At the current rate of global C emissions and without substantial mitigation efforts, atmospheric CO2 is projected to increase by the end of this century to levels not previously experienced on Earth since the onset of our current glacial state. Although Earth has been an icehouse for the past 34 million years, warmer greenhouse conditions have been the ‘typical’ climate state of the past half billion years. Unique insight into how the Earth system will function in such an evolving and high CO2 environment resides in the deep-time analogs of past climate and ecosystem response to greenhouse gas-forced warming of the magnitude comparable to that which we may ultimately face. The deep-time archive, a fully integrated record of climate-ecosystem interactions and feedbacks prompted by high levels of radiative forcing, reveals climate change in the past that was at times far more dynamic than suggested by reconstructions of the past few hundred thousand years. And records of past abrupt change reveal the much slower pace of naturally forced periods of global warming. The association of these periods with critical climate and ecological thresholds provides a context for the future. The inability of numeric climate models to reconstruct surface environmental conditions of past warm periods suggested by proxy records highlights the existence of fundamental processes in the climate system that require further evaluation, and which might indicate that climate projections for our future may well be underestimated. This presentation will discuss – in the context of projected atmospheric CO2 levels – examples of past major transitions that illustrate how greenhouse-gas forced climate change has unfolded in the past and that characterize the fingerprints of climate and ecological thresholds.