The "long summer" is the Holocene era, the [plus or minus]11,000 years of general climate amelioration without which it is doubtful whether Homo Sapiens would ever have emerged from the Stone Age and successfully developed agriculture and colonised almost the entire planet.  But the Holocene has not been uniformly benign, or even stable.  There have been notable cold spells and dry spells, which have spelt disaster for the human societies who lived through them.

This is Fagan's thesis:  that civilizations are far more vulnerable to climate change (not simply global warming) than has been recognised by those civilizations until too late.  Using the resources of scientific archaeology such as archaeobotany, zoology, meteorology, geology, he explores the rise, success, and fall of societies across the world and from the (so far discovered) earliest, in Southwestern Asia, via Mesoamerica, to the final disintegration of the Roman Empire in the 6th century CE.

He argues that there comes a tipping-point, beyond which, however sophisticated technologically these civilizations were (and some of them were amazingly advanced; even the Maya, who didn't have the wheel, were experts in building and at irrigation agriculture), a point is reached where even a minor shift in rains or temperatures can have precipitates food production failure and general collapse.

Fagan's expertise is in historic and prehistoric anthrolopogy; the book is for the general reader. Anyone who wishes to go deeper into his argument can consult the ample references.

304pp paperback; index; notes/references; b/w maps, timelines and charts