Solar Energy and the end of the Harappan Civilization

I first discovered the Harappan civilization while reading H. Beam Piper's excellent short story Omnilingual.  Like many other authors, Piper was fascinated with the concept of Martian civilization.  In this story, the advanced civilization of Mars has vanished while leaving tantalizing clues.  Martha Dane is on the first Earth expedition to Mars, she had previously worked on the Harappa civilization in the Indus valley.

So when I run across stories about Harappa, I pause to think of Piper. Then I plunge into reading the newest story on this mysterious civilization that was in some ways far greater than the better known civilizations that rose alongside the Nile, Euphrates, and Tigris rivers.

After extensive use of satellite data along with years of geologic sampling where the Harappan civilization flourished, the scientists have come to a possible theory on why it collapsed.  The culprit it seems is the sun.  For several thousand years solar radiation in the Northern Hemisphere moderated the monsoon rains which allowed the Harappan civilization to reach great heights.  And then that same solar radiation caused the fall of the same civilization as the monsoon rains dwindled and the land became incapable of supporting the same population density because the land then dried out.  So the people of Harappa migrated east towards the Ganges and its more reliable monsoon rains, leaving behind a mystery that is now just a bit less mysterious.