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
Chasing Sunspot Cycles
|| 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)
| 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
|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)
| 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) (This
was a robust tradition: into the 21st century, enthusiasts
with weird or incomprehensible theories of solar-terrestrial connections,
backed up by selected 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)
| 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 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)
||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)
| 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)
| 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)
Carbon-14 and Jack Eddy TOP
|| 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.
| 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 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.
Read the details in our Interview
| 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)
| 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
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
|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 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)
| 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)
More Sun-Climate Connections (1980s - 1990s)
|| 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)
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 rise and fall along with the sunspot number.
| 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)
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
|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.
|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
The Sun vs. Greenhouse Gases (2000s)
||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*)
=> 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.
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.
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*)
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 broke down after 1960, just when greenhouse gases began to kick in. 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
When Foukal reviewed the question in 2006, he found no decisive
evidence that the Sun had played the central role in any climate change back
to the Little Ice Age. The cold spells of the early modern centuries,
some experts insisted, could be explained by a spate of sky-darkening volcanic
eruptions and other terrestrial factors; in particular, the CO2 level in the atmosphere had dipped during those centuries (perhaps because much farmed land had reverted to carbon-absorbing forest as a result of plagues, including the Black Death in Eurasia and smallpox in the Americas?). However, many experts now found it likely that the Maunder Minimum of solar
activity must 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 stratsophere) 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: 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."(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
be similarly sensitive to greenhouse gas interference with the radiation once
it entered the atmosphere. Some of the scientists who reported evidence
of past connections between solar 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 by the late 21st century a Maunder-Minimum solar effect would be offset with barely a single decade of emissions. As one expert explained, the Little Ice Age "was a mere 'blip'
compared with expected future climatic change."(59)
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.
irradiance (energy received from the Sun) as observed directly by
satellites... minor wiggles while Earth's temperature soared. From the low to high point of a sunspot cycle, the change
in "radiative forcing" (roughly speaking, the change in energy
transferred to our planet) is equivalent to the interference
in radiation passing through the atmosphere caused by 15 years of
human emissions of carbon dioxide.
The Modern Temperature Trend
Past Cycles: Ice Age Speculations
1. This essay is partly based, by permission, on an essay by Theodore S. Feldman, "Solar Variability and Climate Change," rewritten and expanded by Spencer Weart. For additional material, see Feldman's site.
2. Feldman (1993); Fleming (1990); Herschel (1801), pp. 313-16;
on Sun-weather relations see Hufbauer (1991) and Hoyt and Schatten (1997).
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. Douglass (1936); 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
(1990). BACK Angstrom
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). Tinsley's work was stimulated by a correlation reported by Wilcox et al. (1973), which attracted some attention but grew weaker as the next decade of data accumulated.
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
and Mann (2004), p. 20, see p. 7 and passim. See also this
note on the "hockey stick" graph in the essay on the modern
temperature trend. 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. See also materials on the AGU
history site. BACK
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.
17; see also Seitz et al. (1989).
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 studies: Marsh
and Svensmark (2000); Pallé,(2001);
Harrison and Stephenson (2006).
54. Stratosphere and ultraviolet: Haigh
(1994); Rind and Balachandran (1995); Haigh (1996); McCormack
et al. (1997); Shindell et al. (1999);
Labitzke and van Loon (1999); for discussion,
see Wallace and Thompson (2002); more recently, White
(2006). Global electric circuit: Tinsley
(1996); Tinsley (2000). More recent work by Tinsley and others is reviewed by Ram et al. (2009). Arnold (2002) is an example of the complexity of the arguments and provides historical references. Two reviews are Bard and Frank (2006), p. 5; Kirkby
55. For example, correlations of cosmic
rays (as an indicator of solar activity) with Asian monsoons, Neff
et al. (2001); Wang et al. (2005); and
with North Atlantic Ocean events, Bond et al. (2001).
et al. (1999); 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). Know a lot more: Parker
(1999); cf. criticism of Parker by Hoffert
et al. (1999) BACK
56a. Reid (1991); White (1998); White et al. (1997); Lean and Reid (2001); upper atmosphere: van Loon and Shea (2000). BACK
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." Similarly see Wang et al. (2005).
"Over the past 20 years": Lockwood and Fröhlich
(2007); another review: Bard and Frank (2006),
on model sensitivity see p. 7. BACK
57a. Isotopes (carbon-14 and beryllium-10): Bard et al. (2003). Chinese (from a single speleothem, sensitive to monsoon
variations): Zhang et al. (2008). Consensus:
IPCC (2001). "30%": Solanki
et al. (2004), p. 1087; their contention that recent solar activity was unprecedented in the past eight millennia was disputed by Muscheler et al. (2005). BACK
57b. Foukal et
al. (2006). Volcanoes: Shindell et al. (2001); Shindell et al. (2003), Miller et al. (2012); for lower CO2 level Indermühle et al. (1999), Ruddiman (2005); other factors: e.g.,
change of surface brightness due to deforestation, Goosse
et al. (2006). On Sun-weather mechanisms see Meehl et al. (2009). Low solar activity correlates with cold European winters: Lockwood et al. (2010); also, low UV brings cold European and N. American winters: Ineson et al. (2011). Contribute very little: National Research Council (2012). BACK
58. Willson, reporting a brightening of 0.04 percent
between the two most recent solar cycles, Willson
(1997); this was controversial, see Kerr
(1997); similarly Willson and Mordvinov (2003), discussed by
Byrne (2003). Experiments:). A few groups pursued the study of possible mechanisms, for example elaborating theories of how changes in the atmosphere's electric circuit, which varies with the flux of cosmic rays, affected precipitation in the Arctic, Ram et al. (2009), or devising experiments that they hoped would show a direct and strong effect of cosmic rays on clouds, e.g., Svensmark
et al. (2007) (which brought a strong press reaction but proved little). BACK
59. For Gerard Bond's warning about sensitivity
see Pearce (2007c), p. 164; similarly Lockwood
and Fröhlich (2007). National Research
Council (1994), combining statements on pp. 3 and 4; late 21st century: Feulner and Rahmstorf (2010);"blip:" Nelson (1997); similarly see Wigley
and Kelly (1990), p. 558. BACK
© 2003-2013 Spencer Weart & American Institute of Physics