greenhouse limit on surface temperature is higher than that found in one-dimensional models and thus would require a higher amount of carbon dioxide to initiate a moist greenhouse than in one-dimensional models. Other complications include whether the atmosphere is saturated or sub-saturated at some humidity, higher CO2 levels in the atmosphere resulting in a less hot Earth than expected due to Rayleigh scattering, and whether cloud feedbacks stabilize or destabilize the climate system. Complicating the matter, research on Earth's climate history has often used the term "runaway greenhouse effect" to describe large-scale climate changes when it is not an appropriate description as it does not depend on Earth's outgoing longwave radiation. Though the Earth has experienced a diversity of climate extremes, these are not end-states of climate evolution and have instead represented climate equilibria different from that seen on Earth today. For example, it has been hypothesized that large releases of greenhouse gases may have occurred concurrently with the Permian–Triassic extinction event or Paleocene–Eocene Thermal Maximum. Additionally, during 80% of the latest 500 million years, the Earth is believed to have been in a greenhouse state due to the greenhouse effect, when there were no continental glaciers on the planet, the levels of carbon dioxide and other greenhouse gases (such as water vapor and methane) were high, and sea surface temperatures (SSTs) ranged from 40 °C (104 °F) in the tropics to 16 °C (65 °F) in the polar regions. Distant future Most scientists believe that a runaway greenhouse effect is inevitable in the long term, as the Sun gradually becomes more luminous as it ages, and spell the end of all life on Earth. As the Sun becomes 10% brighter about one billion years from now, the surface temperature of Earth will reach , causing the temperature of Earth to rise rapidly and its oceans to boil away until it becomes a greenhouse planet, similar to Venus today. According to the astrobiologists Peter Ward and Donald Brownlee in their book The Life and Death of Planet Earth, the current loss rate is approximately one millimeter of ocean per million years, but the rate is gradually accelerating, as the sun gets warmer, to perhaps as fast as one millimeter every 1000 years. Ward and Brownlee predict that there will be two variations of the future warming feedback: the "moist greenhouse" in which water vapor dominates the troposphere and starts to