Impacts on Arctic Pack Ice Predictions of Extreme Weather and Natural Variability

It is widely accepted that the Arctic Ocean will lose most of its perennial sea ice cover sometime within this century, and perhaps as soon as the decade 2030-2040. Uncertainty as to when a seasonally-ice free state will be reached reflects the unknown future rates of greenhouse gas emissions, the large scatter in projections between different climate models, even when driven with the same scenario of greenhouse gas emissions, and natural climate variability.   It is highly likely, however, that the basic trajectory towards a seasonally ice-free Arctic Ocean will follow the pattern that has been observed over the period of satellite observations, namely, an overall downward trend in ice extent with superimposed year to year fluctuation as well as multi-year periods of both rapid ice loss and temporary recovery.   For example, the (then) record low ice extent observed for September of 2007 was followed by successively higher September ice extents in 2008 and 2009, only to be followed by lower extents in 2010, 2011 and then a plunge to a new record low in 2012.  September of 2013 in turn saw a substantial recover.   This uneven pattern of sea ice decline reflects the natural variability in ocean conditions and weather patterns.    The very low ice extent seen in 2007 was driven in part by an atmospheric pattern in summer favoring high Arctic temperatures.    The strong recovery in 2013 is consistent with a relatively cool and stormy weather pattern that limited summer melt.  It also appears that as the ice cover has thinned, it has become more vulnerable to disruption by extreme weather events.  For example, the very strong storm over the central Arctic Ocean observed in July of 2012 appears to have accentuated summer ice loss, contributing to the record low extent observed that year. The role of ocean circulation changes remains incompletely resolved.  While there is ample evidence of increased transport of warm Atlantic-derived waters into the Arctic Ocean, this warm water is found at depth and it is unclear whether much of this extra heat can be brought to the surface to impact the ice cover.    Other studies point to a role of warmer Pacific waters entering the Arctic Ocean via Bering Strait.  While there has been considerable speculation over the existence of a “tipping point”, which, once crossed, will be followed by a very rapid transition to a seasonally ice free state, recent work argues against this extreme scenario.