Search

During the last ice age, the Northern Hemisphere experienced a series of abrupt millennial-scale climatic changes linked to variations in the strength of the Atlantic Meridional Overturning Circulation and sea-ice extent. However, our understanding of their impacts on decadal-scale climate variability in central Europe has been limited by the lack of high-resolution continental archives. Here, we present a near annual-resolution climate proxy record of central European temperature reconstructed from the Eifel maar lakes of Holzmaar and Auel in Germany, spanning the past 60,000 years. The lake sediments reveal a series of previously undocumented multidecadal climate cycles of around 20 to 150 years that persisted through the last glacial cycle. The periodicity of these cycles suggests that they are related to the Atlantic multidecadal climate oscillations found in the instrumental record and in other climate archives during the Holocene. Our record shows that multidecadal variability in central Europe was strong during all warm interstadials, but was substantially muted during all cold stadial periods. We suggest that this decrease in multidecadal variability was the result of the atmospheric circulation changes associated with the weakening of the Atlantic Meridional Overturning Circulation and the expansion of North Atlantic sea-ice cover during the coldest parts of the last ice age.

Understanding how El Niño‐Southern Oscillation (ENSO) responds to natural variability is of key importance for future climate projections under a warming climate. However, there is no clear consensus on what drives ENSO's variability on centennial timescales. Here, we find that the epikarst in southeastern Alaska is effective at filtering ENSO and solar irradiance signals from the Aleutian Low regional climate, which are subsequently recorded in speleothem proxy data. By applying a correlation test, we find that ENSO was significantly influenced by solar irradiance over the past ∼3,500 years. This relationship dissolved after ∼1970 CE, with ENSO now being dominated by anthropogenic forcing. This implies a new ENSO mean state that will need to be incorporated into future climate projections. Although El Niño‐Southern Oscillation (ENSO) is one of the most important climate phenomena globally, it is currently unknown how ENSO responds to natural variability on timescales over 100 years. Natural variability refers to periodic changes in climate due to either solar forcing, the circulation of the atmosphere and ocean, volcanic eruptions, and other factors, irrespective of human intervention. Here, we help solve this problem by studying deposits in caves (speleothems) from Alaska. The speleothems are excellent at capturing atmospheric conditions over the past 3,500 years, and show that solar forcing was significantly controlling ENSO variability. However, the speleothems also show that humans have altered this natural variability, with ENSO entering a new mean state after ∼1970 CE. Speleothems from southeastern Alaska capture El Niño‐Southern Oscillation (ENSO) variability through a teleconnection with the Aleutian LowENSO variability was strongly influenced by solar forcing before ∼1970 CEENSO significantly changed properties at ∼1970 CE, likely due to anthropogenic forcing Speleothems from southeastern Alaska capture El Niño‐Southern Oscillation (ENSO) variability through a teleconnection with the Aleutian Low ENSO variability was strongly influenced by solar forcing before ∼1970 CE ENSO significantly changed properties at ∼1970 CE, likely due to anthropogenic forcing

The last interglacial (LIG; Marine Isotope Substage 5e, ~127–117 ka) experienced globally warmer than modern temperatures; however, profound differences in regional climate occurred that are relevant to the assessment of future climate change scenarios. Tropical Atlantic sea surface temperature (SST) and hydrology are intrinsic to the spatiotemporal evolution of past and future climate. We present eight monthly resolved coral Sr/Ca and δ18O records (130–118 ka) to reconstruct mean western tropical Atlantic SST and seawater δ18O changes during the LIG. Cooler and fresher than modern surface waters are indicated for the middle of the LIG at ~126 ka. This was followed by a rapid transition to modern‐like SSTs and salinities that characterized the remaining part of the LIG. Our results, which account for differences found among corals, proxies, and SST calibration uncertainties, agree with western tropical Atlantic sediment records. Together, they suggest that an oceanic regime existed that differed from today. The last interglacial is a period of time that occurred approximately ~127 to 117 thousand years ago and experienced globally warmer than modern temperatures, similar to those predicted by computer simulations of future climate change. However, little is known about the temperature and hydrology of the tropical oceans at this time. We analyzed fossil Caribbean corals that lived between 130 and 118 thousand years ago and record within their structures the properties of the seawater they inhabited. From these we reconstructed snapshots of past mean sea surface temperatures and the changing influences of regional ocean currents. Surprisingly, we found cooler and fresher than modern surface waters occurred within the tropical Atlantic at ~126 thousand years ago, a time usually associated with peak global warming. These anomalous conditions were followed by a rapid transition to modern‐like sea surface temperatures and salinities that went on to define the remaining part of the LIG until ~118 ka. Our results agree with other reconstructions of last interglacial climate derived from tropical Atlantic sediment records. Together, they suggest changes in ocean currents that transport waters into the Caribbean and we highlight the complementary use of widely different marine archives to assess past climate change. Last interglacial (LIG) coral Sr/Ca indicates 2.1 ± 0.7 °C cooler than modern tropical Atlantic sea surface temperatures at ~126 kaPaired coral Sr/Ca and δ18O records indicate that fresher tropical Atlantic surface waters also occurred at ~126 kaCoral Sr/Ca and δ18O records complement lower resolution less precisely dated marine sedimentary records of tropical Atlantic LIG climate

The Red Sea is a deep marine basin often considered as small‐scale version of the global ocean. Hydrographic observations and ocean‐atmosphere modeling indicate Red Sea deep water was episodically renewed by wintertime open‐ocean deep convections during 1982–2001, suggesting a renewal time on the order of a decade. However, the long‐term pacing of Red Sea deep water renewals is largely uncertain. We use an annually resolved coral oxygen isotope record of winter surface water conditions to show that the late twentieth century deep water renewals were probably unusual in the context of the preceding ~100 years. More frequent major events are detected during the late Little Ice Age, particularly during the early nineteenth century characterized by large tropical volcanic eruptions. We conclude that Red Sea deep water renewal time is on the order of a decade up to a century, depending on the mean climatic conditions and large‐scale interannual climate forcing. The Red Sea is a deep marine basin often considered as small‐scale version of the global ocean. Hydrographic observations and ocean‐atmosphere modeling indicate Red Sea deep water was episodically renewed by wintertime open‐ocean deep convections in the northern Red Sea during 1982–2001, suggesting a deep water renewal time on the order of a decade. However, the long‐term pacing of Red Sea deep water renewals is largely uncertain, due to lack of hydrographic observations. By using an annually resolved coral oxygen isotope record of winter surface water conditions in the northern Red Sea we show that the late twentieth century deep water renewals were probably unusual in the context of the preceding ~100 years. Our results suggest an absence of major deep water formation events until the 1883 Krakatau volcanic eruption. More frequent major events are detected during the late Little Ice Age, particularly during the early nineteenth century characterized by large tropical volcanic eruptions. From our long‐term perspective we conclude that Red Sea deep water renewal time is on the order of a decade up to a century, depending on the mean climatic conditions and large‐scale interannual climate forcing, which should be considered in management strategies of its unique ecosystems. Observed frequent late twentieth century Red Sea deep water renewals were probably unusual in the context of the preceding ~100 yearsCoral proxy record suggests absence of major deep water formation events until the 1883 Krakatau volcanic eruptionMore frequent major Red Sea deep water formation events are detected during the early nineteenth century and late Little Ice Age

A record of floods from 1500 to 2003 of the River Werra (Germany) is presented. The reconstruction is based on combining documentary and instrumental data. Because both data types have overlapping time intervals, it was possible to apply similar thresholds for flood definition and obtain a rather homogenous flood series. The kernel method yielded estimates of time-dependent flood risk. Bootstrap confidence bands helped to assess the significance of trends. The following was found: (a) the overall risk of floods in winter (November—April) is approximately 3.5 times higher than the summer flood risk; (b) winter flood risk peaked at around 1760 and 1860—it increases again during the past decades; and (c) summer flood risk peaked at around 1760—it shows a long-term decrease from then on. These trends for the Werra contrast with those of nearby River Elbe, reflecting the high spatial variability of orographic rainfall.

The question whether paleoclimatic systems are governed by a small number of significant variables (low-dimensional systems) is of importance for modeling such systems. As indicators for global Plio-/Pleistocene climate variability, four marine, sedimentary oxygen isotope time series are analyzed with regard to their dimensionality using a modified Grassberger-Procaccia algorithm. An artificial, low-dimensional chaotic time series (Hénon map) is included for the validation of the method. In order to extract equidistant data the raw data are interpolated with the Akima-subspline method since this method minimizes the change in variance due to the interpolation. The nonlinear least-squares Gauss-Marquardt regression method is used instead of the linear least-squares fit to the logarithmically transformed points, in order to acquire an unbiased estimate of the correlation dimension. The dependences of the estimated correlation dimension on the embedding dimension do not indicate a small number (i.e., less than 5) of influencing variables on the investigated paleoclimatic system, whereas the low dimension for the Hénon map is verified (dimension 1.22–1.28). Because of the limited amount of data in the oxygen isotope records, dimensions greater than about 5 cannot be examined.

The Mid-Pleistocene Climate Transition (MPT) is the complex climatic change which brought the Late Pleistocene ice ages. We explore the MPT in the time and frequency domains by new methods of time series analysis. High-resolution oxygen isotope records reveal that the ice volume-related increase in δ18O mean (amplitude: 0.29 ± 0.05 (1 − σμ) ‰, transition midpoint: 922 ± 12 ka, duration: 40 ± 9 ka) significantly preceded the abrupt increase in the amplitude of the ∼ 100 ka cycle at 641 ± 9 ka. This finding can be quantitatively simulated using a simple ice-bedrock model in which, due to the additional ice, the calving threshold is exceeded. The simulated calving events prior to ∼ 650 ka are separated by ∼ 77 ka, whereas after ∼ 650 ka they occur pseudo-periodically with a mean period of nearly 100 ka. The cause for the delay of the ∼ 100 ka calving cycle was not a slow bedrock relaxation; rather, the coincidental combination of insolation, existing ice mass, and bedrock depression.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

The “internally” generated intraseasonal variability of the Indian Summer Monsoon is characterized by intermittent periods of enhanced (“active”) and deficient (“break”) precipitation, which produce a quasi east‐west precipitation dipole over the Indian subcontinent. Here we present multicentennial‐length and near annually‐resolved reconstructions of monsoon precipitation, inferred from absolute‐dated and instrumentally calibrated speleothem oxygen isotope records from regions (central and northeast India) that have diametric responses to active‐break monsoon circulation patterns. On centennial timescales (AD 1400–2008), precipitation variability from these two regions exhibit opposing behavior, oscillating between periods with a persistently “active‐dominated” (AD ∼1700 to 2007) and a “break‐dominated” (AD 1400 to ∼1700) regime. The switch between these regimes occurs abruptly (within decades) at a time (AD ∼ 1650–1700) when a proxy record of upwelling intensity from the Arabian Sea suggest an abrupt increase in the monsoon winds. On the basis of these observations, we hypothesize that the frequency distribution of active‐break periods varies on centennial timescales, implying a leading role of internal dynamics in governing the ISM response to slowly‐evolving changes in the external boundary conditions. Internal forcing is the dominant pacemaker of Indian summer monsoon variabilityMonsoon circulation oscillates between persistentlyIndian monsoon exhibits threshold type response