Early investigations on the effect of atmospheric carbon dioxide levels on the runaway greenhouse limit found that it would take orders of magnitude higher amounts of carbon dioxide to take the Earth to a runaway greenhouse state. This is because carbon dioxide is not anywhere near as effective at blocking outgoing longwave radiation as water is. Within current models of the runaway greenhouse effect, carbon dioxide (especially anthropogenic carbon dioxide) does not seem capable of providing the necessary insulation for Earth to reach the Simpson-Nakajima limit.