The carbon dioxide levels necessary to thaw Earth have been estimated as being 350 times what they are today, about 13% of the atmosphere. Since the Earth was almost completely covered with ice, carbon dioxide could not be withdrawn from the atmosphere by release of alkaline metal ions weathering out of siliceous rocks. Over 4 to 30 million years, enough and methane, mainly emitted by volcanoes but also produced by microbes converting organic carbon trapped under the ice into the gas, would accumulate to finally cause enough greenhouse effect to make surface ice melt in the tropics until a band of permanently ice-free land and water developed; this would be darker than the ice, and thus absorb more energy from the Sun—initiating a "positive feedback".