U41B-01 08:00h
Modeling Be-10 in Polar ice: Climatic vs. Solar Effects
The close correlation between the production of the cosmogenic isotope $^{10}$Be and changes in heliomagnetic activity makes ice-core $^{10}$Be an attractive proxy for studying changes in solar output. However, the task of interpreting $^{10}$Be ice-core records on centennial timescales is complicated by the fact that climate-related deposition changes -- which may be unrelated or disproportional to variations in irradiance -- can potentially obscure $^{10}$Be's solar production signal. By using the GISS ModelE GCM to selectively vary climate and production functions, we model $^{10}$Be flux values and then calculate concentration changes at key coring sites. We will present results for solar changes and compare them to variations in $^{10}$Be concentration as functions of (a) ocean changes related to the 8.2 ky event and (b) atmospheric changes due to varying volcanic and greenhouse gas forcings.
U41B-02 INVITED 08:15h
Solar Forcing of Abrupt Glacial Climate Change in a Coupled Climate System 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. Using the coupled climate system model CLIMBER-2 we show that the combined effect of two well-known centennial-scale solar cycles could explain the timing of the Dansgaard-Oeschger events. These pronounced cycles, the Gleissberg and DeVries cycles with almost stable periods near 87 and 210 years, are close to prime factors of 1470 years. Due to the dynamics inherent in the simulated Dansgaard-Oeschger events, i.e. the threshold behavior and the thermal inertia of the thermohaline circulation, the combined effect of the two solar cycles results in a very robust 1470-year timescale of these events for Glacial conditions. [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)
U41B-03 08:30h
Centennial to multi-millennial signals in the atmospheric radiocarbon record during the Holocene - a solar link from tree-rings
Commonly, fluctuations of the atmospheric 14C level are taken as an indicator of solar variability. This concept is based on the well understood physical relation between 14C production from cosmic ray flux and heliomagnetic and geomagnetic shielding of the earth, on one hand, and on the observed close correlation of sunspot numbers (SSN), and other indicators of solar activity, and D14C over the past three centuries, on the other hand. Although there are good arguments for considering solar activity changes to dominate decadal to multi-centennial signals in the 14C record, it remains an open question as to how much carbon system changes, and the changes in the relative distribution of 14C among the major reservoirs contribute to 14C variations. In our contribution we address this questions focussing on the tree-ring based 14C record, now reaching back to 12,400 years BP. We discuss spectral features of the record, and compare them to results obtained from the 10Be record in ice cores. Using possible range of changes in the meridional overturning circulation of the North Atlantic we calculate their contribution to the 14C signals. We show the response of forcing the CLIMBER-2 model with irradiance variations scaled by the 14C anomalies. We report on our ongoing work to relate climate proxies in tree-ring parameters to intervals of strong 14C anomalies in the Holocene.
U41B-04 INVITED 08:45h
Solar Irradiance Variation on Centennial to Millennial Time Scales
Solar irradiance variation observed over the 11 yr sunspot cycle is caused by the changing areas of dark and bright magnetic structures (sunspots, faculae)on the solar disc, but its barely 0.1 % amplitude is insufficient to drive existing climate models. Irradiance reconstructions incorporating an additional slowly varying component of sufficient amplitude to drive such models have been widely used in recent climate studies. But these reconstructions were based on results from photometry of Sun like stars which have now been largely retracted. This changed evidence challenges our understanding how solar luminosity variation could drive climate. Variation of UV flux may play a role, but its correlation with global temperature seems low, at least in the 20th century. The Sun's enormous thermal inertia restricts sources of luminosity variation on centennial to millennial time scales, to relatively superficial layers. This constraint diminishes the likelihood that deeper lying structural changes associated with e.g. the solar dynamo play a significant role. Still, some newly discovered aspects of solar magnetic behavior suggest how luminosity variation on these time scales might conceivably occur with the sign and amplitude implied by the correlations between solar activity and climate. More accurate solar and stellar observations and modeling will be required to investigate such mechanisms at the frontier of our understanding of the Sun.
U41B-05 09:00h
Detection of a secular trend in Total Solar Irradiance of + 0.04 % per decade during solar cycles 21 - 23
A `piecewise' continuous record of total solar irradiance (TSI) has been made by satellite experiments since November 1978. Assembling a contiguous record from this set of observations is a challenging task due to variations in quality of the different data sets, the limitation that a subtle secular trend might be masked by uncertainties in accuracy, precision and traceability and the necessity of relating the ACRIM1 and ACRIM2 results across a two year gap. The ACRIM approach to constructing a composite TSI record retains maximum precision and traceability by using results published by the science teams of the various experiments without alteration and the Nimbus7/ERB comparisons to link ACRIM1 and ACRIM2. The resulting ACRIM composite TSI demonstrates an upward trend of 0.04 (+/- 0.01) % per decade between activity minima during solar cycles 21-23. [Willson & Mordvinov] Another TSI composite, the `PMOD' [Frohlich & Lean], uses the same set of TSI data in a different approach and does not find a significant trend between minima. The PMOD composite uses TSI proxy models to justify modifying published results and relates ACRIM1&2 results using the overlapping data of the ERBS/ERBE experiment. The absence of a trend in the PMOD composite can be shown to be an artifact of uncorrected ERBS/ERBE degradation during the ACRIM gap. It will be further shown that chromospheric TSI regression (proxy) models are not competitive in accuracy, precision or traceability with satellite TSI observations and their use is therefore counter-indicated in construction of TSI composites. [Willson, R.C., A. V. Mordvinov, JGRL 30, pp. 1199-1202, 2003, Frohlich C., J. Lean, JGRL 25, pp. 4377-4380, 1998]
<a href='http://acrim.com' >http://acrim.com
U41B-06 INVITED 09:15h
The Holocene Total Solar Irradiance Based on $^{10}$Be Extracted From Ice Core
The sun is by far the most important energy source for the Earth's climate system. Although its magnetic variability has been revealed by various observations of solar parameters, the solar irradiance was considered for a long time as constant and misleadingly named `solar constant'. Since satellite based radiometers have shown that the solar irradiance varies with the 11-year sunspot cycle, increasing interest has been directed towards the sun's significance as a variable natural climate forcing factor. However, the measurements reveal that the total solar irradiance (TSI) changes over a solar cycle by only approx. 0.1 % what has questioned the relevance of the solar forcing. On the other hand, historical observations of the sun such as the 400 years long sunspot record clearly point to solar magnetic variability larger than observed during the satellite based monitoring period. A longer record of past TSI is needed to determine the full spectra of past variability in the sun's impact on our climate. Cosmogenic radionuclides like $^{10}$Be and $^{14}$C stored in ice cores and tree rings, respectively, provide the only indirect information on the sun's long-term behavior on millennial time scales and thus on the sun's total potential of variability. We present a new method to reconstruct past TSI from $^{10}$Be measurements from the GRIP ice core, which provides an improved basis for the detailed calculation of the effect of solar forcing on the Earth's climate. $^{10}$Be is produced by the interaction of galactic cosmic rays with the Earth's atmosphere. The heliomagnetic and geomagnetic fields modulate the cosmic ray intensity and therefore the production rate. Taking the geomagnetic modulation into account we reconstructed quantitatively the solar activity in terms of the heliospheric modulation parameter $\Phi$ and subsequently the TSI for the past $\sim$9300 years. Based partly on physical models, this method differs widely from previously applied linear regression approaches to reconstruct TSI from cosmogenic radionuclides. The results reveal a longtime solar variability significantly larger than observed so far by direct measurements and point out that the current high activity of the sun is not exceptional regarding the entire Holocene.
U41B-07 INVITED 09:30h
Solar forced Dansgaard/Oeschger events?
Prior to the period of direct satellite-based solar observations (last 25 years) and indirect solar observations based on sunspots (400 years) most evidence for a solar influence on climate is based on the comparison of radionuclide with climate records. For this period radionuclide records represent the most reliable proxies to reconstruct changes in solar activity. Cosmogenic radionuclides are produced in the atmosphere by the interaction of galactic cosmic rays with the atoms of the atmosphere. The solar and geo-magnetic field influence the production rates indirectly by deflecting the galactic cosmic rays. However, radionuclide records are also influenced by climatic changes. Atmospheric radiocarbon concentration, for example, can be influenced by changes in oceanic ventilation. 10Be and 36Cl measured in ice cores can be influenced by changes in atmospheric transport and deposition. It is crucial to identify potential climatic influence on radionuclide records since otherwise the comparison of radionuclide with climate records could feign a solar influence on climate. This is extremely important for the period of the last ice age where the strong climate changes during the Dansgaard/Oeschger events significantly influenced the radionuclide concentration in ice cores. We will discuss if it is possible to assess the solar influence on climate based on radionuclide data sets from the GRIP ice core during the period of the last ice age. Since the radionuclide records from Greenland also reliably record past changes in the Earth magnetic field it is also possible to study the suggested influence of cosmic rays on climate.