Chapter 1 - Climate Change Through Earth's History

Abstract

For at least 3.8billionyears, the Earth has possessed a climate system that continuously maintained a surface environment conducive to life, with liquid water at the surface, interchanging with various amounts of polar ice. The early Earth was warm, perhaps with higher concentrations of greenhouse gases compensating for a solar output smaller than present. Intermittent ice ages began ∼2.5billion years ago, perhaps associated with oxygenation of the Earth's atmosphere and reduction in methane levels; these included more or less worldwide ‘Snowball Earth’ events, with ice extending to low latitudes. From the beginning of the Phanerozoic, the Earth has alternated between ‘icehouse states’ – albeit of lesser severity than the ‘Snowball’ events – and ‘greenhouse states’, such as that of the Mesozoic Era. The Earth is currently in an icehouse state marked by geologically closely spaced fluctuations of climate, largely paced by astronomical variations and amplified by changes in greenhouse gas levels. The present interglacial state of the Holocene is very likely to soon attain levels of climatic warmth not seen for several million years, because of human modification of climate drivers, notably greenhouse gases.