U43A-0737 1340h
Atmospheric radiocarbon fluctuations in the Late Glacial obtained from tree-ring chronologies - aspects of rapid climate change
We built a floating 1382-ring chronology based on pines recovered from alluvial deposits in Germany, Switzerland and Northern Italy. It includes a tree buried in situ by the Laacher See eruption (LSE) providing already a strong constraint about the absolute age of the chronology. We performed high-precision 14C (+-25 years) analyses of decadal resolution samples. The end of the chronology covers the initial 150 years of the Younger Dryas (YD) as evidenced by the strong 14C age drop from 10,950 14C BP to 10,680 BP in less than 50 years, commonly considered marking the onset of YD. Based on these constraints we can calculate the approximate atmospheric 14C level during the Bolling/Allerod interval. We observe several substantial 14C fluctuations that appear synchronous to cooling events documented in Greenland ice cores and the Cariaco basin sediments, among others. In the contribution potential causes of atmospheric 14C fluctuations (solar activity changes, ocean ventilation change, sea ice cover) are discussed and their relation to rapid climate change in the LG and early YD is considered.
<a href='http://www.iup.uni-heidelberg.de/institut/forschung/groups/fa/radiokohlenstoff/radiometrie-web-html' >http://www.iup.uni-heidelberg.de/institut/forschung/groups/fa/radiokohlenstoff/radiometrie-web-html
U43A-0738 1340h
Holocene Secular Variability and 200-Year Dipolar Oscillations in the Atmospheric Circulation Over Antarctica Deduced From ice Core Dust Records
Variability in atmospheric circulation over Antarctica reflects the character of exchanges with the surrounding southern ocean and the complexity of the coupled atmosphere-sea-ice-ocean system. We compared sub-centennial resolution dust records (~1 sample every 50 years) from two 600 km distant East Antarctic ice cores from Vostok and EPICA Dome C, over a period encompassing a large part of the Holocene period. The relative chronology of the two records is well constrained (about +/- 33 years) .by continuous electrical loggings and volcanic markers. The relative proportion of the of coarse or fine aeolian dust transported to the East Antarctic Plateau is taken as a proxy for the atmospheric transport. Based on the size distribution changes observed during volcanic events from distant sources recorded in Antarctic ice, our hypothesis stands that the aeolian dust is differently graded en route according to the altitude of the pathway. The fine-size dust would be associated to the relative contribution from high level air masses subsidence while the coarse-size dust to advection of low troposphere air masses. The two records of the dust size parameters display apparently asynchronous structured variations at secular and millennial frequency. However, the records share common band frequency and one around 200 years. The tighten chronologies allow to built stacked composite records to enhance the signal-to-noise ratio. The stacked row-data composite-sum contains most of energy between 130 and 500 years. Conversely the stacked composite-difference signal displays a prominent 200-yr band oscillation. In the 200-yr band, Vostok and Dome C dust series display a clear correlation but are in opposite phase over the 6 kyr studied period. We suggest the spectrum of the composite-sum represents the variability of the overall meridional atmospheric transport to Antarctica, or the strength of the Antarctic Oscillations coupled to the atmosphere-sea-ice-ocean system. On a other hand, we interpret the 200-yr opposite variations between Vostok and Dome C records as persistent seesaw phenomenon or a dipole in air advection/ subsidence over the two locations, that we associate to the secular variations of the eccentricity of the Antarctic vortex. The marked 200 years band frequency hints to the possible solar influence.
U43A-0739 1340h
Development of the Maunder Type of Solar Activity and Their Climatic Response
The peculiarities of solar activity changes around Maunder (about 350 years ago) and Homeric (around 2700 years ago) minima of solar activity were analyzed. The solar activity changes at the time intervals relating to the solar activity minima mentioned have similarity. Both of these solar activity minima are accompanied by global climate changes. Differences in climatic response to Maunder and Homeric solar activity minima could be related to the development of the geomagnetic "Etrussia-Sterno" excursion around 2700 years ago, which can act on atmospheric circulation during the Homeric minimum of solar activity. Joint analysis of development of solar activity around the Maunder and Homeric minima make possible to understand the direction of development of the long-term and medium-term of solar activity and climate change at the recent time. Analysis shows that the solar activity level has reached its maximum in the vicinity of 20-21 centuries and will decrease in this century. Both of long-term and medium-term cyclical changes of solar activity are accompanied by global climate changes. This work was supported by INTAS (Project 2001-0550 and 03-51-4445), RFBR (projects 03-04-48769 and 03-05-65063), NorFA Grant "Network for Dendroecological and Dendrochronological Research in Northern Europe", General Physics Department of RAS (program "Solar wind"), and Russian Federal Program "Astronomy"
U43A-0740 1340h
Centennial to Millennial Scale Variations in the Indian Summer Monsoon Winds and Solar Variability
The monsoon circulation arises from the seasonal variation in the energy received from the sun, and records of the Indian summer monsoon provide a means to understand the sun-climate connection. The Indian summer monsoon wind speed was reconstructed from G. bulloides shell abundance in foraminifer-bearing nannofossil ooze sediments from the NW Arabian Sea for the past 10,000 years. This study extends our previous work by increasing the sample resolution to 50 years, by calibrating the G. bulloides record in terms of alongshore wind stress, and by quantifying the link between Arabian Sea wind speed and other aspects of the monsoon such as all-India rainfall and the surface pressure gradient. Coarse age control relative to solar variations make stratigraphic correlations uncertain. Nevertheless, we found concentrations of variance in the monsoon wind speed at the century to millennial scale that correspond to solar variations indicated by the Be-10 and C-14 proxies. The correlation is strongest in the early Holocene, and weakest during the late Holocene when the monsoon was weak. The Maunder Minimum was a time of reduced solar output and with low seasonality due to the precession of the axis of Earth rotation, and corresponds to the interval of the weakest monsoon winds observed during the last 10,000 years. At the millennial scale, the monsoon winds correspond not only with solar variability but also with times of cooler conditions in the North Atlantic. The hypothesis of a direct solar link to the tropical monsoon circulation versus a connection via the North Atlantic remains unresolved, both explanations supported by conceptual and numerical models.
U43A-0741 1340h
On The Possible Regional Distribution Of Ca 2.4 Kyr Climatic Cycle
A tree-ring proxy of summer temperature anomalies for northern Finland for the last 7500 years was analysed using both the Fourier and wavelet approaches. A distinct ca 2.0 Kyr periodicity was found in this temperature proxy. This variation is likely a manifestation of 2.0-2.5 Kyr Hallstattzeit cycle, present in many climatic records. Analysis showed that the Hallstattzeit climatic variation (period 2.0-2.5 Kyr) probably is not a global, common rhythm but has a spatial distribution. The origin and cause of this quasi 2.4 Kyr cycle is still unknown, however, some evidence of its possible connection with atmospheric dynamics were obtained
U43A-0742 1340h
Long-Term Sun Climate Connections, Revealed by the Analyses of Historical and Other Proxy Records
The Sun, once considered constant, actually goes through 11-year, decadal, centennial, and even longer cycles. Our analysis of historical sunspot and aurora records, carbon-14 and beryllium-10 abundances from long-lived trees and deep polar ice cores, respectively, shows that it has gone through nine long cycles in the past 1800 years. Although these changes amounted to $<$1% of the total irradiance there is clear evidence they produced corresponding changes in the climate [Pang and Yau, Eos, 83, No. 43, 481, 2002]. For example during the Maunder Minimum (1645-1715) sunspots were rarely seen (about once in ten years from Europe or China). Total solar irradiances, reconstructed from historical sunspot records, were 0.25% lower then. This correlates nicely with an estimated 0.5-degree drop in Northern Hemisphere summer surface temperatures during the Little Ice Age [Lean, GRL 22, 3195, 1995]. We have also analyzed Chinese historical weather records for comparison. Reports of unseasonable cold are classified by the degree of severity: (1) Late (April-June) or early (July-September) killing frosts; (2) Bitter cold/heavy snowfall; and (3) Heavy sustained snowfall, bitter cold with frozen wells, rivers and icebound seas. The latter cases were often widespread and multi-year. All categories occurred most frequently during the coldest part of the Little Ice Age. The Category 3 episodes were in 1652-54, 1656, 1664, 1670-72, 1676-77, 1683, 1688-91, 1716, and 1718-19. For example the Yangtze River and its lakes froze up to 3-4 times in 1650-1700. The coldest period thus coincides with the Maunder Minimum, and is consistent with general circulation model hindcast winter conditions for China [Shindell, Science, 294, 2149, 2001]. There was only one Category 3 episode between the Maunder and Dalton Minima--in 1761 (due to a large volcanic eruption); and two in the Dalton Minimum (1795-1825)--in 1796 and 1814-17. The Sun has gradually brightened since the Dalton Minimum. But the climate of China remained cold through the 19th century, as in the rest of the world, probably due to increased volcanic aerosol loading of the atmosphere [Sato, JGR 98, 22987, 1993]. The climate of China seems to have been warm during the Late 14th-Century Maximum (1350-1410). We have found only one Category 1 episode--in 1393. It then turned cold during the Sporer Minimum (1410-1590). Category 3 episodes occurred in 1453-54, 1493, 1513, 1569, and 1577-78. Lesser ones were also common. Some scientists suggest that the Little Ice Age actually began in the 13th Century, and is comprised of the Wolf (1280-1350), Sporer, Maunder, and Dalton Minima. It and the Little Climate Optimum make up a millennium-long cycle [Broecker, Natural History 101, 6, 4/1992]. The warm Classical Age and cool Dark Ages, the cold Iron Age (1st millennium BC), and warm Bronze Age (2nd and 3rd millennia BC) could be considered still earlier millennial cycles. These trends are generally consistent with carbon-14 deviations from its long-term variations. There is also some historical and archaeological evidence for the early trends, as rhinoceros and elephant herds were abundant along the Yellow River during the Shang dynasty (1600-1100 BC). The tropical fauna and flora have since disappeared, as North China gradually turned cold and arid. We conclude that the climate of China generally follows world trend. The major forcing seems to have been changing solar luminosity on a decadal to millennial timescale. Volcanic eruptions and changing ocean currents also frequently perturbed the climate.
U43A-0743 1340h
Timing of Holocene Glacier Recessions in the Swiss Alps
Alpine glaciers are sensitive climate indicators on time scales as short as decades to centuries. Periods of cold and wet climatic conditions cause glacial advances and deposition of moraines. Subsequent change to warmer and drier conditions cause glacial recessions. Since the cold event of the Little Ice Age glaciers in the Swiss Alps have retreated substantially exposing high walls of lateral moraines which consist of a stack of overlying till units. Previous work focused on such stacked moraines and the related glacial advances. However, such paleoclimatic reconstructions based only on moraines are incomplete, in particular for periods of glacial recession. This study examines Holocene glacier recessions based on pieces of wood and peat occurring in glaciofluvial deposits of outburst flood events. These subfossil remains indicate that (i) glaciers were once smaller than present, (ii) climate conditions allowed vegetation growth and a higher treeline elevation than today, (iii) sequences of glaciofluvial gravels and lodgement tills accumulated in now glaciated basins. We focus in this study on three climaticly different re-gions: i.e. Unteraar Glacier (Central Alps), Tschierva Glacier (Eastern Alps) and Ried Glacier (South Central Alps). The conventional radiocarbon and AMS dating was used to determine the age of more than 140 samples of wood and peat fragments. The results show that glacier recessions occurred in distinct phases throughout the Holocene, synchronously in different Alpine regions. The total duration of recession phases is longer than 5500 yr or $>$ 50 $%$ of the Holocene epoch. The glacial recessions are separated by glacial advances, which occurred in time periods shorter than 400 yr. A comparison of our Holocene record of glacier length variations with the ice rafted debris events in the North Atlantic, the Be-10 record of ice cores and atmospheric $\Delta$ 14C suggest a combination of solar and North Atlantic forcing. Thus, situations of glacial recession, to the extent of smaller glaciers than present, occurred throughout the Holocene.
<a href='http://mypage.bluewin.ch/eljoerin/u-page/activities.htm' >http://mypage.bluewin.ch/eljoerin/u-page/activities.htm
U43A-0744 1340h
North American Terrestrial Climate Connection to Solar, Ocean, and Ice Core Records on Millennial-Scales During the Holocene
A leading hypothesis for millennial-scale climate variability during the Holocene is solar output variations. However, the timing and expression of this variability in individual high-resolution records has proven ambiguous. We discuss a continental-scale North American pollen-based temperature reconstruction that correlates in the frequency domain with marine records from the North Atlantic (IRD, NADW) and cosmogenic nuclide records, proxies for solar output variations (14C, 10Be) as well as other records from around the world. Because our temperature reconstruction is based on hundreds of sites and thousands of dates, the climate signal is more readily identified then from reconstructions based on one site, which are affected by local non-climatic factors. The incorporation of time and space results in a higher probability of extracting the climate signal than in any individual high-resolution record. Coherency in the frequency domain between our temperature time series and other marine, terrestrial and ice records suggests that the climate as a whole varied at a more periodic 1,000 ± 100 years during the Holocene. Indeed, uncovering a common frequency between the different climate records of the Holocene is key to disentangling climate variability on millennial-scales. It appears likely that Holocene climate variations on millennial-scale resulted from an external cause, potentially solar output variations, associated with internal feedbacks acting as amplifiers.
U43A-0745 1340h
A Sun-Climate Connection on Long Time Scales: Pros and Cons
A significent obstacle to advancing our knowledge of sun-climate connections is that solar physicists best understand changes in total solar irradiance (TSI) for the 11 year sunspot cycle, while paleoclimatologists tend to find the most convincing sun-climate correlations among cycles in the centennial to millennial band for which TSI changes are unknown. Indeed, some solar physicists argue that there are no trends longer than 11 years in TSI. Lack of a quantitative value for TSI on the longer time scales appears to be underappreciated by modelers and seriously compromises their efforts to understand how the climate system responds to solar variability. Paleoclimatologists and modelers urgently need an agreed upon number for TSI on timescales greater than 11 years. Further complications arise from uncertainty in the relation between TSI and proxies of solar activity, atmospheric radiocarbon and beryllium 10, which extend the observational record of solar activity back to about 12,000 years ago. This is unfortunate because there is increasing evidence for good matches between paleoclimate and nuclide variations of centennial to millennial duration. Cloud-climate connections proposed to resolve this problem, however, are inconsistent with climate during the Laschamp geomagnetic event about 40,000 years ago. The nuclide-TSI connection is another urgent issue that needs to be addressed by solar physicists and cosmogenic nuclide geochemists. It is unlikely that the weak variations in direct solar forcing could have produced climate changes large enough to be resolved in proxy climate records. Strong amplifiers and feedbacks, such as changes in stratospheric ozone, shifts in planetary wave patterns, changes in thermohaline circulation, and excitation of annular modes, must have operated within the climate system. Such amplifying mechanims are suggested by modeling but need confirmation, where possible, from paleoclimate records. Finally, climate models suggest that during the last one to two millennia ENSO, and/or volcanism may have had a larger influence on climate than solar forcing. Yet recent paleoclimate records spanning most of all of the last 12,000 years suggest that at least on time scales of multi-centuries to millennia, solar forcing may be the dominant agent of climate change. Sorting out attribution on different time scales is an especially important problem that the paleoclimate and solar physics communities need to address.
U43A-0746 1340h
Solar Forcing of Abrupt Glacial Climate Change in a Coupled Climate System Model and in a Simple Conceptual Model
Various climate archives show a quasi-periodicity of about 1470 years in the North Atlantic region. In the last Glacial, this cycle manifests itself in prominent warmings, the Dansgaard-Oeschger events. In the Holocene, a drift-ice cycle of approximately 1500 years coincides with "rapid (100- to 200-year), conspicuously large amplitude variations" in proxies of solar activity [1], which suggests a solar origin of the 1470-year climate cycle. The coupled climate system model CLIMBER-2 is able to reproduce many features of the observed Dansgaard-Oeschger events when forced by two well-known centennial-scale solar cycles, the Gleissberg and DeVries cycles with periods near 87 and 210 years. Due to the model dynamics, i.e. the threshold behavior and the thermal inertia of the thermohaline circulation, the combined effect of these two cycles results in a very robust 1470-year timescale of the model response for Glacial conditions. Here we show how a very simple conceptual model, which incorporates only the threshold character and the thermal inertia of the THC, can be used to clarify the dynamics and the response of CLIMBER-2. Other concepts, e.g. the idea of an intrinsic 1470-year mode, are far less applicable to explain the response of CLIMBER-2. [1] Bond, G. et al., Persistent Solar Influence on North Atlantic Climate During the Holocene, Science 294, 2130-2136 (2001); published online 15 November 2001 (10.1126/science.1065680)