Guest commentary from Georg Hoffmann
Our understanding of the natural carbon cycle has greatly improved since the times of Svante Arrhenius (1859-1927) and Guy Stewart Callendar (1898-1964). We know what the atmospheric background value of CO2 currently is (it passed 380ppm last year, about 100ppm over the pre-industrial level), we know the seasonal/diurnal cycle in different environments, we have been able to put reasonable constraints on terrestrial and marine sources and sinks, and finally we know the impact of fuel combustion both globally and locally in heavily polluted areas.
The most impressive record documenting the human role in atmospheric CO2 concentrations was recently published in the AR4 SPM showing the comparably minor variations of CO2 during the Holocene (as measured in various ice cores) followed by the abrupt increase of greenhouse gases (GHGs) since the start of industrialisation in the 19th century (figure).
It took a lot of long and painful lessons for scientists to come to this level of understanding of the natural carbon cycle and its anthropogenic contributions. Difficulties arose mainly from two problems: First, sampling of air masses and subsequent measuring of CO2 concentrations was in itself a difficult problem. Measuring techniques were based on titration with limewater or baryta (the Pettenkofer process) and needed a significant amount of technical skill and patience for a proper measurement. The regular use of a CO2 standard to permanently check the measurement quality was something of an exception.
Secondly, nearly all early sampling facilities were tested in continental environments often under the sporadic influence of heavily polluted air masses (such as Paris, Parc Montsouris, Copenhagen, Dieppe etc.). How large is the influence of such “CO2 pollution”? A quick tour through my car-traffic-saturated home town, Paris, can give us a good first impression:
- Jardin Luxembourg (major but still tiny green spot in the center of Paris) 425ppm
- Place de la Bastille: 430ppm
- Place de l’Etoile (the crazy huge roundabout around the Arc de Triomphe): 508ppm
- And the winner was Place de la Nation: 542ppm (ie 160ppm over background!).
All these measurements by David Widory and Marc Javoy (reference below) were snapshot measurements, but they show how CO2 concentrations can vary strongly due to nearby fossil fuel combustion. Even in apparently “natural” environments there are many technical pitfalls to avoid. Strong CO2 fluxes due to the breathing (i.e. photosynthesis and respiration) of the biosphere are producing large diurnal cycles. A sampling site too close to the surface or shielded by a surrounding forest can easily bias the CO2 signal by several dozens of ppm.
During Callendar’s time the exact numbers and processes involved in regional low-altitude CO2 measurements were not yet precisely known. Nevertheless, he made a careful selection of reliable data depending on the methodology used and, in particular, depending on the wind direction since he correctly assumed that North Atlantic marine air represents much better something like a Northern Hemisphere background value. Callendar published his results first 1938 and finally (in 1958) gave a “best value” of 289ppm for the late 19th century amazingly confirmed by subsequent ice core work (see Figure 1).
Recently an article by E-G. Beck has been wafting through the Internet and has now been ‘published’ by Energy and Environment which challenges all these findings or, more precisely, ignores the last 50 years of carbon cycle research [Curiously, this journal always seems happy to ennoble even the strangest idea with the scientific label: "peer reviewed"]. Beck’s approach is very simple: He decided from the beginning that Keeling and Callendar obviously are ideological fanatics and that finally all chemical measurements in the 19th and early 20th century actually were fine. Great news of course!
So what does the new CO2 “reconstruction” look like? For example, within 15 years CO2 levels rose from about 290ppm (1925) to about 470ppm (1942). Worse, within only 10 years these huge CO2 levels were absorbed again and came back to boring mainstream values of about 300ppm.
The list of arguments against such variability in the carbon cycle is too long even for a post on RC but here are a few of the main ones:
- The fluxes necessary to produce such variations are just unbelievably huge. Modern fossil fuel emissions are about 7.5GT (Giga Tons) Carbon per year which would correspond to about 3.5ppm increase per year (except that about half is absorbed by natural sinks in the ocean and the terrestrial biosphere). Beck’s supposed 150ppm source/sink in a decade corresponds therefore to a CO2 production/absorption about ten times stronger than the entire global industrial production of 2007 (putting aside for the moment additional complications since such CO2 levels had to be equilibrated at least partly with the ocean and the real CO2 source must even be larger).
- Such huge biospheric fluxes would leave an enormous 13C signal in the atmosphere. Nothing remotely like that is observed in tree ring cellulose data.
- Beck makes an association of some of the alleged huge CO2 peaks with volcanic eruptions. The Mauna Loa CO2 record started by Charles Keeling 1955 (http://cdiac.ornl.gov/, http://cdiac.ornl.gov/trends/co2/graphics/mlo145e_thrudc04.pdf ) however doesn’t show much variability associated with the big eruptions of El Chichon, Agung or Pinatubo. (Readers should know however that on much longer, geologic, timescales, CO2 levels are heavily influenced by volcanic and tectonic activity, but that is not important on the interannual (or even centennial) timescale).
- The paper suggests that the CO2 peak in the 1940 is forced by the first temperature rise in the 20th century. That would make 150ppm due to a temperature shift of 0.4°C. What happened then with the next rise from the 1970s to today? The observed about 0.5°C rise corresponded to “only” 70ppm always assuming that fossil fuel combustion does not leave any remains in the atmosphere…. ;)
- And most importantly, we know from ice core analysis the CO2 concentration from the pre-industrial to modern times. The results of three different Antarctic cores broadly confirm the picture of an accelerating rise of CO2 above levels of natural variability over the last 650.000 years.
This paper has already received some appropriately critical reviews (particularly from Rabett Run here and
The last question to answer is how on earth Beck’s paper could survive a half-decent review process from anyone who knows any of this history. But this is a question best posed to the Editorial board of Energy and Environment and its Editor, Sonja Boehmer-Christiansen.
Widory, D. and Javoy, M. 2003. The carbon isotope composition of atmospheric CO2 in Paris. Earth and Planetary Science Letters 215: 289-298.