 Solution

The mechanisms affect surface energy input, heat supply from below, and the transport and thickness of sea ice. They reinforce one another: atmospheric warming and ocean heat thin the cover; circulation can export old ice; thinner ice melts earlier, increasing open-water absorption and strengthening ice-albedo feedback. Heat below the cold halocline matters only to the extent that mixing delivers it upward. Consequently the seven effects cannot be assigned independent percentages from the information given; their importance varies by region and year.

In a forward-looking answer from the paper's 2009 context, sustained warming would be expected to produce continued Arctic sea ice decline, a longer summer open-water season, less multi-year sea ice, and potentially nearly ice-free late summers within coming decades. Winter darkness permits new first-year sea ice to form, so loss of summer ice does not imply immediate year-round disappearance. Winter ice would tend to become thinner, more seasonal, and slower to recover after enhanced ocean heating. A precise date requires a forcing scenario and model, and internal variability causes year-to-year departures. Wang and Overland's 2009 projection is a contemporary example of such a conditional summer outlook.