The lifetime of habitability on rocky planets orbiting stars that brighten over time, like the Sun, is finite. Once the planet finds itself closer to its star than the inner edge of its habitable zone, any ocean of liquid water on its surface evaporates, and the atmosphere goes into a temporary water-rich phase if enough water was initially present. After millions of years, the planet eventually loses its water. This is the runaway greenhouse effect, the ultimate climate catastrophe that turns potentially temperate planets into hot wastelands with a steam atmosphere overlying a possibly molten magma surface. The advent of the James Webb Space Telescope and the promise of future observatories makes studying these prime examples of failed Earths all the more timely.

I will introduce the field of exoplanet climate modelling in the context of a study that focused on modelling the different ways that such a planet can stop heating up and start cooling down to transition to a post-runaway climate state. I explore the properties that its atmosphere would feature, including the location of potential clouds that could improve or hamper habitability prospects. I also estimate what the JWST could see when observing such planets, assuming that their climate is undergoing the processes that we are modelling.