9/3/14 For the love of graphs

By Mike Johnson

While I have never been good at maths (I lost track somewhere around long division), graphs have always fascinated me, a bit like maps. You have a picture of the world, or at least some aspect of it. Graphs are ideally suited to what I call the ‘quantum capacity’ of the human mind to grasp information almost instantly.

Some graphs, however, are worth lingering over, such as the global average temperature anomaly. The graph below, showing sea level rise and projected sea-level rise in the Auckland area is also worth a good look.

While it is pretty much a standard deviation graph, that is, deviations from the average, one unusual feature is that it has two different referents running vertically up each side instead of the usual one. The left hand vertical measures existing sea-level rises relative to the 1961 – 1990 average, while the right hand vertical measures future sea level rise relative to the 1980 – 1999 average.

The black line tracking from the left side, around 175cc below the average in 1870, to hitting the average in the 1970s, is a ten year smoothed line based on tidal gauges. Where the red line superimposes on the black is where satellite data, coming on-stream in the 1980s, has confirmed data from tidal gauges. The narrowing gray shaded area tracking the black and red lines represents the degree of uncertainty in the data. Note how this increases the further back in time we go. As we go forward, our certainty increases

The lime yellow line jumping up and down is yearly data supplied by the Ports of Auckland. These yearly variations can all too easily obscure the underlying trend. This is where the ‘once in a hundred years’ or ‘once in fifty years’ rhetoric comes from, and the graph reveals how deceptive that is. During the peak tides of 2000, some wise old codger would remember that it was just as bad in the late 1940s, and again in the late 1960s – the implication being that nothing has changed, whereas in fact the ten year averaged data line shows the underlying trend standing out against the natural yearly variation.

It’s just too easy for us to remember the peaks and miss the trend.

The large wedge of pale blue represents globally averaged projected data, not just local data. That is, if Auckland follows the global trend, this is where sea levels will go. The scimitar of sharp green shows where sea levels will go if the Greenland and Antarctic (Ross ice shelf in particular) continues to melt as it has done in the past with expected temperature rises. [With regard to computer modelling, we have had more than 15 years of being able to compare modelling with what happens in real world, and the models are pretty accurate.]

What the graph in total reveals is that despite public perception sea levels have in fact risen and will continue to rise as we continue to put C02 and other gasses into the atmosphere.

Where is the Supercity’s planning for this?