For everything there is a season - tide and time

Over at the warren, Uncle Eli has posted about SLR, a subject I touched on last month, here, and here, including references to Grinsted's excellent material.

It occurs that the Average Josephine (IOW, most of the world) might look at the projections of sea level rise and, whilst registering that a change will have an impact, might reasonably ask why a meter or so of extra sea might be such a big deal. So here's an attempt to place this into a 'human' context.

On its own, a few centimetres of water 'added on' to the water level at any given beach or dockside is no big deal. This is because the other forces which have an effect on sea level at any given location are much stronger (they create more variability) than the underlying 'signal' of sea level.

First and most obvious is the effect of the tides. Whilst in some places sea level fluctuates by a few metres over the course of a day, in others the tidal range can top ten metres and more (up to around 15-16 m for some locations). In practical terms, coastal human habitation and business takes this into account, so in most places human activity takes places above the high tide level, for obvious reasons.

At certain times of year, specifically the Equinoctial Spring Tides, the various forces which interact to create tides and tidal ranges combine to create particularly high and low tides. It is not uncommon already for these to overreach the human adaptation level and to result in localised flooding, which is further worsened if these tidal periods occur in sync with strong weather conditions (in particular, depressions, often related to storms).

Now, the current range of projections for changes in sea level have to be considered in the context of historic tidal ranges and existing infrastructure and human-ocean interfaces.
For example, the 'averaged' sea level range globally hits around 79cm during Spring tides. This means that, overall, the shoreline would experience a few extra inches more or less. But in a local and regional context this 'average' is effectively meaningless, and is not reflected in the real experience of many coastal dwellers. For most people on coasts, the tides go up and down several metres.

If global average sea level rises by, say, half a metre, what does this mean for local impacts? And what effect does this have on local tidal ranges and, in particular, during the upper bound of the ranges (the Equinoctial Springs) and those occasions when these coincide with storm surges?

Well, it should not be difficult to work out that an extra 'average' SLR of 50cm is going to mean a rise of high tides, and of high springs, in the order of 2 - 5 metres of 'extra' sea. Given that a storm surge can increase sea levels (for example, in the Philippines) by another 4-5 metres, and you end up with places which are likely to experience regular (annual or more frequent) tidal surges in the range of 5 -10 metres. We have seen the horrific effects on one part of the world where a tidal surge of 5 metres, added to extreme weather conditions, has resulted in devastation and carnage. Now multiply this by all those places which are 'vulnerable' to such variability.

As an aside (because I'm not certain of the projected effects), it should be noted that a Spring tide normally produces currents twice as fast as Neap tides, but with eight times the power, or force. Anyone who has struggled to get out of the water onto a beach during an ebb in difficult conditions can have a sense of how much power is involved - it is, literally, an overwhelming force. 

It seems rational to presume that higher Springs will be associated with faster currents and therefore greater forces - resulting in more erosion, more localised damage and greater stress on infrastructure (including defences). So, the effect-multiplier of a few centimetres of extra sea level 'on average' produces impacts which can easily be seem to include, for example, the overwhelming of low-lying islands, or the inundation of coastal cites, oil refineries, nations (Bangladesh).

If all of this is the consequence of half a metre of sea level rise, what then is the consequence of a metre or more? Is it likely to be twice as bad? Or, given the 'effect multipiers, are we instead talking about a localised effect with a difference of an order of magnitude?

Finally, for the economically-minded amongst you, remember that an astonishing proportion of the world's trade is conducted across the oceans - around 90% of all trade goods is shipped at some point. Now, consider the impact on shipping of the changes outlined above. The cost of building tougher ships, the cost of building new, relocated shipping hubs (the World's three deepest 'ultratanker' and supercontainer ports are all vulnerable to rising sea level). The risks and losses, all to be paid for by someone.

A very high proportion of the Global population lives in the coastal strip - I think it's about 85% of the population. Not all of these people would be vulnerable in the way I outline above, but with sea level rise must come, inevitably, relocation and mass urban movement inland, fundamentally changing the dynamic geography of our society.

That's why sea level rise matters, and why a metre is more significant than half a metre.

NOTE also Simon Donner's eminently pragmatic contribution to the discussion of adaptation.