2O); chlorofluorocarbons (CFCs)
(Intergovernmental Panel on Climate Change, 1992). If solar
irradiance were to vary over the next century, natural climate
change might also result. Nevertheless, until recently there has
been no proof that variations in the Sun's output do in fact occur
(as evidenced by the term solar "constant", which is still in
widespread use).
Observations of total solar irradiance by spacecraft radiometers
(Willson and Hudson, 1991; Hoyt et al., 1992) have now detected
decadal variations on the order of 0.1 percent in apparent
association with the Sun's 11-year activity cycle (Figure 2.1);
larger variations, of the order of a few tenths percent, occur on
shorter time scales and are associated with the Sun's 27-day
rotation. The magnitude of the 11-year cycle effect
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FIGURE 2.1 Contemporary solar activity
variations as indicated by the sunspot number (top panel) and
changes in total solar radiative output (bottom panel) recorded by
the ERB radiometer on the Nimbus 7 satellite, ACRIM I on the SMM
satellite and ACRIM II on the UARS, and by the ERBE program (NOAA9
and ERBS). Total solar irradiance is increased during times of
maximum solar activity (e.g., 1980 and 1990) and decreased during
the intervening minimum. The differences in irradiance levels
between the different measurements are of instrumental origin and
reflect absolute inaccuracies in the measurements. Proposed future
programs to measure total solar irradiance are indicated. Courtesy
of J. Lean.
is compared in Figure 2.2 with anthropogenic radiative forcing
of climate by increased greenhouse gases and aerosols and by ozone
decreases. During the first half of the 1980s, forcing of the
climate system by declining solar radiative output was more than
sufficient to offset the estimated net anthropogenic forcing.
Despite the similarity of the climate forcings over the decadal
time scales shown in Figure 2.2, the magnitude of the climate
system's response to solar forcing could be greater or less than
its response to anthropogenic forcing. This is because the
translation of radiative forcing to surface
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FIGURE 2.2 Estimated climate forcings during the
three recent decades of the twentieth-century owing to measured
changes in greenhouse gases (solid line), net anthropogenic forcing
from greenhouse gases, aerosols, clouds and ozone changes (dotted
line), and solar irradiance variations associated with the 11-year
solar activity cycle alone (small squares). Combined greenhouse
plus solar (dashed line) and net anthropogenic plus solar (dash-dot
line) forcings are also shown. In each case, the thin lines are
projections. The solar forcing is from the empirical model of
Foukal and Lean (1990), which accounts for irradiance changes
during the 11-year cycle caused by dark sunspots and bright
faculae, but does not include additional variability sources acting
on longer time scales. Zero point of solar forcing is the
1978–1989 mean. Adapted from Hansen and Lacis (1990) and
Hansen et al. (1993). Reprinted with permission from Nature,
Copyright 1990, Macmillan Magazines Limited.
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temperature response is highly specific to the altitude,
latitude, and history of the forcing (Hansen and Lacis, 1990,
Hansen et al., 1993), contrary to the conclusions derived from
earlier general circulation model (GCM) comparisons that doubled
CO2 and a 2 percent increase in
solar irradiance have an equivalent effect (Hansen et al., 1984).
The observed irradiance changes do imply the potential for
additional solar forcing in the future, making it incumbent on
global change research to monitor, understand, and ultimately
predict solar effects on climate. It also makes more compelling the
search for a solar signature in the historical climate record.
Understanding solar influences on climate requires the
interaction of two primary research areas that are currently quite
distinct: the monitoring and assessment of solar irradiance
variations, which is reviewed first in this chapter, and the
perspective of solar variability and climate from both the
paleoclimate record and for future global change, which is
discussed subsequently. Origins of the solar radiative output
variations are addressed in the broader context of the variable Sun
in Chapter 6.
Total Solar Irradiance
Variability
Knowledge of the Sun's radiative energy output at all
wavelengths is ultimately required for global change research.
However, current capabilities for precise determination of this
variation exist only at wavelengths shorter than about 250 nm,
since at longer wavelengths the measurement uncertainties
significantly exceed the amplitudes of the solar variations, which
are thought to be less than 1 percent. Measurements of total
(spectrally integrated) solar irradiance can be made with two
orders of magnitude greater precision and currently provide the
primary record of solar radiative output variations.
Contemporary Measurements
During the first three-quarters of the twentieth century, ground
based observations were unable to detect total irradiance
variations that were unambiguously solar in origin (Frohlich, 1977;
Hoyt, 1979; Newkirk, 1983). The two principal limitations were
uncertainties due to instrument calibration and to atmospheric
interference and attenuation. However, in
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the recent decade, long term solar monitoring by calibrated
experiments flown on spacecraft (to overcome the atmospheric
effects) has succeeded in measuring solar irradiance variability on
the time scales of the Sun's 11-year activity cycle.
Launched in late 1978, and operational until 1993, the Earth
Radiation Budget (ERB) experiment on the Nimbus 7 spacecraft has
provided the longest solar irradiance data base (Hickey et al.,
1988; Hoyt et al., 1992), although its record (Figure 2.1) is
limited by the constraint that the top priority for the Nimbus
7/ERB platform was nadir-looking Earth observations, with only a
few minutes per orbit of solar observational opportunity.
Coincident with the ERB measurements over most of its lifetime are
measurements made by the Active Cavity Radiometer Irradiance
Monitor (ACRIM I), launched on the Solar Maximum Mission (SMM) in
early 1980 (Willson et al., 1981; Willson, 1984; Willson and
Hudson, 1991). This experiment was specifically designed for, and
dedicated to, long term, high precision solar total irradiance
monitoring; it ceased operation in October 1989 when the SMM
spacecraft reentered the Earth's atmosphere.
The Nimbus 7/ERB and ACRIM I results provided the first
unequivocal proof of intrinsic total solar irradiance variability,
and variations have since been detected on every observable time
scale (Figure 2.1). ACRIM's high precision is attributable to its
active, electrically self-calibrating cavity (ESCC) solar
pyrheliometers and its full-time solar pointing, which provided
large numbers of observations. Solar variations measured by ACRIM
have been corroborated by the ERB data, with the agreement between
the two independent data sets improved by accounting for
temperature dependent calibration errors and solar pointing
limitations in ERB (Hoyt et al., 1992).
The successor experiments to the Nimbus 7/ERB were the Earth
Radiation Budget Satellite (ERBS) and the Earth Radiation Budget
Experiment (ERBE) on the National Oceanic and Atmospheric
Administration (NOAA)-9 satellite (Lee III, 1990; Lee III et al.,
1994). The use of an active cavity ESCC mode for solar observations
has improved the quality of the data, but there are operational
constraints on solar viewing similar to those with Nimbus 7/ERB,
with even less frequent data acquisition opportunities. These
latter instruments operate only about every second week and
therefore have limited ability to characterize solar rotational
modulations, which occur over 27-day time scales. ERBE and
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ERBS data (Figure 2.1) both show a decline through the solar
minimum period and an increase with the increasing solar activity
of solar cycle 22. Differences do exist among the different
irradiance data bases in the rate of decrease in cycle 21, the
actual occurrence of minimum activity, and the rate of increase in
cycle 22. These differences are possibly the result of uncorrected
sensor degradation.
In September 1991, ACRIM II was launched on the Upper Atmosphere
Research Satellite (UARS). Preliminary data (Figure 2.1) indicate
that the UARS/ACRIM II irradiance measurements are systematically
lower by about 2 W/m 2 than those
of SMM/ACRIM I, whereas Nimbus 7/ERB data indicate solar variations
of only a few hundredths of a percent. It would have been
preferable to overlap the SMM/ACRIM I and UARS/ACRIM II experiments
to provide direct cross-calibration, but the UARS launch delay made
this impossible. Thus to preserve the continuity of the ACRIM solar
irradiance data base, a third party comparison between ACRIM I and
ACRIM II is needed, using the Nimbus 7/ERB or ERBE/ERBS
experiments. In the latter case, given the infrequent ERBE/ERBS
solar observations, the standard error is estimated to be some 30
times larger than a direct ACRIM I/II comparison.
The data shown in Figure 2.1 indicate that the average solar
irradiance declined systematically from 1980 until mid-1986 at a
mean rate of 0.015 percent per year. The irradiance minimum in 1986
occurs near the activity cycle minimum of September 1986 (as
indicated by the sunspot number data in Figure 2.1). The subsequent
rapid increase, corresponding to the buildup of solar activity in
solar cycle 22, becomes clearly visible in 1988, continuing to the
cycle 22 maximum. Declining values in the latter half of 1992
herald the approach of the next solar activity minimum, expected in
1995–1996. Taken together, the solar radiometer data indicate
that the amplitude of the recent 11-year irradiance cycle is about
0.1 percent, disregarding the high Nimbus 7/ERB values in the early
years of that record, where the uncertainties are large because of
the need to remove significant instrumental effects from the
measurements (see Hoyt et al., 1992).
While the ACRIM I, Nimbus 7/ERB, ERBS, and ERBE sensors indeed
show similar solar cycle variations of about 0.1 percent (aside
from the high Nimbus 7/ERB data in 1978–1979), their absolute
solar irradiance values range over some 6 W/m2, due to absolute calibration
uncertainties.
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The current inaccuracies of the total solar irradiance
measurements, which are typically ± 0.2 percent or larger
(Willson, 1984; Luther et al., 1986), are more than twice the
downward trend seen from 1980 to 1985. That the uncertainties in
measurements made by state-of-the-art solar radiometers are
significantly larger than their long term precision, and than the
changes caused by solar variability, has important consequences for
the continuation of the irradiance data base. In the absence of a
third party comparison between ACRIM I and ACRIM II, a decade of
solar monitoring would have been terminated, since the solar
radiometers lack the accuracies to measure real solar changes
smaller than a few tenths percent, twice the 11-year irradiance
cycle.
Instruments such as ACRIM and ERB record the variation in the
total electromagnetic energy from the Sun without identifying the
wavelengths of the radiation at which the variations are occurring.
About 99 percent of the total solar irradiance signal is from
radiation at wavelengths longer than 300 nm, radiation that
penetrates to the troposphere and the Earth's surface. However,
shorter wavelength, more variable solar UV radiation (Figure 1.1),
which is absorbed primarily above the trophosphere (Figure 1.2),
contributed approximately 20 percent of the decline in the total
solar irradiance from mid-1981 to 1985 (Lean, 1989). It is not
known whether the entire solar spectrum varies in phase with solar
activity, or how energy might be redistributed within the spectrum.
Percentage variations at longer wavelengths are expected to be much
smaller than those at UV wavelengths, on the order of a few tenths
percent and not necessarily in phase with the activity cycle
(Figure 1.1). However, these longer wavelength spectral irradiance
variations have yet to be observationally defined.
Implications from Observations of
Solar Surrogates
The direct correlation of solar radiative output with solar
activity over the 11-year solar cycle is a major discovery from the
ACRIM and ERB long term solar monitoring programs. Variations in
total solar irradiance occur continuously, on time scales of days
to months, in response to episodes of activity throughout the
11-year solar cycle and the modulation of active region emission by
the Sun's 27-day rotation. These variations reflect the
inhomogeneous emission of radiation on the solar disk. Solar
radiation is depleted in active region sunspots and enhanced in
active
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region faculae (Wilson et al., 1981; Sofia et al., 1982; Foukal
and Lean, 1986; Chapman et al., 1986). From the minimum to the
maximum of the 11-year activity cycle there is an increase in
active regions, both sunspots and faculae, on the solar disk. Total
solar irradiance is thought to be positively correlated with the
11-year solar activity cycle because excess facular brightness,
especially from the background active network of bright emission
outside of the largest active regions, more than compensates for
the sunspot deficit (Foukal and Lean, 1988; Willson and Hudson,
1991). Global perturbations in temperature and/ or diameter may
also be occuring (Kuhn et al., 1989; Ribes et al., 1989; Kuhn and
Libbrecht, 1991; Sofia and Fox, 1994).
To understand the forcing of the climate system by solar
irradiance changes, it is necessary to have empirical models
capable of extrapolating the radiative output variations to epochs
beyond present solar cycles. Knowing that total solar irradiance is
enhanced at times of maximum activity, and that these variations
appear to arise from the competing effects of two different types
of active regions (dark sunspots and bright faculae), suggests that
past variations may be reconstructed from historical indicators of
solar activity. Empirical parameterizations have been developed to
investigate this possibility. The most successful models (Chapter
6) are based on regressions between the ACRIM I or ERB results
(corrected for sunspot effects) with specific solar activity
indices (derived from the solar He I 1083 nm, Ca II 393.4 nm, and H
I 121.6 nm lines) that are considered better surrogates for the
total irradiance brightness source than are the classical solar
activity indicators, the Zü rich sunspot number and the 10.7
cm radio flux (Foukal and Lean, 1988; Livingston et al., 1988).
Many of the major features of the irradiance data have been
reproduced by a regression model using the equivalent width (EW) of
the solar He I line; these models do not reproduce the high levels
of irradiance measured by the radiometers in 1979– 1980 near
the maximum of solar cycle 21. Also , there are inconsistencies
between the Nimbus 7/ERB measurements and model around the time of
the cycle 22 activity maximum. Either the empirical relationships
differ between solar minimum and solar maximum, and perhaps from
one solar cycle to the next, or the irradiance observations are too
high because of instrumental artifacts.
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When empirical models of total solar irradiance variability
developed from the extant spacecraft data are extrapolated over the
past century, the long term variations arising from magnetic
sunspot and faculae features alone have been no greater than 0.1
percent (Foukal and Lean, 1990). However, as discussed below,
limits of solar variability, such as inferred from observations of
Sun-like stars, provide circumstantial evidence for a brightness
component that has been slowly increasing the total solar
irradiance since the Maunder Minimum, a time of reduced solar
activity from about 1645 to 1715. With changes in this additional
brightness component superimposed on the 11-year cycle variations,
a reduction of 0.24 percent is estimated for the Maunder Minimum,
relative to the mean of the contemporary 11-year irradiance cycle
(Lean et al., 1992a).
Solar observations made by telescopes in the seventeenth century
also suggest increased solar diameter and equatorial surface
rotation during the Maunder Minimum, compared with the modern Sun
(Eddy et al., 1976; Nesme-Ribes et al., 1993). Using apparent solar
radius as a surrogate for solar irradiance leads to speculation of
a reduction as large as 1 percent during the late seventeenth
century (Nesme-Ribes et al., 1993). In addition to uncertainties
about the amplitude of solar irradiance values in the Maunder
Minimum, there are also differences in reconstructions of the
relative temporal variations in the irradiance since then -- over
the past 300 years. While derivations based on different solar
surrogates -- such as the apparent solar radius record, the length
of the sunspot cycle, the sunspot decay rate, or the mean activity
level of the 11-year cycle -- do agree about the overall increasing
levels of solar activity during the past 300 years, phase
differences in specific episodic increases and decreases of
activity may be as large as 20 years (Hoyt and Schatten, 1993).
Geophysical Proxies
Relatively continuous, direct records of solar activity exist
only since the telescopic discovery of sunspots in the early 1600s.
For estimating changes in solar activity over the past several
thousand years, other indicators have been proposed, such as
variations in cosmogenic 14 C in
tree rings and 10 Be in ice cores
(Beer et al., 1988; Suess and Linick, 1990; Beer et al., 1991;
Stuiver and Reimer, 1993; Stuiver and Braziunas, 1993). Historical
solar activity variations inferred from these cosmogenic
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isotopes prior to the industrial era are similar (McHargue and
Damon, 1991), even though the physical connections between the
proxies and solar activity the indirect. For example the 14 C record is connected to solar
activity as follows. Changes in the solar wind in response to solar
activity variations modulate the heliospheric magnetic topology.
During times of minimum solar activity, cosmic rays are swept out
of the heliosphere less effectively by the solar wind than during
maximum solar activity. Thus at solar minima an increased flux of
galactic cosmic rays reaches the Earth's atmosphere. This leads to
increased production of 14 C,
which accumulates in the biosphere where it is available for uptake
by trees. The similarity between the recent 14 C record and the envelope of the
sunspot record of solar activity is evident in Figure 1.3. Although
many uncertainties exist in interpreting such phenomena, these
records offer the potential for gaining improved understanding of
solar behavior in the extended past, relevant to global change
issues (Wigley and Kelly, 1990; Damon and Sonett, 1991).
Evidence from Observations of Sun-Like
Stars
The sun is a rather common star, and its behavior is thought to
be typified by that of stars of similar age, mass, radius, and
composition. Routine monitoring of the activity of a selection of
Sun-like stars during the past decade has indeed revealed
rotational and activity cycles on time scales similar to those seen
in the Sun (Radick et al., 1990). Also, observations of Ca II
emission in Sun-like stars indicate that 4 out of 13 stars
monitored monthly since 1966 exhibited no activity cycle, implying
that extended periods of inactivity, as exemplified in the modern
solar record by the Maunder Minimum, may be common (Baliunas and
Jastrow, 1990). This conjecture is roughly supported by the
occurrence of minima that punctuate the
14 C geophysical record of solar activity.
In the four Sun-like stars observed to be inactive, Ca II
emissions were almost always lower than in the stars that exhibited
activity cycles (Baliunas and Jastrow, 1990). While et al. (1992)
have shown that the Sun's contemporary Ca II emission corresponds
to that of the brighter half of the cycling stars observed by
Baliunas and Jastrow (1990) and does not overlap the range of lower
Ca II emission typical of noncycling stars. Lean et al. (1992a)
investigated the implications of these stellar observations
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for the Sun's radiative output by utilizing current
understanding of the origin of the variations in total solar
irradiance and in the Ca II emission from the Sun and stars. Their
results, shown in Figure 2.3, suggest that during the Sun's Maunder
Minimum the total solar irradiance might have been about 0.24
percent less than its mean value between 1980 and 1990.
Such a decrease is consistent with inferences about the level of
solar radiative output during the Maunder Minimum reported by
Wigley and Kelly (1990) from the climate record, and also with
stellar observations that provide compelling evidence for
variabilities of 0.2 percent to 0.5 percent in the luminosity of
Sun-like stars (Lockwood and Skiff, 1990; Lockwood et al., 1992).
Foukal (1994) notes that the larger luminosity changes observed in
Sun-like stars do not necessarily imply equally larger changes in
the Sun, at least in the present epoch, since these changes are
actually consistent with current understanding of modulation by
photospheric magnetism. Also, the variability amplitudes detected
in stars likely depend on the observer's viewing angle relative to
the stellar spin axis (Schatten, 1993).
Solar Forcing of Climate Change
Variations in solar irradiance may affect the Earth's climate
through a direct influence on the global mean temperature or in
more subtle ways. The magnitude of climate change that can be
associated directly with the changes in total solar irradiance
measured during the recent solar activity cycle (about 0.1 percent,
see Figure 2.1) is small compared to past climate excursions.
Current GCMs estimate that a 2 percent increase in the solar
irradiance would produce about 4°C global warming (Hansen et
al., 1984). Assuming this result is the right order of magnitude,
and that it scales linearly, the 0.1 percent irradiance variation
observed by spaceborne radiometers in solar cycle 21 would produce
an equilibrium temperature change of about 0.2°C. However, the
change from maximum to minimum activity of the 11-year cycle occurs
over about five years, too little time to allow for full
equilibrium response of the climate system. Furthermore, where
averaged over the solar cycle, the effect is reduced by the
periodic nature of the forcing, the radiative change during the
second half effectively
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Figure 2.4 Solar variability and surface
temperature compared in the upper figure are the 11-year running
mean of the sunspot number with global average sea-surface
temperature anomalies, from G. Reid, J. Geophys. Res., 96, 2835,
1991, copyright by the American Geophysical Union. In a
one-dimensional model of the thermal structure of the ocean,
consisting of a 100m mixed layer coupled to a deep ocean, and
including a thermohaline circulation, a change of 0.6 percent in
the total solar irradiance is needed to reproduce the observed
variation of 0.4°C in the sea-surface temperature anomalies.
Compared in the lower figure are the length of the solar cycle
(plus signs) with Northern Hemisphere land temperature anomalies
(asterisks), calculated as averages over individual ''half" solar
cycles (i.e., solar maximum to solar minimum), from E.
Friis-Christensen and K. Lassen, Science, 254, 698, 1991, copyright
by the American Association for the Advancement of Science.
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time scales are somewhat smaller (Lean et al., 1992a and Figure
2.3) than the 0.4 to 1.5 percent needed to explain the paleoclimate
record. If the climate sensitivity is greater (one inference from
Milankovitch GCM studies; Rind et al., 1989; Phillipps and Held,
1994; discussed below) or the global temperature change smaller
than indicated, the required solar variability would be reduced.
Furthermore, although GCM climate simulations estimate a mean
global temperature reduction of 0.46° C for a solar irradiance
reduction of 0.25 percent (Rind and Overpeck, 1993), some regions
of the Earth's surface may cool and others warm by as much as
1° C as a result of advective changes caused by differential
heating of the land and oceans.
The problem of assessing direct solar radiative forcing of
climate change is additionally complicated because the extent to
which total solar irradiance variability arises from radiative
changes at ultraviolet rather than at visible wavelengths (Lean,
1989) determines the altitude of its direct impact on the global
system. If this impact shifts to altitudes mostly above the
troposphere, total solar irradiance forcing of surface temperature
would be reduced. On the other hand, the amplitude of irradiance
variations in the visible and infrared portions of the solar
spectrum that directly heat the surface, though thought to be small
(e.g., Figure 1.1), is not currently known.
While solar radiative changes are probably not the sole driving
force of the historical climate record, they nevertheless will need
to be understood and quantified in order to unravel the
contribution of solar forcing. Indeed, circumstantial evidence
points to a solar forcing contribution to the temperature changes
observed over the past century (Kelly and Wigley, 1992; Schlesinger
and Ramankutty, 1992) that decreases the predicted temperature
change associated with a doubling of atmospheric CO2 by nearly half (Lacis and Carlson,
1992).
From the perspective of the U.S. Global Change Research Program,
it is important to know how solar irradiance variations can be
expected to vary in the future and, in particular, the likelihood
that events such as another Little Ice Age, will occur in the
coming century. Were the only variations in solar radiative output
an 11-year cycle with peak-to-peak amplitude of about 0.1 percent,
solar forcing could be expected to modulate the net anthropogenic
climate forcing as shown in Figure 2.2. But another scenario is
that additional solar forcing might arise from
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longer-term irradiance variations superimposed on the 11-year
activity cycle, such as the speculated long term increase in
irradiance from the Maunder Minimum to the present Modern
Maximum.
Lacking a detailed modeling capability for, and adequate
knowledge of, solar processes on which to base predictions,
researchers have utilized spectral analysis to develop predictive
tools. Phenomena such as sunspot numbers have periodicities on the
order of 100, 55, and 11 years, along with the solar magnetic cycle
of 22 years (e.g., Berger et al., 1990). Ice core records as well
as other climatic data suggest periods of about 80 and 180 years
(Johnsen et al., 1970), possibly related to solar activity (Otaola
and Zenteno, 1983). Extrapolation into the future of two cycles
evident in the 14 C record, at 208
years (the Suess cycle) and 88 years (the Gleissberg cycle),
suggests that the increasing solar activity that has followed the
Maunder Minimum may continue into the early twenty-first century
(Damon and Sonett, 1991), with a decline commencing around 2040.
But extrapolation of these cycles into the future and prediction of
solar effects is a highly questionable procedure, given our lack of
knowledge of the fundamental processes involved (see Chapter
6).
Wigley and Kelly (1990) have attempted to assess limits on the
role that solar forcing of climate change may play, relative to
that of greenhouse gases, during the next 200 years. Analogous to
their approach, and consistent with their results, the predictions
shown in Figure 2.5 indicate that were the Sun to experience a
period of inactivity such as the Maunder Minimum, commencing in the
year 2000, and accompanied by reduction in its radiative output of
0.25 percent, the resultant climate forcing would indeed modulate,
but not counter, the predicted anthropogenic climate forcing. As
noted previously, determining the actual climate impact of the
forcings shown in Figure 2.5 (and Figure 2.2) is difficult because
of the specific nature expected for the climate system's response
to each of the individual forcings.
Solar Activity Cycles and the
Weather
There have been many studies of the possible relationships
between weather phenomena and the 11-year solar sunspot cycle or
the 22-year solar magnetic cycle. Summaries of the results of these
studies prior to the early 1980s have been published by Herman and
Goldberg (1978) and NAS
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FIGURE 2.5 Climate forcings determined for the
past 140 years (upper bar chart) and a scenario for future climate
forcing (lower) if anthropogenic forcing continues to increase at
its current rate of 1 W/m 2 per
140 years but is partly offset by a solar Maunder Minimum-type
event commencing in 2000, taking 200 years to develop. Courtesy of
J. Hansen, after Wigley and Kelly (1990).
(1982). While statistical relationships have in some cases been
significant, thescientific community as a whole has strongly
resisted accepting the findingsas proof of a causal relationship,
primarily because the mechanisms providingthe linkage have not been
apparent.
The subject has received new impetus in the past decade, due
both to the observation of total and ultraviolet irradiance
variations associated with the 11-year solar activity cycle and to
observations of a distinct 10-to-12-year oscillation (TTO) in
various atmospheric parameters that appear to be in phase with the
solar cycle (e.g., Labitzke and van Loon, 1990;
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1993) and related to the quasibiennial oscillation (QBO) in
tropical stratospheric winds (Figure 2.6). The connection between
the TTO and the 11-year solar cycle remains unproven and the
statistical validity of the relationship has been debated (Salby
and Shea, 1991). Nevertheless, the relationship is considered
sufficiently useful to be incorporated in techniques for seasonal
forecasting of U.S. weather (Barnston and Livezey, 1989).
Other studies have indicated correlations between solar activity
and weather phenomena even when no stratification by QBO phase is
made. For example, the mean latitude of winter storm tracks in the
north Atlantic appears to shift equatorward at times of maximum
solar activity relative to times of activity minima (Tinsley and
Deen, 1991). Periods at 11 and/or 22 years have appeared as
prominent peaks in spectral analyses of the Earth's surface
temperature (Allen and Smith, 1994), sea level pressure (Kelly,
1977), the length of the Atlantic tropical cyclone season (Cohen
and Sweester, 1975), ice accumulation data (Holdsworth et al.,
1989), drought incidence in the western U.S. (Mitchell et al.,
1979), the areal extent of North American forest wildfires
(Auclair, 1992), global northern hemisphere marine temperatures
(Newell et al., 1989), and the separation between annual dust
layers in an ice core from the Guliya Ice Cap (Thompson et al.,
1993).
However, the basic problem remains: without an understanding of
the physical causal connection, the suspicion will persist that the
results are the product of a posteriori choices (e.g., Baldwin and
Dunkerton, 1989, Salby and Shea, 1991) or are simply the product of
natural internal variability (James and James, 1989). Understanding
the implied relationships between the Sun and the weather, and the
role played by the QBO, would be of enormous benefit, both from the
practical standpoint of seasonal forecasting and by enhancing the
ability to model and deduce the sensitivity of the climate system
to a small external perturbation.
Results of a recent set of GCM studies (Rind and Balachandran,
1994; Balachandran and Rind, 1994) indicate that variations in the
middle atmosphere temperature and wind structure associated with
the QBO and solar UV irradiance variations did impact the
troposphere, primarily through alterations in the generation and
propagation of the longest tropospheric planetary waves. The
resulting longitudinal variations in tropospheric temperature,
wind, and geopotential height were similar in
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Figure 2.6 Compared in a) are the 10.7 cm solar
flux and the atmospheric pressure difference [(70°N,
100°W) minus (20°N, 60°W)] in the west years of the
equatorial stratosphere quasibiennial oscillation (QBO) in
January–February. The changes in the differences between the
land (100°W) and sea (60°W) pressures are correlated with
the 11-year solar activity cycle. Shown in b) is the surface air
temperature at Charleston, South Carolina, during
January–February in QBO west years and in c) the number of
lows crossing the 60th meridian west between the latitudes of
40°N and 50°N. From Labitzke and van Loon, Phil. Trans,
Royal Society London, (1990). Permission granted by the Royal
Society of London.
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nature and magnitude to those reported by van Loon and Labitzke
(1988). The major caveat is the exaggerated UV irradiance
variations utilized in the study; nevertheless, knowledge of the
response of the troposphere to solar cycle activities that directly
affect the middle atmosphere is growing.
In addition, the GCM studies demonstrated that dynamical changes
induced by solar cycle variations can affect the radiative
properties of the troposphere by influencing cloud and snow cover.
Furthermore, the effects do not cancel when averaged over the
ascending and descending portions of the cycle. This implies that a
time-integrated solar cycle forcing of the climate system is
possible through its impact on tropospheric dynamics and feedbacks,
rather than through direct insolation perturbation. If so, this
would likely have a very different climate impact than the forcing
associated with increasing greenhouse gases, whose effect on the
middle atmosphere and tropospheric dynamics is entirely
different.
Insolation Changes Due to Orbital
Variations
The study of Hays et al. (1976) showed that the climate record
deduced from deep sea sediments varied with periodicities that
generally matched those of the Earth's orbital variations,
specifically variations in eccentricity, obliquity (axial tilt),
and date of perihelion (Figure 2.7). The theory that orbital
variations are indeed the pacemakers of the ice ages has become
widely accepted. However, the theory does have problems, both from
the observational and the modeling perspectives, that are
instructive for evaluating solar influences on global change, and
which must be also addressed by the USGCRP in the broader context
of the Earth System History USGCRP science element.
The need to understand this issue arises not primarily from the
need to predict future climate based on the orbital configurations,
but rather from the standpoint of what it implies about the
sensitivity of the climate system, and about the ability of climate
models to simulate climate sensitivity, since the forcing can be
quantified. The last ice age was presumably initiated during the
time of strongly reduced summer insolation nearly 110,000 years
before the present (BP). The reductions projected for the next
10,000 years are extremely small in comparison and, from this
perspective, another ice age is unlikely in that time frame. This
is illustrated in Figure 2.7.
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Figure 2.7 Orbital (Milankovitch) forcing of
climate, as illustrated by the schematic at upper right. Shown on
the left are variations in insolation caused by cyclic changes in
the Earth's orbital parameters (centricity, obliquity, and
precession) that are correlated with variations in global ice
volume and in atmospheric carbon (from Earth System Science, A
Closer View, Report of the Earth System Sciences Committee, NASA
Advisory Council, 1988). The calculated changes in northern
hemisphere summer solar radiation since 160,000 years BP (from Rind
et al., 1989), lower right, indicate extremely small reductions for
the next 10,000 years. From this perspective, another ice age is
unlikely in that time frame. Courtesy of NASA Advisory Council,
1988, NASA.
Are the changes in insolation effected by the Earth's orbital
variations sufficient to have initiated ice ages — that is,
are they the real cause of the glacial/interglacial transitions of
the Pleistocene? Melting of the ice sheets occurred 15,000 to
10,000 years BP, coincident with high northern hemisphere summer
insolation, in agreement with this hypothesis. However, the
southern hemisphere climate also experienced rapid warming in this
time interval, when southern hemisphere insolation was at a
minimum. The apparent synchronicity of the two hemispheres in their
responses to orbital variations, which for the precessional cycle
has opposite solar insolation effects in the two hemispheres, has
long been a
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mystery and raises the question of how much of the climate
response is actually associated with orbital forcing.
Spectral analysis of the paleoclimate record shows that the
maximum power lies in the approximately 100,000 year period, which
is of the same order as the Earth's eccentricity variation.
However, the changes in eccentricity, on the order of a few tenths
of a percent over the past 5 million years, produce little change
in net annual solar radiation, so that any possible effects on the
seasonal distribution of radiation must be combined with variations
in tilt and precession of the Earth's rotation axis, which are
larger. Thus it is surprising that the about 100,000 year period
dominates in the climate record. Examples of this mismatch can
easily be found: the peak of the last ice age, about 20,000 years
BP, coincides with a very weak minimum in Northern Hemisphere
summer solar insolation, and the deglaciation Northern Hemisphere
summer maximum at about 12,000 years BP is no larger than a similar
feature at about 30,000 years BP, which did not lead to complete
deglaciation. These facts suggest that processes other than direct
solar forcing may be responsible for the observed climate
record.
Even the timing of the insolation variations relative to the
climatic response has been questioned. Winograd et al. (1988, 1992)
analyzed the oxygen-18 variations found in a calcitic vein in the
southern Great Basin. The uranium series age dates of the calcite
vein indicated that major glacial/interglacial transitions occurred
some 10,000 to 20,000 years before the solar insolation variations;
for example, the peak interglacial in this record appears at
147,000 ± 3,000 years BP, significantly before the
insolation peak. While the relevance of this local record to global
temperature and precipitation changes may be in doubt, high sea
level stands in the period 135,000 to 140,000 years BP have been
found by various researchers (e.g., Moore, 1982). The absolute
dating capability associated with the calcite vein is in contrast
to the approximate dating techniques associated with the deep sea
paleoclimate record, where assumptions about sedimentation rates
are fundamental in matching the orbital periodicities.
When the orbital solar insolation variations are incorporated in
general circulation climate models, the temperature changes are not
sufficient to produce ice sheet growth, especially in regions of
low altitude accumulation, as was apparently the case for the
Laurentide ice sheet (Rind et al.,
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1989; Phillipps and Held, 1994). Either the models are
incomplete or orbitally induced solar insolation variations are at
best only a catalyst for glacial/interglacial changes. Both of
these conclusions have important implications for global change
projections: the former implies that contemporary GCMs might not be
sufficiently sensitive to solar radiative forcing (whether of
orbital or solar activity origin), while the latter emphasizes that
it is the climate system feedbacks that are most important in
producing climate change, invalidating the use of simple transfer
functions between radiative forcing perturbations and climatic
responses.
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Representative terms from entire chapter:
solar activity
