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1—
Introduction
The Coupled Sun-Earth System
The Earth environment as we know it exists because of the energy
it receives from the Sun. Radiant energy from the Sun powers the
atmospheric and oceanic circulations that profoundly influence the
state of the biosphere. Without solar radiation, photosynthesis
would cease. Solar radiation and high energy particles impinge
continually on the envelope of gases and plasma that surrounds and
protects the narrow habitable layer of the Earth's surface. Changes
in the amount of solar energy input to the total Earth system are
caused by three main mechanisms: i) geometric factors related to
the Earth's inclination and orbit around the Sun (which alter the
distribution of radiation incident on the Earth), ii) processes in
the Earth system itself (which regulate the amount of energy
received by the Earth), and iii) variations in the activity of the
Sun (which modulate the energy emitted by the Sun).
Geometric relationships modulate solar inputs to the Earth. The
seasonal progression of weather is controlled by the tilt of the
Earth's axis of rotation relative to the direction normal to the
Earth's orbital plane and by orbital eccentricity and precession.
In addition, small periodic variations in the Earth's orbital
parameters over time scales of tens of thousands of years
(Milankovitch cycles) along with associated feedbacks
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and possible carbon dioxide changes are believed to cause
significant variations in the Earth's climate.
Processes within the Earth system regulate the solar energy
inputs through numerous feedback mechanisms that influence the
greenhouse warming of the Earth. Some of these feedbacks include
variations in cloudiness and ice cover that determine the planetary
albedo and hence affect the portion of the incoming solar radiation
that is available to the Earth system.
Variations in solar energy related to the activity of the Sun
can also generate natural changes in the Earth system: assessing
the extent of this latter effect is the topic of this report.
There is no doubt that solar variability alters the energy input
to the global Earth system, which is considered here in the
broadest sense to extend from the biosphere, where weather and
climate are experienced, to the Earth's near-space environment,
some 1000 km above. Both the short-wavelength ultraviolet (UV)
radiation and the solar wind and energetic particles from the Sun
undergo large changes related to the presence of active regions in
the solar atmosphere. These changes cause dramatic variability in
the Earth's upper atmosphere, ionosphere, and magnetosphere. Only
recently have spacecraft observations revealed that small
variations (about 0.1 percent) also occur in the total
electromagnetic energy radiated by the Sun. These radiative
variations are also connected to the presence of active regions in
the solar atmosphere (dark sunspots and bright faculae), and they
occur on all time scales observed thus far, from minutes to the
Sun's 11-year activity cycle.
The spectrum of the radiant energy incident on the top of the
Earth's atmosphere and the change in this radiation during the
solar activity cycle are shown in Figure 1.1. Some of the Sun's
radiant energy is reflected back into space by the Earth's surface,
by clouds, and by aerosols; the remaining portion is absorbed by
the Earth's surface and within the Earth's atmosphere. Figure 1.2
illustrates the altitude of unit optical depth. This is the mean
altitude at which solar spectral energy is reduced by the Earth's
atmosphere to roughly 1/e of its value at the top of the
atmosphere. This curve is determined by the concentrations of
radiatively absorbing gases in the Earth's atmosphere. Figures 1.1
and 1.2 indicate that the more variable, shorter wavelength solar
energy is absorbed at higher altitudes in the atmosphere. Radiation
at wavelengths shorter than
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Figure 1.1 (a) The Sun's spectral irradiance
(solid line, typical of solar minimum conditions) is compared with
the spectrum of a black body radiator at 5770 K (dashed line). The
broad spectral bands identified along the top of this figure are
the ultraviolet (UV), visible (VIS), and infrared (IR). Not shown,
at wavelengths longer than the IR, is the microwave or radio
portion of the solar spectrum. (b) Approximate amplitude of the
Sun's spectral irradiance variation from the maximum to minimum of
the 11-year activity cycle. The solar cycle variation in the
spectrally integrated, or total, solar irradiance is indicated by
the dot-dash line. From J. Lean, Reviews of Geophysics, 29, 506,
1991, copyright by the American Geophysical Union.
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Figure 1.2 Shown on the left is the altitude at
which the solar irradiance is attenuated by 1/e (unit optical
depth) in the Earth's atmosphere, for an overhead Sun. Also
indicated are the primary atmospheric absorbing species of the
radiation within different spectral bands and the wavelength
regions that dominate ozone production and absorption. Adapted from
Meier (1991). Shown on the right is the standardized temperature of
the Earth's atmosphere from the surface to 250 km. Atmospheric
regions, called spheres, are defined by boundaries based on
inflections in the temperature profile (at approximately 15 km, 50
km, and 100 km) determined largely by solar radiative heating
through gaseous absorption. Reprinted by permission of Kluwer
Academic Publishers.
about 160 nm is mostly absorbed above about 100 km (in the
thermosphere), where solar variability generates temperature
variations of hundreds of degrees. Solar radiation at wavelengths
from about 150 to 310 nm is absorbed primarily in the middle
atmosphere, which is conventionally defined as being that
atmospheric region from about 15 to 100 km, between the troposphere
and the thermosphere.
Deposition of the Sun's energetic particle input to the global
Earth system is more complicated. Here, interactions with the
Earth's magnetic field are important since the charged particles in
the solar wind are guided
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Representative terms from entire chapter:
solar variability
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along magnetic field lines. Thus, low latitudes are shielded
from much but not all of the incoming charged particles, with most
of the energetic particles being guided into the Earth's atmosphere
in the polar regions.
Numerous statistical studies have reported variations in
atmospheric and hydrospheric parameters that are attributed to
solar variability effects on many time scales. Generally speaking,
it is much easier physically to relate variations in the Earth's
upper atmosphere to known solar activity than is the case for the
lower atmosphere and hydrosphere. This is because those energetic
inputs from the Sun that show the largest amount of variability
(associated with higher energy photons, solar wind, and energetic
particles) are usually absorbed in the Earth's upper atmosphere.
Solar forcing of the upper atmosphere is thus well recognized and
has been verified by the agreement between atmospheric observations
and theoretical assessments of the upper atmosphere's response to
known solar energy inputs.
Although energetically viable mechanisms for significant solar
variability influences on the lower atmosphere and surface of the
Earth have yet to be identified, some very interesting associations
between solar variability and weather and climate have been
suggested. Among these are the cited coincidence between the time
of the Maunder Minimum (1645 to 1715) in sunspot activity and the
coldest temperatures of the Little Ice Age Figure 1.3). Another
example is the recent work by Labitzke and van Loon (1990, 1993)
which suggests an association between solar activity, as measured
by the 10.7 cm solar radio flux, and atmospheric temperature
changes, with an important role being played by the phase of the
quasibiennial oscillation (QBO) in the zonal wind in the tropical
lower stratosphere.
The main problem in quantitatively explaining statistical
associations between solar variability parameters and sizable
climate and weather effects is that the amount of energy in
variable solar energy inputs is small compared both to the incoming
solar energy itself and to lower atmosphere energetics. Table 1.1
compares the magnitudes of various solar and magnetospheric energy
inputs to the Earth system. The total solar radiative energy input
per unit area is about 1368 Watts per square meter (W/m2) in an averaged sense. Observed
11-year cycle variations in total solar irradiance (often referred
to as the solar ''constant") are about 1.3 W/m2. This is two orders of magnitude
larger than the averaged soft
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Figure 1.3 Relationship between winter severity
in Paris and London (top curve) and long-term solar activity
variations (bottom curve). The shaded portions of this curve denote
the times of the Spörer and Maunder minima in sunspot
activity. The dark circles indicate naked-eye sunspot observations.
Details of the solar activity variation since 1700 are indicated in
the bottom curve by the sunspot number data. The winter severity
index has been shifted 40 years to the right to allow for cosmic
ray-produced 14 C assimilation
into tree rings. From J. Eddy, Science, 192, 1189, 1976, copyright
by the American Association for the Advancement of Science.
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TABLE 1.1 Comparative energy inputs from the Sun to the
Earth system and the change in these energy inputs over the 11-year
solar cycle. Also indicated are the approximate regions of the
Earth system where the energy is deposited.
Source
Energy (W/m2)
Solar Cycle Change (W/m2)
Deposition Altitude
Solar Radiation
total solar irradiance
1368
1.3
surface
UV 200–300 nm
16
0.15
0–50 km
UV 120–200 nm
0.1
0.015
50–120 km
EUV 0–120 nm
0.003
0.005
100–500 km
Particles
Solar protons
0.002
30–90 km
Proton aurora
0.001 – 0.036
90–130 km
Visual aurora
0.0006 – 0.6
90–130 km
Galactic cosmic rays
0.000007
0–90 km
Joule Heating of Thermosphere
E= 1 mV/m
0.000014
100–500 km
E=100 mV/m
0.14
100–500 km
Solar Wind
0.0003
above 500 km
Downward Heat Conduction from Magnetosphere
0.00003
above 500 km
X-ray and extreme ultraviolet (EUV) radiation inputs from the
Sun and about one order of magnitude less than the solar
ultraviolet (UV) radiation that enters into middle atmosphere ozone
photochemistry. Solar UV radiation from 200 to 300 nm is believed
to vary by a few percent, which implies that the energy associated
with changes in this radiation is a factor of 10 or so less than
that associated with total irradiance variations. Energetic
particle sources posses still less energy.
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The direct solar forcing of climate associated with the energy
changes in Table 1.1 is currently thought to be smaller than is
inferred from some of the observed statistical associations. This
makes it difficult to develop viable quantitative models, since
more complicated, indirect, amplifying or coupling mechanisms must
be invoked. Nevertheless, the more energetic photons and particles
that have the largest percentage variations are important
candidates for forcing, since they can affect the concentrations of
chemical constituents that can possibly redistribute larger amounts
of energy. Energy from the Sun, whether as photons, energetic
particles, or from solar wind-magnetosphere interactions, and
whether deposited at low or high latitudes, is eventually
distributed over the entire globe by the continuous motions of the
Earth's atmosphere and oceans. Because of this, chemical,
radiative, and dynamical perturbations generated by solar
variability may be transported to different latitudes and
altitudes; this absence of specific spatial and altitude boundaries
within the global Earth system means that direct solar forcing of
atmospheric regions remote from the biosphere may nevertheless
affect it indirectly to some extent.
Global Change Research
The goal of the United States Global Change Research Program
(USGCRP) is to establish the scientific basis for national
and international policymaking relating to natural and
human-induced changes in the global Earth system (Committee
on Earth Sciences, 1989). To achieve this goal, the committee
defined three specific objectives: i) establish an integrated,
comprehensive, long term program of documenting the Earth system on
a global scale, ii) conduct a program of focused studies to improve
our understanding of the physical, geological, chemical,
biological, and social processes that influence Earth system
processes and trends on global and regional scales, and iii)
develop integrated conceptual and predictive Earth system
models.
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Solar Influences on Global Change: A
Major
Scientific Research Element of the USGCRP
The need to understand solar variability influences in the study
of global change arises because solar-driven global change
complicates the detection, understanding, and prediction of
anthropogenic forcing. Research efforts toward achieving this
understanding might be conceptualized thus:
Monitoring : The solar energy inputs to the
Earth system must be measured continuously. Solar phenomena (e.g.,
sunspots and faculae) that are thought to affect these energy
inputs must also be measured. Changes in Earth system parameters
must be monitored so that associations can be detected.
Understanding : Once associations between
different aspects of solar behavior are established, hypotheses are
developed about their physical causes. The predictions of theories
based on these hypotheses are then tested against observations in
an effort to either prove or disprove the theories. Theories may
have to be reformulated in light of new observations. The same
intellectual process needs to be followed in formulating theories
of the response of the Earth system to variable solar energy
inputs. Agreement between theory and observation suggests that a
good level of understanding has been achieved.
Predicting : Two types of prediction are
possible. One is statistical prediction. In this case, statistical
associations are quantified by some sort of regression function
which is then used to predict the future. A deeper level of
understanding is required to make physical predictions. In this
case, quantitative physical laws are formulated and their
predictive capability is verified with retrospective data as well
as by predictions into the future.
The goal of research in solar influences on global change then
is the development of the necessary data bases and understanding of
the physical processes that lead to the ability to assess and
predict the behavior of the Sun and its influence on the Earth
system.
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Objectives of the Report
This report deliberately focuses first on the most obvious and
immediate solar forcing of that part of the Earth's environment
where life exists, where understanding solar influences on global
change is most important to human welfare and which must thus have
high priority. Chapters 2 and 3, therefore, concentrate on solar
influences on temperature and composition of the lower layers of
the Earth's atmosphere. Chapters 4 and 5 assess solar forcing of
higher atmospheric layers and of the Earth's near-space environment
and the possible coupling of this forcing to the biosphere.
Chapters 4 and 5 do not attempt an exhaustive discussion of all
solar-terrestrial connections; this is left, for the most part, to
other studies. Chapter 6 discusses knowledge of solar variability
itself. Chapter 7 covers strategies for research in solar
influences on global change, and recommendations appear in Chapter
8.
The Working Group on Solar Influences on Global Change met
twice, in November 1990 and March 1991. Since then the topic has
been the focus of three meetings: a Workshop on Solar-Terrestrial
Impacts of Global Change, sponsored by the High Altitude
Observatory in Boulder, CO, in May 1991; an international symposium
on The Sun as a Variable Star: Solar and Stellar Irradiance
Variations, International Astronomical Union, Colloquium No. 143,
in Boulder in June 1993; and a NATO Advanced Research Workshop on
The Solar Engine and its Influence on Terrestrial Atmosphere and
Climate, in Paris in October 1993. Proceedings of these three
meetings are in preparation. Significant effort has been made to
include in this report the relevant results reported at these
meetings and in the scientific literature, as of June 1994.