David L. Kidder*, Thomas R. Worsley**

Dept. of Geological Sciences, Ohio University, Athens, Ohio 45701, USA

Abstract

Hothouse climate has been approached or achieved more than a dozen times in Phanerozoic history. Geologically rapid onset of hothouses in 104–105 yr occurs as HEATT (haline euxinic acidic thermal transgression) episodes, which generally persist for less than 1 million years. Greenhouse climate preconditions conducive to hothouse development allowed large igneous provinces (LIPs), combined with positive feedback amplifiers, to force the Earth to the hothouse climate state. The two most significant Cenozoic LIPs (Columbia River Basalts and much larger Early Oligocene Ethiopian Highlands) failed to trigger a hothouse climate from icehouse preconditions, suggesting that such preconditions can limit the impact of CO₂ emissions at the levels and rates of those LIPs.

Human burning of fossil fuels can release as much CO₂ in centuries as do LIPs over 10⁴–10⁵ yr or longer. Although burning fossil fuels to exhaustion over the next several centuries may not suffice to trigger hothouse conditions, such combustion will probably stimulate enough polar ice melting to tip Earth into a greenhouse climate. Long atmospheric CO₂ residence times will maintain that state for tens of thousands of years.

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**Present address: 1044 Thunderbird Court, Eagle, Idaho 83616, USA,

Manuscript received 14 June 2011; accepted 22 Nov. 2011.

DOI: 10.1130/G131A.1