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CO2, 14C, the Sun and Climate Change
over the Last Millennium
1. CO2 and climate variability (Gerber et al.,2003)
2. 14C and solar activity (Muscheler et al., 2005)
Fortunat Joos
Climate and Environmental Physics
University of Bern
[email protected]
• Can we evaluate climate-carbon cycle models over the
decadal-to-century time scale?
• How big is the carbon cycle‘s response to climate
• What is the role of solar forcing and solar forcing
The ice core CO2 record: a new quantitative
Reconstructed ranges for low frequency variations
in NH temperature are between 0.3oC and 1oC
Reconstructed solar forcing varies by
up to a factor of five
Preindustrial atmospheric CO2 variations:
a joint constraint on carbon cycle - climate feedbacks
and past climate variations
(Etheridge al.)
(Neftel al.)
Siegenthaler et al., 2004
How self-consistent are the
climatic records when interpreted
within a carbon cycle-climate
Simulations with the
Bern Carbon Cycle Model
All simulations:
solar+volcanic +anthropogenic forcing
Bern CC: T2xCO2= [1.5 K – 4.5 K].
Varying solar forcing:
a) “Low Solar Forcing” : Bard et al., 2000, low solar forcing
( Maunder Minimum Reduction: 0.25%; Lean et al., 95)
b) “High Solar Forcing” : Bard et al, high
(Maunder Minimum Reduction: 0.65% ; ~ Reid, 97)
Low solar forcing: Simulated atmospheric
CO2 versus ice core data
Model results: smoothed with DML age distribution
High Solar Forcing
Large low frequency
variations are not
compatible with the
ice core CO2 record
How big is the carbon cycle-climate
feedback in current models ?
Millennium simulations with the Bern model shows
that a low frequency NH temperature change of 1 K
induces a CO2 change of 12 ppm
d CO2/d T(NH) = 12 ppm K-1
The 20th century simulations with the ten
C4MIP models yield a range of
~4 to 16 ppm K-1
(Friedlingstein et al., 2005)
CO2: difference between a simulation
with and without climate change
How big is the carbon cycle-climate
feedback in current models ?
The ice core CO2 and NH temperature records
constrain the carbon cycle-climate feedback
Preindustrial CO2 variations, CO2:
~ 6-10 ppm
Low-frequency NH temperature variations T:
~ 0.4 to 0.9 K
The carbon cycle-climate feedback:
d CO2/d T(NH) = 7-25 ppm K-1
Conclusions: Last Millennium CO2
• The ice core CO2 record and the (low-frequency)
NH temperature records constrain the carbon
cycle-climate feedback to 7 - 25 ppm/K (for
modest climatic variations), comparable to the
C4MIP range (4 to 16 ppm/K).
• A high low-frequency variability in NH
temperature and a high solar forcing or solar
forcing amplification are not compatible with the
CO2 record within the framework of carbon
cycle - climate models. This is consistent with
new evidence from solar like stars.
How Unusual is Today‘s Solar Activity?
• Solanki et al. (Nature, 2004) suggest that
today’s solar activity is unprecedented
during the Holocene
The tree-ring 14C record: a solar proxy
Radiocarbon production depends on the shielding
by the earth and solar magnetic fields.
Reconstructing solar magnetic activity from the
tree ring 14C record
Tree ring radiocarbon record
 carbon cycle model
Deduced radiocarbon production rate
 production model for radiocarbon
+geomagnetic field intensity
Deduced solar modulation /solar magnetic activity
Normalisation to neutron monitor and
ionisation chamber data
Today‘s solar activity is not unusual in the
context of the last millennium
Muscheler et al., 2005
Normalisation to Neutron and Ionisation
Chamber Data
• Solar magnetic activity can be reconstructed
from the radiocarbon tree ring record.
• Today‘s solar magnetic activity appears not to be
unusual in the context of the last millennium
• Different solar proxies yield broadly consistent
?? Is there a link between solar magnetic activity
and irradiance ??