The Piora Oscillation, an abrupt cold and wet period in the climate history of the Holocene Epoch, is generally dated to the period of c. 3200 to 2900 BCE.

The phenomenon is named after the Val Piora or Piora Valley in Switzerland, where it was first detected; some of the most dramatic evidence of the Piora Oscillation comes from the region of the Alps, where glaciers advance, apparently for the first time since the Holocene climatic optimum; the Alpine tree line drops by 100 meters.

Yet the climate change extends far beyond the Alps and Europe, affecting what is now the New England region of North America, where hemlock and elm trees suffer a dramatic decline.

Similar evidence comes from California and elsewhere; some changes in flora prove permanent.

In the Middle East, the surface of the Dead Sea rises nearly 100 meters (300 feet), then recedes to a more usual level.