"Back in 1994 a U.S. National Academy of Sciences panel had estimated
that if solar radiation were to weaken as much as it had during the 17th-
century Maunder Minimum, the entire effect would be offset by another two
decades of accumulation of greenhouse gases. A 2010 study reported that
with the growing rate of emissions, by the late 21st century a Maunder-
Minimum solar effect would be offset in a single decade. As one expert
explained, the Little Ice Age "was a mere 'blip' compared with expected
future climatic change."(60)"
***
Changing Sun, Changing Climate?
"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
civilization. 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 was 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 was far outweigh any solar effects.(1)
Subsections: Chasing Sunspot Cycles - Searching for a Mechanism (1950s -
Early 1970s) - Carbon-14 and Jack Eddy - More Sun-Climate Connections
(1980s - 1990s) - The Sun vs. Greenhouse Gases (2000s)
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.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 a connection
between Sun and weather, 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, he remarked,the price of wheat had
been high, presumably reflecting spells of drought.(2)
- LINKS -
More discussion in
<=>Simple models
Chasing Sunspot Cycles TOP OF PAGE
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 from one century to the next. That still
left room for modest 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)
<=Simple models
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*)
=>Rapid change
<=>Modern temp's
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.
According to his calculations, over a period of years when the Sun was more
active it was 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)
<=Aerosols
=>Chaos theory
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.
<=>Modern temp's
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.
=>Climate cycles
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) (This was a robust tradition: into the
21st century, enthusiasts with weird or incomprehensible theories of solar
influences, backed up by selected weather data and intricate graphs,
continued to show up at scientific meetings of meteorological societies.) 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 a connection did exist, lurking somewhere in the data.(12)
=>Climatologists
=>Climate cycles
<=>Chaos theory
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
causes."(14*)
<=Simple models
=>Climate cycles
Such thinking was still in circulation in the 1950s. 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) 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)
Searching for a Mechanism (1950s - Early 1970s)
TOP OF PAGE
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 that 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. Therefore, 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)
=>Carbon dates
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. But the authors admitted that 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 cap, 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)
=>Rapid change
<=Modern temp's
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)
=>Modern temp's
(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.)
By now it was clear that if you applied powerful statistical techniques to
enough tree ring samples, you would sometimes turn up the 11-year solar
cycle. Solar activity definitely had some kind of effect on climate in some
places — but nothing obviously strong or consistent. For exaample, the
1970s saw 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.
=>Public opinion
=>Modern temp's
<=The oceans
All this helped to guarantee that scientists would continue to scrutinize any
way that solar activity might 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)
Carbon-14 and Jack Eddy TOP OF PAGE
Evidence was accumulating 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 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 Dating.
<=Carbon dates
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*)
<=Public opinion
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 most scientists.(31)
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 electric charges that cosmic rays generated in the
atmosphere somehow affected how dust and other 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 physically conceivable that changes
in sunspots could have something to do with changes in climate. Most
experts, however, continued to believe the idea was not only unproven but
preposterous. Interest might be piqued when someone reported a new
correlation between solar changes and weather, but nobody was surprised
when further data and analysis knocked it down.(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.
<=>Climatologists
Jack Eddy
Jack Eddy
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 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 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, not solar physics. Meanwhile,
however, it called into doubt the theoretical reasoning that said the Sun
could not be a variable star.(35)
<=External input
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 was true.
Pushing farther, Eddy drew attention to a spell of low carbon-14, and thus
high 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 and benign."(36)
=Milestone
(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 much more definite, but it too had many local variations, not
everywhere as obvious as around the North Atlantic. 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 had imagined.)(36a)
=>Aerosols
=>Public opinion
<=>Modern temp's
=>Climatologists
Eddy worked hard to "sell" his findings. At a 1975 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 roughly track 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)
<=Aerosols
<=>Models (GCMs)
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 possibility."(43)
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)
<=>Modern temp's
=>Aerosols
More Sun-Climate Connections (1980s - 1990s)
TOP OF PAGE
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)
<=Government
sunspot number
Sunspot numbers, compiled by European observatories. The roughly 11-year
cycle has variable intensity, peaking in the 1780s, 1850s and 1960s. The
solar magnetic field, ultraviolet radiation, and other features that may affect
climate are found to rise and fall along with the sunspot number. Courtesy
NASA
Satellite measurements pinned down precisely how solar brightness varied
with the number of sunspots. Over a sunspot cycle the energy radiated
varied by barely 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? 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)
=Milestone
=>Public opinion
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 — and 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)
Proposed Sun-climate correlation 1870-1990
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. Later studies and reanalysis of
the data found severe errors, and the authors themselves shifted from
claiming an effect on high-level clouds to claiming an effect on low-level
clouds. But the study did serve to stimulate new thinking.
<=International
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.
Meanwhile others speculated about mechanisms through which the
powerful electric circuit that circles the planet, and which varies in response
to solar activity, might influence cloudiness.(54)
While the physics of how solar activity could affect clouds remained
obscure, it was now undeniable that possible mechanisms could exist. And
while the data were noisy, a growing variety of evidence, some of it going
back thousands of years, showed credible correlations between solar
activity and one or another feature of the climate. Whatever the exact form
solar influences took, most scientists were coming to accept that the
climate system was so unsteady that many kinds of minor external change
could trigger a shift. It might not be necessary to invoke exotic cosmic ray
mechanisms, for the system might be sensitive even to the tiny variations in
the Sun's total output of energy, the solar constant. The balance of scientific
opinion tilted. Many experts now thought there was indeed a solar-climate
connection.(55)
The Sun vs. Greenhouse Gases (2000s) TOP OF PAGE
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 rise of
average global temperature over that period? As the 21st century began,
most experts thought it likely that the Sun had 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 changes in solar activity. How far the solar
changes had influenced climate, however, remained speculative. The
temporary cooling had probably been at least partly related to an increase in
smoke from smoggy haze, dust from farmlands, volcanic eruptions, and
other aerosols. It was also possible that the climate system had just swung
randomly on its own. 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*)
=>Models (GCMs)
=> Public opinion
By the early 21st century, however, evidence of connections between solar
activity and weather was strengthening. Extremely accurate satellite
measurements spanning most of the globe revealed a distinct correlation
between sea-surface temperatures and the eleven-year solar cycle. The
effect was tiny, not even a tenth of a degree Celsius, but it was undeniable.
Similarly weak but clear effects were detected in the atmosphere near the
surface and, somewhat stronger, in the thin upper atmosphere.(56a) The
practical significance of these effects was minor — after all, if the sunspot
cycle had a truly powerful effect on weather, somebody would have proved
it much earlier. The new findings, however, did pose an important challenge
to computer modelers. A climate model could no longer be considered
entirely satisfactory unless it could reproduce these faint, but theoretically
significant, decade-scale cycles.
Rough limits could now be set on the extent of the Sun's influence. For
average sunspot activity decreased after 1980, and on the whole, solar
activity had not increased during the half-century since 1950. As for cosmic
rays, they had been measured since the 1950s and likewise showed no
long-term trend. The continuing satellite measurements of the solar
constant found it cycling within narrow limits, scarcely one part in a
thousand. Yet the global temperature rise that had resumed in the 1970s
was accelerating at a record-breaking pace, chalking up a total of 0.8°C of
warming since the late 19th century. It seemed impossible to explain that
using the Sun alone, without invoking greenhouse gases. "Over the past 20
years," a group reviewing the data reported in 2007, "all the trends in the
Sun that could have had an influence on the Earth's climate have been in the
opposite direction to that required to explain the observed rise in global
mean temperatures." It was a stroke of good luck that the rise of solar
activity since the 19th century halted in the 1960s. For if solar activity had
continued to rise, global temperatures might have climbed slightly faster —
but scientists would have had a much harder job identifying greenhouse
gases as the main cause of the global warming.
<=Modern temp's
=Milestone
The most advanced computer modeling groups did manage to reproduce
the faint influence of the sunspot cycle on climate. Their calculations
showed that since the 1970s that influence had been overtaken by the rising
effects of greenhouse gases. The modelers got a good match to maps of
the climate changes observed over the past century, but only if they
included the effects of the gases, and not if they tried to attribute it all to the
Sun. For example, if they put in only an increase of solar activity, the results
showed a warmer stratosphere. Adding in the greenhouse effect made for
stratospheric cooling (since the gases trapped heat closer to the surface).
And cooling was what the observations showed.(57*)
<=Models (GCMs)
What about global Sun-climate correlations farther back through time?
Paleontologists' studies of isotopes stemming from cosmic rays continued
to show a rough connection with the Medieval and Little Ice Age climate
anomalies. And an especially neat study of deposits in a cave in China found
a solid correlation between weather and solar activity spanning the past two
millennia. However, the correlation had broken down after 1960, just when
greenhouse gases began to kick in — evidently overwhelming weaker
influences. Painstaking studies simply failed to find any significant
correlation between cosmic rays and cloudiness. The consensus of most
scientists, arduously hammered out in a series of international workshops,
flatly rejected the argument that the soaring temperatures since the 1960s
could be dismissed as a consequence of changes on the Sun. 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."(57a)
<=International
When Foukal reviewed the question in 2006, he found no decisive evidence
that the Sun had played the central role in any climate change, not even the
Little Ice Age. The cold spells of the early modern centuries, experts were
beginning to realize, could be at least partly explained by other influences.
For one, a spate of sky-darkening volcanic eruptions that had triggered a
period of increased sea ice which reflected sunlight from the North Atlantic
region. Even more striking was evidence that the CO2 level in the
atmosphere had dipped during those centuries — perhaps because so
much farmland had reverted to carbon-absorbing forest as a result of
plagues, including the Black Death in Eurasia and smallpox in the Americas.
The greenhouse effect, even back then, looked like the dominant influence.
Still, many experts thought it likely that the Maunder Minimum of solar
activity could have had something to do with the early modern climate
anomalies, contributing perhaps a couple of tenths of a degree of cooling.
One theory, for example, held that the changes in ozone (less
ultraviolet=less ozone=less warming in the stratosphere) would have had a
particularly strong effect on the Northern Hemisphere jet stream. This
particularly affected the weather in Europe, the classic location of Little Ice
Age cold spells: perhaps low solar activity did make for colder winters there.
Whatever the mechanism, a group convened in 2012 concluded that solar
ultraviolet variations had mainly regional effects and could "contribute very
little to global temperature variations."(57b*)
A few scientists persevered in arguing that much smaller solar changes
(which they thought they detected in the satellite record) had driven the
extraordinary warming since the 1970s. But even among these outlying
groups, leaders admitted that in the future, "solar forcing could be
significant, but not dominant." Nevertheless the argument that solar activity
was the true cause of global warming continued to circulate. It was one
example of the indestructible "zombie" theories that plagued discussions.
As it happened, solar activity sank to historic lows after 2005. Some
prominent figures among the opposition to regulating greenhouse gases
publicly predicted rapid global cooling. When temperatures climbed to a
new record in 2014 (and a higher record in 2015, higher still in 2016, etc.)
while solar activity remained unusually low, only the ignorant or
disingenuous could persist in denying that greenhouse gases were the only
plausible cause.(58*)
By the 2010s the study of "solar-terrestrial relations" (as scientists called
the topic) had settled down to teasing out the subtle ways solar activity
might influence specific weather patterns. Such research required, first,
assembling and standardizing vast collections of weather data, and second,
adapting one or another of the elaborate supercomputer models of the
atmosphere to test hypotheses for complex mechanisms like ozone
interactions."(59)
The research was of interest for the perpetual enterprise of improving short-
term weather predictions, but barely relevant to climate change
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 was at least a bit sensitive to almost
imperceptible changes in the total radiation arriving from the Sun? The
planet would surely react no less strongly to changes in the interference by
greenhouse gases with the radiation after it entered the atmosphere. Some
of the scientists who reported evidence of past connections between the
Sun and climate changes warned explicitly that their data did not show that
the current global warming was natural — it only showed the extreme
sensitivity of the climate system to small perturbations.
Back in 1994 a U.S. National Academy of Sciences panel had estimated that
if solar radiation were to weaken as much as it had during the 17th-century
Maunder Minimum, the entire effect would be offset by another two
decades of accumulation of greenhouse gases. A 2010 study reported that
with the growing rate of emissions, by the late 21st century a Maunder-
Minimum solar effect would be offset in a single decade. As one expert
explained, the Little Ice Age "was a mere 'blip' compared with expected
future climatic change."(60)
For more on temperature changes over the past millennium or so, see the
conclusion and figure captions in the essay on The Modern Temperature
Trend.
=>Modern temp's
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