© 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?
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.
Reference