Sea ice data from UIUC shows that there has been no change in date of the Antarctic minimum, the date of the maximum, or the length of the Antarctic winter/summer.
Atmospheric CO2 has increased by 50 ppm during that time period.
There is also no indication that the ozone hole is having any effect.
If I had to guess, I’d say the length of the melt season has more to do with the orbital parameters of the earth than with CO2 levels, which, as far as I know have no influence on the equinoxes. CO2 comes into play in the intensity of the melt season.