Scientific assessment of solar induced climate change

Summary

The climate is far from constant. Many factors of climate change have been reported in the literature. We may distinguish internal variability and external forcing. The former also referred to as autogenic forcing is caused by interactions between the components of the climate system.

The quantification of the human influence on climate depends strongly on our understanding of the climate system including climate variability driven by internal variations as well as by external forcings. One of the external forcings is due to the ever changing activity of the sun. Before the industrial revolution (set at 1750 to 1850) the human factor can be excluded and until 1950 it may be neglected at least at global scale. Studying the climate change before 1950 may therefore reveal the impact of natural climate factors alone.

In order to assess the understanding of the solar terrestrial relation, the Netherlands Scientific Assessment and Policy Analysis for Climate Change programme (WAB) asked the Royal Netherlands meteorological Institute (KNMI) to coordinate a project, which should include an assessment on the knowledge of the solar dynamo, the proxy records as well as on the understanding of the possible and probable mechanisms of climate change due to changes in solar variability. Three specialists on the issue of solar dynamics, proxy records and climate science present their views on the subject of the solar terrestrial link. This report provides a consensus view of the experts involved in this assessment.

Sun & the earth's climate

The major sun-climate relationship is the absorption of solar radiation by the atmosphere and by the earth surface. Moreover, atmospheric flow is largely influenced by the distribution of the absorbed solar radiation as well as of the outgoing infrared radiation in the climate system. Even with a constant level of solar activity climate processes are influenced. Clear examples are the diurnal and seasonal cycle. On the very long timescale the interaction between the earth's cryosphere and the periodic changes in the earth orbital parameters, modifying the integrated seasonal solar insolation at high latitudes, results in the occurrence of ice ages.

Variations of solar activity may expose the climate system to an additional forcing which induces either climate changes on the global scale or a pattern of responses on the regional scale, depending on the physical mechanism. Besides the direct mechanism of variations in the total solar irradiance changing the radiative balance of the climate system, the search is for indirect mechanisms, which enhance variations in solar activity parameters into significant impact on climate. At least three competing mechanisms may be considered:

1. Small variations in the visible part of the solar irradiance, affecting the atmosphere from below.
2. Variations of UV - radiation, affecting directly the stratosphere and the ozone distribution, thus influencing the lower atmosphere from above.
3. Variations of particle radiation of the sun and/or variations in cosmic radiation, having an effect on the electrical and magnetic properties of the earth's atmosphere, eventually causing a change in the atmospheric composition (either by aerosol or cloud formation, or an influence on the concentration of ozone).

Long term variation of solar activity

Comparing reconstructions of solar activity parameters and climate records may reveal solar-terrestrial relationships, but in general there is no solid evidence to which extent these correlations are caused by changes in solar activity. The origin of these controversies lies in the fact that possible solar signals are not easily distinguished from other sources of climate variability, such as volcanic forcing, ENSO and long term internal variability. Also, uncertainties in reconstructions and climate records hamper unequivocal proof of the solar terrestrial link. Although correlations between solar activity and climate parameters do not establish cause-effect relationships, they may give indications for underlying mechanisms of climate change due to solar activity.

Research with climate models

The impacts of climate factors, like changes in solar activity, can be further studied with coupled atmosphere-ocean general circulation models (GCMs). These are the state-of-the-art tool for understanding the present climate and estimating the effects of natural as well as anthropogenic climate perturbations. Such comprehensive models include many physical processes and their mutual interactions. The evaluation with observational data is essential for getting at least some confidence in the present generation of climate models, although it is difficult or even impossible to get correspondence with the real world in all aspects. At present, the GCM simulations differ in many aspects on the regional scale. Therefore, it is difficult to be conclusive about cause-effect relations at least on the regional scale.