The reason, in one word, is RUNAWAY.
Runaway is a descriptive term for what the scientists call abrupt irreversible rapid global warming, which would be global climate catatsrophe. It involves tipping points.
A 2012 paper By Prof C Duarte says The Arctic could Trigger Domino Effect Around the World.
The science says it can happen (IPCC 2007), but it is not included in the linear projecting climate models.
Abrupt climate change encompasses two extreme results of Arctic warming- abrupt cooling can happen (thermohaline circulation change) and abrupt warming (+ve Arctic feedbacks). This page covers the warming process. Abrupt climate warming could be over 10 years or more than100 years.
The major risks to society and environment from climate change are posed primarily by abrupt and extreme climate phenomena. Potential forms of abrupt change include [...] widespread melting of permafrost leading to large-scale shifts in the carbon cycle. Abrupt and extreme phenomena can exceed the thresholds for ecological and societal adaptation through either the rapid rate or magnitude of the associated climate change [IPCC 2007].
The US is conducting research abrupt warming situations under the Investigation of the Magnitudes and Probabilities of Abrupt Climate Transitions (IMPACTS) Project out of the Lawrence Berkeley National Laboratory. This includes the following topics.
Rapid destabilization of methane hydrates in Arctic Ocean sediments; and Mega droughts in North America, including the role of biosphere-atmosphere feedbacks.
The background information confirms these are all real risks with continued warming, What is not addressed is that these would be mutually reinforcing.
The Arctic responds to global warming by increasing the rate of warming through several feedback processes, which, if allowed to become established, will inevitably lead to uncontrollable accelerating global warming or what for many years has been called "runaway" climate change. This is not to be confused with the scientific term "runaway greenhouse effect" or Venus syndrome.
There are two general, very large feedback processes in the Arctic that definitely will increase as global warming continues. One is melting Arctic ice and the other is emitting Arctic methane. The loss of ice will definitely increase the emission of Arctic methane to the atmosphere, which makes the Arctic sea ice meltdown the big planetary emergency.
"Runaway" is an apt description as runaway climate change is the result of the combined three following inevitabilities:
Increased radiative forcing and inertia from multiple feedbacks
We all know about the rapid meltdown of the Arctic summer sea ice. It has long been known that the vast expanse (2.5 million square miles before industrial atmopsheric GHG pollution) of the year-round Arctic sea ice acts in the summer as a cooling influence to the Arctic region, northern hemisphere and to some extent the whole global climate. Its loss in the summertime will lead to additional warming.
This emergency to our planet's biosphere comes from multiple positive Arctic climate feedback processes, each of which affects the whole biosphere and each of which will increase the rate of global warming / temperature increase. Atmospheric temperatures are rising faster in the Arctic than in other regions.
Already today, all the potentially huge Arctic positive climate feedbacks are operating.
The Arctic summer sea ice is in a rapid, extremely dangerous meltdown process. The Arctic summer ice albedo loss feedback (i.e., open sea absorbs more heat than ice, which reflects much of it) passed its tipping point in 2007 – many decades earlier than models projected, and scientists now agree the Arctic will be ice free during the summer by 2030. However, that is not to say it couldn't happen very much earlier.
Models of sea ice volume indicate a seasonally ice-free Arctic likely by 2015, and possibly as soon as the summer of 2013.
Such a collapse will inexorably lead to an accelerated rate of Arctic carbon feedback emissions of methane from warming wetland peat bogs and thawing permafrost.
The retreat of sea ice appears to be leading to the most catastrophic feedback process of all. This is the venting of methane to the atmosphere from frozen methane gas hydrates on the sea floor of the Arctic continental shelf.
At the Fall Meeting of the American Geophysical Union in San Francisco from 5-9 December 2011, there was a session on Arctic Gas Hydrate Methane Release and
Climate Change at which Dr. Igor Semiletov of the Far Eastern branch of the Russian Academy of Sciences reported dramatic and unprecedented plumes of methane – a greenhouse gas that is over 70 times more potent than carbon dioxide for 20 years after emission – were seen bubbling to the surface of the Arctic Ocean by scientists undertaking an extensive survey of the region. This has been reported by UK's Independent newspaper and copied by news agencies around the world and in a number of online blogs.
All of these Arctic feedbacks are described in detail in the 2009 World Wildlife Fund (WWF) report, Arctic Climate Feedbacks: Global Implications.
If methane release from Arctic sea floor hydrates happens on a large scale — and this year's reports suggest that it will — then this situation can start an uncontrollable sequence of events that would make world agriculture and civilization unsustainable. It is a responsible alarm, not alarmist, to say that it is a real threat to the survival of humanity and most life on Earth.
What to do
There are several ways to tackle the problem if action is not delayed: they may be grouped together as geo-engineering solutions. However, they do require rapid mobilisation on national and international scales: first, to verify the science, and second, to implement the necessary counter-measures. There is an almost impossible challenge to implement the counter-measures quickly enough to prevent the possible collapse of the Arctic sea ice in summer 2013, but this challenge has to be faced as an international emergency.