The decrease in p50 in Oxygen Dissociation curve is due to increase in

Conversely, a fall in temperature or a rise in pH shifts the curve to the left, and a lower Po2 is required to bind a given amount of O2. A convenient index for comparison of such shifts is the P50, the Po2 at which hemoglobin is half saturated with O2. The higher the P50, the lower the affinity of hemoglobin for O2. Reference: Berne& Levy Physiology updated 6th ed, p 462