SH51E-01 INVITED 08:00h
Observations of Total Solar Irradiance indicate a + 0.04 % per decade trend during solar cycles 21 - 23
Continuous Total solar irradiance (TSI) observations have been made by satellite experiments since late 1978. A precise, contiguous composite TSI can be derived for the past 26 years by relating these results through consecutive overlapping observations. A crucial issue for a composite is the relationship between ACRIM1 and ACRIM2 results across the two year gap between them. This can be established by using one of the two overlapping data sets, the Nimbus7/ERB or ERBS/ERBE. The choice is important because the effects on the composite are significantly different. The ACRIM composite uses unaltered published results and the Nimbus7/ERB overlapping comparisons to link ACRIM1 and ACRIM2. The most significant feature of the ACRIM composite for climate change is an upward trend of 0.04 (+/- 0.01) % per decade between activity minima during solar cycles 21-23. [Willson & Mordvinov] TSI composites using the ERBS/ERBE data to link ACRIM1&2 results, such as the well known PMOD of Frohlich & Lean, use the same TSI data sets in a different approach but do not find a significant trend between minima. The absence of a trend in such ERBS/ERBE-based composites can be shown to be an artifact of uncorrected ERBS/ERBE degradation during the ACRIM1-ACRIM2 `gap'.There are other differences between the ACRIM and PMOD composites, driven principally by the PMOD's use of TSI proxy models to justify modifying the published results of ACRIM1 and Nimbus7/ERB. TSI regression (proxy) models based on chromospheric spectral features are not competitive in accuracy, precision or traceability with satellite TSI observations and are therefore likely to cause spurious effects when used in the construction of TSI composites. The TSI record has been sustained by overlapping, redundant experiments using their level of measurement precision to sustain longer term traceability. This TSI monitoring strategy is essential for continuity in the future because the uncertainty of current satellite sensors (~ 0.1 %) is an order of magnitude too large to detect subtle long term TSI variations of potential climate change significance. [Willson, R.C., A. V. Mordvinov, JGRL 30, pp. 1199-1202, 2003, Frohlich C., J. Lean, JGRL 25, pp. 4377-4380, 1998]
http://www.acrim.com
SH51E-02 08:15h
Finding the Sources of Irradiance Variation at Sunspot Minimum
In 2006-2007 the Solar Bolometric Imager (SBI) and the Multi-Spectral Imager (MSI) will operate in the polar stratosphere where near-space conditions can be attained for 10 to 20 days. The instruments will provide bolometric (wavelength-integrated light) and color temperature images of the Sun. At the upcoming sunspot minimum, SBI observations will be able to detect subtle sources of solar irradiance variation with the least confusion by signals from the magnetic fields. This is the best observational approach to characterizing potential causes of the long-term irradiance variations. Possible predicted sources of secular variability include torsional waves and meridional flow variations. SBI uses a 30-cm diameter F/12 Dall-Kirkham telescope with uncoated mirrors, and neutral density filters to provide broadband (bolometric) sensitivity that varies only by 7 percent over the wavelengths from 0.28 microns to 2.6 microns. The MSI is a CCD-based imager that will provide diagnostics of solar magnetic and thermal structures while SBI assesses their radiance. Sunspots, faculae and magnetic network will be identified from the MSI images. Sonic filtering of the MSI images will isolate the oscillatory signal. That signal will be used to remove oscillations from SBI averages to reduce the solar noise. Inferred solar irradiance variations will be compared with SORCE/TIM and ACRIMSAT measurements. The images and data products will be openly available via the Web.
http://sd-www.jhuapl.edu/SBI/
SH51E-03 INVITED 08:30h
Climate sensitivity to solar activity: The contribution of solar cycles 21-23 to global mean surface warming
We study the solar signature on global temperature time series and use the corrected ACRIM satellite total solar irradiance composite data of Willson and Mordvinov. These data present a +0.04% per decade trend during solar cycles 21-23 (1980-2002). We use these data to estimate the climate sensitivity to solar activity at different time scales. After discussing certain limitations of multiple linear regression analysis in analyzing nonstationary time series, we introduce a stochastic linear wavelet transfer methodology that uses the influence of the Schwabe (11 years) and the Hale (22 years) solar cycles on the global surface temperature. The direct total solar irradiance forcing during the 21-23 solar cycles might have contributed from a minimum of 7-12% (by evaluating the upward trend of the ACRIM data with a step function) to a possible 10-20% (according to a linear increase) of surface warming during the same period. However, the Sun's variability on climate, the net solar forcing, might have minimally contributed $\sim$10-30% of the global temperature trend over 1980-2002. This finding suggests the presence of strong climate feedbacks to solar dynamics.
SH51E-04 08:50h
Interdecadal Variability of Sea-Surface Temperatures: The Case for Solar Irradiance Forcing
The record of global, hemispheric, and basin-wide sea-surface temperatures during the era of shipboard measurements provides some of the best evidence for a sun-climate linkage on interdecadal time scales. On these long time scales the upper mixed layers of the oceans can reach a quasi-equilibrium thermal balance with the external forcing, unlike the situation for interannual and even decadal-scale forcing, for which the thermal inertia of the oceans causes significant attenuation of the response. The interdecadal-scale SST data available happens to coincide roughly with the so-called Modern Minimum of solar activity, centered in the first decade of the 20th century. The data will be presented and discussed, together with some conclusions about the global climate sensitivity to solar irradiance variations.
SH51E-05 INVITED 09:05h
Physical Origin of the Variations of the Total Solar Irradiance (TSI) on Decadal Timescales or Longer
A variable (dynamo) magnetic field in the solar interior induces structural changes that result in variations of the global solar parameters (luminosity, radius, and temperature). Of course, as the luminosity changes, so does the basic level of the TSI, which is additionally modulated by surface magnetic activity (spots, faculae, and network). Since the energy reservoir for this internal process is very large, the mechanism can produce TSI modulation over very long timescales, depending only on the properties of the driving magnetic field. In other words, if the dynamo field undergoes secular changes (e.g. Gleissberg cycle or Maunder-type minima), the TSI will also undergo modulation having the same timescales. This paper will describe the current evidence in support of the operation of this mechanism, and how it is currently modeled. In addition, it will describe what observational and theoretical improvements need to be made to produce a model sufficiently robust to explain past change, and to forecast its future effect on climate.
SH51E-06 INVITED 09:25h
Recent Advances in Solar Influences on Earth's Climate Recent Advances in Solar Influences on Earth's Climate
This talk reviews recent studies on the impact of solar variability on the Earth's climate system. Some attention will be given to recent reports drawn from paleoclimatology and others from climate modeling. The paleoclimatological studies include attempts to explain the surface temperature record over the last 1000 years with some attention to reconciling several of the records. Some modeling studies have been applied to these long records, but most of the modeling attention will be focused on attempts to detect the 11 year cycle in the large scale temperature fields. After reviewing recent successful detection studies by the author's group utilizing the last century's record, several more recent studies will be examined in an attempt to understand the underlying mechanisms involved. The more recent studies make use of shorter records but take into account vertical information as well as data taken at the surface. The tentative conclusion is that the response over the last few cycles is rather large at some latitudes, and the cause might originate in dynamical alterations of the general circulation rather than merely the often-suggested energy balance considerations. The response to the 11-year cycle is likely to be a roughly equal combination of both dynamical and simple energy balance effects at least at some latitudes.
SH51E-07 09:45h
Long-Term Total Irradiance Composites
Accurate long-term total and spectrally resolved solar irradiance measurements are required for full understanding of the response of Earth's atmosphere and climate to irradiance changes. Space-based irradiance observations over the last two and a half solar cycles span a time interval too short to reveal secular changes and/or to establish conclusively whether there are significant changes in the amplitude or the character of irradiance variations on longer time scales. Since the time period of interest far exceeds the lifespan of any single experiment, composite irradiance time series must be compiled from data of several irradiance experiments. Because the absolute accuracy of the current measurements is limited (about $\pm$0.2% in case of total irradiance), overlapping and redundant measurements are needed to ensure that the resulting composite data sets represent the ``true'' solar behavior. However, the largest obstacle in creating the current long-term total irradiance composite is the two-year gap between the SMM/ACRIM~I and UARS/ACRIM~II measurements. Adjustment of the ACRIM~I and ACRIM~II data now must be made through the Nimbus-7/ERB and/or the ERBE measurements. While using the published Nimbus-7/ERB data set, Willson and Mordvinov (2003) concluded that the minimum of cycle 22 was higher than the minimum of cycle 21, Fr\"ohlich (2004) claims that no trend can be seen in total irradiance within the current measurement accuracy. In this paper we compare various total irradiance time series, to better understand the differences between the two total irradiance composites. We will apply a new approach to adjust the ACRIM~I and ACRIM~II time series to further study the possible secular trend in total irradiance. Fr\"ohlich, C. 2004, In the Solar Variability and Its Effect on Earth's Climate, Eds. J. Pap and P. Fox, AGU Monograph, No. 141, p. 97. Willson, R.C. and Mordvinov, A.V.: 2003, Geophys. Res. Lett. Vol. 30, 3-1.