The effects of changing solar activity on climate: contributions from palaeoclimatological studies

J. Space Weather Space Clim. 2 (2012) A09 DOI: 10.1051/swsc/2012009 Ó Owned by the authors, Published by EDP Sciences 2012 The effects of changing solar activity on climate: contributions from palaeoclimatological studies Stefan Engels1,* and Bas van Geel1 1 Institute for Biodiversity and Ecosystem Dynamics, University of Amsterdam, Science Park 904, P.O. Box 94248, NL-1090 GE Amsterdam, The Netherlands *corresponding author: e-mail: Received 6 March 2012 / Accepted 9 July 2012 ABSTRACT Natural climate change currently acts in concert with human-induced changes in the climate system. To disentangle the natural variability in the climate system and the human-induced effects on the global climate, a critical analysis of climate change in the past may offer a better understanding of the processes that drive the global climate system. In this review paper, we present palaeoclimatological evidence for the past influence of solar variability on Earth’s climate, highlighting the effects of solar forcing on a range of timescales. On a decadal timescale, instrumental measurements as well as historical records show the effects of the 11-year Schwabe cycle on climate. The variation in total solar irradiance that is associated with a Schwabe cycle is only ~1 W m 2 between a solar minimum and a maximum, but winter and spring temperatures on the Northern Hemisphere show a response even to this small-scale variability. There is a large body of evidence from palaeoclimatic reconstructions that shows the influence of solar activity on a centennial to millennial timescale. We highlight a period of low solar activity starting at 2800 years before present when Europe experienced a shift to colder and wetter climate conditions. The spatial pattern of climate change that can be recognized in the palaeoclimatological data is in line with the suggested pattern of climate change as simulated by climate models. Millennial-scale climate oscillations can be recognized in sediment records from the Atlantic Ocean as well as in records of lake-level fluctuations in southeastern France. These oscillations coincide with variation in 14C production as recognized in the atmospheric 14C record (which is a proxy-record for solar activity), suggesting that Earth’s climate is sensitive to changes in solar activity on a millennial timescale as well. Key words. solar activity – sunspot – paleoclimatology – proxies 1. Introduction Predictions of future climate change that can result from a human-induced increase in atmospheric carbon dioxide and methane levels are alarming (IPCC 2007). Natural climate change is currently acting in concert with human-induced changes in the climate system and it is important to separate the effects of different forcing agents in order to determine to what extent the late 20th-century changes may be unusual in the light of preindustrial natural climate variability (Luterbacher et al. 2004). The idea of solar forcing of Earth’s climate dates back to the early 19th century (Herschel 1801; Gray et al. 2010), but the role of solar variability as a forcing mechanism is still poorly understood. To disentangle the natural variability in the climate system and the human-induced effects on the global climate, a critical analysis of climate change in the past may offer a better understanding of the processes acting on the Earth’s surface and driving the global climate system. Recent periods of low solar variability such as the Maunder Minimum (Eddy 1976) and its effects on climate (the Maunder Minimum was part of the so-called Little Ice Age) have indeed been documented in historical records. However, the period for which there are historical records of changes in, for instance, precipitation or temperature is (in geological terms) relatively short. In most regions instrumental records do not start earlier than the 19th century. In order to reconstruct processes driving climate change on a longer timescale, we have to use indirect measurements of relevant parameters of the climate system, viz. proxyindicators for climate change and for changing solar activity. A wide range of proxies and techniques are available to study past changes in the climate system, and sediments, peat deposits, accumulated ice and even tree rings provide natural archives in which these proxies are preserved. These natural archives are available on large parts of the globe, covering the continents, the oceans and the ice caps of Greenland and Antarctica. This offers the potential to reconstruct spatial patterns in past climate change. The knowledge of both pattern and timing of climate change in the past is a prerequisite in order to understand the causes of changing climate at various timescales (Vandenberghe et al. 1998). In this review, we discuss different natural forcing mechanisms that affect climate at the Earth’s surface as well as some of the hypotheses that are proposed to explain amplification of the relatively small variability in total solar irradiance to large-scale processes that can influence the climate system. We then review palaeoclimatological evidence for past influence of solar variability on the climate, showing the effects of changing solar forcing on a range of timescales. We conclude this review with some remarks on the potential of climate model simulations. This is an Open Access article distributed under the terms of creative Commons Attribution-Noncommercial License 3.0 J. Space Weather Space Clim. 2 (2012) A09 2. Forcing factors 2.1. Orbital forcing The amount of solar radiation that reaches the top of the atmosphere depends on the position of the Earth in relation to the Sun as well as on the activity of the Sun itself. The effect of the poly-cyclic behavior in the Earth’s position relative to the sun on Earth’s climate was first described by Milankovitch (1941). The effects of the Milankovitch cycles on Earth’s climate are well known as orbital forcing. Variations in eccentricity (departure from a circular orbit, varying with a 100 and 400 kyr periodicity), obliquity (axial tilt, varying with a 41 kyr period) and precession (rotation of the Earth’s axis, varying with a 19–23 kyr periodicity) of the Earth’s orbit determine major climatic changes on Earth (e.g. Berger 1988). On a Quaternary timescale (i.e. the last 2.6 Myr), the combined effects of changing eccentricity, obliquity and precession have resulted in a dynamic climate that is characterized by variations between long glacial periods and relatively short interglacials. The onset of the current interglacial, the Holocene, occurred during an increase in solar insolation received by the upper atmosphere. Northern hemispheric summer insolation has shown a steady decline during the past 11 kyr, resulting in gradually declining temperatures over large parts of the Northern hemisphere (e.g. Renssen et al. 2009). Variation in orbital forcing is a large-scale, gradual mechanism which is superimposed by shorter variations in other (...truncated)