Cool stars, the sun and climate variability: Is there a connection? Ulrich Cubasch 1, E. Zorita 2, F. Gonzales-Rouco 3, H. von Storch 2, I. Fast 1 1 Institut für Meterologie, Freie Universität, Berlin 2 GKSS, Geesthacht 3 Universidad Complutense, Madrid

Outline Introduction The ECHO-G climate model Experimental set-up Results Discussion

The climate system External Forcing

Temperature-Reconstruction (treerings, corals, ice and sediment cores, historical evidence) of the temperature of the northern hemisphere from the year 1000 bis 1999 and instrumental temperature from 1902 to 1999 The „Mann et al“- curve

Scientific questions To what extend does external forcing (sun, volcanoes) influence the climate? –Can a climate model simulate the historic climate variability? –Is the currently observed climate change unique or just part of the natural climate variability? –Can we determine the climate sensitivity of the system? –Can the model be used to improve the reconstruction of the historic climate?

The climate model

The ECHO-G Model coupled atmosphere ocean model ECHAM-HOPE-G atmosphere: ECHAM 4: –vertical resolution: 19 levels –horizontal resolution: T30 (approx. 3,75°) ocean: HOPE-G –T42 Arakawa E-Grid (approx. 2,8°), equator refinement –vertical resolution: 20 levels

Land sea mask global

Experimental set-up

The solar forcing reconstructed via 3 different methods

+ = Volcanism Solar Radiation Effective Forcing

Experiments 1. Erik starting at the year Columbus starting at the year 1500

Zorita et al, 2004 The solar and volcanic forcing and the model response

3d-models treering (Esper et al) Multiproxy Mann et al bore hole EBM treering (Jones et al)

A comparison with the Hadley-centre simulation HADCM nat. forc. Columbus Erik

Simulated near surface-temperature anomaly (The late Maunder Minimum)

reconstructed modelled The temperature anomaly during the late Maunder Minimum ( )

climate sensitivity relates the forcing D Q and the temperature response D T  Q = 1/ ŝ  T +  F = l  T +  F  Q:= radiative forcing  T:= transient climate response (TCR)  F:= flux into the surface (dominated by ocean) ŝ:= climate sensitivity  feedback parameter

equilibrium climate sensitivity historic measure based on equilibrium experiments  D F = 0 standard measure is the equilibrium global mean temperature change for doubling of CO 2

climate sensitivity the global climate sensitivity is an important climate parameter, because it is a feature of climate models and the climate system it characterizes the response to forcing it is used to compare models, rescale results, calibrate simpler models etc.

Estimate of climate sensitivity from historic data and model simulations (Zorita et al, 2003). black ; blue:

Can models help to reconstruct climate?

Temporal Changes of Available Proxy Series Available proxy-data (tree-ring) for Europe

Northern Europe Southern Europe Temperature reconstruction by blending model and treering data Fast, 2004

Scientific questions To what extend does external forcing (sun, volcanoes) influence the climate? Substantially - but what is dominant? –Can a climate model simulate the historic climate variability? Yes – there are questions about the amplitude –Is the currently observed climate change unique or just part of the natural climate variability? Seems to be pretty unique –Can we determine the climate sensitivity of the system? It gives an estimate –Can the model be used to improve the reconstruction of the historic climate? In some regions

Open Questions Can we get an estimate about the solar output for the last million years? How important are the volcanoes? Is there a link between solar variability and volcanism? Do treerings underestimate long periodic variabiliy? So far, the UV-B shift and the spatial distribution of volcanoes has been neglected – how important are these factors? The interstellar particle flux – how important is it really, and can it be parameterized?