IS THERE EVIDENCE FOR SOLAR FORCING OF CLIMATE IN THE GISP2 OXYGEN-ISOTOPE RECORD

Changes in solar constant over an 11 yr cycle suggest a certain, but l imited, degree of solar forcing of climate. The high-resolution climat e (oxygen isotope) record of the Greenland GISP2 (Greenland Ice Sheet Project 2) ice core has been analyzed for solar (and volcanic) influen ces. The atmospheric C-14 record is used as a proxy of solar change an d compared to the oxygen isotope profile in the GISP2 ice core. An ann ual oxygen isotope profile is derived from centimeter-scale isotope me asurements available for the post-A.D. 818 interval. Associated extrem e summer and winter isotope ratios were found to yield similar climate information over the last millennium. The detailed record of volcanic aerosols, converted to optical depth and volcanic explosivity change, was also compared to the isotope record and the oxygen isotope respon se calibrated to short-term volcanic influences on climate. This calib ration shows that century-scale volcanic modulation of the GISPZ oxyge n isotope record can be neglected in our analysis of solar forcing. Th e timing, estimated order of temperature change, and phase lag of seve ral maxima in C-14 and minima in O-18 are suggestive of a solar compon ent to the forcing of Greenland climate over the current millennium. T he fractional climate response of the cold interval associated with th e Maunder sunspot minimum (and C-14 maximum), as well as the Medieval Warm Period and Little Ice Age temperature trend of the past millenniu m, are compatible with solar climate forcing, with an order of magnitu de of solar constant change of similar to 0.3%. Even though solar forc ing of climate for the current millennium is a reasonable hypothesis, for the rest of the Holocene the century-scale events are more frequen t in the oxygen isotope record than in the C-14 record and a significa nt correlation is absent. For this interval, oceanic/atmospheric circu lation forcing of climate may dominate. Solar forcing during the surpr isingly strong 1470 yr climate cycle of the 11,000-75,000 yr B.P. inte rval is rather hypothetical. (C) 1997 University of Washington.