= Solution
Invert the <Lebedev ice-growth law> to obtain the original accumulated <freezing degree-days>:
$$
N_0=\left(\frac{1.5}{0.0133}\right)^{1/0.58}\simeq3454\,\mathrm{K\,day}.
$$
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 $4\times275=1100$ degree-days. Hence
$$
N_1\simeq2354\,\mathrm{K\,day},\qquad\boxed{H_1=0.0133N_1^{0.58}\simeq1.20\,\mathrm m.}
$$
The 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.
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