Water Vapor Amplifies CO2 Warming but Is Uncontrollable

Water vapor is a powerful greenhouse gas that amplifies CO2 warming by more than half through a vicious cycle—more CO2 warms the air, a warmer atmosphere holds more water vapor, which warms it further—but because atmospheric water content is set by temperature over a 70%-ocean planet, it is not a lever we can control, with the one wrinkle that jet aircraft deliver water vapor to the stratosphere where it lingers and warms longer.