True, but they release it each year when the ice refreezes. Look at various years in this plot http://ocean.dmi.dk/arctic/meant80n.uk.php and you will see generally more warmth than average in the fall and less in the spring. The spring cooling is the absorption of heat as you point out. The fall warmth is from the refreeze.
Scientific American, Nuclear Fission Confirmed as Source of More than Half of Earth’s Heat
Excerpt: "The new measurements suggest radioactive decay provides more than half of Earth’s total heat, estimated at roughly 44 terawatts based on temperatures found at the bottom of deep boreholes into the planet’s crust. The rest is leftover from Earth’s formation or other causes yet unknown, according to the scientists involved. Some of that heat may have been trapped in Earth’s molten iron core since the planet’s formation, while the nuclear decay happens primarily in the crust and mantle"
If you call -40 “warmth”...