From CAWSES

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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?

open questions to be attacked

• For century-scale climate change, we need a better estimate of TSI secular trend from pre-industrial / Maunder Minimum times to the present.

• For decadal climate change, we need to understand UV variations; what is happening during this current solar minimum; what will happen in future cycle(s)

• Are we using the most appropriate solar proxies in regression studies?

• Many datasets are too short for robust statistical analysis. Need careful analysis in attribution of solar signals

• Need to better understand the solar signal in the stratosphere, including the role of the QBO. Summer signal as well as winter.

• Need more climate models to include stratospheric processes.

• Need a better understanding of stratosphere-troposphere coupling (planetary waves, synoptic scale waves). Does coupling with mesosphere / thermosphere have any influence?

• What are the relative contributions of ”bottom-up” and ”top-down” mechanisms to the solar impact on the troposphere?

• Do solar energetic particles have any influence on tropospheric climate?

• Need improved understanding of processes whereby cosmic rays may influence 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.

Current activities

Here we list some current activities directly related to Task 1 of CAWSES-II.

Projects

• ISSI Team: “Solar Influence on Climate”, 2006-2009, (leader Lesley Gray)

• ISSI Project: "Geospace coupling to Polar Atmosphere", 2009-2011 (leader Annika Seppala)

• ISSI Team: “Study of cosmic ray influence upon atmospheric processes”, accepted for 2010-2011, (leader I. Mironova)

• EU COST project: “Towards a sounder assessment of the impact of solar variability on the Earth’s climate”, submitted, (leader T. Dudok de Witte)

• WMO SPARC CCMVal: Structured intercomparison of coupled chemistry-climate models, includes solar forcing, report 2010

• WMO SPARC Solaris: Coordinated activities in solar effects on stratosphere-troposphere coupling, ongoing, (leaders K. Matthes, K. Kodera)

• WCRP CMIP5: Structured intercomparison of coupled climate-ocean models, in preparation for IPCC AR5 report (2013), includes solar forcing on century timescales.

• UK NERC consortium project: ”Solar Influences on Climate and the Environment”, 2008-2013 (leader J. Haigh)

International Research Collaborations (money-free)

• CRII (Cosmic Ray Induced Ionization) model is extended toward the upper atmosphere, including SEP. (Finland, Russia)

• Case study of SEP influence upon aerosol properties in the upper troposphere and lower stratosphere (cooperation of Finland, Russia, USA)

• Cosmogenic isotope tracers of atmospheric large scale and regional dynamics (cooperation of Brazil, Finland, Russia, Sweden)

• Detailed model of production and transport of cosmogenic isotopes (Finland, USA, Russia, Switzerland)

• Long-term reconstruction of TSI/SSI based on a physical model (Germany, Finland, UK)

• Sensitivity of atmospheric response to variability in solar spectrum (UK, USA, Germany)

• Coupled modelling of solar spectral influence on stratosphere and downward coupling (Germany, Japan, USA, UK)

Meetings and Conferences

• Aspen Global Change Institute Interdisciplinary Workshop: “Global Change and the Solar Terrestrial Environment”, Aspen, June 2010 (coordinators Lesley Gray, Charles Jackman, Paul Kintner, Peter Pilewskie; Howard Singer)

• European Cosmic Ray Symposium, Finland, August 2010 (SOC Chair I. Usoskin) [for the first time a terrestrial and planetary session is included in the Program].

• HAO Smposium: "Eddy Cross-Disciplinary Symposium on Sun-Climate Research" Aspen October 2010 (Phil Judge)

• Fourth International Space Climate Symposium: India, January 2011 (D. Nandy, K. Mursula, I. Usoskin, D. Marsh, J. Beer)

• IAU Symposium: “Comparative magnetic minima: characterizing quiet times in the Sun and stars” Mendoza, Argentina, Oct 2011 (Sarah Gibson)

Topical publications

• Gray, L. J., J. Beer, M. Geller, J. D. Haigh, M. Lockwood, K. Matthes, U. Cubasch, D. Fleitmann, G. Harrison, L. Hood, J. Luterbacher, G. A. Meehl, D. Shindell, B. van Geel, and W. White (2010), SOLAR INFLUENCES ON CLIMATE, Rev. Geophys., 48, RG4001, doi:10.1029/2009RG000282

(available as open access due to CAWSES-II at http://www.agu.org/journals/rg/rg1004/2009RG000282/2009RG000282.pdf ).

• Special issue of J. Atmos. Solar-Terr. Phys. “Space Climate”, in press 2010, editors D. Marsh, K. Mursula, D. Nandi, I. Usoskin.

• Special issue of J. Adv. Space Res. “Solar Variability, Cosmic Rays and Climate” (tentative title), planned for 2011, editor I. Usoskin.