Geoscience Reference
In-Depth Information
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Background
THE POTENTIAL SUN-CLIMATE CONNECTION
The Sun varies over a broad span of timescales, from its brightening over its lifetime to the
fluctuations commonly associated with magnetic activity over days to years. The latter activity includes
most prominently the 11-year sunspot cycle and its modulations. The September 2011 workshop
summarized in this report explored the connection between this kind of activity and Earth's climate
variability.
Research on the connection between solar magnetic activity and Earth's climate spans the
subdisciplines involved in heliophysics, climate science, and the science and engineering of space-borne
observatories. Intended to briefly introduce the workshop's topics, the background information provided
here is not a product of the workshop itself.
Variations in the total solar irradiance (broad spectral band irradiance: TSI) incident on Earth's
atmosphere can cause imbalances in Earth's radiation budget that can induce temperature shifts near the
surface. The temperature of Earth can be understood to a first approximation as controlled by the balance
between the radiative energy received from the Sun and Earth's thermal emission of radiative energy to
space.
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Thermal emission increases with increasing temperature, and Earth can be thought of as settling
into an equilibrium by adjusting its temperature so that this thermal radiation balances the solar energy
absorbed by the planet. An increase or a decrease in the TSI is expected on this basis to increase or
decrease the temperature of Earth. For example, the TSI changes over an 11-year cycle in step with the
cycle of sunspots with an amplitude of nearly 0.1 percent, and this variation's small effects (perhaps an
amplitude of a few hundredths of a degree centigrade) on temperatures can be detected, albeit with
considerable imprecision, in climate records.
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The sunspot cycle amplitude varies, and during the 17th century the Sun was virtually without
spots for about 70 years. This observation has bolstered research on the theoretical underpinning of the
solar cycle and the roles played by different features of the Sun's face (photosphere). These features
include the sunspots themselves, the faculae (bright regions surrounding the spots), and the network of
magnetic field features over the Sun's surface.
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Interest is focused on the quantitative relationship
between these features and the TSI.
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How can the TSI be extrapolated to past (or future) periods when
the Sun appeared to be (or might be) more or less active than now? Related studies include the
