GP54A-01 INVITED
Does the earth's magnetic field influence climate?
The main agents which are invoked are solar variability, changes in atmospheric greenhouse gas content, or internal variability of the coupled ocean-atmosphere system. Evidences for connections between climate and magnetic field variations have received less attention and will be reviewed. On the 10-100yr timescale, that of recent secular variation, there appears to be a rather good correlation between decadal changes in amplitude of geomagnetic variations of external origin, solar irradiance and global temperature. The correlation applies until the 1980's, suggesting that solar irradiance may be a key forcing function of climate until then, when the correlation breaks and (anomalous?) warming may emerge from the signal (this is the subject of separate, ongoing work). Indeed, only solar flux of energy and particles can jointly explain such parallel variations in temperature and external magnetic field. On the 100-5000yr timescale, that of historical and archeomagnetic change, intriguing features are the recently proposed archeomagnetic jerks, i.e. fairly abrupt (~100 yr long) geomagnetic field variations found at irregular intervals over the past few millennia, using the archeological record from Europe to the Middle East. These seem to correlate with significant climatic events in the eastern North Atlantic region. A proposed mechanism involves variations in the geometry of the geomagnetic field (f.i. tilt of the dipole to lower latitudes), resulting in enhanced cosmic-ray induced nucleation of clouds. On the 103-106 yr timescale, that of excursions and reversals, evidence for correlations in field intensity changes, excursions and reversals, which invoke Milankovic forcing in the core, either directly or through changes in ice distribution and moments of inertia of the Earth, is proposed but is still rather tenuous. In conclusion, no forcing factor, be it changes in CO2 concentration in the atmosphere or changes in cosmic ray flux modulated by solar activity and geomagnetism, or possibly other factors, can at present be neglected or shown to be the overwhelming single driver of climate change in the past century. Intensive data acquisition is required to further probe indications that the Earth's and Sun's magnetic fields may have significant bearing on climate change at time scales going from years to millennia, and more.
GP54A-02 INVITED
Sun's Direct and Indirect Role in Climate Change
Broadly, the Earth's climate is driven by the Sun's output, the Earth's reflectance and the Earth's thermal emission. Of these three fundamental climate variables, the Earth's reflectance is the least well-studied. In fact, variations in reflectance are being implicitly ignored when solar cycle variables are treated as proxies for the net sunlight reaching Earth. Variations in the solar irradiance have been precisely measured for more than a quarter century combining observations from various satellites, and here we review the physical reasons why the Sun's irradiance variations over a solar cycle, as well as over longer historical times have climatologically insignificant variations. So, why are there terrestrial signatures of the solar cycle in climate records, or periods like the Maunder Minimum corresponding to times when the Sun was less active? If the variations of irradiance over the most recent solar cycles were typical, then changes in the net sunlight reaching Earth is a logical source of the terrestrial signatures of solar variability. Here, the relevant component of the net sunlight is the much less well-studied global reflectance of the Earth. Small variations in the Sun's output could be amplified in the much less well-studied terretrial albedo. We review our knowledge of the Earth's reflectance from terrestrial measurements of the earthshine and discuss future plans for measurements of terrestrial reflectance.
GP54A-03 INVITED
Solar Activity Earth Rotation Rate and Global Surface Temperature Long-term Variations.
Empirical evidences of the relationship that does exist between solar activity, Earth rotation rate and global surface temperature long-term modulations and the mechanisms able to explain such a relationship are reviewed. In particular we discuss the possibility that these modulations are excited either externally by planetary spin-orbit interaction, or internally to the sun , so controlling the intensity and frequency of geomagnetic storm time variations that in turn excite length of day and secular geomagnetic variations by electromagnetic induction.
GP54A-04 INVITED
Century-Scale Variations of Solar and Geomagnetic Activity and Their Possible Role in Climate Change.
Modeling of the effects of solar variability on atmospheric temperatures indicates that the response of the ocean is an important factor. The thermal inertia of the ocean tends to damp changes in atmospheric temperature on time scales of the solar cycle and less. However, over longer time scales the damping is less effective. This suggests that systematic solar variations on longer time scales are more effective in changing climate. We will discuss evidence that century scale variations in the Sun and the geomagnetic activity, such as the 88-year cycle, and the Grand Maxima and Minima have resulted in observed climate change.
GP54A-05
Reconstruction of the Long-Term Irradiance Variations
Solar irradiance variations have been recorded only since 1978. Clearly, there is a need to extend these records into the past in order to evaluate their possible influence on the Earth's climate. Here, a reconstruction of solar irradiance back to the Maunder minimum from the surface magnetic flux is presented. The reconstruction is based on a simple physical model that builds on the sunspot number records and sunspot areas where available. Since the sunspot area records generally consist of a compilation of data from multiple observatories, a proper cross-calibration is essential. The use of data of different sources directly combined can lead to errors in estimating the increase of solar irradiance during the past centuries. Thus, a brief description of the cross-calibration of sunspot areas is also presented.
GP54A-06 INVITED
Solar Activity Effects on Long-Term Trends in the Upper Atmosphere
The long-term continuous increase of greenhouse gas concentration in the atmosphere and other anthropogenic influences excite long-term trends and changes in the atmosphere-ionosphere system. The observed long-term trends in the 20th century might be, however, influenced by contribution of Sun's origin, and the process of determination of anthropogenic trends from observational data may be "spoilt" by the 11-year solar cycle. After brief presentation of global trend pattern, the role of solar/geomagnetic activity in long-term trends in various regions of the atmosphere/ionosphere system is reviewed. The ways of avoiding or at least diminishing the effect of solar cycle on trend determination are mentioned. As for the possible solar and geomagnetic activity responsibility for part of the observed long-term trends, the two main conclusions are as follows: (i) The role of solar and geomagnetic activity in the observed long-term trends decreases with decreasing altitude from the F- region ionosphere down to the troposphere. (ii) In the 20th century the role of solar and geomagnetic activity in the observed long-term trends/changes was decreasing from its beginning towards its end. While at the beginning of the 20th century changes of the solar activity played some role even in the troposphere, at present only in the F2 region ionosphere the geomagnetic activity seems to play important and/or dominant role in trends. The trends in the E region ionosphere turned from the geomagnetic control to the dominant anthropogenic control around 1970.
GP54A-07
Multidecadal Oscillations in Climate and Earth Rotation: Signatures of Internal Core Variability?
Multidecadal oscillations have been well documented in the Earth's climate. We use a simple energy-balance model with greenhouse gas, sulphate aerosol and parameterized solar irradiance forcing to account for external effects in the global mean surface temperature record since 1850. The residual temperature variation shows a pronounced 60-70 year oscillation, which is further clarified through the use of singular spectral analysis (SSA). The record of length-of-day (LOD) variation since 1835 shows oscillations of similar period, with the SSA- reconstructed LOD signal robustly tracking the residual temperature variation over more than two complete cycles. Similar variability is found in a record of geomagnetic (aa) variations since 1870 following removal of the 11-year solar cycle, and in reconstructed series of core angular momentum. This study supports the notion that multidecadal variability in the Earth's core may affect both solid Earth rotation through angular momentum exchange with the mantle, and surface temperature through modulation of terrestrial magnetic shielding of charged particle fluxes into the atmosphere.