From CAWSES

Revision as of 10:41, 22 July 2010

What is the solar influence on climate?

Co-leaders:

• Ilya Usoskin (FIN)
• Joanna Haigh (UK)

Introduction

Knowledge of its natural variability is crucially important for understanding the Earth’s climate, in particular for estimating the anthropogenic factor in climate change. Natural sources of the climate variability during the last century are poorly known, which affects the evaluation of the man-made effects because of the inherent complicated links and feedbacks in the climate system. Because of its complexity, empirical studies of individual factors provide only part of the picture and indicate the necessity for models of the system as a whole, with all the driving factors and internal links operating simultaneously. Some forcing factors are known better than the others so that available estimates may contain large uncertainties. For instance, a detailed account of the direct solar forcing has entailed a reassessment of the causes of early 20-th century warming (IPCC AR4). Natural forcing still remains a largely unknown component of the climate modeling, in particular the effect of solar variability.

Solar forcing may occur by several different means, with varying degrees of uncertainty. The most direct impact is by changes in the total solar irradiance (TSI), whose climate effect is relatively straightforward to model. Variations of TSI are, however, experimentally known only for the last 30 years, while its reconstructions to the past remain largely uncertain and controversial, leading to a significant uncertainty in the direct solar effect upon climate. Nevertheless the likely range suggests that its contribution to global climate change over the past two centuries is weak compared to the anthropogenic influence, but dominant on longer time scales in the pre-industrial epoch. Thus, a better estimate of the possible long-term trend in TSI is a crucial issue for studies of modern climate history and, potentially, longer term predictions.

Solar forcing of climate can be produced indirectly by two other routes: the effective downward transport of physical/chemical changes induced by solar/geomagnetic activity in stratosphere, and atmospheric changes initiated within the troposphere by solar activity. The former includes the direct effects of spectral irradiance or precipitating energetic particles on the upper atmosphere that can in some way induce effects lower in the atmosphere. We note that the impact on the upper atmosphere is quite well understood, and the uncertainties associated with these effects are largely related to the mechanisms of coupling between the upper and lower atmospheres. The other route relates to the possible influence of cosmic rays and/or geomagnetic activity on the properties (clouds, aerosols, water vapor) of the troposphere. Although some evidence exists that such a causal relation can be real, a quantitative estimate of this effects is unknown, with the “very low” level of understanding, according to IPCC AR4. Presently, there is no reliable model (physical or empirical), which could provide even a simple quantitative estimate of this effect. Such big uncertainties in the natural climate variability give rise to often speculative public debates, trumpeted by the mass media, between “mainstreamers”, arguing the dominant role of man-made effects in the global warming, and “sceptics”, claiming that it is solely due to natural variability. Objective scientific investigations are required and we accept the challenge of disentangling the effects and increasing levels of understanding of the natural variability of climate.

Scientific issues

In this Project we plan to put special emphasis upon several key questions to attack the problem.

What is the effect of transient solar events on the middle and lower atmosphere?

What are the uncertainties in establishing the long-term direct solar effect upon climate?

How to quantify and numerically test indirect solar effects upon climate?

Milestones

• Performing case studies of extreme transient solar events, in particular those involving energetic particles, and their possible immediate and/or delayed effect upon physical/chemical parameters of the atmosphere;
• Parameterizing effects that are currently not well represented in GCMs;
• Estimating realistic uncertainties in the external drivers, including solar and spectral solar irradiance, and the related ambiguities in the climate response.