 An alternative theory suggests instead that the jet stream shifted northward in response to the changing topographic forcing of the melting North American ice sheet, which brought more rain to the North Atlantic, which freshened the ocean surface enough to slow the thermohaline circulation.There is also some evidence that a solar flare may have been responsible for the megafaunal extinction, but that cannot explain the apparent variability in the extinction across all continents.A hypothesized Younger Dryas impact event, presumed to have occurred in North America about 12,900 years ago, has been proposed as the mechanism that initiated the Younger Dryas cooling.Among other things, findings of melt-glass material in sediments in Pennsylvania, South Carolina and Syria have been reported.