Read here. Earth's climate is constantly changing as it oscillates from one extreme to another. As the researchers documented, this oscillation also affects the Arctic area and its sea ice. During ancient periods, the earth was a lot warmer and produced periods where Arctic sea ice decline was substantial.
"Since the change in sea-ice cover observed at the end of the 20th century (which climate alarmists claim to be unnatural) was far exceeded by changes observed multiple times over the past several thousand years of relatively stable atmospheric CO2 concentrations (when values never strayed much below 250 ppm or much above 275 ppm), there is no compelling reason to believe that the increase in the air's CO2 content that has occurred since the start of the Industrial Revolution has had anything at all to do with the declining sea-ice cover of the recent past..."