It doesn't matter if it's 0.4mm or 4mm, still negligible.

Yea a 20 degree temperature rise would sure have an effect. But as you said it'd kill so much life that sea level would be the least of our worries.

Going underground wouldn't help because the heat would travel into the ground over time. If we had colder winters than we do now, it could give the underground a chance to
cool off and maintain a lower temperature.