Mi(ni)Geo

Fig. 5 in D. L. Kidder & T. R. Worsley, GSA Today, 22, 2 (2012). From the paper’s conclusions (highlights are mine):

Humans can raise global atmospheric CO2 to levels known from much warmer ancient climates (e.g., Hay, 2011; Kiehl, 2011). Conditions in some of those warm climates will probably be achieved if current levels of carbon emissions continue, although precise prediction of the degree and rate of warming is difficult. A cool greenhouse similar to the MMCO in which the tropics and deep sea warm, most northern ice melts, and perhaps half of the Antarctic ice disappears appears possible within centuries. A warm greenhouse is also possible, although reaching it faces steeper precondition hurdles.

We suspect it will be difficult for humans to force Earth from the current icehouse to a hothouse. The likely cool greenhouse in which about half of Antarctica is still ice-covered means devastation from the tens of meters sea level is likely to rise (e.g., Ward, 2010), and poleward shifting of warm climate belts. Although a hothouse may not occur because economic crises or intentional climate-mitigating efforts by humans or fossil-fuel exhaustion limit greenhouse gas emissions, even a cool greenhouse climate will severely disrupt many societies and economies.

Feedbacks and still-unknown amplifiers will ultimately control just how far humans can force climate toward a hothouse. Uncertainties over these feedbacks should not distract us from the likelihood that a cool greenhouse seems imminent within perhaps a century or two. Long atmospheric CO2 residence times will probably keep Earth from returning to an icehouse for centuries to millennia unless active removal of CO2 from the atmosphere is undertaken.