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Changing Sun, Changing Climate?
This essay is partly based,
by permission, on an essay by Theodore S. Feldman (PSDI, Bedford, Mass), "Solar
Variability and Climate Change," rewritten and expanded by Spencer Weart.
For additional material, see Feldman's
Since it is the Sun's energy that drives the weather system, scientists
naturally wondered whether they might connect climate changes with solar
variations. Yet the Sun seemed to be stable over the timescale of human
lifetimes. Attempts to discover cyclic variations in weather and connect
them with the 11-year sunspot cycle, or other possible solar cycles ranging
up to a few centuries long, gave results that were ambiguous at best.
These attempts got a well-deserved bad reputation. Jack Eddy overcame
this with a 1976 study that demonstrated that irregular variations in
solar surface activity, a few centuries long, were connected with major
climate shifts. The mechanism remained uncertain, but plausible candidates
emerged. The next crucial question was whether a rise in the Sun's activity
could explain the global warming seen in the 20th century? By the 1990s,
there was a tentative answer: minor solar variations could indeed have
been partly responsible for some past fluctuations... but future warming
from the rise in greenhouse gases would far outweigh any solar effects.
| The Sun so greatly dominates the skies that
the first scientific speculations about different climates asked only
how sunlight falls on the Earth in different places. The very word
climate (from Greek klimat, inclination or latitude) originally
stood for a simple band of latitude. When scientists began to ponder
the possibility of climate change, their thoughts naturally turned
to the Sun. Early modern scientists found it plausible that the Sun
could not burn forever, and speculated about a slow deterioration
of the Earth's climate as the fuel ran out.(1)
In 1801 the great astronomer William Herschel introduced the idea
of more transient climate connections. It was a well-known fact that
some stars varied in brightness. Since our Sun is itself a star, it
was natural to ask whether the Sun's brightness might vary, bringing
cooler or warmer periods on Earth? As evidence of such a connection,
Herschel pointed to periods in the 17th century, ranging from two
decades to a few years, when hardly any sunspots had been observed.
During those periods the price of wheat had been high, he pointed
out, presumably reflecting spells of drought.(2)
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More discussion in
| Speculation increased in the mid-19th century, following the discovery
that the number of spots seen on the Sun rose and fell in a regular
11-year cycle. It appeared that the sunspots reflected some kind of
storminess on the Sun's surface violent activity that strongly
affected the Earth's magnetic field. Astronomers also found that some
stars, which otherwise seemed quite similar to the Sun, went through
very large variations. By the end of the century a small community
of scientists was pursuing the question of how solar variability might
relate to short-term weather cycles, as well as long-term climate
changes.(3) Attempts to correlate weather patterns with the sunspot cycle
were stymied, however, by inaccurate and unstandardized weather data,
and by a lack of good statistical techniques for analyzing the data.
Besides, it was hard to say just which of many aspects of weather
were worth looking into.
| At the end of the 19th century, most meteorologists
held firmly that climate was stable overall, about the same in one
century as in the last. That still left room for cycles within the
overall stability. A number of scientists looked through various data
hoping to find correlations, and announced success. Enthusiasts for
statistics kept coming up with one or another plausible cycle of dry
summers or cold winters or whatever, in one or another region, repeating
periodically over intervals ranging from 11 years to several centuries.
Many of these people declined to speculate about the causes of the
cycles they reported, but others pointed to the Sun. An example was
a late 19th-century British school of "cosmical meteorology," whose
leader Balfour Stewart grandly exclaimed of the Sun and planets, "They
feel, they throb together."(4)
| Confusion persisted in the early decades of the 20th century as
researchers continued to gather evidence for solar variation and climate
cycles. For example, Ellsworth Huntington, drawing on work by a number
of others, concluded that high sunspot numbers meant storminess and
rain in some parts of the world, resulting in a cooler planet. The
"present variations of climate are connected with solar changes much
more closely than has hitherto been supposed," he maintained. He went
on to speculate that if solar disturbances had been magnified in the
past, that might explain the ice ages.(5)
| Meanwhile an Arizona astronomer, Andrew Ellicott
Douglass, announced a variety of remarkable correlations between the
sunspot cycle and rings in trees. Douglass tracked this into past
centuries by studying beams from old buildings as well as Sequoias
and other long-lived trees. Noting that tree rings were thinner in
dry years, he reported climate effects from solar variations, particularly
in connection with the 17th-century dearth of sunspots that Herschel
and others had noticed. Other scientists, however, found good reason
to doubt that tree rings could reveal anything beyond random regional
variations. The value of tree rings for climate study was not solidly
established until the 1960s.(6*)
| Through the 1930s the
most persistent advocate of a solar-climate connection was Charles
Greeley Abbot of the Smithsonian Astrophysical Observatory. His predecessor,
Samuel Pierpont Langley, had established a program of measuring the
intensity of the Sun's radiation received at the Earth, called the
"solar constant." Abbot pursued the program for decades. By the early
1920s, he had concluded that the solar "constant" was misnamed: his
observations showed large variations over periods of days, which he
connected with sunspots passing across the face of the Sun. Over a
term of years the more active Sun seemed brighter by nearly one percent.
Surely this influenced climate! As early as 1913, Abbot announced
that he could see a plain correlation between the sunspot cycle and
cycles of temperature on Earth. (This only worked, however, if he
took into account temporary cooling spells caused by the dust from
volcanic eruptions.) Self-confident and combative, Abbot defended
his findings against all objections, meanwhile telling the public
that solar studies would bring wonderful improvements in
weather prediction.(7*) He and a few others at the Smithsonian pursued the topic
single-mindedly into the 1960s, convinced that sunspot variations
were a main cause of climate change.(8)
| Other scientists were quietly skeptical.
Abbot's solar constant variations were at the edge of detectability
if not beyond. About all he seemed to have shown for certain was that
the solar constant did not vary by more than one percent, and it remained
an open question whether it varied anywhere near that level. Perhaps
Abbot was detecting variations not in the solar constant, but in the
transmission of radiation through the atmosphere.(9) Still, if that varied with the sunspot
cycle, it might by itself somehow change the weather.
| Despite widespread skepticism, the study of
cycles was popular in the 1920s and 1930s. By now there were a lot
of weather data to play with, and inevitably people found correlations
between sunspot cycles and selected weather patterns. Respected scientists
and over-enthusiastic amateurs announced correlations that they insisted
were reliable enough to make predictions.
| Sooner or
later, every prediction failed. An example was a highly credible forecast
that there would be a dry spell in Africa during the sunspot minimum
of the early 1930s. When that came out wrong, a meteorologist later
recalled, "the subject of sunspots and weather relationships fell
into disrepute, especially among British meteorologists who witnessed
the discomfiture of some of their most respected superiors." Even
in the 1960s, he said, "For a young [climate] researcher to entertain
any statement of sun-weather relationships was to brand oneself a
crank."(10) Specialists in solar physics felt much
the same. As one of them recalled, "purported connections with...
weather and climate were uniformly wacky and to be distrusted... there
is a hypnotism about cycles that... draws all kinds of creatures out
of the woodwork."(11) By
the 1940s, most meteorologists and astronomers had abandoned the quest
for solar cycles in the weather. Yet some respected experts continued
to suspect that they did exist, lurking somewhere in the data.(12)
| Less prone to crank
enthusiasm and scientific scorn, if equally speculative, was the possibility
that the Sun could affect climate on much longer timescales. During
the 1920s, a few people developed simple models that suggested that
even a modest change in solar radiation might set off an ice age,
by initiating self-sustaining changes in the polar ice. A leading
British meteorologist, Sir George Simpson, believed the sequence of
ice ages showed that the Sun is a variable star, changing its brightness
over a cycle some 100,000 years long.(13) "There has always been a reluctance among scientists to
call upon changes in solar radiation... to account for climatic changes,"
Simpson told the Royal Meteorological Society in a Presidential address
of 1939. "The Sun is so mighty and the radiation emitted so immense
that relatively short period changes... have been almost unthinkable."
But none of the terrestrial causes proposed for ice ages was at all
convincing, he said, and that "forced a reconsideration of extra-terrestrial
| Such thinking was still in circulation two decades later. The eminent
astrophysicist Ernst Öpik wrote that none of the many explanations
proposed for ice ages was convincing, so "we always come back to the
simplest and most plausible hypothesis: that our solar furnace varies
in its output of heat." Öpik worked up a theory for cyclical
changes of the nuclear reactions deep inside the Sun. The internal
fluctuations he hypothesized had a hundred-million-year timescale
that seemed to match the major glacial epochs. Manwhile,within a given
glacial epoch "a kind of 'flickering' of solar radiation" in the Sun's
outer shell would drive the expansion and retreat of ice sheets.(15)
In the 1950s, when reviews and textbooks listed various possible explanations
of ice ages and other long-term climate changes, ranging from volcanic
dust to shifts of ocean currents, they often invoked long-term solar
variation as a particularly likely cause. As a U.S. Weather Bureau
expert put it, "the problem of predicting the future climate of Planet
Earth would seem to depend on predicting the future energy output
of the sun..."(16)
| Meanwhile some people continued to pursue the exasperating hints
that minor variations in the sunspot cycle influenced present-day
weather. Interest in the topic was revived in 1949 by H.C. Willett,
who dug out apparent relationships between changes in the numbers
of sunspots and long-term variations of wind patterns. Sunspot variations,
he declared, were "the only possible single factor of climatic control
which might be made to account for all of these variations." Others
thought they detected sunspot cycle correlations in the advance and
retreat of mountain glaciers. Willett admitted that "the physical
basis of any such relationship must be utterly complex, and is as
yet not at all understood." But he pointed out an interesting possibility.
Perhaps climate changes could be due to "solar variation in the ultraviolet
of the sort which appears to accompany sunspot activity." As another
scientist had pointed out a few years before, ultraviolet radiation
from the explosive flares that accompany sunspots would heat the ozone
layer high in the Earth's atmosphere, and that might somehow influence
the circulation of the lower atmosphere.(17)
| In the 1950s and 1960s, instruments on rockets that climbed above
the atmosphere managed to measure the Sun's ultraviolet radiation
for the first time. They found the radiation did intensify during
high sunspot years. However, ultraviolet light does not penetrate
below the stratosphere. Meteorologists found it most unlikely that
changes in the thin stratosphere could affect the layers below, which
contain far more mass and energy. Still, the hypothesis of atmospheric
influence remained alive, if far from healthy.
| A few scientists speculated more broadly.
Maybe weather patterns were affected by the electrically charged particles
that the Sun sprayed out as "solar wind." More sunspots throw out
more particles, and they might do something to the atmosphere. More
indirectly, at times of high sunspot activity the solar wind pushes
out a magnetic field that tends to shield the Earth from the cosmic
rays that rain down from the universe beyond. When these rays penetrate
the upper reaches of the atmosphere, they expend their energy producing
sprays of charged particles so more sunspots would mean fewer
of these particles. Either way there might be an influence on the
weather. Meteorologists gave these ideas some credence.(18*) But the solar wind and ultraviolet
carried only a tiny fraction of the Sun's total energy output. If
they did influence weather, it had to be through a subtle triggering
mechanism that remained altogether mysterious. Anyway variations connected
with sunspots seemed likely to bear only on temporary weather anomalies
lasting a week or so (the timescale of variations in sunspot groups
themselves), not on long-term climate change.(19)
| People continued to report weather features that varied with the
sunspot cycle of 11 years, or with the full solar magnetic cycle of
22 years (the magnetic polarity of sunspots reverses from one 11-year
cycle to the next). There were also matches to possible longer solar variation cycles.(20) It was especially
scientists in the Soviet Union who pursued such correlations. In the
lead was a team under the Leningrad meteorologist Kirill Ya. Kondratyev,
who sent balloons into the stratosphere to measure the solar constant.
In 1970 his group claimed that the Sun's output varied along with
the number of sunspots by as much as 2%. This drew cautious notice
from other scientists. As the authors admitted, the conclusion would
remain in doubt unless it could be verified by spacecraft entirely
above the atmosphere.(21)
| Another tentatively credible
study came from a team led by the Danish glaciologist Willi Dansgaard.
Inspecting layers of ancient ice in cores drilled from deep in the
Greenland ice sheet, they found cyclical variations. They supposed
the Sun was responsible. For the cycle that they detected, about 80
years long, had already been reported by scientists who had analyzed
small variations in the sunspot cycle.(22*) Another cycle with a length of about
180 years was also, the group suspected, caused by "changing conditions
on the Sun." The oscillations were so regular that in 1970 Dansgaard's
group boldly extrapolated the curves into the future. They began by
matching their results with a global cooling trend that, as others
reported, had been underway since around 1940. The group predicted
the cooling would continue through the next one or two decades, followed
by a warming trend for the following three decades or so.(23)
| The geochemist Wallace Broecker was impressed.
He "made a large leap of faith" (as he later put it) and assumed that
the cycles were not just found in Greenland, but had a global reach.(24)
He calculated that the global cooling trend since around 1940 could
be explained by the way the two cycles both happened to be trending
down. His combined curve would bottom out in the 1970s, then quickly
head up. Greenhouse effect warming caused by human emissions of carbon
dioxide gas ( CO2) would come on top of this
rise, making for a dangerously abrupt warming.(25)
| (Later studies failed to find Dansgaard's cycles globally. If they
existed at all, the cause did not seem to be the Sun, but quasi-cyclical
shifts in the North Atlantic Ocean's surface warmth and winds. This
was just another case of supposed global weather cycles that faded
away as more data came in. It was also one of several cases where
Broecker's scientific instincts were sounder than his evidence. The
downturn in temperature since the 1940s, whether due to a variation
in the Sun's radiation or some other natural cause, could indeed change
to a natural upturn that would add to greenhouse warming instead of
subtracting from it. In fact that happened, beginning in the 1970s.)
|The 1970s also brought controversial claims that weather data and tree
rings from various parts of the American West revealed a 22-year cycle
of droughts, presumably driven by the solar magnetic cycle. Coming
at a time of severe droughts in the West and elsewhere, these claims
caught some public attention.(26*) Scientists were beginning to understand, however, that
the planet's climate system could go through purely self-sustaining
oscillations, driven by feedbacks between ocean temperatures and wind
patterns. The patterns cycled quasi-regularly by themselves on timescales
ranging from a few years (like the important El Niño Southern
Oscillation in the Pacific Ocean) to several decades. That might help
to explain at least some of the quasi-regular cycles that had been
tentatively associated with sunspots.
| All this helped to guarantee that scientists would continue to
scrutinize any possibility that solar activity could influence climate,
but always with a skeptical eye. If meteorologists had misgivings,
most astronomers dismissed outright any thought of important solar
variations on a timescale of hundreds or thousands of years. Surface
features like sunspots might cycle over decades, but that was a weak,
superficial, and short-term effect. As for the main energy flow, improved
theories of the nuclear furnace deep within the Sun showed stability
over many millions of years. Alongside this sound scientific reasoning
there may have been a less rational component. "We had adopted a kind
of solar uniformitarianism," solar physicist John (Jack) Eddy suggested
in retrospect. "As people and as scientists we have always wanted
the Sun to be better than other stars and better than it really is."(27)
| Evidence was accumulating, however, that
the Sun truly does change at least superficially from one century
to another. Already in 1961 Minze Stuiver had moved in the right direction.
Stuiver was concerned about peculiar variations in the amount of radioactive
carbon-14 found in ancient tree rings. Carbon-14 is generated when
cosmic rays from the far reaches of the universe strike the atmosphere.
Stuiver noted how changes in the magnetic field of the Sun would change
the flux of cosmic rays reaching the Earth.(28) He had followed this up in collaboration with the carbon-14
expert Hans Suess, confirming that the concentration of the isotope
really had varied over past millennia. They were not suggesting that
changes in carbon-14 (or cosmic rays) altered climate; rather, they
were showing that the isotope could be used to measure solar activity
in the distant past. For the development of this important technique,
a good example of laboratory work and its attendant controversies,
see the supplementary essay on Uses of Radiocarbon
| In 1965 Suess tried correlating
the new data with weather records, in the hope that carbon-14 variations
"may supply conclusive evidence regarding the causes for the great
ice ages." He focused on the bitter cold spell that historians had
discovered in European writings about weather from the 15th through
the 18th century (the "Little Ice Age"). That had been a time of relatively
high carbon-14, which pointed to low solar activity. Casting a sharp
eye on historical sunspot data, Suess noticed that the same centuries
indeed showed a low count of sunspots. In short, fewer sunspots apparently
made for colder winters. A few others found the connection plausible,
but to most scientists the speculation sounded like just one more
of the countless correlations that people had announced over the past
century on thin evidence.(29*)
| Meanwhile carbon-14 experts refined their understanding of how
the concentration of the isotope had varied over past millennia. They
could not decide on a cause for the shorter-term irregularities. Solar
fluctuations were only one of half a dozen plausible possibilities.(30) The early 1970s also brought claims
that far slower variations in the Earth's magnetic field correlated
with climate. In cores of clay drawn from the seabed reaching back
a million years, colder temperatures had prevailed during eras of
high magnetism. The magnetic variations were presumably caused by
processes in the Earth's interior rather than on the Sun, but the
correlation suggested that cosmic rays really did influence climate.
As usual the evidence was sketchy, however, and it failed to convince
| In 1975, the respected meteorologist Robert Dickinson, of the National
Center for Atmospheric Research (NCAR) in Boulder, Colorado, took
on the task of reviewing the American Meteorological Society's official
statement about solar influences on weather. He concluded that such
influences were unlikely, for there was no reasonable mechanism in
sight except, maybe, one. Perhaps the electrical charges that
cosmic rays brought into the atmosphere somehow affected how aerosol
particles coalesced. Perhaps that somehow affected cloudiness, since
cloud droplets condensed on the nuclei formed by aerosol particles.
This was just piling speculation on speculation, Dickinson hastened
to point out. Scientists knew little about such processes, and would
need to do much more research "to be able to verify or (as seems more
likely) to disprove these ideas." For all his frank skepticism, Dickinson
had left the door open a crack. One way or another, it was now at
least scientifically conceivable that changes in sunspots could have
something to do with changes in climate. Most experts, however, continued
to consider the idea discredited if not preposterous.(32*)
In 1976, Eddy tied all
the threads together in a paper that soon became famous. He was
one of several solar experts in Boulder, where a vigorous community
of astrophysicists, meteorologists, and other Earth scientists had
grown up around the University of Colorado and NCAR. Yet Eddy was
ignorant of the carbon-14 research an example of the poor
communication between fields that always impeded climate studies.
He had won scant success in the usual sort of solar physics research,
and in 1973 he lost his job as a researcher, finding only temporary
work writing a history of NASA's Skylab. In his spare time he pored
over old books. Eddy had decided to review historical naked-eye
sunspot records, with the aim of definitively confirming the long-standing
belief that the sunspot cycle was stable over the centuries.
Read the details in our Interview with
| Instead, Eddy found evidence that the Sun was by no means as constant
as astrophysicists supposed. Especially intriguing was evidence suggesting
that during the "Little Ice Age" of the 16th-17th centuries, sky-watchers
had observed almost no sunspot activity. People clear back to Herschel
had noticed this prolonged dearth of sunspots. A 19th-century German
astronomer, G.W. Spörer, had been the first to solidly document
it, and a little later, in 1890, the British astronomer E. Walter
Maunder drew attention to the discovery and its significance for climate.
Other scientists, however, thought this was just another case of dubious
numbers at the edge of detectability. Maunder's publications sank
into obscurity. It was only by chance that while Eddy was working
to prove the Sun was entirely stable, another solar specialist told
him about Maunder's work.(33*)
|"As a solar astronomer I felt certain that it could never have happened,"
Eddy later recalled. But hard historical work gradually persuaded
him that the early modern solar observers were reliable the
absence of sunspot evidence really was evidence of an absence. Digging
deeper, he found the inconstancy confirmed by historical sightings
of auroras and of the solar corona at eclipses (both of which reflected
heightened activity on the Sun's surface). Once his attention was
drawn to the carbon-14 record, he saw that it too matched the pattern.
All the evidence pointed to long-sustained minimums and at least one
maximum of solar activity in the past two thousand years. It was "one
more defeat in our long and losing battle to keep the Sun perfect,
or, if not perfect, constant, and if inconstant, regular. Why we think
the Sun should be any of these when other stars are not," he continued,
"is more a question for social than for physical science."(34)
| As it happened, the ground had already been
prepared by developments in astrophysics in the early 1970s. Physicists
had built a colossal particle detector expressly to observe the elusive
neutrinos emitted by the nuclear reactions that fueled the Sun. The
experiment failed to find anywhere near the flux of neutrinos that
theorists insisted should be reaching the Earth. Was it possible that
deep within the Sun, production of energy was going through a lull?
Perhaps the output of stars like the Sun really could wander up and
down, maybe even enough to cause ice ages? The anomaly was eventually
traced to neutrino physics rather than solar physics. Meanwhile, however,
it called into doubt the theoretical reasoning that said the Sun could
not be a variable star.(35)
| Eddy's announcement of a solar-climate connection nevertheless
met the customary skepticism. He pushed his arguments vigorously,
stressing especially the Little Ice Age, which he memorably dubbed
the "Maunder Minimum" of sunspots. The name he chose emphasized that
he was not alone with his evidence. It is not unusual for a scientist
to make a "discovery" that others had already announced fruitlessly.
A scientific result cannot flourish in isolation, but needs support
from other evidence and ideas. Eddy had gone some distance beyond
his predecessors in historical investigation. More important, he could
connect the sunspot observations with the carbon-14 record and the
new doubts about solar stability. It also mattered that he worked
steadily and persuasively to convince other scientists that the thing
| Pushing farther,
Eddy drew attention to a spell of high carbon-14, and thus low solar
activity, during the 11th-12th centuries. Remarks in medieval manuscripts
showed that these centuries had been unusually warm in Europe. It
was far from proven that those were times of higher temperatures all
around the globe. However, scientists were (as usual) particularly
impressed by evidence from the North Atlantic region where most of
them lived and where the historical record was best known. Especially
notable was the mild weather that had encouraged medieval Vikings
to establish colonies in Greenland — colonies that endured for
centuries, only to perish from starvation in the Little Ice Age. Eddy
warned that in our own times, "when we have observed the Sun most
intensively, its behavior may have been unusually regular
Decades later, after painstaking
studies developed much fuller series of data covering the entire
globe, these data showed a complex variety of periods of warmth
and periods of cold. The so-called "Medieval Warm Period"
when Iceland and Greenland were settled was a group of regional
variations, significant but not as universal and extreme as the
steep temperature rise felt around the world since the 1980s. The
"Little Ice Age" was more definite, but it too had many
local variations, not everywhere as important as in the North Atlantic
region. As one pair of experts remarked in 2004, "If the development
of paleoclimatology had taken place in the tropical Pacific, Africa,...
or Latin America, the paleoclimatic community would almost certainly
have adopted other terminology." Instead of a Little Ice Age
and Medieval Warm Period, scientists of the 1970s might have talked,
for example, about great periods of drought. Still, Eddy's central
point would stand: regional climates were more susceptible to perturbing
influences, including small changes on the Sun, than most scientists
Eddy worked hard to "sell" his findings. At a 1976 workshop
where he first presented his full argument, his colleagues tentatively
accepted that solar variability might be responsible for climate
changes over periods of a few hundreds or thousands of years.(37) Eddy pressed on to turn up more evidence
connecting temperature variations with carbon-14, which he took
to measure solar activity. "In every case when long-term solar activity
falls," he claimed, "mid-latitude glaciers advance and climate cools."(38)
|Already while Eddy's
sunspot figures were in press, other scientists began to explore how
far his idea might account for climate changes. Adding solar variability
to the sporadic cooling caused by dust from volcanic eruptions did
seem to give a better match to temperature trends over the entire
last millennium.(39) Peering closer at the more accurate
global temperatures measured since the late 19th century, a group
of computer modelers got a decent match using only the record of volcanic
eruptions plus greenhouse warming from increasing carbon dioxide,
but they improved the match noticeably when they added in a record
of solar variations. All this proved nothing, but gave more reason
to devote effort to the question.(40)
| Meanwhile Stuiver and others confirmed the connection between solar
activity and carbon-14, and this became a standard tool in later solar-climate
studies.(41) An example was a study that reported a match between carbon-14
variations and a whole set of "little ice ages" (indicated by advances
of glaciers) that had come at random over the last ten thousand years.(42) Other studies, however, failed to find such correlations.
As a 1985 reviewer commented, "this is a controversial topic... the
evidence relating solar activity and carbon-14 variations to surface
temperatures is equivocal, an intriguing but unproven
| Scientists continued
to report new phenomena at the border of detectability. In particular,
Ronald Gilliland (another NCAR scientist) followed Eddy's example
in analyzing a variety of old records and tentatively announced slight
periodic variations in the Sun's diameter. They matched not only the
11-year sunspot cycle but also the 80-year cycle that had long hovered
at the edge of proof. Adding these solar cycles on top of greenhouse
warming and volcanic eruptions, Gilliland too found a convincing match
to the temperature record of the past century. He calculated that
the solar cycles were currently acting opposite to the rise in carbon
dioxide, so as to give the world an equable climate until about the
year 2000. This might lead to complacency about greenhouse warming,
he feared, which "could be shattered" when the relentlessly increasing
carbon dioxide added onto a solar upturn. Most of his colleagues awaited
more solid proof of the changes in diameter and the long-term cycle
(and they continue to await it).(44)
| Yet how could changes in the number of sunspots affect climate?
The most direct influence would come if the change meant a rise or
fall in the total energy the Sun radiated upon the Earth, the so-called
"solar constant." The development of highly accurate radiometers in
the 1970s raised hopes that variations well below one percent could
be detected at last. But few trusted any of the measurements from
the ground or even from stratospheric balloons. Rockets launched above
the atmosphere provided brief observations that seemed to show variation
over time, but it was hard to rule out instrumentation errors. Nor
were many convinced by Peter Foukal when he applied modern statistical
methods to Abbot's huge body of old data, and turned up a faint connection
between sunspots and the amount of solar energy reaching the Earth.
Even if that were accepted, was it because the Sun emitted less energy?
Or was it because ultraviolet radiation from solar storms somehow
changed the upper atmosphere, which in turn somehow influenced climate,
and thus affected how much sunlight Abbot had seen at the surface?(45)
|To try to settle the question, NASA included
an instrument for measuring the solar constant on a satellite launched
in 1980. The amazingly precise device was the work of a team at the
Jet Propulsion Laboratory led by Richard C. Willson. Soon after the
satellite's launch, they reported distinct if tiny variations whenever
groups of sunspots passed across the face of the Sun. Essential confirmation
came from an instrument that John Hickey and colleagues had previously
managed to insert in the Nimbus-7 satellite, a spacecraft built to
monitor weather rather than the Sun.(46)
Both instruments proved stable and reliable. In 1988, as a new solar
cycle got underway, both groups reported that total solar radiation
did vary slightly with the sunspot cycle.(47)
| Satellite measurements pinned down precisely how solar brightness
varied with the number of sunspots. The radiation varied by only about
one part in a thousand; measuring such tiny wiggles was a triumph
of instrumentation.(48) A
single decade of data was too short to support any definite conclusions
about long-term climate change, but it was hard to see how such a
slight variation could matter much.(49) Since the 1970s, rough calculations
on general grounds had indicated that it should take a bigger variation,
perhaps half a percent, to make a serious direct impact on global
temperature. However, if the output could vary a tenth of a percent
or so over a single sunspot cycle, it was plausible to imagine that
larger, longer-lasting changes could have come during the Maunder
Minimum and other major solar variations. That could have worked a
real influence on climate.
| Some researchers carried
on with the old quest for shorter-term connections. Sunspots and other
measures plainly showed that the Sun had grown more active since the
19th century. Was that not linked somehow to the temperature rise
in the same decades? Some people persevered in the old effort to winkle
out correlations between sunspots and weather patterns. For example,
according to a 1991 study, Northern Hemisphere temperatures over the
past 130 years correlated surprisingly well with the length
of the sunspot cycle (which varied between 10 and 12 years). This
finding was highlighted the following year in a widely publicized
report issued by a conservative group. The report maintained that
the 20th-century temperature rise might be entirely due to increased
solar activity. The main point they wanted to make was less scientific
than political: "the scientific evidence does not support a policy
of carbon dioxide restrictions with its severely negative impact on
the U.S. economy."(50)
Critics of the report pointed out that the new finding sounded
like the weary old story of sunspot work if you inspected
enough parameters, you were bound to turn up a correlation. As it
happened, already by 2000 the correlation of climate with cycle
length began to break down. Moreover, a reanalysis published in
2004 revealed that from the outset the only pattern had been a "pattern
of strange errors" in the key study's data. Little more could
be said without further decades of observations, plus a theory to
explain why there should be any connection at all between the sunspot
cycle and weather. The situation remained as an expert had described
it a century earlier: "from the data now in our possession, men
of great ability and laborious industry draw opposite conclusions."(51)
The most straightforward correlation, if it could be found, would
connect climate with the Sun's total output of energy. Hopes of
finding evidence for this grew stronger when two astronomers reported
in 1990 that certain stars that closely resembled the Sun showed
substantial variations in total output. Perhaps the Sun, too, could
vary more than we had seen in the decade or so of precise measurements?
In fact, studies a decade later showed that the varying stars were
not so much like the Sun after all. Still, it remained possible
that the Sun's total luminosity had climbed enough since the 19th
century to make a serious impact on climate — if anyone could
come up with an explanation for why the climate should be highly
sensitive to such changes.(51a)
|A more promising approach pursued the possibility
of connections between climate shifts and the slow changes in the
Sun's magnetic activity that could be deduced from carbon-14 measurements.
A few studies that looked beyond the 11-year sunspot cycle to long-term
variations turned up indications, as one group announced, of "a more
significant role for solar variability in climate change... than has
previously been supposed."(52) In 1997 a pair of scientists
drew attention to a possible explanation for the link. Scanning a
huge bank of observations compiled by an international satellite project,
they reported that global cloudiness increased slightly at times when
the influx of cosmic rays was greater. Weaker solar activity apparently
meant more clouds. A later reanalysis of the data found severe errors,
but the study did serve to stimulate new thinking.
|The proposed mechanism roughly resembled the speculation that Dickinson
had offered, with little confidence, back in 1975. It began with the
fact that in periods of low solar activity, the Sun's shrunken magnetic
field failed to divert cosmic rays from the Earth. When the cosmic
rays hit the Earth's atmosphere, they not only produced carbon-14,
but also sprays of electrically charged molecules. Perhaps this electrification
promoted the condensation of water droplets on aerosol particles?
If so, there was indeed a mechanism to produce extra cloudiness. A
later study of British weather confirmed that at least regionally
there was "a small yet statistically significant effect of cosmic
rays on daily cloudiness."(53)
| Other studies meanwhile revived the old idea that increased ultraviolet
radiation in times of higher solar activity might affect climate by
altering stratospheric ozone. While total radiation from the Sun was
nearly constant, instruments in rockets and satellites found the energy
in the ultraviolet varying by several percent over a sunspot cycle.
Plugging these changes into elaborate computer models suggested that
even tiny variations could make a difference, by interfering in the
teetering feedback cycles that linked stratospheric chemistry and
particles with lower-level winds and ocean surfaces. By the end of
the 1990s, many experts thought it was possible that changes in the
stratosphere might affect surface weather after all.(54)
| Whatever the exact form solar influences took, most scientists
were coming to accept that the climate system was so unsteady, in
general, that many kinds of minor external change could trigger a
shift. With somewhat plausible mechanisms to back up the evidence
for a solar-climate connection, the long-wavering balance of scientific
opinion tilted. Many experts now thought the connection might be real.
|When a 1999
study reported evidence that the Sun's magnetic field had strengthened
greatly since the 1880s, it brought still more attention to the key
question: was increased solar activity the main cause of the temperature
rise over that period? Whether any of the proposed solar mechanisms
did in fact produce a noticeable effect on global climate was still
no more than speculation. But as the 21st century began, most experts
thought it plausible that the Sun might have driven at least part
of the previous century's warming. Most convincingly, the warming
from the 1880s to the 1940s had come when solar activity had definitely
been rising, while the carbon dioxide buildup had not yet been large
enough to matter much. A cooling during the 1950s and 1960s followed
by the resumption of warming also correlated loosely with solar activity.
How far the solar changes had influenced climate, however, remained
speculative. An increase in smoggy haze, dust from farmlands, and
other aerosols had probably had something to do with the cooling.
It was also possible that the climate system had just swung randomly
on its own.(55*)
|By now it was evident that the old dream of
predicting climate change directly from solar variations was hopeless.
Even if solar physicists could predict long-term changes of the Sun
(which they could not), so many other interactions pushed and pulled
the climate system that no single force would explain it. One senior
solar physicist insisted, "We will have to know a lot more about the
Sun and the terrestrial atmosphere before we can understand the nature
of the contemporary changes in climate."(56)
However, rough limits could
be set on the extent of the Sun's influence. Average sunspot activity
did not increase after 1980, and overall solar activity during the
period since 1950 looked little different from earlier periods.
The satellite measurements of the solar constant found it cycling
within narrow limits (less than one part in a thousand). As for
cosmic rays, they had been measured since the 1950s and likewise
showed no long-term trend. Yet the global temperature rise that
had resumed in the 1970s was accelerating at a record-breaking pace.
It seemed impossible to explain that using the Sun alone, without
invoking greenhouse gases. For one thing, the stratosphere was cooling,
which was exactly what models predicted would result from the greenhouse
effect, but was the reverse of what should result from a solar influence(57*)
|The consensus of most
scientists, arduously hammered out in a series of international workshops,
flatly rejected the argument that the global warming of the 1990s
could be dismissed as a mere effect of changes on the Sun. The pioneer
of historical solar influences, Jack Eddy, wrote that if the Sun were
"the only agent of climatic change, we would live in a world
where the mean global surface temperature varied, in any century,
through limits of at most about 0.5°C." Similarly, in 2004
when a group of scientists published evidence that the solar activity
of the 20th century had been unusually high, they nevertheless concluded
that "even under the extreme assumption that the Sun was responsible
for all the global warming prior to 1970, at most 30% of the strong
warming since then can be of solar origin." When Foukal reviewed
the question in 2006, he agreed that there was no good evidence that
the Sun had played a role in any climate change back to the Little
Ice Age. (Meanwhile, new historical evidence suggested that the cold
of the early modern centuries might have been partly due to a spate
of volcanic eruptions.)(57a)
|Some experts persevered in arguing that slight solar changes (which
they thought they detected in the satellite record) had driven the
extraordinary warming since the 1970s. Most scientists expected that
these correlations would follow the pattern of every other subtle
solar-climate correlation that anyone had reported over the past century
— fated to be disproved by the next decade or two of data. A
few scientists persevered in studying possible mechanisms, for example
devising experiments that they hoped would show how cosmic rays could
affect climate. Yet even if somebody did finally manage to show an
influence on climate from changes in the Sun, it could not be very
great. Greenhouse warming was bound to swamp any solar effects as
the quantities of the gases in the atmosphere soared ever higher.
Willson, the leader of the satellite experts, explained that in the
future,"solar forcing could be significant, but not dominant."(58*)
|The import of the claim that solar variations
influenced climate was now reversed. Critics had used the claim to
oppose regulation of greenhouse gases. But what if the planet really
did react with extreme sensitivity to almost imperceptible changes
in the radiation arriving from the Sun? The planet would surely also
be sensitive to greenhouse gas interference with the radiation once
it entered the atmosphere. A U.S. National Academy of Sciences panel
estimated that if solar radiation were now to weaken as much as it
had during the 17th-century Maunder Minimum, the effect would be offset
by only two decades of accumulation of greenhouse gases. As one expert
explained, the Little Ice Age "was a mere 'blip' compared with expected
future climatic change."(59)
For recent work on temperature changes over the past millenium
or so, probably related to solar variations plus volcanic eruptions
and perhaps other factors as well as the recent rise of greenhouse
gases, see the conclusion and figure captions in the essay on The
Modern Temperature Trend.
Modern Temperature Trend
Past Cycles: Ice Age Speculations
1. Feldman (1993); Fleming (1990).
2. Herschel (1801), pp. 313-16;
see Hufbauer (1991).
3. Notably, for variations related to the evolution of the Sun and
stars, Dubois (1895); for sunspot cycles Czerney (1881) .
4. See for example, Brückner
(1890), chapter 1; translated in Stehr and von Storch
(2000), pp. 116-121; Stewart: Gooday (1994).
5. Huntington (1914), quote p.
480; Huntington (1923); summarized in Huntington and Visher (1922) .
6. Webb (2002),
chapter 3; Webb (1986); Fritts (1962)
pioneered accurate use of tree rings. Fritts
(1976) notes the skepticism (page v) and shows how it was overcome.
Climate periods of 11-12 years as well as longer cycles also appeared
in annual layers of clay laid down in lake beds (varves), Bradley (1929); for references and summary,
see Brooks (1950). BACK
7. Abbot and Fowle (1913);
similarly A. Ångström, using Abbot's data, said the solar constant varied with
sunspot number, although decades later he retracted. Ångström (1922); Ångström (1970); historical studies are Hufbauer (1991), p. 86; DeVorkin
8. Abbot (1967); Aldrich and Hoover (1954).
9. Fröhlich (1977).
10. Lamb (1997), pp. 192-93.
11. J. Eddy, interview by Weart, April 1999, AIP, p. 6.
12. Nebeker (1995), p. 95.
13. Simpson (1934); Simpson (1939-40). Simpson cited A. Penck, who argued that the
entire world had cooled and only solar changes could explain this.
14. Simpson (1939-40), p. 210.
15. Öpik (1958);
"flickering" (due to uncertain convective changes): Öpik
(1965), p. 289.
16. E.g., Brooks (1949), ch. 1;
Shapley (1953); Wexler (1956),
quote p. 494, adding that turbidity (from volcanoes) was equally important.
17. Willett (1949), pp. 34, 41,
50; see Lamb (1997), p. 193; the earlier hypothesis (not cited
by Willett) is in Haurwitz (1946); glacier papers are cited by
Wexler (1956), p. 485.
18. A possible connection between cosmic rays and clouds was
already established at the end of the 19th century by the inventor of the cloud chamber, Wilson (1899); it was admittedly "speculation" that ionization in
the upper troposphere affected storminess. Ney (1959); the
ideas found some favor with, e.g., Roberts (1967), pp. 33-34.
19. Sellers (1965), pp. 220-23.
20. Lamb (1977), pp. 700-704.
21. Kondratyev and Nikolsky
(1970); Fröhlich (1977).
22. Johnsen et al. (1970);
similarly, Dansgaard et al. (1971), same quote p. 44; the period
they reported was precisely 78 years, and Schove (1955) had
reported a 78-year variation between long and short sunspot cycles as well as a possible 200-year
period; in addition, not noted by the glaciologists, a roughly 80-year modulation in the amplitude
of the sunspot cycle was reported by Gleissberg (1966);
weather correlations with the 80-year cycle were reported in 1962 by B.L. Dzerdzeevski as cited
by Lamb (1977), p. 702.
23. Johnsen et al. (1970); see
also Dansgaard et al. (1971); Dansgaard et al. (1973).
24. Broecker (1999).
25. Broecker (1975).
26. Roberts and Olson (1975)
(admitting that "A mere coincidence in timing... will not, of course, constitute proof of a physical
relationship"); Mock and W.D. Hibler (1976) (a "pervasive" but
only "quasi-periodic" 20-year cycle); Mitchell et al. (1979)
(tree-ring data analysis "strongly supports earlier evidence of a 22 yr drought rhythm... in the
U.S.... in some manner controlled by long-term solar variability..." ).
27. Eddy (1977), p. 92.
28. Stuiver (1961).
29. Suess (1968), p. 146; in the
best review of sunspot history available to Suess at this time, D.J. Schove took no notice of any
anomaly such as the early-modern minimum, although it is visible in his data. Schove (1955); a tentative longer-term correlation of climate
(glacier advances) with C-14 was shown by Denton and
Karlén (1973), who suggest that "climatic fluctuations, because of their close
correlation with short-term C14 variations, were caused by varying solar activity," p. 202; for the
Little Ice Age, see Fagan (2000); Lamb (1995), ch. 12.
30. Ralph and Michael
31. Wollin et al. (1971); Gribbin (1982), ch. 7.
32. Dickinson (1975);
a similar speculation, connecting cosmic rays with storminess, was offered
by Tinsley et al. (1989); see also the solar activity-atmosphere
connection reported by Wilcox et al. (1973).
Another weather-Sun correlation was laid out in Herman
and Goldberg (1978), which met strong resistance including attempts
to suppress publication, according to Herman (2003),
ch. 18. BACK
33. Maunder (1890) attributes
the discovery to Spörer; some authors now refer to a 17th-century Maunder Minimum and
a 15th-century Spörer Minimum. Eddy chose "Maunder" to make a phrase that would be
memorable: Eddy, interview by Weart, April 1999, AIP, p. 11. For history and references, see Eddy (1976); examples of neglect of Maunder: he was cited, but
only for other work, in Abetti (1957); Kuiper (1953); Menzel (1949);
the 17th-century paucity of sunspots was noted without any reference by Willett (1949), p. 35.
34. The first published statement was an abstract for the March
1975 meeting of the American Astronomical Society Eddy
(1975); and next at a Solar Output Workshop in Boulder, Colo., Eddy (1975); the famous publication was Eddy (1976), "defeat" p. 1200; "felt certain," Eddy (1977), pp. 80-81. See Eddy, interview by Weart, April 1999,
35. Hufbauer (1991), pp.
36. "benign," Eddy (1977),
p. 69. BACK
36a. Jones and
Mann (2004), p. 20, see p. 7 and passim. BACK
37. White (1977), see Mitchell
p. 21, Hays p. 89; note also the earlier, more doubting response of Mitchell (1976), p. 491. "Salesman": Eddy, interview by Weart,
April 1999, AIP, p. 14.
38. Eddy (1977), quote p. 173;
for more extensive speculations and reflections, see Eddy
39. Schneider and Mass
(1975); similarly, Schneider and Mass (1975).
40. Hansen et al. (1981), using
what was admittedly a "highly conjectural" (p. 93) measure of variability by D.V. Hoyt.
41. Stuiver and Quay (1980).
42. Wigley and Kelly (1990).
43. Bradley (1985), p. 69.
44. Gilliland (1981), reporting
11- and 76-year variations in solar size; Gilliland (1982); Gilliland (1982), quote p. 128.
45. Hufbauer (1991), pp.
278-80; for example, a 1978 workshop concluded that changes in stratospheric ozone due to
ultraviolet radiation might influence climate McCormac and Seliga
(1979), pp. 18, 20.
46. Hickey et al. (1980); Willson et al. (1981); Hufbauer
(1991), pp. 280-92.
47. Willson and Hudson
(1988); Hickey et al. (1988).
48. Lee et al. (1995).
49. Hoyt and Schatten (1997).
50. Seitz (1992), p. 28, see p.
51. Friis-Christensen and
Lassen (1991); Kerr (1991); Young
(1895), p. 162. Errors: Damon and Laut (2004).
51a. Baliunas and Jastrow
(1990); Foukal (2003).
52. "More significant" (an "admittedly crude" analysis): Cliver et al. (1998), p. 1035.
53. Svensmark and Friis-Christensen
(1997); Friis-Christensen and Svensmark (1997);
the effect was also reported, less convincingly, by Pudovkin and Verentenenko (1995); Pudovkin and Veretenenko (1996). Errors: Damon
and Laut (2004). Later study: Harrison and
Stephenson (2006). BACK
54. Haigh (1994); Haigh (1996); McCormack et al.
(1997); Shindell et al. (1999); for discussion, see Wallace and Thompson (2002).
55. Lockwood et al. (1999);
Marsh and Svensmark (2000). Reviewing various
claims, including some based on observations of variations in supposedly
Sun-like stars, three experts concluded in 2004 that "Any relationship"
between long-term solar variations and climate "must remain speculative,"
Foukal et al. (2004). BACK
56. Parker (1999); cf. criticism
of Parker by Hoffert et al. (1999).
57. Tett et al.
(1999); stratosphere: IPCC (2001),
p. 709. Benestad (2005) reports that "...comparison
with the monthly sunspot number, cosmic galactic rays and 10.7 cm absolute
radio flux since 1950 gives no indication of a systematic trend in the
level of solar activity that can explain the most recent global warming." BACK
57a. Consensus: IPCC
(2001). John Eddy, "Editor's summary," Consequences
2:1 (Winter 1996), online at the US
Global Change Research Information Office site. "30%," Solanki
et al. (2004), p. 1087. Foukal et al. (2006).
58. Willson reported a brightening of 0.04 percent
between the two most recent solar cycles, Willson
(1997); this was controversial, see Kerr
(1997); similarly and more recently, Willson and Mordvinov (2003); discussed by
Byrne (2003). Experiments: e.g., Svensmark
et al. (2007) (which brought a strong press reaction but proved little).
"Mere 'blip':" Nelson (1997).
59. National Research Council
(1994), combining statements on pp. 3 and 4; blip: Wigley
and Kelly (1990), p. 558. BACK
© 2003-2007 Spencer Weart & American Institute of Physics