Solar influences on climate

The Sun and climate change

The role that the Sun may have played in past and current climate change remain a controversial issue. By now, little doubt remains that the global warming observed over the past three decades or so is primarily due to human activity, namely the release into the atmosphere of various greenhouse gases. The situation is not so clear, however, upon going further back in time. Various lines of evidence suggest a causal link between solar activity and climate. Among these, the most striking are arguably (1) the temporal coincidence between the Maunder Minimum, an epoch of strongly suppressed solar activity spanning the second half of the seventeenth century, and the so-called "little ice age" well-known in climatology; a similar temporal coincidence between the so-called Medieval Maximum, an extended epoch of enhanced overall solar activity levels, and a well-documented warming trend in the eleventh and twelfth centuries; and (3) the striking correlation between solar activiy and mean temperature trends in the first three quarters of the twentieth century, and in particular the conspicuous "plateau" extending from 1940 to 1980, a time during which ithe emission of greenhouse gases was steadily on the rise.

Many physical mechanism originating in the Sun can potentially affect Earth's climate, all of them being associated with the solar magntic activity cycle. The 0.1% cycle-related variations in the total solar irradiance measured from space since 1978 are too low to have had a significant effect on climate (assuming that TSI variations remained at that level in the more distant past). Another mechanism under investigation is the indirect modulation of cloud coverage via the effect of solar activity on the flux of cosmic ray entering the Earth's upper atmosphere.

Another mechanism, arguably the most promising, invokes the large cycle-related variations of the solar flux of ultraviolet radiation. While contributing very little to the overall radiative energy budget, solar UV radiation is the primary driver of heating and chemistry in the stratophere and above. The impact of solar activity on these atmospheric layers is now well-documented. However, the nature of the coupling with the underlying tropospheric layers (0 to 10 km in altitude) remains poorly understood.

Figure 1: A portion of the solar spectrum in the far ultraviolet. The yellow line indicates the general level of variability betwen high and low and activity phases. Note however than for certain spectral lines, such as Lyman-alpha of CaH+K, the variability can be larger by over an order of magnitude. Data from the Atlas-1 mission, courtesy of G. Thuillier (Service d'Aéronomie/CNRS, France).

Modelling the solar UV flux

The first component of this project centers on the development of a physical model for the changes of the solar UV flux associated with varying overall activity levels, with emphasis on the spectral bands important for stratospheric dynamics and chemistry. We plan to feed the output of our dynamo models of the solar activity cycle to a extension of our current model of the total solar irradiance, modified to produce time series of UV radiative fluxes in the spectral bands of interest. This will require the parallel development of improved models for the formation of faculae and supergranular magnetic network, for which our recently developped DLA model of supergranulation offers an excellent starting point. Knowing the photospheric coverage of these various classes of UV-emitting photospheric structures, the final step will be to construct global UV spectra using extant empirical spectral data.

Climate response

The climate's response to variations in the solar UV flux occurs most likely through ozone, the primary UV absorber in the stratosphere. The key question, targeted in the second component of the project, is the nature and efficiency of the coupling between the stratosphere and upper troposphere. This coupling is highly dynamical (exchange of air masses, turbulent mixing, etc), and so its modelling requires a coupled climate+chemistry model. The first step will be to use the model FASTOC, a very computationally efficient model of chemistry and (reduced) atmospheric dynamics, for a systematic investigation of tropospheric response to varying solar activity levels, through a large set of simulations on timescales spanning years to centuries. The idea here is to produce ensemble averages that are statistically meaningful. This will be complemented by more focused, physically complex simulations using the Canadian Middle Atmosphere Model (CMAM).

This project is partly supported by a research grant from the program "projets de recherche en équipe" of FQRNT (Québec).

Who in the group works on this:

Paul Charbonneau, Kim Thibault, Cassandra Bolduc, in collaboration with Michel Bourqui (Dept. of atmospheric and oceanic sciences, McGill U.), and Stella Melo (Canadian Space Agency).