A method of drilling a subsea wellbore, comprising: drilling the wellbore by injecting drilling fluid through a tubular string extending into the wellbore from an offshore drilling unit and rotating a drill bit disposed on a bottom of the tubular string, wherein: the drilling fluid exits the drill bit and carries cuttings from the drill bit, and the returns flow to a floor of the sea via an annulus defined by an outer surface of the tubular string and an inner surface of the wellbore, and while drilling the wellbore: mixing lifting fluid with the returns at a flow rate proportionate to a flow rate of the drilling fluid, thereby forming a return mixture, wherein: the lifting fluid has a density substantially less than a density of the drilling fluid, the return mixture has a density substantially less than the drilling fluid density, the returns flow from the seafloor, through a subsea wellhead, and into a pressure control assembly connected to the subsea wellhead, a marine riser is connected to the PCA and connected to the ODU by an upper marine riser package (UMRP), the lifting fluid is mixed with the returns by injection into the UMRP and down the marine riser, and the return mixture flows to the ODU via a conduit; measuring a flow rate of the returns or the return mixture; and comparing the measured flow rate to the drilling fluid flow rate to ensure control of a formation being drilled. drilling the wellbore by injecting drilling fluid through a tubular string extending into the wellbore from an offshore drilling unit and rotating a drill bit disposed on a bottom of the tubular string, wherein: the drilling fluid exits the drill bit and carries cuttings from the drill bit, and the returns flow to a floor of the sea via an annulus defined by an outer surface of the tubular string and an inner surface of the wellbore, and the drilling fluid exits the drill bit and carries cuttings from the drill bit, and the returns flow to a floor of the sea via an annulus defined by an outer surface of the tubular string and an inner surface of the wellbore, and while drilling the wellbore: mixing lifting fluid with the returns at a flow rate proportionate to a flow rate of the drilling fluid, thereby forming a return mixture, wherein: the lifting fluid has a density substantially less than a density of the drilling fluid, the return mixture has a density substantially less than the drilling fluid density, the returns flow from the seafloor, through a subsea wellhead, and into a pressure control assembly connected to the subsea wellhead, a marine riser is connected to the PCA and connected to the ODU by an upper marine riser package (UMRP), the lifting fluid is mixed with the returns by injection into the UMRP and down the marine riser, and the return mixture flows to the ODU via a conduit; measuring a flow rate of the returns or the return mixture; and comparing the measured flow rate to the drilling fluid flow rate to ensure control of a formation being drilled. mixing lifting fluid with the returns at a flow rate proportionate to a flow rate of the drilling fluid, thereby forming a return mixture, wherein: the lifting fluid has a density substantially less than a density of the drilling fluid, the return mixture has a density substantially less than the drilling fluid density, the returns flow from the seafloor, through a subsea wellhead, and into a pressure control assembly connected to the subsea wellhead, a marine riser is connected to the PCA and connected to the ODU by an upper marine riser package (UMRP), the lifting fluid is mixed with the returns by injection into the UMRP and down the marine riser, and the return mixture flows to the ODU via a conduit; the lifting fluid has a density substantially less than a density of the drilling fluid, the return mixture has a density substantially less than the drilling fluid density, the returns flow from the seafloor, through a subsea wellhead, and into a pressure control assembly connected to the subsea wellhead, a marine riser is connected to the PCA and connected to the ODU by an upper marine riser package (UMRP), the lifting fluid is mixed with the returns by injection into the UMRP and down the marine riser, and the return mixture flows to the ODU via a conduit; measuring a flow rate of the returns or the return mixture; and comparing the measured flow rate to the drilling fluid flow rate to ensure control of a formation being drilled.