It is sigmoidal because of positive cooperativity
*Deranged Physiology:* once an oxygen molecule is bound to it, the oxygenated subunit increases the oxygen affinity of the three remaining subunits

LEFT shift = higher affinity to oxygen = ↓ oxygen delivery

RIGHT shift = reduced affinity to oxygen = ↑ oxygen delivery to tissues

deoxygenated hemoglobin (Hb) binds CO2 with greater affinity than oxygenated hemoglobin (HbO2)
hence oxygen induces a rightward shift of the CO2 dissociation curve, which is called the Haldane effect
in patients with severe COPD who cannot increase minute ventilation, the Haldane effect accounts for about 25% of the total PaCO2 increase due to O2 administration
https://derangedphysiology.com/main/cicm-primary-exam/respiratory-system/Chapter-1142/haldane-effect