By comparing today’s Nature paper to earlier versions I found just a few months old, it looks like some blame revisionism occurred after early discussions of this paper at NOAA in May 2012.
Over at Australian Climate Madness, Simon points out the coverage of the ABC for this new paper in Nature. He writes:
Just as we must get rid of the Medieval Warm Period, the inconvenient Roman Warm Period must also be dealt with, and here’s a novel way of doing it: claim that it was man-made. In a single stroke, the RWP is scrubbed from the list of “natural warmings” that the planet has experienced in recent history, helping the Cause by demonstrating that it too was anthropogenic. The ABC reports:
A period covering the heyday of both the Roman Empire and China’s Han dynasty saw a big rise in greenhouse gases, according to a new study.
The finding challenges the view that human-made climate change only began around 1800.
A record of the atmosphere trapped in Greenland’s ice found the level of heat-trapping methane rose about 2000 years ago and stayed at that higher level for about two centuries.
Methane was probably released during deforestation to clear land for farming and from the use of charcoal as fuel, for instance to smelt metal to make weapons, says lead author Celia Sapart of Utrecht University in the Netherlands.
“Per capita they were already emitting quite a lot in the Roman Empire and Han Dynasty,” she says of the findings by an international team of scientists published today in the journal Nature (link to abstract). (source)
Only one problem. Versions of this paper and slide presentation by the lead author in mid May 2012 make no mention of the Romans or Han dynasty whatsoever. Here’s the original abstract compared to the current one:
ORIGINAL – May 15th, 2012 at NOAA’s ESRL: (Source: http://www.esrl.noaa.gov/gmd/annualconference/abs.php?refnum=110-120409-A)
Isotope Variations in Atmospheric Methane Over the Last Two Millenia
T. Röckmann1, C. Sapart1, G. Monteil1, M. Prokopiou1, R.V.D. Wal1, P. Sperlich2, J. Kaplan3, K. Krumhardt3, C.V.D. Veen1, S. Houweling1, M. Krol1, T. Blunier2, T. Sowers4 and P. Martinerie5
1Institute for Marine and Atmospheric Research, Utrech University, Utrecht, Netherlands; 303-497-4988, E-mail: firstname.lastname@example.org
2Centre for Ice and Climate, Niels Bohr Institute, University of Copenhagen, København DK-2100, Denmark
3Ecole Polytechnique Federale de Lausanne, Route Cantonale, Switzerland
4Earth and Environmental Systems Institute, Geosciences, Penn State University, University Park, PA 16802
5Laboratoire de Glaciologie et Géophysique de lEnvironement, University of Grenoble, Grenoble, France
Methane (CH4) is an important greenhouse gas that is emitted from multiple natural and anthropogenic sources. Atmospheric levels of CH4 have varied on various timescales in the past, but in many cases the causes of these variations are not understood. Analysis of the isotopic composition of CH4 preserved in ice cores provides evidence for the environmental drivers of variations in CH4 mixing ratios, because different sources and sinks affect the isotopic composition of CH4 uniquely. We have analyzed (δ13C) of CH4 in air trapped in Greenland ice cores over the last 2 millennia and find that the carbon isotopic composition underwent pronounced centennial-scale variations between 200 BC and 1600 AD without clear corresponding changes in CH4 mixing ratios. The long-term CH4 increase observed over this period is accompanied by a small overall δ13C decrease. Two-box model calculations suggest that the long-term CH4 increase can only be explained by an increase in emissions from biogenic sources. The centennial-scale variations in isotope ratios must be primarily due to changes in biomass burning, which are correlated with both natural climate variability including the Medieval Climate Anomaly, and with changes in human population, land-use and important events in history.
Now compare that original abstract presented to NOAA to the abstract of the paper in Nature being touted by the press on October 3-4, 2012:
Natural and anthropogenic variations in methane sources during the past two millennia
C. J. Sapart, G. Monteil, M. Prokopiou, R. S. W. van de Wal, J. O. Kaplan, P. Sperlich, K. M. Krumhardt, C. van der Veen, S. Houweling, M. C. Krol, T. Blunier, T. Sowers, P. Martinerie, E. Witrant, D. Dahl-Jensen & T. Röckmann
Nature 490, 85–88 (04 October 2012) doi:10.1038/nature11461
Methane is an important greenhouse gas that is emitted from multiple natural and anthropogenic sources. Atmospheric methane concentrations have varied on a number of timescales in the past, but what has caused these variations is not always well understood1, 2, 3, 4, 5, 6, 7, 8. The different sources and sinks of methane have specific isotopic signatures, and the isotopic composition of methane can therefore help to identify the environmental drivers of variations in atmospheric methane concentrations9. Here we present high-resolution carbon isotope data (δ13C content) for methane from two ice cores from Greenland for the past two millennia. We find that the δ13C content underwent pronounced centennial-scale variations between 100 bc and ad 1600. With the help of two-box model calculations, we show that the centennial-scale variations in isotope ratios can be attributed to changes in pyrogenic and biogenic sources. We find correlations between these source changes and both natural climate variability—such as the Medieval Climate Anomaly and the Little Ice Age—and changes in human population and land use, such as the decline of the Roman empire and the Han dynasty, and the population expansion during the medieval period.
Note that the two abstracts start out identically (highlighted in blue), and have similar language throughout presenting the isotope data, but that the Nature abstract has that added part about Roman empire and the Han dynasty.
In this slideshow presentation of the paper, http://www.esrl.noaa.gov/gmd/annualconference/slides/110-120409-A.pdf this graph from page 5 is quite telling:
As Simon points out on his blog:
The population, as the article goes on to say, was about 300 million, barely 4% of what it it today, and without any industrialisation apart from burning charcoal. I will leave it to you to consider the likelihood of such a tiny agrarian population having a significant effect on the climate.
The ABC’s coverage is similarly disingenuous. I’m not going to pay thirty bucks for the full article in Nature (if anyone has access, I would be grateful for a PDF), but eyeballing the tiny graphics published with the abstract (see above) seems to indicate that centennial scale changes in CH4 mixing ratio in the Roman period were in the order of a 20-40 parts per billion (that’s billion with a b). How the ABC can call this a “big rise in greenhouse gases” is unfortunately yet more evidence of agenda-driven journalism. It’s a tiny fraction compared with the industrial rise in CH4, which took mixing ratios to over 1800 ppb, yet the paper claims it is responsible for the significant warming that occurred around the time of the Roman empire?
The graph of CH4 compared to land use change seems like a good case of correlation:
But as we so often learn, when it comes to correlation, that does not always imply causation. Check out this multipanel graph from page 11 of the slide show:
Note graph “f” in red, which are temperature reconstructions from Moberg et al., 2005, Ljungquist et al., 2011, and try to find a correlation with Ch4 emissions in graph “b”.
From my view, there certainly doesn’t seem to be one that holds past 1000AD, when temperature started going down, but world population and land use increased. Likewise, correlation with transformed charcoal in “c” is weak as well.
The conclusion page 13 from the presentation seem pretty wishy-washy, especially the last point, where no specific blame is placed:
•Pronounced centennial-scale δ13C(CH4 ) variability in pre- industrial period
•Highly likely caused by changes in pyrogenic sources
•Correlation with NH charcoal index and anthropogenic land use rate of change
•Long term CH4 rise due to biogenic sources, and correlates well with land use data
•Both natural variability and anthropogenic activities may have influenced the CH4 budget in the pre-industrial period
The claim about the Romans and Han Dynasty seems quite a stretch when you actually look at the data/graphs. But as you can see in the ABC article, they don’t dare show you those things lest you draw conclusions of your own that don’t fit their narrative.
This might help you understand the motivation to start blaming the Romans and the Asians:
Atmospheric Physics and Chemistry Group
Dr Celia Julia Sapart
Master in “Climate Change”, University of East Anglia, Norwich (UEA), UK, 2006-2007
Perhaps she got “Jonesed” into adding the part about the Romans and Han dynasty?