Throughout most of human history, the Arctic served a vital function in maintaining a stable climate – acting as a giant "air conditioner" for the planet by regulating air and ocean currents. The extent and volume of ice in the region stayed relatively unchanged from ancient times until the early modern era.

During the mid-20th century, however, as the world's population expanded rapidly, man-made emissions of CO2 and other greenhouse gases began to increase at a speed rarely seen in the natural geological record. By the early 21st century, total carbon emissions were exceeding 10 gigatons annually, ten times faster than at any point since the extinction of the dinosaurs.*

Combined with a loss of carbon sinks – through deforestation, soil erosion and other habitat destruction – the resulting accumulation of heat-trapping greenhouse gases in the atmosphere led to a clear warming trend around the globe. This was especially apparent in the Arctic, where temperatures were increasing twice as fast as the world's average.*

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http://www.futuretimeline.net/21stcentury/2020.htm#arctic-sea-ice-future