PETM has several characteristics that are important for a meaningful comparison with our present situation.
1. It was a transient event with a rapid onset
2. It was associated with a large and fast carbon input
Nonetheless, it is important to keep in mind that the climatic and carbon cycle conditions before the PETM were different from those today.
Zeebe 2012 compared a PETM scenario with a Business-as-Usual scenario of fossil fuel emissions (See figure 4 for the PETM versus Anthropocene scenario). The results show that if the proposed PETM scenario roughly resembles the actual conditions during the onset of the event, then the effects on ocean chemistry, including surface-ocean saturation state, were less severe during the PETM than would be expected for the future.
The timescale of the anthropogenic carbon input is so short that the natural capacity of the surface reservoirs to absorb carbon is overwhelmed. As a result of a 5,000-Pg C (carbon), the surface-ocean calcite saturation state would drop approximately 5.4 to less than 2 within a few hundred years (figure 4). In contrast, the PETM scenario suggests a corresponding decline of saturation state from 5.5 to ~4 within a few thousand years.
The destruction of marine ecosystems
Ecologically, the perturbation of the surface-water saturation state across the PETM was not detrimental to the survival of the majority of calcareous species. In contrast, the Paleocene-Eocene boundary marks a major extinction event of benthic foraminifera, affecting 30 to 50% of species globally.
However, it is not clear whether the benthic extinction was caused by changes in oxygenation, bottom water temperatures, carbonate undersaturation or other factors. The reality is that the ocean acidification event that humans may cause over the next few centuries is unprecedented in the geological past. Therefore, simulation models using past events need to be evaluated with care. Nonetheless, the consequence of high carbon dioxide emission rates has a nefarious effect on ocean chemistry irrefutable