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

Abstract

The response of atmospheric CO 2 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 CO 2 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 CO 2 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 CO 2 and NH mean surface temperature yields a CO 2 increase of about 12 ppm for a temperature increase of 1°C and associated precipitation and cloud cover changes. Then, the CO 2 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 d 13 C are small compared to the uncer-tainties in ice core d 13 C data. Simulations with natural forcings only suggest that atmospheric CO 2 would have remained around the preindustrial concentration of 280 ppm without anthropogenic emissions. Sensitivity experiments show that atmospheric CO 2 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 CO 2 has been determined.

Solar influence on climate during the past millennium: Results from transient simulations with the NCAR Climate System Model

“…Although a solar contribution can be isolated in the model results, its relative role diminishes with reduced amplitudes and the contribution of solar forcing to natural variability becomes small. A relatively small climate change (Ͻ1°C) of NH surface temperatures is also consistent with the small preindustrial variations in atmospheric CO 2 , CH 4 , and N 2 O and an independent estimate based on atmospheric greenhouse gas changes over the past millennium (32,48). This result is particularly robust because of the low sensitivity of the model.…”

“…Possible transient effects of solar irradiance changes on climate have been investigated with computationally efficient models, such as Energy Balance Models (15,32) and Models of Intermediate Complexity (33,34) as well as with Atmosphere-Ocean-General Circulation Models for the 20th century (35) and the past (36,37). Here, we employ a coupled Atmosphere-Ocean General Circulation Model in experiments covering the period of the last 1,150 years.…”

Present state of global wetland extent and wetland methane modelling: methodology of a model inter-comparison project (WETCHIMP)

“…LPJ-Bern is a subsequent development of the Lund-PotsdamJena dynamic global vegetation model Joos et al, 2004;Gerber et al, 2003) that combines processbased, large-scale representations of terrestrial vegetation dynamics, soil hydrology (Gerten et al, 2004;Wania et al, 2009a), human induced land use changes (Strassmann et al, 2008;Stocker et al, 2011), permafrost and peatland establishment (Wania et al, 2009a,b) and simulation of biogeochemical trace gas emissions, such as CH 4 (Wania et al, 2010;Spahni et al, 2011;Zürcher et al, 2013). LPJ-Bern uses a different ebullition mechanism for CH 4 emissions from peatlands, which includes variations in partial pressure of CO 2 .…”

A revised 1000 year atmospheric δ13C‐CO2 record from Law Dome and South Pole, Antarctica

“…Models of the global C cycle that include the stable isotopes of C [Joos et al, 1999;Trudinger et al, 2002b;Gerber et al, 2003;Köhler et al, 2006] have found an additional constraint in the companion record of ı 13 C from the same site in Antarctica [Francey et al, 1999, hereinafter F99]. This is a high precision data set, due to meticulous examination of all aspects of the procedure, representing the state of the art in 1999 for measured ı 13 C changes in air trapped in ice cores.…”