Holocene Millennial-Scale Solar Variability and the Climatic Responses on Earth

The solar impact on Earth’s climate is both a rich and open-ended topic with intense debates. In this study, we use the reconstructed data available to investigate periodicities of solar variability (i.e., variations of sunspot numbers) and temperature changes (10 sites spread all over the Earth) as well as the statistical inter-relations between them on the millennial scale during the past 8640 years (BC 6755–AD 1885) before the modern industrial era. We find that the variations of the Earth’s temperatures show evidence for the Eddy cycle component, i.e., the 1000-year cyclicity, which was discovered in variations of sunspot numbers and believed to be an intrinsic periodicity of solar variability. Further wavelet time-frequency analysis demonstrates that the co-variation between the millennium cycle components of solar variability and the temperature change held stable and statistically strong for five out of these 10 sites during our study interval. In addition, the Earth’s climatic response to solar forcing could be different region-by-region, and the temperatures in the southern hemisphere seemed to have an opposite changing trend compared to those in the northern hemisphere on this millennial scale. These findings reveal not only a pronounced but also a complex relationship between solar variability and climatic change on Earth on the millennial timescale. More data are needed to further verify these preliminary results in the future.

Transient Extratropical Response to Solar Ultraviolet Radiation in the Northern Hemisphere Winter

An intermediate complexity General Circulation Model is used to investigate the transient response of the NH winter stratosphere to modulated ultraviolet (UV) radiation by imposing a step-wise, deliberately exaggerated UV perturbation and analyzing the lagged response. Enhanced UV radiation is accompanied by an immediate warming of the tropical upper stratosphere. The warming then spreads into the winter subtropics due to an accelerated Brewer Dobson Circulation in the tropical upper stratosphere. The poleward meridional velocity in the subtropics leads to an increase in zonal wind in midlatitudes between 20N and 50N due to Coriolis torque. The increase in mid-latitude zonal wind is accompanied by a dipole in Eliassen-Palm flux convergence, with decreased convergence near the winter pole and increased convergence in mid-latitudes (where winds are strengthening due to the Coriolis torque); this dipole subsequently extends the anomalous westerlies to subpolar latitudes within the first ten days. The initial radiatively-driven acceleration of the Brewer-Dobson circulation due to enhanced shortwave absorption is replaced in the subpolar winter stratosphere by a wave-driven deceleration of the Brewer-Dobson circulation, and after a month the wave-driven deceleration of the Brewer-Dobson circulation encompasses most of the winter stratosphere. Approximately a month after UV is first modified, a significant poleward jet shift is evident in the troposphere. The results of this study may have implications for the observed stratospheric and tropospheric responses to solar variability associated with the 27-day solar rotation period, and also to solar variability on longer timescales.

Global surface temperature response to 11-year solar cycle forcing consistent with general circulation model results

The 11-year solar cycle is associated with a roughly 1Wm-2 trough-to-peak variation in total solar irradiance and is expected to produce a global temperature response. The sensitivity of this response is, however, contentious. Empirical best estimates of global surface temperature sensitivity to solar forcing range from 0.08 to 0.18 K [W m-2 ]-1. In comparison, best estimates from general circulation models forced by solar variability range between 0.03-0.07 K [W m-2]-1, prompting speculation that physical mechanisms not included in general circulation models may amplify responses to solar variability. Using a lagged multiple linear regression method, we find a sensitivity of globalaverage surface temperature ranging between 0.02-0.09 K [W m-2]-1, depending on which predictor and temperature datasets are used. On the basis of likelihood maximization, we give a best estimate of the sensitivity to solar variability of 0.05 K [W m-2]-1 (0.03-0.09 K, 95% c.i.). Furthermore, through updating a widely-used compositing approach to incorporate recent observations, we revise prior global temperature sensitivity best estimates of 0.12 to 0.18 K [W m-2]-1 downwards to 0.07 to 0.10 K [W m-2]-1. The finding of a most-likely global temperature response of 0.05 K [W m-2]-1 supports a relatively modest role for solar cycle variability in driving global surface temperature variations over the 20th century and removes the need to invoke processes that amplify the response relative to that exhibited in general circulation models.