Using extinct civilizations such as Easter Island (Rapa Nui) as models, a study conducted in 2018 by Adam Frank "et al." posited that climate change induced by "energy intensive" civilizations may prevent sustainability within such civilizations, thus explaining the paradoxical lack of evidence for intelligent extraterrestrial life. According to his model, possible outcomes of climate change include gradual population decline until an equilibrium is reached; a scenario where sustainability is attained and both population and surface temperature level off; and societal collapse, including scenarios where a tipping point is crossed.