Without stopping it, sooner or later, one way or another, the loss of the Arctic summer sea ice would lead to runaway global warming.
Meehl et al., 2007:
In other words, both the loss of Arctic summer sea ice albedo due to global warming and the inevitable (already happening) Arctic feedback carbon emissions (mainly methane) will increase the rate of Arctic and global warming.
What would be the effects of losing all the Arctic summer sea ice?
How much would the loss of Arctic summer sea ice increase the Arctic regional rate of warming?
Scientists now believe that the main factor in the accelerating loss of Arctic summer sea ice is the loss of albedo cooling and the increased heat absorption it causes. Loss of Arctic summer sea ice increases the rate of loss (positive feedback effect).
There is one published indication of how the loss of Arctic summer sea ice will increase the rate of Arctic warming, from David Lawrence in 2008.
He found that "during episodes of rapid sea-ice loss, the rate of Arctic land warming is 3.5 times greater than the average 21st century warming rates predicted in global climate models. While this warming is largest over the ocean, the simulations suggest that it can penetrate as far as 900 miles inland. The simulations also indicate that the warming acceleration during such events is especially pronounced in autumn. The decade during which a rapid sea-ice loss event occurs could see autumn temperatures warm by as much as 5 degrees C (9F) along the Arctic coasts of Russia, Alaska, and Canada. This is highly significant to the increase in methane emissions from thawing permafrost and the warming of methane hydrates."
What about the Northern Hemisphere?
The Arctic is the air conditioner for the entire northern hemisphere so the hemispheric climate will change along with further accelerated warming.
"Make no mistake, the Arctic is the Northern Hemisphere’s air conditioner. If the ice starts disappearing in the summer, it will have a significant impact on our climate."
"There will be knock-on effects in the whole of the northern hemisphere. Everything changes – evaporation from the ocean, changes in wind patterns and the wetness of the winds, and amount of snowfall in the area – because you’re replacing an ice-covered centre to the northern hemisphere with an ocean-covered centre."
Dr. Jennifer Francis of Rutgers University, at the AGU meeting in December 2011 (Does Arctic Amplification Fuel Extreme Weather in Mid-Latitudes?), presented new research in review that shows that Arctic sea ice loss may significantly affect the upper-level atmospheric circulation, slowing its winds and increasing its tendency to make contorted high-amplitude loops. Such high-amplitude loops in the upper level wind pattern (and associated jet stream) increase the probability of persistent (that is, longer-duration) weather patterns in the Northern Hemisphere, potentially leading to extreme weather due to longer-duration cold spells, snow events, heat waves, flooding events, and drought conditions.
All of these spell bad news for Northern hemisphere agriculture and therefore world food security.
Research published in 2008 by C. Archer and K. Caldeira reported that the jet stream has moved northwards 270 miles in 22 years, presumably due to global climate change.
The Arctic summer sea ice loss would mean a huge amount of extra Arctic and subarctic warming, and it will undoubtedly affect the northern hemisphere, especially midcontinental regions. The northern hemisphere is warming faster than the southern hemisphere because there is more land in the north and more water in the south. Midcontental regions tend to warm fastest.
What will that do? There is little published research on this, but scientists say it will increase droughts in drought-prone and marginal dry regions — including in the United States.
Jennifer A. Francis, Winter Northern Hemisphere weather patterns remember summer Arctic sea-ice extent: , "Mechanisms by which the atmosphere 'remembers' a reduction in summer ice cover include warming and destabilization of the lower troposphere, increased cloudiness, and slackening of the poleward thickness gradient that weakens the polar jet stream. This ice atmosphere relationship suggests a potential long-range outlook for weather patterns in the northern hemisphere."
Effect on the jet stream and planetary weather patterns (http://www.wunderground.com/climate/SeaIce.asp):
NRDC: Global Warming Puts the Arctic on Thin Ice (www.nrdc.org/globalwarming/qthinice.asp):
"A warmer Arctic will also affect weather patterns and thus food production around the world. Wheat farming in Kansas, for example, would be profoundly affected by the loss of ice cover in the Arctic. According to a NASA Goddard Institute of Space Studies computer model, Kansas would be 4 degrees warmer in the winter without Arctic ice, which normally creates cold air masses that frequently slide southward into the United States. Warmer winters are bad news for wheat farmers, who need freezing temperatures to grow winter wheat. And in summer, warmer days would rob Kansas soil of 10 percent of its moisture, drying out valuable cropland."
How much will the loss of Arctic summer sea ice boost the rate of global warming?
There is no published research that answers this question, though for many years scientists have been saying that this will increase the global warming rate.
Sea ice loss is not accounted for in future temperature increase projections by the IPCC. The climate sensitivity used is a single linear metric that does not account for non linear changes in the climate system.