Constraining temperature variations over the last millennium by comparing simulated and observed atmospheric CO₂

Abstract.

The response of atmospheric CO₂ and climate to the reconstructed variability in solar irradiance and radiative forcing by volcanoes over the last millennium is examined by applying a coupled physical–biogeochemical climate model that includes the Lund-Potsdam-Jena dynamic global vegetation model (LPJ-DGVM) and a simplified analogue of a coupled atmosphere–ocean general circulation model. The modeled variations of atmospheric CO₂ and Northern Hemisphere (NH) mean surface temperature are compatible with reconstructions from different Antarctic ice cores and temperature proxy data. Simulations where the magnitude of solar irradiance changes is increased yield a mismatch between model results and CO₂ data, providing evidence for modest changes in solar irradiance and global mean temperatures over the past millennium and arguing against a significant amplification of the response of global or hemispheric annual mean temperature to solar forcing. Linear regression (r = 0.97) between modeled changes in atmospheric CO₂ and NH mean surface temperature yields a CO₂ increase of about 12 ppm for a temperature increase of 1 °C and associated precipitation and cloud cover changes. Then, the CO₂ data range of 12 ppm implies that multi-decadal NH temperature changes between 1100 and 1700 AD had to be within 1 °C. Modeled preindustrial variations in atmospheric δ¹³C are small compared to the uncertainties in ice core δ¹³C data. Simulations with natural forcings only suggest that atmospheric CO₂ would have remained around the preindustrial concentration of 280 ppm without anthropogenic emissions. Sensitivity experiments show that atmospheric CO₂ closely follows decadal-mean temperature changes when changes in ocean circulation and ocean-sediment interactions are not important. The response in terrestrial carbon storage to factorial changes in temperature, the seasonality of temperature, precipitation, and atmospheric CO₂ has been determined.