 Solution

Invert the Lebedev ice-growth law to obtain the original accumulated freezing degree-days:The average cold over $275$ days is therefore approximately $12.56\mathrm{K}$ below the reference. Assuming the warmed days remain below the freezing-degree-day threshold, a $4\mathrm{K}$ increase removes $4\times 275=1100$ degree-days. HenceThe estimated end-of-winter thickness falls by about $0.30$ m, or $20\%$. This applies the same empirical growth law to the altered temperature forcing; it does not add changes in snow, winds or ocean heat.