The graph above overlays the NSIDC Arctic graph (red) on their Antarctic graph (blue) with both axes normalised to the same scale. Note that there is almost twice as much sea ice in the southern hemisphere as there is in the northern hemisphere. The northern hemisphere is about 1.5 million km^2 below the mean, and the southern hemisphere is about 1.5 million km^2 above the mean.
The next graph shows the Y-axis shifted to line up the deviations. Note that they are equal and opposite.
The Antarctic anomaly has a large effect on the Earth’s radiative balance for two reasons.
- Solar radiation is near its peak in Antarctica this time of year
- The excess ice is at low latitudes, meaning it gets hit by a lot of direct sunlight.
By contrast, the Arctic receives almost no solar radiation this time of year, so it has little impact on the Earth’s radiative balance.
The northern hemisphere solar peak occurs between April 20 and July 20, when ice extent is always close to normal. At the time of the ice minimum in September, there is very little sunshine, so it has minimal effect on the radiative balance.
Conclusion : the excess ice in the southern hemisphere has more impact on the Earth’s radiative balance than the ice deficiency in the northern hemisphere. I ran this idea by Walt Meier at NSIDC two years ago, and he did not disagree.
Peter Sinclair got this backwards in his video below, where he claims the exact opposite.