A climate scientists view. Climate Change: The Next Generation: Andrew Glikson: Planetary Boundaries (CO2 equivalent of 460 ppm has been surpassed In my view an upper limit of 450 ppm CO2, proposed by a range of reports by government organizations, including the Garnaut Review  and the Australian Government White Paper , can not be sustained, for the following reasons: A. The atmosphere has already transcended the CO2-equivalent (including the forcing of methane) level of 460 ppm. B. A level of 450 ppm CO2 is a mere ~40 ppm below the upper boundary of ~500 ppm, which is the upper limit of stability of the Antarctic ice sheet, formed about 34 million years ago. In the Pliocene, 3 million years ago, a CO2 level of 400 ppm led to temperature rise of about 23 oC and sea level rise of 25+/-12 meters. C. There is no evidence that the climate can be stabilized at such high level of greenhouse-induced forcing. Due to carbon cycle feedback loops and feedbacks related to ice melt/water interaction, CO2 level of 450 ppm may lead to yet higher greenhouse levels, high temperature levels and possible tipping points. D. Not taken into account in many projections are looming emissions of methane, which are already taking place under atmospheric CO2 levels of 388 ppm, or CO2-e levels of 460 ppm.