Wednesday, March 23, 2016

The fifth and last step - drowning!

Yes, there is fossil fuel carbon dioxide building up in the air; it might be contributing to the warming of the world; it is almost certainly not wishing wild weather upon us; so there remains the last question - will the seas rise and submerge huge swathes of real estate?

First it needs to be recognized that the sea level is rising. It has been rising for about the past 25 000 years. However, for the past 7 millennia it has been rising slower than ever before:

The critical question, however, is whether the observed slow rate of rise has increased as a result of the warming climate. There are several lines of evidence that it has not.  One is the long-term data from tide gauges. These have to be treated with caution because there are areas where the land is sinking (such as the Gulf of Mexico, where the silt carried down the Mississippi is weighing down the crust), and others where it is rising (such as much of Scandinavia, relieved of a burden of a few thousand metres of ice about 10 000 years ago). A typical long-term tide gauge record is:

The 1860-1950 trend was 2.47-3.17mm/a; the 1950-2014 trend was 2.80-3.42mm/a, both at a 95% confidence level. The two trends are statistically indistinguishable. There is <5% probability that they might show any acceleration after 1950.

Another line of evidence comes from satellite measurements of sea level.  The figure below shows the latest available satellite information – it only extends back until 1993. Nevertheless, the 3.3±0.3mm/a rise in sea level is entirely consistent with the tide gauge record:

Thus several lines of evidence point to the present rate of sea level rise being about 3mm/a or 30cm per century. Our existing defences against the sea have to deal with diurnal tidal changes of several metres, and low-pressure-induced storm surges of several metres more.  The average height of our defences above mean sea level is about 7m, so adding 0.3m in the next century will reduce the number of waves that occasionally overtop the barrier.

The IPCC predicts (using its rather suspect models) that the sea level will rise by between 0.4 and 0.7m during this century. It claims:
"It is very likely that the mean rate of global averaged sea level rise was 1.7 [1.5 to 1.9] mm/yr between 1901 and 2010 and 3.2 [2.8 to 3.6] mm/yr between 1993 and 2010"
There are many tide gauges where the land does not appear to rising or falling. They all show a change in sea level the region of 2.5-2.9mm/a over the 20th century. On the other hand there are some stations, such as Simonstown, that do indeed seem to show an acceleration in the 21st century; while, surprisingly, many tide gauges on Pacific and Indian Ocean islands show no sea-level rise at all e.g. Guam, Eniwetok, Johnston Atoll.

A very recent paper notes that there was probably a fall of the order of 0.2mm/a as the world cooled after the Medieval Warm Period, and that as we recover from the Little Ice Age (which ended about 1850) the sea has been warming and expanding and, of course, is fed by melting glaciers. They estimate the 20th century sea-level rise to have been only about 0.15m, but predict that the 21st century could see a rise of between 0.24 and 1.31m.

Given the wide range of the prediction, there is a possibility they could be right. Importantly, of course, is the fact that even a 1.3m rise is not likely to be a disaster, in the light of the fact that our defences are already metres high – adding 20% to them would be costly, to be sure, but we would have decades to make the change, and should have more than adequate warning of any significant increase in the rate of sea level rise.


Our five steps have shown:
  • the combustion of ever increasing quantities of fossil fuel has boosted the carbon dioxide concentration of the atmosphere.  
  • the impact of that increase is not demonstrable in a scientific way. There may be some warming of the atmosphere, but at present any warming is almost certainly hidden in the natural variation of temperatures. 
  • there is no evidence for any increase in the frequency or magnitude of weather phenomena, and that includes droughts and floods.  
  • there is a small possibility that sea level rise may make some real estate uninhabitable, but the rise over the coming century is not likely to present any insuperable challenge.
 It can only be concluded that our world will remain perfectly habitable in the face of rising carbon dioxide levels.

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