Discussion on climate oscillations: CMIP5 general circulation models versus a semi-empirical harmonic model based on astronomical cycles.

Power spectra of global surface temperature (GST) records (available since 1850) reveal major periodicities at about 9.1, 10–11, 19–22 and 59–62years. Equivalent oscillations are found in numerous multisecular paleoclimatic records. The Coupled Model Intercomparison Project 5 (CMIP5) general circulation models (GCMs), to be used in the IPCC Fifth Assessment Report (AR5, 2013), are analyzed and found not able to reconstruct this variability. In particular, from 2000 to 2013.5 a GST plateau is observed while the GCMs predicted a warming rate of about 2°C/century. In contrast, the hypothesis that the climate is regulated by specific natural oscillations more accurately fits the GST records at multiple time scales. For example, a quasi 60-year natural oscillation simultaneously explains the 1850–1880, 1910–1940 and 1970–2000 warming periods, the 1880–1910 and 1940–1970 cooling periods and the post 2000 GST plateau. This hypothesis implies that about 50% of the ~0.5°C global surface warming observed from 1970 to 2000 was due to natural oscillations of the climate system, not to anthropogenic forcing as modeled by the CMIP3 and CMIP5 GCMs. Consequently, the climate sensitivity to CO 2 doubling should be reduced by half, for example from the 2.0–4.5°C range (as claimed by the IPCC, 2007) to 1.0–2.3°C with a likely median of ~1.5°C instead of ~3.0°C. Also modern paleoclimatic temperature reconstructions showing a larger preindustrial variability than the hockey-stick shaped temperature reconstructions developed in early 2000 imply a weaker anthropogenic effect and a stronger solar contribution to climatic changes. The observed natural oscillations could be driven by astronomical forcings. The ~9.1year oscillation appears to be a combination of long soli–lunar tidal oscillations, while quasi 10–11, 20 and 60year oscillations are typically found among major solar and heliospheric oscillations driven mostly by Jupiter and Saturn movements. Solar models based on heliospheric oscillations also predict quasi secular (e.g. ~115years) and millennial (e.g. ~983years) solar oscillations, which hindcast observed climatic oscillations during the Holocene. Herein I propose a semi-empirical climate model made of six specific astronomical oscillations as constructors of the natural climate variability spanning from the decadal to the millennial scales plus a 50% attenuated radiative warming component deduced from the GCM mean simulation as a measure of the anthropogenic and volcano contributions to climatic changes. The semi-empirical model reconstructs the 1850–2013 GST patterns significantly better than any CMIP5 GCM simulation. Under the same CMIP5 anthropogenic emission scenarios, the model projects a possible 2000–2100 average warming ranging from about 0.3°C to 1.8°C. This range is significantly below the original CMIP5 GCM ensemble mean projections spanning from about 1°C to 4°C. Future research should investigate space-climate coupling mechanisms in order to develop more advanced analytical and semi-empirical climate models. The HadCRUT3 and HadCRUT4, UAH MSU, RSS MSU, GISS and NCDC GST reconstructions and 162 CMIP5 GCM GST simulations from 48 alternative models are analyzed. [ABSTRACT FROM AUTHOR]