An ice-free summer Arctic in years or decades heralds methane catastrophe.
Catastrophic climate change for many years has been regarded as runaway climate change and has been presented in terms of passing 'tipping points' and as' abrupt 'or 'rapid' climate change. Large constant carbon feedback emissions resulting from global warming would obviously be catastrophically dangerous would at some point be irreversibly progressive (the ultimate vicious cycle). This for a great many years has been the feat of global warming, but there is little in the published science on the runaway combination of feedbacks.
Abrupt warming has definitely happened in the past and of the most concern is the abrupt warming that ended the abrupt Younger Dryas 11,500 years ago- research shows this was 10C in a few decades and due to NH methane emissions .
A expert publication on tipping points is by Tim Lenton. In this 2008 paper the Arctic planetary tipping points of terrestrial permafrost and of methane hydrate were included but not rated in the list of priorities, because of lack of research data.
A safe operating space for humanity, Nature, August 2011 by J. Rockström, Will Steffen, K. Noone, Åsa Persson, F. Chapin, E. Lambin, T. Lenton, M. Scheffer, C. Folke, Hans Schellnhuber et al, found that atmospheric CO2 had passed the safety limit:
Climate Change We have reached a point at which the loss of summer polar ice is almost certainly irreversible. From the perspective of the Earth as a complex system, this is one example of the sharp threshold above which large feedback mechanisms could drive the Earth system into a much warmer, greenhouse gas-rich state with sea levels metres higher than present. Recent evidence suggests that the Earth System, now passing 387 ppmv CO2, has already transgressed this Planetary Boundary.
The IPCC 2007 Assessment acknowledged the catastrophic risk from Arctic summer sea ice loss and methane carbon feedback, and that multiple cascading Arctic feedbacks will occur from global warming.
Risk of Catastrophic or Abrupt Change
IPCC AR 4 WG 3 2.2.4
The possibility of abrupt climate change and/or abrupt changes in the earth system triggered by climate change, with potentially catastrophic consequences, cannot be ruled out. Positive feedback from warming may cause the release of carbon or methane from the terrestrial biosphere and oceans which would add to the mitigation required.
IPCC AR 4 WG 2 15
In both polar regions, components of the terrestrial cryosphere and hydrology are increasingly being affected by climate change (very high confidence). These changes will have cascading effects on key regional bio-physical systems and cause global climatic feedbacks (very high confidence).
What is global climate catastrophe?
There is no formal definition in climate change science of dangerous, disastrous, or catastrophic global climate change. The impression given from media reports is that it means the loss of the Greenland and West Antarctic ice sheets, raising the global sea level. It is much more than those dramatic but slow future events.
The loss of the Arctic summer sea ice has not been recognized as catastrophic danger. Arctic climate change has been recognized by James Hansen in a 2007 paper Dangerous human-made interference with climate: a GISS model study.
Though many say "dangerous interference with the climate system," the objective of the 1992 UN Framework Convention on Climate Change, is not defined, a full reading of the Convention shows it is clearly and specifically defined in terms of the effects of climate change on food and health security in the most climate-change-vulnerable regions and populations.
A common sense definition would be based on human population health. If water security is added in, this is indeed a common sense definition of "dangerous interference with the climate system."
Such a definition does exist in Climate Change and Public Health, Brian S. Schwartz, MD, MS, September 2008 from the proceedings of the 135th Annual Meeting of the American Public Health Association (APHA):
Climate Change, Peak Petroleum, and Public Health
Global climate change health impact
The projected implications of climate change for human health are many, diverse, and wide-ranging in magnitude. Several speakers [5-7, 10] defined the commonly used term "catastrophic climate change" which was used by climatologist James Hansen and others. 
Catastrophic climate change is defined as climate change that would result in
- extinction of up to 50% of terrestrial and marine species,
- sea-level rise leading to the displacement of tens to hundreds of millions of people, and
- changes in regional climate that would cause profound disruption to regional food production and the hydrologic cycle (eg, worse droughts, more severe rain events with flooding and consequent damage). 
There was recognition and agreement [5-7, 10] with the many climatologists who have maintained that there are less than 10 years remaining to change "business as usual" with respect to reducing carbon emissions, or else catastrophic climate change will be unavoidable.
Dangerous climate change therefore means "catastrophically dangerous," and the bottom line of catastrophic climate change is profound disruption to regional food production and water availability to regional populations. Profound disruption would inevitably follow from any sustained losses at any time, because of the basic climate change science: climate change commitment (due to lags in the climate system), the combined "multiplier" effect, and cumulative impact.