55–85 °C during the period of , followed by cooling to more mild temperatures of between 10-40 °C by . Reconstructed proteins from Precambrian organisms have also provided evidence that the ancient world was much warmer than today. However, other evidence suggests that the period of was generally colder and more glaciated than the last 500 million years. This is thought to be the result of solar radiation approximately 20% lower than today. Solar luminosity was 30% dimmer when the Earth formed 4.5 billion years ago, and it is expected to increase in luminosity approximately 10% per billion years in the future. On very long time scales, the evolution of the sun is also an important factor in determining Earth's climate. According to standard solar theories, the sun will gradually have increased in brightness as a natural part of its evolution after having started with an intensity approximately 70% of its modern value. The initially low solar radiation, if combined with modern values of greenhouse gases, would not have been sufficient to allow for liquid oceans on the surface of the Earth. However, evidence of liquid water at the surface has been demonstrated as far back as . This is known as the faint young sun paradox and is usually explained by invoking much larger greenhouse gas concentrations in Earth's early history, though such proposals are poorly constrained by existing experimental evidence. See also Climate state Global warming Global cooling Instrumental temperature record Ocean heat content Satellite temperature measurements Sea surface temperature Thermal history of the Earth Timeline of glaciation List of periods and events in climate history References Historical geology Paleoclimatology Paleoceanography