Sun-climate Connection Research Articles

Millennial-Scale Solar Variability in Tree Rings of Northern Fennoscandia at the End of the Holocene

To investigate the possible Sun–climate connection during the Holocene, the Finnish super-long tree-ring chronology covering the period from 5634 B.C. to A.D. 2004 was analyzed. As an indicator of solar activity, we used a reconstruction of total solar irradiance (TSI) covering 9300 years, which is based on a composite using the cosmogenic radionuclide 10Be measured in polar ice cores, and also on neutron monitor data (Steinhilber et al. 2009). The Multiple Taper Method (MTM) wavelet decomposition and wavelet coherence analyses were applied to the time-series. The MTM spectral analysis identifies the main solar cycles at ca. 200 yr (de Vries or Suess), ca. 350 yr (unnamed) and ca. 900 years (Eddy). The strongest cross-wavelet correlation was discovered between the millennium-cycle components of TSI and tree-ring width variations. This Eddy cycle, which was recently discovered in solar activity, remains both strong and stable through almost the entire Holocene, and it reappears again at lower frequencies (ca. 1300 years) after ca. A.D. 200. Our results raise questions regarding the end of the Holocene and transition to the next glacial period and confirm the complex and nonlinear nature of the Sun–climate relationship during the Holocene Epoch.

Solar activity imprints in tree ring-data from northwestern Russia

The Sun's role in climate variability is now a subject of debates, especially in the context of understanding contribution of solar forcing to modern global warming. Besides, there are some evidences of the approaching new Grand Solar Minimum with Little Ice Age climatic conditions. This expectation is based on the occurrence of the extended solar minimum of 2006–2009. To investigate the possible Sun-climate connection the regional tree-ring chronology covering the period from 1445 to 2005 was analyzed. A total of 36 timber cores of pine Pinus sylvestris L. were sampled near the northern tree-line at Loparskaya station (68.6 N, 33.3 E), including the oldest living pine with more than 560 years of age. The data were processed using modern methods adopted in dendrochronology (cross-dating and standardization) with the help of COFECHA and ARSTAN programs. The analysis revealed significant cooling events, coinciding with the Spoerer (1400–1540), Maunder (1645–1715), Dalton (1790–1830), and Gleissberg (1880–1910) Grand Solar Minima. The application of MTM-spectrum and wavelet decomposition analysis identified the existence of the main cycles of solar activity (5.4, 11.7 and 22 years) in tree-ring width variations. As possible extraterrestrial forcings of climate change we consider here variations in solar irradiance and cosmic ray intensity modulated by the interplanetary magnetic field. As solar and cosmic ray activity indicators we used the annual sunspot number, geomagnetic aa index and Be^10 cosmogenic isotope records. To examine the relationship in time-frequency scale between tree-ring growth and solar activity, the cross wavelet transform and wavelet coherence analysis were applied to the time series. The wavelet coherence analysis identified that the 11 yr and 22 yr periodicities were clearly manifested in the all solar-tree rings connections during and around the Grand Minima of solar activity including the Maunder minimum, when, as is known, sunspots were practically absent. These results confirm the existence of solar activity effect on climate and tree growth above the Arctic Circle and are important for understanding the modern climatic processes.

Causes of non-stationary relationships between geomagnetic activity and the North Atlantic Oscillation

The North Atlantic Oscillation (NAO) is known to be influenced by internal variability of the atmosphere and the ocean and respond to natural or anthropogenic external forcing. However, there is no consensus on the exact mechanisms. The NAO correlates with geomagnetic activity (considered as the parameter for the solar wind intensity) positively during one period (i.e. 1951–1996) but negatively in another period (i.e.1870–1950), making the Sun-climate connection a controversial subject. We try to explain this non-stationary relationship and to find the causes why the correlation had changed the sign during the past 148 years. At the times of low geomagnetic activity before 1950 and after 1997 the correlation between geomagnetic activity and the NAO was relatively weak. The variability of the NAO during those periods was mainly due to other processes. In order to answer the question why a more positive NAO phase has prevailed over the last 30 years of the past century, we study the geomagnetic signal near the surface conditioning upon the strength, shape and location of the stratospheric polar vortex and examine the immediate effect of geomagnetic storms in the troposphere. At times of prevailing high geomagnetic activity (1951–1996) the polar vortex strengthened. The effect of the solar wind was mainly over northern Europe in association with the positive phase of the NAO; a strengthened mid-latitude westerly jet kept the cold air in the Arctic and northern mid-latitudes became milder than average. When geomagnetic activity decreased, the stratospheric polar vortex also weakened. The solar wind signal prevailed over Canada in association with the negative NAO index and more of Arctic air was able to penetrate North America and Eurasia. We finally show that the geomagnetic storms may play a role in the acceleration of the downward penetration of pressure anomalies from the upper stratosphere into the troposphere.

Plausible modulation of solar wind energy flux input on global tropical cyclone activity

Studies on Sun-climate connection have been carried out for several decades, and almost all of them focused on the effects of solar total irradiation energy. As the second major terrestrial energy source from outer space, the solar wind energy flux exhibits more significant long-term variations. However, its link to the global climate change is rarely concerned and remains a mystery. As a fundamental and important aspect of the Earth's weather and climate system, tropical cyclone activity has been causing more and more attentions. Here we investigate the possible modulation of the total energy flux input from the solar wind into the Earth's magnetosphere on the global tropical cyclone activity during 1963–2012. From a global perspective, the accumulated cyclone energy increases gradually since 1963 and starts to decrease after 1994. Compare to the previously frequently used parameters, e.g., the sunspot number, the total solar irradiation, the solar F10.7 irradiation, the tropical sea surface temperature, and the south oscillation index, the total solar wind energy flux input exhibits a better correlation with the global tropical cyclone activity. Furthermore, the tropical cyclones seem to be stronger with more intense geomagnetic activities. A plausible modulation mechanism is thus proposed to link the terrestrial weather phenomenon to the seemingly-unrelated solar wind energy input.

Searching for the 27-day solar rotational cycle in lightning events recorded in old diaries in Kyoto from the 17th to 18th century

Abstract. A solar rotational period of approximately 27 days has been detected in cloud and lightning activities, although the mechanism of the sun–climate connection remains unclear. In previous studies, lightning activity in Japan showed a significant signal of the solar rotational period, especially around the maxima of the decadal solar cycles. Here we analyze the time series of lightning activity in the AD 1668–1767 period, extracted from old diaries in Kyoto, Japan, and search for the signal of solar rotational cycles. The 27-day cycles were detected in the lightning data and occurred only around the maxima of the decadal sunspot cycles. The signal disappeared during AD 1668–1715, which corresponds to the latter half of the Maunder Minimum when both radiative and magnetic disturbances were thought to have been weak. These findings provide insight into the connection between solar activity and the Earth's climate.

A regime shift in the Sun-Climate connection with the end of the Medieval Climate Anomaly

Understanding the influence of changes in solar activity on Earth’s climate and distinguishing it from other forcings, such as volcanic activity, remains a major challenge for palaeoclimatology. This problem is best approached by investigating how these variables influenced past climate conditions as recorded in high precision paleoclimate archives. In particular, determining if the climate system response to these forcings changes through time is critical. Here we use the Wiener-Granger causality approach along with well-established cross-correlation analysis to investigate the causal relationship between solar activity, volcanic forcing, and climate as reflected in well-established Intertropical Convergence Zone (ITCZ) rainfall proxy records from Yok Balum Cave, southern Belize. Our analysis reveals a consistent influence of volcanic activity on regional Central American climate over the last two millennia. However, the coupling between solar variability and local climate varied with time, with a regime shift around 1000–1300 CE after which the solar-climate coupling weakened considerably.

Mid-Holocene emergence of a low-frequency millennial oscillation in western Mediterranean climate: Implications for past dynamics of the North Atlantic atmospheric westerlies

The nature and tempo of Holocene climate variability is examined in the record of forest vegetation from western Mediterranean marine core MD95-2043. Episodes of forest decline occurred at 10.1, 9.2, 8.3, 7.4, 5.4–4.5 and 3.7–2.9 cal. ka BP, and between 1.9 cal. ka BP and the top of the record (1.3 cal. ka BP). Wavelet analysis confirms a ~900 yr periodicity prior to and during the early Holocene and the dominance of a ~1750 yr periodicity after 6 cal. ka BP. The ~900 yr periodicity has counterparts in numerous North Atlantic and Northern Hemisphere palaeoclimate records, and in solar irradiance proxies (Δ14C and 10Be), and may relate to a Sun–climate connection during the early Holocene. Comparisons between the MD95-2043 forest record and strategically located records from Morocco, Iceland, Norway and Israel suggest that the ~1750 yr mid- to late-Holocene oscillation reflects shifts between a prevailing strong and weak state of the zonal flow, with impacts similar to the positive and negative modes of the present-day North Atlantic Oscillation (NAO). The mid- to late-Holocene millennial oscillation in zonal flow appears closely coupled to North Atlantic surface ocean circulation dynamics, and may have been driven by an internal oscillation in deep-water convection strength. The findings suggest that the mid-Holocene transition in western Mediterranean climate was accompanied by a shift in the fundamental tempo of millennial-scale variability, reflecting contrasting sensitivity of the North Atlantic climate system to different forcing factors (solar versus oceanic) under deglacial and fully interglacial conditions.

Variation in surface air temperature of China during the 20th century

The 20th century surface air temperature (SAT) records of China from various sources are analyzed using data which include the recently released Twentieth Century Reanalysis Project dataset. Two key features of the Chinese records are confirmed: (1) significant 1920s and 1940s warming in the temperature records, and (2) evidence for a persistent multidecadal modulation of the Chinese surface temperature records in co-variations with both incoming solar radiation at the top of the atmosphere as well as the modulated solar radiation reaching ground surface. New evidence is presented for this Sun–climate link for the instrumental record from 1880 to 2002. Additionally, two non-local physical aspects of solar radiation-induced modulation of the Chinese SAT record are documented and discussed. Teleconnections that provide a persistent and systematic modulation of the temperature response of the Tibetan Plateau and/or the tropospheric air column above the Eurasian continent ( e.g., 30°N–70°N; 0°–120°E) are described. These teleconnections may originate from the solar irradiance-Arctic–North Atlantic overturning circulation mechanism proposed by Soon (2009). Also considered is the modulation of large-scale land–sea thermal contrasts both in terms of meridional and zonal gradients between the subtropical western Pacific and mid-latitude North Pacific and the continental landmass of China. The Circum-global teleconnection (CGT) pattern of summer circulation of Ding and Wang (2005) provides a physical framework for study of the Sun–climate connection over East Asia. Our results highlight the importance of solar radiation reaching the ground and the concomitant importance of changes in atmospheric transparency or cloudiness or both in motivating a true physical explanation of any Sun–climate connection. We conclude that ground surface solar radiation is an important modulating factor for Chinese SAT changes on multidecadal to centennial timescales. Therefore, a comprehensive view of local and remote factors of climate change in China must take account of this as well as other natural and anthropogenic forcings.

Temporal derivative of Total Solar Irradiance and anomalous Indian summer monsoon: An empirical evidence for a Sun–climate connection

Identifying the pattern of natural climate variability is of immense importance to delineate the effects of anthropogenic climate changes. Global and regional climates are suspected to vary, in unison or with delays, with the Total Solar Irradiance (TSI) at decadal to centennial timescales. Here we show that the Indian summer monsoon rainfall correlates well with the temporal derivative of TSI on multi-decadal timescales. This linkage between the temporal derivative of TSI and the Indian summer monsoon is tested and corroborated both for the instrumental period (1871–2006) and for the last ∼300 years using a speleothem δ 18O record representing rainfall in southwestern India. Our analyses indicate that anomalous dry periods of the Indian monsoon are mostly coincident with negative TSI derivative. This study thus demonstrates the potential of ‘TSI derivative’ as an important indicator of natural monsoon variability on an interdecadal timescale.

Natural climatic oscillations driven by solar activity

Many climatic parameters (ground and ocean surface temperatures, pressure, atmospheric precipitation, etc.) have temporal variations with characteristic periods from several to several tens of years or more. The unknown cause of these oscillations, together with the similarity of some of them to known solar cycles, has stimulated attempts to relate these two phenomena. The basic arguments against the existence of such a relationship are that variations in climatic parameters do not always occur synchronously with the corresponding 11- and 22-year solar cycles: the phase shift between climatic and solar variations is inconstant and changes with time from 0° to 180°. In addition, the energy of terrestrial manifestations of solar activity seems insufficient to stimulate the considered weather-climatic processes, at least within the limits of the linear approach. In the present work, it is shown that in some cases, these contradictions can be removed for variations with a period more than 11 years under the assumption that climatic variations are forced oscillations driven by an external force (for example, a force related to solar activity), that implies the existence of intrinsic (natural) climatic oscillations. The result serves as an additional argument in favor of the reality of a sun-climate connection and probably points to its probable nonlinear mechanism.

Role of wave–mean flow interaction in sun–climate connections: Historical overview and some new interpretations and results

Quasi-decadal variations in solar irradiance – termed the 11-year solar cycle (SC) – have been linked to variations in a variety of atmospheric circulation features, including the polar vortex, the Brewer–Dobson circulation, and the quasi-biennial oscillation. These features share an underlying commonality: they are all rooted in wave–mean flow interaction. The purpose of this paper is to provide a historical overview of the connection between the SC and wave–mean flow interaction and to propose a more complete theoretical framework for solar modulated wave–mean flow interaction that includes both zonal-mean and zonally asymmetric ozone as intermediaries for communicating variations in solar spectral irradiance to the climate system. We solve a quasi-geostrophic model using the WKB formalism to highlight the physics connecting the SC to planetary wave-drag. Numerical results show the importance of the zonally asymmetric ozone field in mediating the effects of solar variability to the wave-driven circulation in the middle atmosphere.

Synchronized Northern Hemisphere climate change and solar magnetic cycles during the Maunder Minimum

The Maunder Minimum (A.D. 1645-1715) is a useful period to investigate possible sun-climate linkages as sunspots became exceedingly rare and the characteristics of solar cycles were different from those of today. Here, we report annual variations in the oxygen isotopic composition (δ(18)O) of tree-ring cellulose in central Japan during the Maunder Minimum. We were able to explore possible sun-climate connections through high-temporal resolution solar activity (radiocarbon contents; Δ(14)C) and climate (δ(18)O) isotope records derived from annual tree rings. The tree-ring δ(18)O record in Japan shows distinct negative δ(18)O spikes (wetter rainy seasons) coinciding with rapid cooling in Greenland and with decreases in Northern Hemisphere mean temperature at around minima of decadal solar cycles. We have determined that the climate signals in all three records strongly correlate with changes in the polarity of solar dipole magnetic field, suggesting a causal link to galactic cosmic rays (GCRs). These findings are further supported by a comparison between the interannual patterns of tree-ring δ(18)O record and the GCR flux reconstructed by an ice-core (10)Be record. Therefore, the variation of GCR flux associated with the multidecadal cycles of solar magnetic field seem to be causally related to the significant and widespread climate changes at least during the Maunder Minimum.

On possible drivers of Sun-induced climate changes

We tested the validity of two current hypotheses on the dependence of climate change on solar activity. One of them states that variations in the tropospheric temperature are caused directly by changes of the solar radiance (total or spectral). The other suggests that cosmic ray (CR) fluctuations, caused by the solar/heliospheric modulation, affect the climate via cloud formation. Confronting these hypotheses with seven different sets of the global/hemispheric temperature reconstructions for the last 400 years, we found that the former mechanism is in general more prominent than the latter. Therefore, we can conclude that in so far as the Sun–climate connection is concerned tropospheric temperatures are more likely affected by variations in the UV radiation flux rather than by those in the CR flux.

Evidence for a link between the flux of galactic cosmic rays and Earth's climate during the past 200,000 years

The possibility that galactic cosmic rays (GCR) influence the Earth's cloud cover and therefore have an important impact on the Earth's radiative climate forcing has become a leading candidate to explain the observed sun–climate connection. A correlation between GCR and low cloud cover has been ascertained in models and observations over the past few years. The deposition of cosmogenic radionuclides in ice cores and deep sea sediments can be used as a proxy for the past GCR-flux and provides an important tool to study the supposed GCR–climate connection on glacial–interglacial timescales. In this study, a record of geomagnetic paleointensity based on 10 Be from deep sea sediments is used as proxy for GCR-flux over the past 200,000 years. It is compared with climate records from marine, terrestrial and ice core archives. Our results are consistent with the GCR–climate theory and suggest the existence of a GCR–climate connection over the past 200,000 years.

Variations in solar magnetic activity during the last 200 000 years: is there a Sun–climate connection?

The production of 10Be in the Earth’s atmosphere depends on the galactic cosmic ray influx that, in turn, is affected by the solar surface magnetic activity and the geomagnetic dipole strength. Using the estimated changes in 10Be production rate and the geomagnetic field intensity, variations in solar activity are calculated for the last 200 ka. Large variations in the solar activity are evident with the Sun experiencing periods of normal, enhanced and suppressed activity. The marine δ18O record and solar modulation are strongly correlated at the 100 ka timescale. It is proposed that variations in solar activity control the 100 ka glacial–interglacial cycles. However, the 10Be production rate variations may have been under-estimated during the interval between 115 ka and 125 ka and may have biased the results. Future tests of the hypothesis are discussed.

Comparison of cosmogenic radionuclide production and geomagnetic field intensity over the last 200 000 years

The production rate of cosmogenic radionuclides such as 10Be or 14C is known to vary as a function of the geomagnetic field intensity. It should, therefore, be possible to extract a record of palaeofield intensity from the deposition record of these radionuclides in marine or terrestrial sediments and ice cores. Field intensity variations, however, are not the only factor that has influenced the cosmogenic radionuclide records. In the case of 14C, variations of the global carbon cycle, caused by reorganization of the ocean circulation patterns from the last glacial to the present interglacial, are superimposed. 10Be is not affected by these variations because it is not part of the carbon cycle, but its deposition rates in marine sediments vary as a function of lateral sediment redistribution and boundary scavenging intensity. A global stacked record of 10Be deposition rates, corrected for sediment redistribution by normalizing to 230Thex, was shown to remove most of the disturbances, and provides a record of 10Be production rate variations over the last 200 000 years, which translates into geomagnetic field intensity variations. This dataset is compared with palaeofield intensities reconstructed from marine sediments by palaeomagnetic methods, from variations in atmospheric 14C/12C derived from independent calibrations of 14C ages, such as U/Th dating and tree ring chronology, and from 36Cl and 10Be fluxes in polar ice cores. Potential influences of the Earth’s orbital parameters and insufficient correction for orbitally triggered climate variations on the palaeointensity reconstructions are assessed. It is argued that the palaeointensity records derived from marine sediments are not significantly affected by these factors.

What do we really know about the Sun-climate connection?

The Earth's climate has always been changing. This is documented in historical as well as in geological records. The reasons for these changes, however, have always been subject to discussions and are still not well understood. In addition to natural climate changes the risk of human influence on climate has recently been seriously considered by the Intergovernmental Panel on Climate Change. Any factor that alters the radiation received from the Sun or lost to Space will affect climate. The Sun's output of energy is known to change over an 11-year cycle, and variations over longer periods occur as well. A number of correlations between solar activity variations and climate changes, some more significant than others, have been reported but they have traditionally been accompanied by a considerable skepticism among scientists because a plausible physical mechanism to account for these correlations has not yet been found. The most immediate cause of climate changes would be changes in the total irradiance of the Sun. This, however, would either imply unrealistically large variations in total solar irradiance or a higher climate sensitivity to radiative forcing than normally accepted. Therefore other mechanisms have to be invoked. The most promising candidate is a change in cloud formation because clouds have a very strong impact on the radiation balance and because only little energy is needed to change the cloud formation process. One of the ways to influence cloud formation might be through the cosmic ray flux that is strongly modulated by the varying solar activity.