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carbon cycle boundary conditions before the PETM
were different from today's—including a different
continental coni guration, absence of continental ice,
and a different base climate. Moreover, ocean carbon-
ate chemistry prior to the event was different from
modern conditions and the sensitivity to carbon per-
turbation was probably reduced (Goodwin et al. 2009 ;
Stuecker and Zeebe 2010). These aspects limit the
suitability of the PETM as a perfect future analogue.
Nevertheless, the PETM provides invaluable infor-
mation on the response of the carbon cycle, climate,
and ocean carbonate chemistry to massive carbon
input in the past. It also allows us to estimate the
timescale over which carbon was removed from the
ocean-atmosphere system by natural sequestration.
Different carbon input scenarios have been pro-
posed for the PETM (e.g. Dickens et al. 1995 ; Panchuk
et al. 2008 ; Zeebe et al. 2009). For example, the scenario
proposed by Zeebe et al. (2009) requires an initial car-
bon pulse of about 3000 Pg C over ~6000 yr in order
to be consistent with the timing and magnitude of
stable carbon isotope records and deep-sea dissolu-
tion patterns. We have compared this PETM scenario
with a business-as-usual scenario of fossil fuel emis-
sions of 5000 Pg C over ~500 yr (Fig. 2.5). Our results
show that if the proposed PETM scenario roughly
resembles the actual conditions during the onset of
the event, then the effects on ocean chemistry, includ-
ing surface-ocean saturation state, were less severe
during the PETM than expected for the future (Zeebe
and Zachos, pers. comm. 2007; Ridgwell and Schmidt
2010). As pointed out in Section 2.3, not only the mag-
nitude but also the timescale of the carbon input is
critical for its effect on ocean carbonate chemistry. The
timescale of the anthropogenic carbon input is so
short that the natural capacity of the surface reser-
voirs to absorb carbon is overwhelmed (Fig. 2.1A). As
a result of a 5000 Pg C input over ~500 yr, the surface-
ocean saturation state of calcite (Ω c ) would drop from
about 5.4 to less than 2 within a few hundred years. In
contrast, the PETM scenario suggests a correspond-
ing decline of Ω c from 5.5 to only about 4 within a few
thousand years. We emphasize, however, that the
PETM scenario may be subject to revision, depending
on the outcome of future studies that will help to bet-
ter constrain the timescale of the carbon input.
(A)
25
PETM Scenario
Business as Usual
20
15
5000 Pg C
10
5
0
(B)
6
4
2
PETM Scenario
Business as Usual
0
0
2000
4000
6000
8000
10000
Year
Figure 2.5 Palaeocene-Eocene Thermal Maximum (PETM) versus the Anthropocene. (A) Carbon emission scenarios as projected for the future (5000 Pg
C over ~500 yr; Zeebe et al. 2008) and the PETM (3000 Pg C over 6 kyr; Zeebe et al. 2009). The onset of the PETM has been aligned with the onset of
industrialization. (B) Changes in surface-ocean saturation state of calcite simulated with the LOSCAR model in response to the carbon input shown in (A).
 
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