CO 2, 14 C, the Sun and Climate Change over the Last Millennium 1. CO 2 and climate variability (Gerber et al.,2003) C and solar activity (Muscheler et al., 2005) Fortunat Joos Climate and Environmental Physics University of Bern

Motivation? Can we evaluate climate-carbon cycle modelsover the decadal-to-century time scale? How big is the carbon cycle‘s response to climate change? What is the role of solar forcing and solar forcing amplification? The ice core CO 2 record: a new quantitative constraint.

Reconstructed ranges for low frequency variations in NH temperature are between 0.3 o C and 1 o C

Reconstructed solar forcing varies by up to a factor of five

Preindustrial atmospheric CO 2 variations: a joint constraint on carbon cycle - climate feedbacks and past climate variations Siegenthaler et al., 2004 (Etheridge al.) (Neftel al.)

How self-consistent are the climatic records when interpreted within a carbon cycle-climate model?

Simulations with the Bern Carbon Cycle Model All simulations: solar+volcanic +anthropogenic forcing Bern CC:  T 2xCO2 = [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 CO 2 versus ice core data Model results: smoothed with DML age distribution

Large low frequency temperature variations are not compatible with the ice core CO 2 record High Solar Forcing

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 CO 2 change of 12 ppm d  CO 2 /d  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)

How big is the carbon cycle-climate feedback in current models ?  CO2: difference between a simulation with and without climate change

The ice core CO 2 and NH temperature records constrain the carbon cycle-climate feedback Preindustrial CO 2 variations,  CO2: ~ 6-10 ppm Low-frequency NH temperature variations  T : ~ 0.4 to 0.9 K The carbon cycle-climate feedback: d  CO 2 /d  ppm K -1

C4MIP range: (20th century simulations: ΔCO2 / ΔT = [4 – 16 ppm/K] Linking variations in CO 2, in NH temperature, and the carbon cycle-climate feedback:

Conclusions: Last Millennium CO 2 The ice core CO 2 record and the (low-frequency) NH temperature records constrain the carbon cycle-climate feedback to 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 CO 2 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

Radiocarbon (14C) in the climate system Radiocarbon is produced in the atmosphere by cosmic rays The strength of the cosmic ray flux and of radiocarbon production depends on the shielding by the earth and solar magnetic fields. Atmospheric radiocarbon reflects carbon cycle interactions. 14C is taken up by the ocean and the biosphere and decays with a life time of 8267 years.

The tree-ring 14 C 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   carbon cycle model Deduced radiocarbon production rate Tree ring radiocarbon record   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 Modulation Parameter and Group Sun Spot Number

Conclusions 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 results. ?? Is there a link between solar magnetic activity and irradiance ??