Scientists have discovered the reason why the earth wasn't covered with a layer of ice four billion years ago, when the Sun's radiation was much less than it is today. The question of this "missing ice age" had haunted scientists for over 40 years. Professor Minik Rosing says: "What prevented an ice age back then was not high CO2 concentration in the atmosphere, but the fact that the cloud layer was much thinner than it is today. In addition to this, Earth's surface was covered by water. This meant that the Sun's rays could warm the oceans unobstructed, which in turn could layer the heat, thereby preventing Earth's watery surface from freezing into ice."