Pervasive solar influence on Greenland temperature over the past 4000 years

During the past decades, Greenland climate has undergone rapid warming and ice sheet ablation in coastal region with a nearly 1 mm/y sea level contribution. For sea level projection, it is critical to understand the mechanisms of Greenland temperature variability. Greenland temperature is known to be affected by the North Atlantic Oscillation (NAO), and it is also highly correlated with North Atlantic average temperature. Using the reconstructed Greenland temperature from argon and nitrogen isotopes in occluded air in GISP2 ice core (Kobashi et al., 2011), we found Greenland temperature deviated negatively (positively) from North Hemispheric (NH) temperature trend during stronger (weaker) solar activity over the past 800 years (Kobashi et al., 2013). Climate modeling suggests that the deviation was caused by solar induced atmospheric circulation changes (like NAO). The model also suggests that Atlantic meridional circulation weakens during the stronger sun by similar processes as enhanced greenhouse effect (Kobashi et al., 2013). For the past 4000 years, we reconstructed northern NH temperature using a 1-dimensional energy balance model with reconstructed climate forcing (volcanic, solar, greenhouse gas, orbital forcings). The model exhibits a clear cooling trend over the past 4000 years as observed for the reconstructed Greenland temperature through decreasing annual average insolation. Considering the negative solar influence on Greenland temperature, the modeled and observed Greenland temperatures agree with correlation coefficients of r = 0.34-0.36, p = 0.1-0.04 in a multidecadal time scale (21-yr RMs) and r = 0.38-0.45, p = 0.1-0.05 in a centennial time scale (101-yr RMs), explaining 14% to 20% of variance of observed temperature in multidecadal to centennial time scales with a 90-96% confidence interval. This is smaller than 25% for European winter and spring temperatures, or 35% for annual temperatures in 11 yr smoothed data over the past five centuries (Hegerl et al., 2011) possibly because of less accurate forcing reconstruction. The weak but pervasive solar influence on Greenland temperature over the past 4000 years provides important implications on current and future Greenland temperatures. Hegerl, G., Luterbacher, J., González-Rouco, F., Tett, S. F., Crowley, T., and Xoplaki, E.: Influence of human and natural forcing on European seasonal temperatures, Nat. Geosci., 4, 99-103, 2011. Kobashi, T., Kawamura, K., Severinghaus, J. P., Barnola, J.-M., Nakaegawa, T., Vinther, B. M., Johnsen, S. J., and Box, J. E.: High variability of Greenland surface temperature over the past 4000 years estimated from trapped air in an ice core, Geophys. Res. Lett., 38, 10.1029/2011GL049444, 2011. Kobashi, T., Shindell, D. T., Kodera, K., Box, J. E., Nakaegawa, T., and Kawamura, K.: On the origin of Greenland temperature anomalies over the past 800 years, Clim. Past, 9, 583-596, 2013.