Very high atmospheric CO2 levels can explain the high temperatures on the still young Earth three to four billion years ago. At the time, our Sun shone with only 70 to 80 percent of its present intensity. Nevertheless, the climate on the young Earth was apparently quite warm because there was hardly any glacial ice. This phenomenon is known as the 'paradox of the young weak Sun." Without an effective greenhouse gas, the young Earth would have frozen into a lump of ice. Whether CO2, methane, or an entirely different greenhouse gas heated up planet Earth is a matter of debate among scientists.