The Sun shone 20% less brightly on early Earth, and yet fossil evidence shows that our planet had warm shallow seas where stromatolites – microbial mats – thrived. Now a study may have solved the “faint young Sun paradox”, showing that saltier oceans could have prevented Earth from freezing over during Archean times, 3bn years ago. We all know that the composition of the atmosphere (particularly the abundance of greenhouse gases) plays a crucial role in tempering Earth’s climate, but what about the composition of the oceans? To answer this question researchers used an ocean-atmosphere general circulation model to investigate the impact of salinity. They show that saltier oceans result in warmer climates, partly because the salt depresses the freezing point of seawater and inhibits sea-ice formation, but mostly because the greater density of salty water alters ocean circulation patterns and aids heat transport to the poles. Under their Archean scenario they show that present-day levels of salinity produce a severely glaciated world with only a narrow strip of open water at the equator. But pushing salinity up to 40% greater than today revealed a warmer Archean world, with average surface temperatures of more than 20C, and ice only appearing seasonally at the poles. Their findings are reported in Geophysical Research Letters.