The reasons for the trends in sea ice are still being debated. However, for the Arctic it is estimated that anthropogenic forcing through the emission of greenhouse gases has contributed 50–60% of the long-term decline of Arctic sea ice. The remaining contribution is believed to come from natural variability. But in the Antarctic the reason for the increase in ice is less clear. The pattern of sea ice change around the Antarctic is dominated by a decrease to the west of the Antarctic Peninsula and an increase across the Ross Sea, which can be attributed to more storm activity between these two areas. The extent of sea ice is strongly influenced by the strength and direction of the winds and the increase in storms has given more warm, northerly flow over the Bellingshausen Sea and greater cold, southerlies over the Ross Sea. This pattern of change is consistent with the increasing temperatures observed over the Antarctic Peninsula and west Antarctica, where temperatures have risen as much as anywhere in the southern hemisphere.
It's currently not clear why there has been an increase in the number and intensity of storms over the southern South Pacific since the late 1970s. However, this area is where the ozone hole has a large impact on the atmospheric circulation and where signals of the El Nino-Southern Oscillation, one of the largest climatic cycles on Earth, are felt most in the Antarctic. It's also where the greatest natural variability in atmospheric circulation is observed in the southern hemisphere, which has been attributed to the fact that the large Antarctic ice sheet is displaced slightly from the pole.
Sea ice extent has a large natural variability in both polar regions because of the amplifying effect of interactions between the atmosphere, ocean and sea ice. In the Arctic there is now increasing evidence that rising greenhouse gas emissions are playing a significant part in the loss of sea ice. However, in the Antarctic the increase in annual mean sea ice extent is only just over 1% per decade, making it impossible at present to separate natural variability from any human influence.