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Bizarrely, climate science deniers are touting a new study that finds we might return to the rapid global warming of the Palaeocene-Eocene thermal maximum (PETM) with much lower levels of CO2 than previously thought.

The PETM’s climate would be quite inhospitable to human civilization.  A February Nature article concluded (see “The Garden of Eden had a 40-foot, 1-ton snake plus 90°F average temperatures“):

If our Palaeocene estimates are correct, tropical temperatures at the slightly younger (55.8 Myr ago) Palaeocene-Eocene thermal maximum (PETM) could have reached 38-40°C, resulting in widespread equatorial heat-death as recent models and other proxy data have predicted.

A 2006 Nature analysis of deep marine sediments beneath the Arctic found Artic temperatures during the PETM almost beyond imagination-above 23°C (74°F)-temperatures more than 18°F warmer than current climate models had predicted (see “A methane feedback from the past strikes again“). The three dozen authors of the 2006 paper concluded that existing climate models are missing crucial feedbacks that can significantly amplify polar warming – as opposed to the imaginary negative feedbacks deniers like Lindzen claim while will magically save humanity from catastrophic warming (see Study: Water-vapor feedback is “strong and positive,” so we face “warming of several degrees Celsius”).

Now a new PETM study is out (click here), which deniers like Swift Boat smearer Marc Morano are touting as evidence climate models don’t accurately model the climate – but which rational climate science activists understand is yet more evidence that most climate models underestimate likely future warming.  Here is the Union of Concerned Scientists press release on the study:

Several climate contrarian Web sites are misrepresenting the findings of a peer-reviewed study published in the July 13 issue of the journal Nature Geoscience. The study – by scientists from Rice University, the University of Hawaii and the University of California at Santa Cruz – provides evidence that current climate models are underestimating the amount of warming that an increase in atmospheric carbon dioxide can cause. In other words, the potential consequences of global warming are likely worse than what scientists are predicting.

The study examined the extent to which increased carbon dioxide levels could explain a 5 to 9 degree Celsius increase the Earth experienced 55.5 million years ago. The authors concluded that current estimates of how much carbon dioxide increases the average Earth temperature only explains 3.5 degrees of warming.

In a commentary published with the study, David Beerling, a paleobiologist at the University of Sheffield in England, writes: “The upshot of the study. is that forecasts of future warming could be severely underestimating the extent of the problem that lies in store for humanity as greenhouse gases accumulate in the atmosphere.

According to Melanie Fitzpatrick, a climate scientist at the Union of Concerned Scientists (UCS), carbon dioxide-induced warming can lead to changes that exacerbate the problem. For example, increasing CO2 concentrations:
– melt tundra, which then releases methane and other heat-trapping gases into the atmosphere;
– warm the air, which then can hold more water vapor, another heat-trapping gas; and
– melt white ice, which exposes the ocean and land, which, because they are darker in color, absorb more heat from the sun and reflect less of it back into space.

Scientists are still trying to precisely quantify the effect of such “positive feedback cycles” that took place millions of years ago as well as the ones that are happening today, Fitzpatrick said. The scientific literature, including the new Nature Geoscience study, indicates that positive feedbacks greatly outweigh negative ones and that current climate models are likely underestimating potential temperature increases from overloading the atmosphere with carbon dioxide and other heat-trapping gases.

Precisely.

I would add that the study’s authors themselves note in the conclusion:

Possible causes of the excess warming include increased production and levels of trace greenhouse gases as a consequence of the climatic warming (such as CH4).

Given that some of the fastest warming on the planet is occurring right where the most methane is stored (see here), the methane feedback remains the biggest worry in the entire carbon cycle.

So go ahead, deniers, make my day and direct more people to read this valuable study.