of hypothesized external climate forcing (colored lines) and standardized proxy
Arctic-wide summer-weighted annual temperature (gray lines, plotted as sigma units: see
paper for explaination)
atmospheric CH4 (T. Nikazawa et al., Geophys. Res. Lett. 20, 943 (1993))
b) reconstructed atmospheric CO2 (D.M. Etheridge et al., Geophys. Res. Lett.
22, 4115 (1996))
c) solar irradiance (J. Lean et al., Geophys.
Res. Lett. 22, 3195 (1995))
d) standardized Greenland (GISP2) ice-core
volcanic sulfate (Zielinski et al., Science 264, 948 (1994))
GISP2 Ice Core drill dome at the summit of Greenland. The Greenland Ice Sheet Project 2
(GISP2) ice cores have revealed much about past environmental change, including the nature
of Arctic stratospheric volcanic aerosol loading over the past four centuries. Photo
credit: Mark Twickler.
Eruptions known to be over-represented in the GISP2 record are marked with an asterisk.