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WIREs Clim Change
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Cycles and trends in solar irradiance and climate

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How—indeed whether—the Sun's variable energy outputs influence Earth's climate has engaged scientific curiosity for more than a century. Early evidence accrued from correlations of assorted solar and climate indices, and from recognition that cycles near 11, 88 and 205 years are common in both the Sun and climate.1, 2 But until recently, an influence of solar variability on climate, whether through cycles or trends, was usually dismissed because climate simulations with (primarily) simple energy balance models indicated that responses to the decadal solar cycle would be so small as to be undetectable in observations.3 However, in the past decade modeling studies have found both resonant responses and positive feedbacks in the ocean‐atmosphere system that may amplify the response to solar irradiance variations.4, 5 Today, solar cycles and trends are recognized as important components of natural climate variability on decadal to centennial time scales. Understanding solar‐terrestrial linkages is requisite for the comprehensive understanding of Earth's evolving environment. The attribution of present‐day climate change, interpretation of changes prior to the industrial epoch, and forecast of future decadal climate change necessitate quantitative understanding of how, when, where, and why natural variability, including by the Sun, may exceed, obscure or mitigate anthropogenic changes. Copyright © 2010 John Wiley & Sons, Ltd.

Figure 1.

Shown in the upper panel is a record of total solar irradiance obtained as an average of three different observational composites. In the second panel are irradiance variations estimated from an empirical model that combines the two primary influences of facular brightening and sunspot darkening. The symbols indicate direct observations made by the TIM instrument of the SORCE mission, used to determine the relative sunspot and facular components in the model, shown separately in the middle panel. Annual mean sunspot numbers shown in the bottom panel indicate overall levels of solar activity in cycles 21, 22 and 23, with times of minima indicated by the dashed lines. [Ref 57 provides details and sources of the various time series].

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Figure 2.

Compared in (a) are observed monthly mean global temperatures (black) and an empirical model (orange) that combines four different influences. In (b) the individual contributions of these influences are shown, namely ENSO (purple), volcanic aerosols (blue), solar irradiance (green) and anthropogenic effects (red). Together the four influences explain 76% (r2) of the variance in the global temperature observations. Future scenarios are shown as dashed lines. Anthropogenic and solar contributions are extended into the future using knowledge of past behavior, assuming the same linear trend for anthropogenic gases of the recent past and future irradiance cycles similar to cycle 23. In lieu of the ability to forecast ENSO and volcanic activity, possible scenarios are based on past events [from Ref 52].

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Figure 3.

Geographical response patterns in surface temperature are shown for the 1997–98 ‘super’ ENSO, the Pinatubo volcano, solar cycle 23 and anthropogenic influences from 1980 to 2006, derived from the monthly surface temperatures on a 5” × 5” grid. Gray regions indicate lack of data [based on Ref 22].

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Figure 4.

Compared in the upper panel are the observed monthly mean global temperatures (black) of Earth's lower and middle tropospheres, and lower stratosphere with empirical models (orange) that combine four different influences, shown separately in the bottom four panels. Each of the four different sources contributes different modulation of Earth's atmospheric temperature at different heights above the surface. Relative strengths are deduced from multiple regression analysis. The ENSO, volcanic, solar and anthropogenic time series are described in Ref 22. The National Oceanic and Atmospheric Administration produced the Quasi Biennial Oscillation time series, available at http://www.cpc.noaa.gov/data/indices/. The Microwave Sounding Unit measured the temperature anomaly data,58 which are available at http://vortex.nsstc.uah.edu/data/msu/.

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Figure 5.

Reconstructions of total solar irradiance with different assumptions about the strength of the background component that underlies the activity cycle are compared since the Maunder Minimum.[57 provides details].

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Figure 6.

Compared with the CRU monthly mean global temperature time series (hadcrut3vcgl) in (a) is an empirical mean global temperature time series (hadcrut3vcgl) is an empirical model obtained from multiple regression for the period from 1889 to 2008, inclusive.22 The value of r is the correlation coefficient for the global temperature observations and empirical model. Reconstructions of the contributions to the monthly mean global surface temperatures by individual (b) ENSO, (c) volcanic, (d) solar, and (d) anthropogenic influences (at appropriate lags) are also shown. The periodograms on the right illustrate cycles present in the monthly mean values of each of the four sources of global temperature variance.

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Figure 7.

Superimposed on the geographical response pattern of observed surface temperature to solar irradiance changes in cycle 23 (Figure 3) are the responses evident in published analyses of paleoclimate records from specific sites. Gray regions indicate lack of data.

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