Most of you have probably seen the news already that a recent study can’t explain all the warming that we know happened in the geologic past purely from atmospheric CO2 emissions. What’s that–there’s Something Else that could be causing warming besides CO2??? Is it time to raid the palatial headquarters of the IPCC and Al Gore’s home and forcefully take back their Nobel Prizes? Have the climate scientists got it all wrong???
Well, no.
Despite the histrionics this paper has created among the deniers, it means no such thing.
The best treatment I’ve seen of the situation comes from the Union of Concerned Scientists’ news posting, Study Says Climate Models May Underestimate Warming | Union of Concerned Scientists (emphasis added):
Several climate science 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.
Notice the subtle detail that many of the people showing up in my new feeds are “somehow” missing: The study said that the level of CO2 in the atmosphere could not explain the warming; it did not relate to the level of all greenhouse gases. From the paper’s abstract (emphasis added):
Richard Zeebe and colleagues used analyses of marine sediments and a carbon-cycle model to estimate the amount of carbon dioxide released during the Palaeocene-Eocene Thermal Maximum, which lasted for about 100,000 years. They found that, using current estimates of climate sensitivity to increasing levels of atmospheric carbon dioxide, the carbon release could only explain up to 3.5 °C of the warming. They concluded that as yet unknown warming feedbacks must have caused the additional rise in temperature.
If we assume that their technique–using marine sediments plus carbon cycle modeling as a proxy for atmospheric CO2 levels–is deadly accurate, it still leaves open the possibility, as the UCS points out, of a massive, sustained methane release from defrosting permafrost (and/or, presumably, methane hydrate deposits). Large amounts of methane, which has roughly 20 times the warming potential of CO2, could make an enormous difference. And as we found out recently, there is about twice as much carbon in the permafrost as we thought, roughly 1.5 trillion tons.
This finding only strengthens my belief that the potential for massive releases from permafrost and methane hydrates is the scariest unknown in our current situation.
(I suspect that in time most of us will agree that a key determinant of the stability of the Earth’s climate is the presence of a large amount of land at just the right distance from one of the poles (in conjunction with other climate patterns like ocean and air currents). This lets biomass grow during warm periods, lock up CO2 until a cooling cycle is triggered (in part by the sequestration of that CO2), and then when a very moderate amount of warming is triggered by some other mechanism that carbon is re-introduced as CO2 and methane which greatly amplifies the warming. I’ve looked for evidence that anyone has investigated or talked seriously about this potential mechanism, but I’ve come up empty, so any stupidity herein is solely my fault.)
For some background on the events of 55 million years ago, see Wikipedia: Paleocene-Eocene Thermal Maximum.
