Search

The Indian Ocean Dipole (IOD) is a major mode of interannual climate variability, but its response to external climate forcings (i.e. solar forcing, volcanic radiative forcing (VRF) and greenhouse gas (GHG) radiative forcing) remains elusive. To improve our understanding of the variability of the IOD, it is necessary to investigate the IOD’s response to external forcings through multi-model simulations. Here a Granger causality test is used to examine the impact of external forcings on the IOD from past 1000 years simulations (850–1850 Common Era) derived from Coupled Model Intercomparison Project Phase 5 (CMIP5) models. The results show significant causal effects of VRF on the IOD in preindustrial times of the past 1000 years from the MPI-ESM-P, MRI-CGCM3, GISS-E2-R and CCSM4 models and uncertainties in the IOD’s responses to volcanic eruptions from other six models. Additionally, the phase responses (i.e. positive or negative) of the IOD to large volcanic eruptions remain unclear even from models showing significant causal impacts of VRF on the IOD. This result shows that the IOD exhibits a more complex response to volcanic forcing than the El Niño-Southern Oscillation. The causal impact of solar forcing on the IOD is more likely to be weak in most models. The IOD’s response to GHG variations is not significant across all the models due to minor fluctuations in GHGs occurring during preindustrial times of the past 1000 years. Further analyses based on new, improved and higher resolution models might further our understanding of the IOD’s responses to external forcing.

This study presents an analysis of the Indian summer monsoon (ISM) rainfall variations for a 1460-year period (1720–3180, Before Present BP: 1950 AD), based on a long record of stable isotopic variations (δ18O) with high temporal resolution (~annual) obtained from a U-Th dated stalagmite from the Kadapa cave in peninsular India. This stalagmite proxy record captures variations associated with wet and dry monsoons on decadal to centennial time-scales, together with a general declining trend in the ISM during the 1460-year period. It is noted that the declining trend of the ISM follows the northern hemispheric summer insolation, which is known to influence the location and strength of the Inter tropical convergence zone (ICTZ). The stalagmite record also indicates an abrupt climate change, characterized by the decline of ISM around 2800 yr BP, as manifested in the enrichment of18O values. Furthermore, the enriched18O values around 2800 yr BP are corroborated by changes in the stalagmite growth rate, its trace elemental ratios (Sr/Ca, Ba/Ca and U/Ca) and crystallographic structure. In addition, the decline of ISM around 2800 yr BP coincides with a sudden rise in the atmospheric Δ14C, indicative of reduced solar activity. This period around 2800 yr BP is widely reported as the cold European climate associated with ice debris events in the North Atlantic (also known as the Iron Age Cold Epoch), which were reportedly forced by low solar activity. Syntheses of other available stalagmite records from the Indian region, during the common time-frame, show coherent variations with the Kadapa stalagmite and also the Dongge cave stalagmite (southern China), pointing to synchronous variations of the Indian and the East Asian monsoon systems.

A 1200-year-long tree-ring width record from living and subfossil mountain hemlock wood is used to reconstruct February through August temperatures for the Gulf of Alaska, providing a record of past climate variability for the Northeast Pacific sector that captures interannual to centennial timescales. The moderate elevation at the tree-ring sites has allowed these trees to retain their temperature signal without evidence of the so-called divergence effect, or underestimation of tree-ring inferred temperature trends, which is observed at many northern latitude forest locations. This ‘divergence-free’ reconstruction reveals centennial trends that include a warm interval centered on ad 950 for coastal Alaska that occurs around the time of the ‘Medieval Warm Period’, a warming that is only rivaled by recent decades. Spectral analysis of this reconstruction supports the centennial pacing identified as a 170–220-year cadence consistent with solar variability. On the decadal to bidecadal scale, the reconstruction reveals ~10- and 18-year cycles, which have been observed elsewhere in climate records for western North America and are linked to solar and lunar tidal forcing, respectively. Temperature minima that occur at ad 969–970 and 1698–1700 correspond with the timing of major volcanic events. This tree-ring reconstruction supports centennial modes of solar forcing as a driver of surface air temperatures in the Gulf of Alaska, with lunar tidal, solar variability, internal variability, and volcanism, impacting climate on annual to decadal timescales.

It has been argued that Central European lake levels were driven by solar activity. This interpretation rests on the comparison of a score record of radiocarbon dates with the 14C residual curve. This score record is a variant of a cumulative probability density function (CPF). In this paper it is argued that this method is not valid because the shape of the CPF is determined by the calibration curve. This hypothesis is tested by a set of null models. An alternative interpretation is given according to which only few episodes of Holocene lake-level changes were climatically driven, while most of the time non-climatic factors were dominating. On a more general level some methodological issues of CPFs are discussed and exemplified by the comparison of dendrochronological results from archaeological settlements and a CPF created using radiocarbon dates from the same settlements.

We report on a sediment record from a small lake within the subarctic wetland complex Stordalen in northernmost Sweden covering the last 1000 years. Variations in the content of minerogenic material are found to follow reconstructed variations in the activity of the Sun between the 13th and 18th centuries. Periods of low solar activity are associated with minima in minerogenic material and vice versa. A comparison between the sunspot cycle and a long instrumental series of summer precipitation further reveals a link between the 11 yr solar cycle and summer precipitation variability since around 1960. Solar minima are in this period associated with minima in summer precipitation, whereas the amount of summer precipitation increases during periods with higher solar activity. Our results suggest that the climate responds to both the 11 yr solar cycle and to long-term changes in solar activity and in particular solar minima, causing dry conditions with resulting decreased runoff.

Several studies indicate that changes in solar activity may have driven Holocene subtropical monsoon variability on decadal and centennial timescales, but the strength and nature of this link remains debated. In this study, we combine a recent mapping of the Holocene solar-cycle activity with four subtropical speleothem δ18O records, which allows a strong test of the link between solar activity, monsoon activity (or intensity), and the hydrological cycle. This is possible because the speleothem δ18O records mainly reflect changes in local rainfall composition, which is controlled by changes in total moisture loss along the atmospheric transport path and monsoon intensity. We find that the spectral density distributions of the speleothem records exhibit particularly significant ~210 yr cyclicities that tend to coincide in time with the three Suess solar-cycle bursts, i.e. intervals around 1850–3200 BP, 4500–5700 BP, and 7750–8850 BP when the ~210 yr solar cycle was particularly strong. The speleothems from Dongge Cave (China) and Sofular Cave (Turkey) appear to have recorded all three Suess bursts, whereas the speleothems from Heshang Cave (China) and Pink Panther Cave (southwestern USA) only recorded the first and last Suess bursts, and the middle Suess burst, respectively. The temporal relationship between the Suess solar cycle and particularly significant 210 yr oscillations in the speleothem δ18O records therefore supports the notion that solar variability played a significant role in driving centennial-scale changes in the hydrological cycle in the subtropics during the Holocene.

On the basis of sediment and pollen analyses of a radiocarbon-dated sediment sequence from Lake Joux in the Swiss Jura Mountains (west-central Europe), this paper presents a high-resolution record of lake-level and vegetation changes for the last millennium. The lake-level record makes it possible to determine that low-stands of the water-table dominated at c. AD 1200—1400, and from 1720 onward, with interruptions by short-lived rise events at c. AD 1340 and 1840. Highstands prevailed at c. AD 1100, and around AD 1450, 1550 and 1700. The comparison of the Joux lake-level record with a solar irradiance record based on cosmogenic nucleides supports the hypothesis of a major solar forcing of climate variations in west-central Europe over the last millennium. In agreement with other palaeohydrological records established in Western Europe and in the central North Atlantic Ocean, the Joux lake-level record suggests that, on a multicentennial scale, the period of the `Little Ice Age' coincided in the European mid-latitudes with wetter climatic conditions, and probably with an increase in summer precipitation. Variations in the hydrological cycle appear to have been associated with changes in the general atmospheric circulation pattern, probably coupled with variations in oceanic circulation and solar activity. The Joux record also points to changes in water resources expected to result from ongoing global warming. Finally, despite the severe climatic conditions which prevail in the Joux Valley, the human impact history at Lake Joux over the last millennium seems to reflect political choices and economic context more than climatic changes.

Holocene glacier fluctuations prior to the `Little Ice Age' in Garibaldi Provincial Park in the British Columbia Coast Mountains were reconstructed from geomorphic mapping and radiocarbon ages on 37 samples of growth-position and detrital wood from glacier forefields. Glaciers in Garibaldi Park were smaller than at present in the early Holocene, although some evidence exists for minor, short-lived advances at this time. The first well-documented advance dates to 7700—7300 14C yr BP. Subsequent advances date to 6400—5100, 4300, 4100—2900 and 1600—1100 14C yr BP. Some glaciers approached their maximum Holocene limits several times during the past 10 000 years. Periods of advance in Garibaldi Park are broadly synchronous with advances elsewhere in the Canadian Cordillera, suggesting a common climatic cause. The Garibaldi Park glacier record is also broadly synchronous with the record of Holocene sunspot numbers, supporting previous research that suggests solar activity may be an important climate forcing mechanism.

Multiproxy palaeohydrological records from three raised bogs in Northern Ireland indicate that a major shift to wetter/cooler climatic conditions postdated the rapid decrease in solar activity at 2800 cal. BP by ~100 years. This event is bracketed by two wiggle-match radiocarbon-dated cryptotephra layers in each profile, enabling a high degree of chronological precision. These replicated data corroborate previous findings based on Irish peat humification profiles, and may indicate spatial complexity in the climatic response to solar activity between oceanic and continental areas.

In his comment on Bleicher (2013) Magny presents a revised version of his score method and reaffirms that it can prove a possible influence of solar activity on Central European lake levels. He also stresses the diagnostic value of archaeological lake site settlements. This paper points at methodological problems that are not yet resolved in the revised version. Since there is great consensus that not only is the the Sun a primary forcing factor in the climate’s system, but also that its effect cannot be expected to be linear for all of the Holocene, the use of the score record is questioned. The proposed relation between archaeological lake site settlements and solar activity is shown to be doubtful.

There has been confirmation in the last two decades, through instrumental measurements onboard satellites, that the ‘solar constant’ does, as has long been hypothesized, vary over different timescales and with identifiable periodicities. This being the case, it is necessary to explore how these variations play a role in terrestrial climate change. While there is no consensus as to the best method for estimating past variations in solar output, it seems likely that over the last 500 years the sun has played a role in the changing climate. However, there is little evidence to suggest that changes in irradiance are having a large impact on the current warming trend.

The focus of this study is the possible correlation between changes in the climate of Scandinavian and changes in solar irradiation. Reliable information about Holocene climatic change in Sweden and Norway is currently available from two main sources: the 14C dating of pine wood retrieved from above the present pine-tree limit and studies of glacier variations based on proglacial lacustrine sediments and on moraines. The reconstructed alpine tree-limit reveals that summer temperature in general was warmer during the early Holo cene than it was during the late Holocene. Superimposed on this general trend are several fluctuations of a few hundred years' duration. Relatively cold periods with a duration of the order of 300-600 years occurred frequently during the Holocene. In this paper, dates of the major climatic events are compared with an index of solar activity, the so-called δ14C anomalies. For most of the last 9000 years a good correspondence is demonstrated between the timing of cold events in Scandinavia and the timing of major δ14 C anomalies (low solar irradiation). The general Holocene cooling trend is believed to be partly a result of land uplift following deglaciation and partly a result of orbitally forced changes in irradiation. Large fluctuations in Scandinavian summer temperature can be reconciled with the pattern of climatic change presented in several recent studies in the North Atlantic region. A link between these areas could be provided by changes in the production of North Atlantic Deep Water.