Concerning the issue of whether storms (Sandy and the nor’easter) are signals of the impact of climate change on extreme weather:
- No significant increase in Atlantic hurricanes since the late 1880s has been observed.
- The number of hurricanes that make U.S. landfall has not significantly increased or decreased.
- In the future, it is likely that global frequencies of tropical cyclones will either decrease or remain essentially the same as a result of projected greenhouse gas induced climate change, though rainfall rates related to such storms are likely to increase.
- There is low scientific confidence that overall storminess has changed, however, it is likely that there has been a human-induced increase in coastal extreme sea level events due to overall sea level rise.
- Near Sandy’s landfall, sea level has risen over one foot since the mid-19 Century, mostly (but not solely) due to the increase in volume of the ocean attributable to its warming resulting from climate change.
- It is very likely that further sea level rise will contribute to increased coastal high water levels in the future, conditions that led to Sandy’s primary impacts on coastal New York and New Jersey.
Concerning the issue of whether storms (Sandy and the nor’easter) are signals of the impact of sea ice on extreme weather:
- Arctic sea ice extent has significantly diminished in a manner inconsistent with internal variability alone and anthropogenic forcing has likely been a contributing factor.
- There is low confidence on changes in either the number or the intensity of mid-latitude storms, and there is also low confidence on the role played by sea ice forcing.
- Scientific understanding remains controversial whether there is an appreciable or detectable impact of Arctic sea ice loss on subarctic weather during Fall and early winter.
- The immediate cause for the severe U.S. impacts induced by Hurricane Sandy is the fatal, albeit random, merger of two transitory weather systems. It is very unlikely that either of these weather systems individually was appreciably affected by Arctic sea ice loss. The case of the unusual merger of two weather systems into a single potent and destructive force along the eastern seaboard in late October 2012 thus is most likely an example of a great event having little underlying cause.
- Although there was evidence of local atmospheric changes due to record sea ice loss in 2012, the large-scale pattern of atmospheric circulation over the North Atlantic, which was dominated with rapidly progressing weather systems rather than by stationary climate anomalies, was unlikely a detectable signal of depleted Arctic sea ice.