Feedback from PETM could explain sustained warming

© 2004 Eugene S. Takle

An abrupt global warming 55 million years ago, often referred to as the Palaeocene/Eocene thermal maximum (PETM), occurred in connection with a massive release of 1,000-2,000 Gt of carbon in the form of methane hydrate from the ocean floor (Bowen et al., 2004). Although long-lived by human-lifetime standards, carbon dioxide (lifetime ~ 100 years) and methane (lifetime ~ 10 years) have short lifetimes by geological standards. So the reason for the sustained warming of the PETM remains a mystery. Increases in tropospheric water vapor and decreased soil sequestration of carbon, both of which will occur quickly with the stratospheric build up of carbon dioxide and methane, likely contributed as positive feedbacks to the warming. On the other hand, delayed negative feedbacks, such as weathering of silicates and delivery of nutrients and alkalinity to oceans would have contributed negative feedbacks that gradually (over 70,000 years) allowed the climate to recover from the 5 to 10 degree Celsius warming.

The noteworthy message of Bowen et al. (2004) is that not all feedbacks have the same time scale and that the carbon-induced warming of the PETM likely triggered some kind of positive feedback that amplified and sustained the abrupt rise in temperatures. The negative feedback, although possibly equally as effective, was slower to become established and lasted well beyond the effective period of the positive feedback. Will there be an analogous and abrupt positive feedback to amplify the current warming?

Reference

Bowen, G.J., D.J. Beerling, P.L. Koch, J.C. Zachos, and T. Quanttlebaum, 2004: A humid climate state during the Palaeocene/Eocene thermal maximum. Nature, 432, 495-499.