Glaciers grow for two reasons: snowfall and cold. Warmer sea temperatures for incoming storms off the Pacific coast or Gulf of Alaska result in more snowfall - sometimes a helluva lot more snowfall. Those in snow country have seen this — too cold often results in not much snow, temperatures just below freezing often results in lotsa snow. As long as the temperatures on the glacier prevent too much melting (only below 32 F, not below 0 F) so that more snow accumulates than melts, the glaciers will grow. In normally extremely cold areas, a slight increase in temperature results in a helluva lot more snow — more than enough to make up for warmer summers.
I recently read a National Geographic from 30 years ago reporting on the phenomenon of glacier growth in Alaska where warmer weather was resulting in heavier snowfall further down on the side of the mountains. During the Ice Age extreme cold it was often very dry.