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] www.climate.unibe.ch/~joos Motivation? • Can we evaluate climate-carbon cycle models over 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 CO2 record: a new quantitative constraint. 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 model? 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 temperature 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 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 ??