Sandy : not caused by Global Warming, but certainly influenced.

There is a sharp division over the superstorm: what is the connection between Hurricane/tropical storm Sandy and global warming?

Contrarians are rubbishing any claims of a connection, but they are using a Straw Man Fallacy, saying that environmentalists are claiming that global warming CAUSED Sandy.

Anthropogenic Global Warming did not cause Sandy. Hurricanes, even in October, are part of the weather scenery in the USA, and have been so well before before our influence on climate became apparent.

However, it is perfectly reasonable to claim that global warming affected the path and intensity of Sandy.

It is always necessary to put events into the fullest possible context.

Climate warming theory predicts that the numbers of hurricanes may decrease, but that the intensity of hurricanes will increase. (IPCC 2007)

The records bear this out:

Source Dr Ryan N Maue

The top of the graph is of all hurricanes, and the lower line is major hurricanes. The red lines are trends. So this record tends to bear out the theory in a general kind of way.

Here is the global temperature, just for comparison:

Here is a match for Atlantic Meridional Oscillation (the Atlantic version of the more well-known Pacific ENSO, the El Nino/La Nina oscillation):

That, again, is suggestive of some kind of correlation. As indeed there ought to be, since warmer waters should give hurricanes more energy. There are five other factors that contribute to the formation of hurricanes, including atmospheric instability and lack of change in wind direction with altitude. If all of these factors could be compounded together, the match would no doubt be closer.

Here is another graph, again from Dr Maue:

It shows an  increase in all hurricanes as well as major hurricanes in the warmer period from 1995-2012.

And in case anyone wants to say "Well it's all due to natural variation of ocean currents", the ocean generally is getting warmer, and you can gain a visual impression of this from the ENSO charts, with more red warm spikes showing in the more recent records:

I did a plot from some original data for the ENSO and it showed an upward trend, but unfortunately I lost the link between the graph and the source, so I cannot show it.

The increasing frequency of positive phases of the oscillations is to be expected given the increase in total ocean heat content:

In short, it is to be expected that with global warming, we must an expect an increase in frequency of severe storms and hurricanes. Which is not the same as saying that global warming CAUSED Sandy. It made it more probable. It also steered Sandy onshore.

Global warming is taking place in the Arctic much faster than it is taking place globally. This has caused a blocking high pressure event, which causes the jet stream to wander further south, which has caused the hot USA and cold UK summer. This block caused Sandy to veer coastwards.

In conclusion - it is safe to say that Sandy was not caused by was exacerbated by Man-made Global Warming.