COMMENTARY ON “THE ANTHROPOGENIC GREENHOUSE ERA
BEGAN THOUSANDS OF YEARS AGO”
MICHEL CRUCIFIX1, MARIE-FRANCE LOUTRE2 and ANDR ´E BERGER2
1Met Office, Hadley Centre for Climate Prediction and Research, FitzRoy Road, Exeter,
Devon EX1 3PB , U.K.; E-mail: michel.crucifix@metoffice.gov.uk
2Institut d’Astronomie et de Ge´ophysique G. Lemaˆitre, Universite´ catholique de Louvain,
Louvain-la-Neuve, Belgium
Abstract. Bill Ruddiman (Climatic Change, 61, 261–293, 2003) recently suggested that early civil-
isations could have saved us from an ice age because land management over substantial areas caused
an increase in atmospheric CO2 concentration. Ruddiman suggests a decreasing “natural course”
of the Holocene greenhouse gases concentrations and sea-level by referring to analogous situations
in the past, namely the last three interglacials. An examination of marine isotopic stage 11 would
perhaps make Ruddiman’s argument even more thought-challenging. Yet, the hypothesis of a natural
lowering of CO2 during the Holocene contradicts recent numerical simulations of the Earth carbon
cycle during this period. We think that the only way to resolve this conflict is to properly assimilate the
palæoclimate information in numerical climate models. As a general rule, models are insufficiently
tested with respect to the wide range of climate situations that succeeded during the Pleistocene. In this
comment, we present three definitions of palæoclimate information assimilation with relevant exam-
ples. We also present original results with the Louvain-la-Neuve climate-ice sheet model suggesting
that if, indeed, the Holocene atmospheric CO2 increase is anthropogenic, a late Holocene glacial
inception is plausible, but not certain, depending on the exact time evolution of the atmospheric CO2
concentration during this period.
The debate about the reason for the steady increase in CO2 concentration between
the Early Holocene (260 ppmv, 8000 years ago) and the pre-industrial era (280
ppmv, 300 years ago), recently spiced up by Bill Ruddiman’s hypothesis that this
increase could be due to early land management (Ruddiman, 2003), illustrates well
two conceptions of understanding and forecasting climate dynamics.
On the one hand, experts in biogeochemical cycles have shown that the in-
crease in atmospheric CO2 concentration during the Holocene is easily explained
by a set of reasonable hypotheses about natural changes in land carbon sequestra-
tion, lysocline depth and sea-surface temperature in response to the orbital forcing.
Seemingly, the only debate is about the respective contributions of these three fac-
tors (Indermu¨hle et al., 1999; Brovkin et al., 2002; Joos et al., 2004). The MoBidiC
model (Crucifix and Joos, 2004) simulates for its part no more than a 4 ppmv in-
crease in CO2 concentration in response to the orbital forcing during the Holocene
(Figure 1, previously unpublished results). Unfortunately, these estimates are as-
sociated with considerable uncertainties. Thorne et al. (2004, submitted) recently
termed ‘structural uncertainties’ errors associated with the hypotheses used in a
model-based analysis process (it covers, in this case, aspects as various as inferring
Climatic Change (2005) 69: 419–426 c© British Crown Copyright 2005

420 MICHEL CRUCIFIX ET AL.
Figure 1. Time evolution of atmospheric CO2 concentration and δ13C ratio simulated by MoBidiC
in response to the orbital forcing during the Holocene. Initial conditions are atmosphere, vegetation,
ocean circulation and carbon balance in equilibrium with an atmospheric CO2 concentration of 260
ppmv, δ13C = 6.5 per mil and orbital forcing of 10,000 years ago. The model carbon cycle includes
equations for vegetation productivity and growth, ocean soft tissue carbon production (particulate and
dissolved) and remineralisation at depth, biogenic carbonate production and dissolution, and air-sea
gas exchanges. The ocean alkalinity is constant. The CO2 concentration data obtained from the Taylor
Dome ice core by Indermu¨hle et al. (1999) are displayed as a comparison.
past alkalinity from preservation records, using zonally averaged equations and a
certain tracer diffusion scheme, or neglecting coral-reef build-up). They are, by def-
inition, difficult to identify and to quantify. Given our present level of understanding
of global carbon cycling, it is certainly fair to argue that the sign of the natural trend
in CO2 during the pre-industrial Holocene is not sufficiently constrained by models
alone.
The other approach, followed by Bill Ruddiman, is to scrutinise the past and
look for situations analoguous to the present-day. The sledgehammer argument is
“Why does a CO2 rise similar to the Holocene one fail to occur on all three previous
interglacials?” But are the previous interglacials real analogues?
The choice of an analogue is made by reference to a set of variables estimated to
be relevant for describing the evolution of the global climate system. Continental
ice volume is surely one of these. Observations have shown that the Last Glacial
Maximum – during which the orbital forcing was very similar to today (Berger,
1978) – is associated with a reduced CO2 concentration (Petit et al., 1999), a
substantial cooling of the ocean surface (Sarnthein et al., 2003), a reduction of the
ventilation of the North Atlantic deep Ocean (Duplessy et al., 1988), absence of
boreal forests (Bigelow et al., 2003) and large permafrost areas (implying frozen
wetlands) (Renssen and Vandenberghe, 2003; Kondratjeva et al., 1993). Computer
experiments have also shown that large ice sheets modify the dynamics of the
atmosphere considerably (Kageyama et al., 1999); the sea-level drop impacts on the
primary productivity of coastal ecosystems (Giraud et al., 2003) as well as on coral
reefs (Ridgwell et al., 2003). There are thus many sources of interaction between
the carbon cycle and ice volume. On the other hand, both ice volume and CO2 are
ultimately driven by the orbital forcing. Milankovitch (1941) already pointed out
that to use one insolation curve (e.g. June insolation at 65◦N) has an arbitrary nature.