Answer:
The arctic melting would be an example of a positive effect on the warming of our planet. With less ice we would have less reflecting material (albedo effect) and therefore the ability to absorb more heat.
To date this is not a serious issue as the total melting of the Arctic is less then 4% and made up for by the increases in ice at the Antarctic. The issue does warrant further observation though. Since 2007 we have seen a 26% increase in summer ice, the most telling sign of the strength of melting in that area.