The distinction between the short-run and long-run aggregate supply curve is necessary because: the long-run aggregate supply curve is horizontal while the short-run aggregate supply curve is upward sloping changes in the price level affect real GDP in the short run but not in the long run. the price level is always higher in the short run than in the long run. the price level can change in the short run but is fixed in the long run. the price level is always higher in the long run than in the short run