Well not exactly.  First difficult wouldn't adjust downward until 2,016 blocks are completed and then it only adjusts downward by a factor of 4.  So just to get back to difficulty 1 would require more than 32,000 blocks to be completed.  The first would be at the current difficulty and difficulty would only decline slowly.  Mining even the genesis block by hand is simply not possible.    Difficulty one requires an average of 4 billion hashes.  Completing even a single sha256d hash by hand is measured in hours (more realistically a day or more).

Still the good news is that only half of the earth is facing the sun at any given time so destruction of the entire planet's computing resources would multiple solar storms on a massive scale.  The other thing to consider is that under any civilization ending solar storm you will be to busy fighting and killing for clean water and sufficient food to worry about the bitcoin network halting.