Final answer:
The two gases that are effective at absorbing Earth's longwave radiation are water vapor and carbon dioxide (CO₂), both of which contribute significantly to the greenhouse effect.
Step-by-step explanation:
The two gases that are good absorbers of longwave radiation emitted from the Earth are water vapor and carbon dioxide (CO₂). These gases contribute to the greenhouse effect, effectively trapping heat within the atmosphere and maintaining global temperatures suitable for life. While water vapor is responsible for absorbing a significant portion of infrared radiation, CO₂ also plays a critical role.
It is CO₂'s absorption features that cause the sharp cutoff of longwave radiation on the right-hand side of the water window, as depicted in the referenced figure. Though other gases like methane and ozone are also greenhouse gases, they are less concentrated in the atmosphere and their contributions to the effect, while still important, are relatively smaller in comparison to water vapor and CO₂.