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Page 1
Executive Summary
Is the Sun an agent of global change? Whether variable energy
inputs from the Sun have anything to do with the Earth's weather
and climate has been debated contentiously for more than a century.
In 1982 a National Academy of Sciences Panel on Solar Variability,
Weather and Climate studied the issue in detail, concluding that
it is conceivable that solar variability plays a role in
altering weather and climate at some yet unspecified level of
significance. In the decade since, monitoring of the Sun
and the Earth has yielded new knowledge essential to this debate.
There is no doubt that the total radiative energy from the Sun that
heats the Earth's surface changes over decadal time scales as a
consequence of solar activity. Observations indicate as well that
changes in ultraviolet radiation and energetic particles from the
Sun, also connected with solar activity, modulate the layer of
ozone that protects the biosphere from solar ultraviolet radiation.
This report reassesses solar influences on global change in the
light of this new knowledge of solar and atmospheric variability.
Moreover, the report considers climate change to be encompassed
within the broader concept of global change; thus the biosphere is
recognized to be part of the larger, coupled Earth system.
Implementing a program to continuously monitor solar irradiance
over the next several decades will provide the opportunity to
estimate solar influences on global change, assuming continued
maintenance of observations of climate and other potential forcing
mechanisms (e.g., greenhouse gases, aerosols, clouds, ozone). In
the lower atmosphere, an increase in
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Page 1
Executive Summary
Is the Sun an agent of global change? Whether variable energy
inputs from the Sun have anything to do with the Earth's weather
and climate has been debated contentiously for more than a century.
In 1982 a National Academy of Sciences Panel on Solar Variability,
Weather and Climate studied the issue in detail, concluding that
it is conceivable that solar variability plays a role in
altering weather and climate at some yet unspecified level of
significance. In the decade since, monitoring of the Sun
and the Earth has yielded new knowledge essential to this debate.
There is no doubt that the total radiative energy from the Sun that
heats the Earth's surface changes over decadal time scales as a
consequence of solar activity. Observations indicate as well that
changes in ultraviolet radiation and energetic particles from the
Sun, also connected with solar activity, modulate the layer of
ozone that protects the biosphere from solar ultraviolet radiation.
This report reassesses solar influences on global change in the
light of this new knowledge of solar and atmospheric variability.
Moreover, the report considers climate change to be encompassed
within the broader concept of global change; thus the biosphere is
recognized to be part of the larger, coupled Earth system.
Implementing a program to continuously monitor solar irradiance
over the next several decades will provide the opportunity to
estimate solar influences on global change, assuming continued
maintenance of observations of climate and other potential forcing
mechanisms (e.g., greenhouse gases, aerosols, clouds, ozone). In
the lower atmosphere, an increase in
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solar radiation, like the greenhouse gas increase, is expected
to cause global warming. In the stratosphere, however, the two
effects produce temperature changes of opposite sign. A monitoring
program that would augment long term observations of tropospheric
parameters with similar observations of stratospheric parameters
could separate these diverse climate perturbations and perhaps
isolate a greenhouse footprint of climate change. Monitoring global
change in the troposphere is a key element of all facets of the
United States Global Change Research Program (USGCRP), not just of
the study of solar influences on global change. The need for
monitoring the stratosphere is also important for global change
research in its own right because of the stratospheric ozone
layer.
There are no firm plans at present to implement the primary
recommendation of this report, a program of continuous monitoring
of solar irradiance to provide the data needed to diagnose and
interpret solar influences on climate change. Because current solar
radiometric techniques are insufficiently accurate, ensuring data
continuity over many decades will require a series of space based
observations with sufficient temporal overlap for calibration
transfer and prevention of data loss from instrument failure. This
measurement program may well be precluded by the dearth of access
to space.
Scientific Conclusions
Q: Do solar variations directly force global surface
temperature?
A: Yes.
Inexorable change is predicted for the biosphere, that sphere of
the terrestrial global environment where life exists. It is
imperative to reliably detect, understand, and predict climate
change arising from increasing greenhouse gases and aerosols in the
Earth's atmosphere. This requires that natural climate forcing,
particularly solar variability, also be detected and understood. In
the study of solar influences on global change, determining the
extent to which solar influences modify global surface temperature
is a matter of the highest priority.
Energy from the Sun sustains life on Earth. By far the dominant
energy input is the visible solar radiation that heats the Earth's
land
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surfaces and the oceans. The atmospheric composition and the
distribution of oceans and land masses combine with the solar
energy input to determine the radiative balance, and hence the
climate, of the Earth's biosphere. A change,
DS W/m2, in solar radiation
received by the Earth causes climate forcing of 0.7DS/4 W/m2
which perturbs directly the equilibrium global temperature by an
amount DT=l-1(0.7DS/4)°C where the climate sensitivity,
l-1, is
currently estimated to be in the range 0.3 to 1.0°C/(W/m2) (e.g., Wigley and Raper, 1990). While
the primary radiative perturbation of ± 3 percent is
indisputably the changing insolation generated by the annual cycle
of the Earth's elliptical orbit around the Sun, intrinsic changes
in the Sun's radiative output also occur on decadal and possibly
longer time scales.
The Sun's total radiative input to Earth decreased by about 0.1
percent during 1980–1986 then increased by about the same
amount during 1986–1990 (Willson and Hudson, 1991). A 0.1
percent solar irradiance change produces a climate forcing of 0.24
W/m2. For comparison, the climate
forcing by increasing greenhouse gases from 1980 to 1986 was about
0.25 W/m 2 (Hansen et al., 1990).
Concomitant increases in atmospheric aerosols may have reduced the
net anthropogenic climate forcing to almost half that arising from
greenhouse gases alone (Hansen et al., 1993). Thus, during the
recent descending phase of the 11-year solar activity, solar
forcing canceled much of the net anthropogenic forcing.
The climate system's response to various forcings depends on the
history, altitude, and latitude of the forcing and the climate
sensitivity, l-1. While the change in equilibrium
global surface temperature associated with a steady climate forcing
of 0.24 W/m2 is estimated to be in
the range 0.1° to 0.2°C, the transient response to a
periodic 11-year forcing of the same magnitude is assumed to be
much less than the equilibrium response because the response time
of the climate system is of the order of decades or more (Hansen
and Lacis, 1990). However, the true extent to which the climate
system's response diminishes or amplifies solar forcing compared
with anthropogenic forcings is uncertain.
As records of paleoclimate and historical solar activity have
improved, the possibility that variations in solar radiative
forcing played a role in past climate change continues to be raised
(see, for example, The Royal Society, 1990). There is now clear
corroborating evidence from 14C in
tree rings and 10Be in ice cores
that solar activity during past millennia
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exhibited a series of minima, each of 40 to 100 years duration,
roughly every 200 to 210 years, and that these minima appear to be
associated with colder-than-average global temperatures on Earth
(Eddy, 1976; Wigley and Kelly, 1990). The coincidence of the Sun's
Maunder Minimum with the lowest temperatures of the Little Ice Age
is the best documented of such associations in the recent past.
That a physical mechanism might be responsible for the
similarity of the historical climate and solar activity records has
become more plausible because of observational proof that the Sun's
radiative output varied throughout the only 11-year activity cycle
during which it has thus far been monitored (Willson and Hudson,
1991). Furthermore, circumstantial evidence suggests that solar
irradiance variations may not be limited to the 0.1 percent change
detected by contemporary solar monitoring. Observations of Sun-like
stars indicate a greater range of activity levels than yet detected
in the contemporary Sun (Baliunas and Jastrow, 1990; Lockwood et
al., 1992). Also, decreases in irradiance in the range 0.2 percent
to 0.3 percent, consistent with the stellar data, are simulated for
the Sun's Maunder Minimum by altering the distribution of magnetic
features in the solar atmosphere, known to cause much of the
11-year cycle change, within limits defined by independent,
spatially resolved solar observations (White et al., 1992; Lean et
al., 1992a).
Taken collectively, the above evidence, although circumstantial,
does suggest that solar variability could influence future global
change, which requires that solar irradiance be properly monitored,
understood, and, if possible, predicted. Lack of knowledge of solar
influences will limit the certainty with which anthropogenic
climate change can be detected. But it is unlikely that solar
influences on global change will be comparable to the expected
anthropogenic influences. Were solar irradiance to decrease by 0.25
percent over the next 200 years, a value speculated for the Maunder
Minimum, the equilibrium global surface temperature is estimated by
a general circulation model to decrease 0.46°C (Rind and
Overpeck, 1993). This decrease would be too small to offset
greenhouse forcing which, by the mid-twenty first century, is
expected to have caused a global temperature increase in the range
1.5 to 4.5°C.
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Q: Do solar variations modify ozone and the middle
atmosphere structure?
A: Yes.
The biosphere, a fragile region in the troposphere where weather
and climate are experienced, is protected from solar ultraviolet
radiation by a layer of ozone that resides about 30 km above it in
the Earth's middle atmosphere. The middle atmosphere is significant
for global change not just because of the ozone layer embedded in
it; this region is also the upper boundary of the troposphere.
Changes in the middle atmosphere, which are known to occur in
response to variable solar energy inputs, are suspected of
impacting the weather and climate. Determining the extent to which
solar variability modifies ozone and the middle atmosphere is
therefore the second highest priority in the study of solar
influences on global change.
The Earth's middle atmosphere absorbs the Sun's ultraviolet (UV)
radiation. Were this radiation able to penetrate to the biosphere,
it would damage life on Earth; instead it creates our protective
ozone shield. Ozone forms when solar UV radiation (at wavelengths
less than 242 nm) dissociates molecular oxygen into oxygen atoms
that combine with molecular oxygen to make ozone. Extending outward
from the Earth's surface to about 100 km, the ozone layer has its
peak concentration at about 30 km. The Sun's UV radiation also
creates many of the radical species that subsequently destroy
ozone. Most notably, the chlorine (Cl) atom is a product of UV
photodissociation of chlorofluorocarbons (CFCs) that have risen to
the lower stratosphere following their release near the Earth's
surface. Ozone is also destroyed by solar radiation at longer
wavelengths and by catalysts produced by energetic particle
precipitation. Both the solar ultraviolet radiative energy and the
energetic particle output are modulated by solar activity.
The Sun's UV radiation is an order of magnitude more variable
than the visible solar radiation that penetrates to the Earth's
surface, and these variations generate natural changes in the ozone
layer. Specifying natural ozone variability is essential for
untangling anthropogenic effects in the long term ozone record.
Observational studies (Stolarski et al., 1991; Hood and McCormack,
1992; Randel and Cobb, 1994) signify the response of ozone to solar
forcing. From 1986 to 1990 the increase in
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solar UV radiation in concert with the Sun's 11-year activity
cycle is estimated to have increased global total ozone by about
1.8 percent. This approximately offset the suspected anthropogenic
decrease of 1.35 percent over the same period (-0.27 percent/year).
Other studies suggest that changes in the precipitation of
relativistic electrons that penetrate into the middle atmosphere
may also play a role in natural ozone variations (Callis et al.,
1991).
Current atmospheric loading of chlorine and other anthropogenic
radicals is expected to deplete global ozone until the beginning of
the twenty-first century. Elimination of CFCs is expected to
reverse this downward trend. Determining whether an observed ozone
recovery in the twenty-first century is the consequence of
successful CFC mitigation or, instead, of increased solar activity
will require continuous, reliable monitoring of solar energy inputs
to the middle atmosphere. As well as the UV irradiance, sporadic
solar influences, such as energetic particles that may destroy
ozone for periods of days to many months, must also be understood.
This has been clearly demonstrated by a series of high surges of
solar activity throughout 1989 (near the peak of the current
activity cycle) that may have depleted ozone in the Antarctic
(Stephenson and Scourfield, 1991) and at lower latitudes (Reid et
al., 1991).
Q: Do solar variability effects in the Earth's upper
atmosphere couple to the middle atmosphere and the
biosphere?
A: Possibly.
The Earth's lower and middle atmospheres are surrounded by the
neutral and ionized medium of the upper atmosphere and its embedded
ionosphere, which shelters the biosphere from highly energetic,
dramatically varying solar radiation and particles. In the upper
atmosphere, temperature, density, and winds are highly responsive
to variations in solar energy input. Furthermore, adjacent layers
of the Earth's atmospheric envelope are intimately connected.
Highly variable solar inputs to the Earth's upper atmosphere in
the form of energetic photons at wavelengths of less than 180 nm
and energetic particles cause the global mean exospheric
temperature of the thermosphere to vary by about 700 K, from about
600 K during solar cycle minimum to about 1300 K during solar cycle
maximum. Physical
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processes by which this profound solar influence on the Earth's
upper atmosphere might impact the biosphere are not established,
but radiative, chemical, dynamical, and electrical mechanisms have
been identified that couple the upper atmosphere with the middle
and lower atmospheres. For example, large variations in both solar
high energy photons (X-rays and extreme ultraviolet radiation) and
energetic particles initiate significant changes in upper
atmosphere odd-nitrogen, which can destroy ozone if transported
down to the high-latitude middle atmosphere by the mean circulation
pattern (e.g., Huang and Brasseur, 1993). Also, the ionosphere is
connected to the troposphere via the global electric circuit.
Solar influences on the upper atmosphere do affect our society
in other ways. Activities related to navigation and rescue,
defense, and communication rely increasingly on spacecraft
technology. Solar energy inputs control the state of the Earth's
upper atmosphere where spacecraft orbit, and efficient use of space
requires operational understanding of the variability of the upper
atmosphere, a part of the global Earth system that is highly
sensitive to solar forcing.
In the context of global change within the upper
atmosphere, knowledge of solar forcing is essential. Since the
emission of carbon dioxide infrared radiation is the dominant
cooling mechanism throughout the mesosphere and lower thermosphere,
releases of trace greenhouse gases from human activity potentially
could cause significant changes in the structure of the Earth's
upper atmosphere. Whereas the greenhouse effect will cause the
troposphere to warm by a few degrees, the global mean thermosphere
has been predicted to cool by as much as 50 K in response to
projected doublings of carbon dioxide (CO2) and methane (CH4) concentrations from present levels
(Roble and Dickinson, 1989). Accompanying redistributions of major
and minor constituents may decrease satellite drag by up to 40
percent; affect the propagation of atmospheric tides, gravity
waves, and planetary waves into the thermosphere from the lower
atmosphere; modify the thermospheric circulation; and change the
electrodynamic structure. Such anthropogenic effects on the upper
atmosphere will be superimposed on large natural variability caused
by solar forcing. It is unknown whether these anthropogenic changes
could alter the couplings between the upper, middle, and lower
layers of the atmosphere. Monitoring the upper atmosphere in the
light of its natural variability is therefore important.
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Q: Do solar variability effects in the Earth's near-space
environment couple to the biosphere?
A: We don't know.
Surrounding the Earth and its atmosphere is the geospace
environment of the magnetosphere, composed of solar and space
plasmas and energetic particles. Shaped primarily by the Earth's
magnetic field and its interaction with the solar wind, the
magnetosphere is the primary receptor of the highly variable mass,
momentum, and energy from the solar wind. It is tightly coupled to
the upper atmosphere and is also involved in the global electric
circuit. Any study of solar influences on global change must
therefore consider potential coupling to the biosphere.
The relatively self-contained magnetosphere extends from the
upper atmosphere to altitudes of about 10 Earth radii on the Sunlit
side of the Earth and to more than 1000 Earth radii on the
nightside. The global topology of this region is organized by the
dipole magnetic field intrinsic to the Earth, which extends far
into space and serves to deflect the onrushing plasma, or solar
wind, that emanates from the solar corona. The solar wind flows
continually over, around, and into the terrestrial magnetosphere,
and in so doing continually imparts mass, momentum, and energy to
the system. The added energy must then be dissipated either
continuously or sporadically. An example of such dissipation is
geomagnetic storms, major disturbances in the magnetosphere that
manifest themselves by large variations (for periods of hours to
days) in the magnetic (and electric) fields surrounding the
Earth.
Whether solar forcing of the Earth's near-space environment
couples through the upper atmosphere to the biosphere in a way that
would be important for global change remains unknown. The extent to
which energetic particles couple into the Earth's system depends on
the geospace medium through which they must travel. High energy
solar protons have been observed to modify ozone concentrations in
the middle atmosphere; relativistic electrons precipitating from
the magnetosphere may also play a role (Baker et al., 1987). Human
activities, such as navigation and resource exploration, can be
significantly affected by magnetic field variations associated with
geomagnetic storms. In particular, bursts of
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geomagnetic activity precipitated by solar events can induce
current surges that may disable power grids, as was the case in
March 1989 (Allen et al., 1989).
Q: Do we need to improve our knowledge of the variable Sun
to understand and predict solar influences on global
change?
A: Yes.
Solar variability potentially can influence global surface
temperatures and middle atmosphere ozone concentrations. The goal
of ultimately predicting this influence makes it essential that we
learn how and why the Sun varies as it does. Relatively high
priority must be given to acquiring knowledge of the origin of the
solar energy variations that force global change in the Earth's
lower and middle atmospheres.
Our Sun is one of many variable stars in the cosmos. Changes in
both its radiative and particle outputs originate in what is
actually rather common stellar behavior: a cycle in the emergence
of magnetic activity with, in the case of the Sun, a period of 11
years. It is the Sun's magnetic flux that generates the dark
sunspots and bright faculae that modulate total solar irradiance,
providing radiative forcing of climate change. Extensions of
magnetic active regions into higher layers of the Sun's atmosphere
are enhanced in shorter wavelength, higher energy radiation. The
appearance and disappearance of bright active regions throughout
the 11-year activity cycle control the solar radiative output
variations that perturb the Earth's ozone layer and also, more
dramatically, the upper regions of the Earth's atmosphere.
Energetic particles traveling from the Sun to the Earth are guided
by lines of the solar and terrestrial magnetic fields, while the
solar wind continually transports plasmas and magnetic fields to
the Earth's near-space environment.
For studying solar influences on global change, a continuous
record of the variable energy input that reaches the Earth from the
Sun is essential. The observational record is, however,
intermittent and extends over only a few solar cycles. Direct
measurements, which must be made from space, are difficult and
frequently devalued by instrumental uncertainties. Ideally, our
record of the Sun's variable energy input to the Earth should
extend over all possible time scales and be predictable into the
future. It will ultimately be extended through reliable
understanding of how and why
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the Sun, as a star, varies as it does. This knowledge is needed
to unravel possible solar forcing in the paleoclimate record and to
assess future solar forcing of global change. It may be obtained
through analysis and interpretation of solar images and surrogates
that connect the changes in global solar energy output to the
fundamental physical parameter changes underlying all solar
variability, the magnetic field.
Records of solar activity during the past few thousand years can
serve as surrogates for solar energy inputs to Earth, providing the
physical connections are adequately understood. Preliminary
analysis of these records indicates that extrema of solar radiative
output variations may indeed have been larger than the changes
during the few recent cycles of activity for which direct
measurements exist. Over time scales of stellar evolution,
observations of Sun-like stars can help to provide limits on solar
variations that might have occurred in the past and may be expected
in the future. We cannot presume from our limited monitoring of the
contemporary Sun over little more than a decade, and during an
epoch of relatively high solar activity, that we have yet sampled
the range of variability of which the Sun is capable. But we must
nevertheless comprehend this variability to reliably determine
solar influences on global change.
Recommendations
The highest priority and most urgent activity for determining
solar influences on global change is to:
1.
Monitor the total and spectral solar irradiance from an
uninterrupted, overlapping series of spacecraft radiometers
employing in-flight sensitivity tracking.
So that the long term value of present solar monitoring is not
lost, adequate temporal overlap to permit cross-calibration with
future observations is critical. This goal must be achieved in an
era of decreasing access to space.
In addition, the following activities will be needed to properly
monitor, understand, and predict solar influences on global
change.
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Pursuit of recommendations 2 to 6 is essential to the
interdisciplinary research effort needed to provide an adequate
scientific basis for global change policymaking. The actions of
recommendations 7 to 12 are essential to ensure a complete
understanding of all potential coupling mechanisms.
2.
Conduct exploratory modeling and observational studies to
understand climate sensitivity to solar forcing.
3.
Understand and characterize, through analysis of solar
images and surrogates, the sources of solar spectral (and hence
total) irradiance variability.
4.
Monitor, without interruption, the cycles exhibited by
Sun-like stars and understand the implications of these
observations for long term solar variability.
5.
Monitor globally, over many solar cycles the middle
atmosphere's structure, dynamics, and composition, especially ozone
and temperature.
6.
Understand the radiative, chemical, and dynamical
pathways that couple the middle atmosphere to the biosphere, as
well as the middle atmosphere processes that effect these
pathways.
7.
Monitor continuously, with improved accuracy and long
term precision, the ultraviolet radiation reaching the Earth's
surface.
8.
Understand convection, turbulence, oscillations, and
magnetic field evolution in the solar plasma so as to develop
techniques for assessing solar activity levels in the past and to
predict them in the future.
9.
Monitor continuously the energetic particle inputs to the
Earth's atmosphere.
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10.
Monitor the solar extreme ultraviolet spectral irradiance
(at wavelengths less than 120 nm) for sufficiently long periods to
fully assess the long term variations.
11.
Monitor globally over long periods the basic structure of
the lower thermosphere and upper mesosphere so as to properly
define the present structure and its response to solar forcing.
12.
Conduct observational and modeling studies to understand
the chemical, dynamical, radiative and electrical coupling of the
upper atmosphere to the middle and lower atmospheres.
Representative terms from entire chapter:
solar influences
