effect is considerably less than movements in the earth's crust in many places. The lateral movement of the earth's tectonic plates is typically 50–100 mm per year; in some places much more. Claims that some islands and other places are being inundated by sea level rise are more likely due to tectonics (in the case of coral atolls – as this is how they form) or in places like Venice, due to excessive groundwater use, there may be dozens of other factors before sea level rise becomes an issue. It may even be a positive factor in some locations like the Turkish coast, where once coastal villages are now inland.
I have been catching the ferry in Sydney for various periods spanning 40 years and I see no perceptible change in the sea level in Sydney Harbour (or over harbour side seawalls or pools) but I do accept that I have trouble discerning a 12 cm rise over that period, as the tidal variation is vastly larger. Even the ripples on a calm day would make this difference hard to confirm.
That faster inundation might happen in future relies on the tipping point theory - that at some point soon there will be a cataclysmic climate change due to positive feedback to temperature rise - resulting in the remaining land ice (principally Greenland and Antarctica) melting, together with ocean warming and expansion. This will be troublesome for some cities and very serious for delta dependent countries like Bangladesh.
On the other hand melt water from receding glaciations is an extremely important (if globally short term) resource and is already feeding more people than ever before.
Since I wrote the original paper a credible scientific view has evolved and generally agrees that if global warming accelerates there may be a half metre rise in sea level over the next 50 years. Hardly grounds for selling your harbour frontage just yet.
Ridiculous projections about vastly greater inundation provide succour for those opposed to doing anything; or just tilting at windmills, as in Copenhagen; shades of Don Quixote.