henryk svensmark: cosmoclimatology, a new perspective
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SvenSmark:CosmoClimatology
1.18 A&G•February2007•Vol.48
Data on cloud cover rom satellites, com-
pared with counts o galactic cosmic
rays rom a ground station, suggested
that an increase in cosmic rays makes the world
cloudier. This empirical nding introduced a
novel connection between astronomical and
terrestrial events, making weather on Earth
subject to the cosmic-ray accelerators o super-
nova remnants in the Milky Way. The result wasannounced in 1996 at the COSPAR space science
meeting in Birmingham and published as “Varia-
tion o cosmic-ray fux and global cloud coverage
– a missing link in solar-climate relationships”
(Svensmark and Friis-Christensen 1997).
The title refected a topical puzzle, that o how
to reconcile abundant indications o the Sun’s
infuence on climate (e.g. Herschel 1801, Eddy
1976, Friis-Christensen and Lassen 1991), with
the small 0.1% variations in the solar irradiance
over a solar cycle measured by satellites. Clouds
exert (on average) a strong cooling eect, and
cosmic-ray counts vary with the strength o thesolar magnetic eld, which repels much o the
infux o relativistic particles rom the galaxy.
The connection oers a mechanism or solar-
driven climate change much more powerul than
changes in solar irradiance.
During the past 10 years, considerations o the
galactic and solar infuence on climate have pro-
gressed so ar, and have ound such widespread
applications, that one can begin to speak o a
new paradigm o climate change. I call it cosmo-
climatology and in this article I suggest that it is
already at least as secure, scientically speaking,
as the prevailing paradigm o orcing by variablegreenhouse gases. It has withstood many attempts
to reute it and now has a grounding in experi-
mental evidence or a mechanism by which cosmic
rays can aect cloud cover. Cosmoclimatology
already interacts creatively with current issuesin solar–terrestrial physics and astrophysics and
even with astrobiology, in questions about the ori-
gin and survival o lie in a high-energy universe.
All these themes are pursued in a orthcoming
book (Svensmark and Calder 2007).
How do cosmic rays help make clouds?
The comparisons o data on clouds and cosmicrays, with which the story began, continued to
pay o. They conrmed that cloudiness is more
clearly linked with solar-modulated galactic cos-
mic rays than with other solar phenomena such
as sunspots or the emissions o visible light, ultra-
violet and X-rays (Svensmark 1998). A big step
orward came with the realization that the lowest
clouds, below about 3 km in altitude, respond
most closely to variations in the cosmic rays
(Marsh and Svensmark 2000), a counter-intui-
tive nding or some critics (e.g. Kristjansson and
Kristiansen 2000). Figure 2 compares data rom
the International Satellite Cloud ClimatologyProject and the Huancayo cosmic-ray station.
There is no correlation at high and middle alti-
tudes, but an excellent match at low altitudes.
In gure 3, the correspondence between low
clouds and cosmic rays is seen to persist over
a longer timescale. A simple interpretation isthat there are always plenty o cosmic rays high
in the air, but they and the ions that they liber-
ate are in short supply at low altitudes, so that
increases or decreases due to changes in solar
magnetism have more noticeable consequences
lower down.
The involvement o low-level clouds providedan experimental opportunity. The chie objection
to the idea that cosmic rays infuence cloudiness
came rom meteorologists who insisted thatthere was no mechanism by which they could
ChangesintheintensityogalacticcosmicraysaltertheEarth’scloudiness.Arecentexperimenthasshownhowelectrons
liberatedbycosmicraysassistinmakingaerosols,thebuildingblocksocloudcondensationnuclei,whileanomalousclimatictrendsinAntarcticaconrmtheroleocloudsinhelpingtodriveclimatechange.Variationsinthecosmic-rayinfuxduetosolarmagneticactivityaccountwellorclimaticfuctuationsondecadal,centennialandmillennialtimescales.Overlongerintervals,thechanginggalacticenvironmentothesolarsystemhashaddramaticconsequences,includingSnowballEarth
episodes.AnewcontributiontotheaintyoungSunparadoxisalsoonoer.
abStract
Cosmoclimatology: a
1: Cosmic rays (relativistic electrons) stirring in the Cassiopeia A supernova remnant make the wispyblue lines o energetic X-ray emissions seen by NASA’s Chandra X-ray Observatory. (NASA/CXC/UMassAmherst/M D Stage et al.)
Henrik Svensmark drawsattention to an overlookedmechanism o climate change:clouds seeded by cosmic rays.
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SvenSmark:CosmoClimatology
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do so. On the other hand, some atmosphericphysicists conceded that observation and theory
had ailed to account satisactorily or the ori-
gin o the aerosol particles without which water
vapour is unable to condense to make clouds. A
working hypothesis, that the ormation o these
cloud condensation nuclei might be assisted by
ionization o the air by cosmic rays, was open to
microphysical investigation by experiment.
Experimental tests
In 1998 Jasper Kirkby at the CERN particlephysics lab in Geneva proposed an experiment
called CLOUD to investigate the possible roleo cosmic rays in atmospheric chemistry. Theidea was to use a beam o accelerated particles
to simulate the cosmic rays, and to look or aero-
sols produced in a reaction chamber containing
air and trace gases. The temperature and pres-
sure would be adjustable to simulate conditions
at dierent levels in the atmosphere. Kirkby
assembled a consortium o 50 atmospheric,
solar–terrestrial and particle physicists rom
17 institutes to implement it (CLOUD proposal
2000), but regrettably there were long delays in
getting the project approved and unded. The
go-ahead eventually came in 2006 and the ull
experiment at CERN should begin taking data
in 2010.
Meanwhile, in Copenhagen, the discovery
that low-level clouds are particularly aected
by cosmic-ray variations suggested that a sim-
pler experiment, operating only at sea-level
temperature and pressure, might capture some
o the essential microphysics. Instead o a par-
ticle beam, we used natural cosmic rays, sup-plemented by gamma rays when we wanted to
check the eect o increased ionization o the air.
Our team set up the experiment in the basemento the Danish National Space Center, with a large
plastic box containing puried air and the trace
gases that occur naturally in unpolluted air over
the ocean. Ultraviolet lamps mimicked the Sun’s
rays. During experiments, instruments traced the
chemical action o the penetrating cosmic rays in
the reaction chamber. We called the experiment
SKY, which means “cloud” in Danish.
By 2005 we had ound a causal mechanismby which cosmic rays can acilitate the produc-
tion o clouds (Svensmark et al. 2007). The data
revealed that electrons released in the air by
cosmic rays act as catalysts. They signicantlyaccelerate the ormation o stable, ultra-small
clusters o sulphuric acid and water molecules
which are building blocks or the cloud conden-
sation nuclei. Figure 4 shows a typical run. Vast
1985 1990 1995 2000 2005
year
27
28
29
30
l o w
c l o u d a m o u n t ( % )
–20
–10
0
10
c o s m i c r a y ( % )
new theory emerges
c l o u d a n o m a l i e s ( % )
1980 1985 1990 1995
1980 1985 1990 1995
1980 1985 1990 1995
year
c o s m i c r a y s ( % )
high
middle
low
c l o u d a n o m a l i e s ( % )
c l o u d a n o m a l i e s ( % )
c o s m i c r a y s ( % )
c o s m i c r a y s ( % )
1.5
1.0
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–0.5
–1.0
–1.5
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–0.5
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–0.5
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–1.5
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–5
–10
0
–5
–10
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–5
–10
2: At dierent levels in theatmosphere (high >6.5 km,middle 6.5–3.2 km andlow <3.2 km) the blue lineshows variations in globalcloud cover collated bythe International SatelliteCloud Climatology Project.The red line is the recordo monthly variations incosmic-ray counts at theHuancayo station. Whilethere is no match at thehigher altitudes, a closecorrespondence between
cosmic rays and clouds lowin the atmosphere is plain tosee. (Marsh and Svensmark2000)
3: As in gure 2, the low-cloud comparison extendsover a longer period.
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SvenSmark:CosmoClimatology
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numbers o such microscopic droplets appeared
in the air o the reaction chamber, and their pro-
duction increased proportionately when we used
gamma rays to induce more ionization (gure 4,
bottom). The speed and eciency with which
the electrons do their work o stitching molecu-
lar clusters together took us by surprise. It is a
mechanism previously unknown in meteorologyand it brings the cosmos into climate studies in a
precise microphysical way.
Do clouds really drive climate change?
Low-level clouds cover more than a quarter o
the Earth and exert a strong cooling eect at the
surace. (For clouds at higher altitudes there is
a complicated trade-o between cooling andwarming.) The 2% change in low cloud during
a solar cycle, as seen in gure 3, will vary theinput o heat to the Earth’s surace by an aver-
age o about 1.2 W m–2, which is not trivial. It
can be compared, or example, with 1.4 W m–2 attributed by the Intergovernmental Panel on Cli-
mate Change or the greenhouse eect o all o
the additional carbon dioxide in the air since the
Industrial Revolution (Houghton et al. 2001).
I cosmic-ray counts merely went up and down
with the 11-year cycle o solar activity, there
would be no trend in the climate. Systematic
records o infux to the Earth’s surace go back
to 1937. Cosmic-ray changes beore then can
be seen in the rate o ormation o radioactive
isotopes such as beryllium-10, or inerred rom
the Sun’s open coronal magnetic eld. As seen
in gure 5, the various methods agree that there
was a pronounced reduction in cosmic rays in
the 20th century, such that the maximal fuxes
towards the end o the century were similar
to the minima seen around 1900. This was in
keeping with the discovery that the Sun’s coronal
magnetic eld doubled in strength during the
20th century (Lockwood et al. 1999).
Here is prima acie evidence or suspecting
that much o the warming o the world dur-
ing the 20th century was due to a reduction in
cosmic rays and in low-cloud cover. But distin-
guishing between coincidence and causal actionhas always been a problem in climate science.
The case or anthropogenic climate changeduring the 20th century rests primarily on the
act that concentrations o carbon dioxide and
other greenhouse gases increased and so didglobal temperatures. Attempts to show that cer-
tain details in the climatic record conrm the
greenhouse orcing (e.g. Mitchell et al. 2001)have been less than conclusive. By contrast, the
hypothesis that changes in cloudiness obedient
to cosmic rays help to orce climate change pre-
dicts a distinctive signal that is in act very easily
observed, as an exception that proves the rule.Cloud tops have a high albedo and exert their
cooling eect by scattering back into the cosmos
much o the sunlight that could otherwise warm
the surace. But the snows on the Antarctic ice
l o w
c l o u d c o v e r ( % )
0
1700 1750 1800 1850 1900 1950 2000
year
c o s m i c r a y f l u x c h a n g e ( % )
30
10
20
40
30
27
28
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29
a e r o s o l s ( c m
– 3 )
H 2
S O
4
c o n c e n t r a t i o n
( 1 0 7 c
m – 3 )
0 1 2 3 4
t (hours)
a e r o s o l s ( c m – 3 )
ion density (cm–3)
n u c l e a t i o n r a t e ( s – 1 c
m – 3 )
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0 1000 2000 3000 4000 5000 6000 7000
4: Creation o thebuilding blocks o cloudcondensation nuclei (top)under the infuence ogalactic cosmic rays is seenin a typical run in the SKYexperiment in Copenhagen(Svensmark et al. 2007).
At time 0 a burst o UVlight (simulating solar UV)triggers the ormation,rom trace gases in the air,o sulphuric acid moleculesshown by the blue areaand the right-hand scale.Within 10 minutes, greatnumbers o molecularclusters o sulphuric acidand water molecules largerthan 3 nm appear in thereaction chamber (blackcurve and let-hand scale).The red curve is the t o
a simple model combiningrates o production oclusters and their loss onthe walls o the chamber.The lower diagram showsthe linear relation betweenion density and aerosolnucleation. Red stars arethe experimental data. Inthe real atmosphere, inthe absence o walls, theclusters would grow in amatter o hours to becomecloud condensation nuclei.
5: Changes in the fux ogalactic cosmic rays since1700 are here derived romtwo independent proxies,10Be (light blue) and opensolar coronal fux (blue)(Solanki and Fligge 1999).Low cloud amount (orange)rom gure 3, is scaled andnormalized to observationalcosmic-ray data rom Climax(red) or the period 1953 to2005 (3 GeV cut-o). Notethat both scales are invertedto correspond with risingtemperatures. The long-termchange in the average fuxis as large as the temporaryvariation within one solarcycle. The change in radiativeorcing by a 3% change in lowcloud amount over this periodcan be estimated to ~2 W m–2.
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sheets are dazzlingly white, with a higher albedo
than the cloud tops. There, extra cloud coverwarms the surace, and less cloudiness cools it.
Satellite measurements show the warming eect
o clouds on Antarctica, and meteorologists at
ar southern latitudes conrm it by observa-
tion. Greenland too has an ice sheet, but it is
smaller and not so white. And while conditionsin Greenland are coupled to the general climate
o the northern hemisphere, Antarctica is largely
isolated by vortices in the ocean and the air.
The cosmic-ray and cloud-orcing hypothesis
thereore predicts that temperature changes in
Antarctica should be opposite in sign to changes
in temperature in the rest o the world. This is
exactly what is observed, in a well-known phen-
omenon that some geophysicists have called the
polar see-saw, but or which “the Antarctic cli-
mate anomaly” seems a better name (Svensmark
2007). To account or evidence spanning many
thousands o years rom drilling sites in Antarc-tica and Greenland, which show many episodes
o climate change going in opposite directions,
ad hoc hypotheses on oer involve major reor-
ganization o ocean currents. While they might
be possible explanations or low-resolution cli-
mate records, with error-bars o centuries, they
cannot begin to explain the rapid operation o
the Antarctic climate anomaly rom decade to
decade as seen in the 20th century (gure 6).
Cloud orcing is by ar the most economical
explanation o the anomaly on all timescales.
Indeed, absence o the anomaly would have
been a decisive argument against cloud orcing
– which introduces a much-needed element o
reutability into climate science.
Does climate ollow cosmic-rays?
Figure 5 takes the climate record back 300
years, using rates o beryllium-10 production
in the atmosphere as long-accepted proxies or
cosmic-ray intensities. The high level at AD 1700
corresponds with the Maunder Minimum (1645–
1715) when sunspots were extremely scarce
and the solar magnetic eld was exceptionally
weak. This coincided with the coldest phase o what historians o climate call the Little Ice Age
(Eddy 1976). Also plain is the Dalton Minimum
o the early 19th century, another cold phase.
The wobbles and the overall trend seen in gure
5, between cold 1700 and warm 2000, are just a
high-resolution view o a climate-related switch
between high and low cosmic-ray counts, o a
kind that has occurred repeatedly in the past.
Iciness in the North Atlantic, as registered by
grit dropped on the ocean foor rom driting and
melting ice, is a good example o the climate data
now available. Gerard Bond o Columbia Univer-
sity and his colleagues showed that, over the past12 000 years, there were many icy intervals like
the Little Ice Age – eight to ten, depending on how
you count the wiggles in the density o ice-rated
debris. These alternated with warm phases, o
which the most recent were the Medieval Warm
Period (roughly AD 900–1300) and the Modern
Warm Period (since 1900). A comparison with
variations in carbon-14 and beryllium-10 pro-
duction showed excellent matches between high
cosmic rays and cold climates, and low cosmic
rays and the warm intervals (Bond et al. 2001).
For these authors, here was persuasive evi-dence or what they called “a persistent solarinfuence”. But like many other investigators o
astronomical actors in climate change, Bond et
al. regarded the cosmic rays merely as indica-
tors o the magnetic state o the Sun, varyingin a quasi-periodic ashion and aecting solar
irradiance. High cosmic rays signied a ainter
Sun. Although the problem o how small changes
in irradiance could exert so big an infuenceremained, the proposition that the cosmic rays
themselves act on the climate more powerully,
by governing cloudiness, was set aside.Two o Bond’s co-authors, Jurg Beer and
Raimund Muscheler o the Swiss Federal Insti-
tute o Environmental Science and Technology,
use radionuclides to explore solar and climatic
cosmic rays coming from exploding stars in the Milky Way
highest energy intermediate energy lowest energy
Sun’s
magnetic
field
Earth’s
magnetic
field
Earth’s
atmosphere
60% from
high energies
37% from
intermediate energies
3% from
low energies
muons at low altitudes
1900 1920 1940 1960 1980 2000
year
1
0
–1
T ( n o r t h e r n h e m i s p h e r e d e g . )
T ( 6
4 S – 9 0 S d e g . )
0
–1
1
6: The Antarctic climate anomalyduring the past 100 years isapparent in this comparison othe annual surace temperatureanomalies or the northernhemisphere and Antarctica(64°S–90°S), rom the NASA-GISScompilations. The Antarctic data
have been averaged over 12 yearsto minimize the temperaturefuctuations. The blue and red linesare ourth-order polynomial ts tothe data. The curves are oset by1 K or clarity, otherwise they wouldcross and re-cross three times.(Svensmark 2007a)
7: The most important cosmic-rayparticles that assist in cloud-making in the lower atmosphere(<2 km), the muons, originatemainly rom particles that arriverom the stars with very highenergy. The magnetic deences othe Sun and the Earth have littleeect on them. The Sun’s magneticeld infuences the supply o alarge minority o muons, but eware obedient to changes in theEarth’s magnetic eld (adaptedrom Svensmark and Calder 2007).Ionization o the air by cosmic raysbelow 2 km altitude is due mainlyto penetrating showers romhigh-energy primary cosmic rays,
according to calculations usingthe Karlsruhe CORSIKA program(Svensmark and Svensmark2007). The boxes are only intendedto illustrate the energy rangethat the Earth’s magnetic eldcan modulate. For example, atpolar regions there is only smallinfuence and i the geomagneticeld disappears, the expectedincrease in low-altitude ionizationis only 3%. Solar activity infuencesthe primaries responsible or about37% o the ionization. The entirerange o primary energies is subject
to large changes rom the galacticenvironment o the solar system,including local supernovae, spiral-arm passages, and variations in thestar-ormation rate.
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SvenSmark:CosmoClimatology
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variations in the past. Although the climate
changes o the last 12 000 years have indeed
ollowed the cosmic-ray variations, Beer andMuscheler were already co-authors o a paper
arguing strongly that the cosmic rays were not
the driver (Wagner et al. 2001). They had striking
evidence rom 40 000 years ago, in the Laschamp
Event when the geomagnetic eld became veryweak, in what may have been a ailed reversal o
the eld. Without the screening eect o the geo-
magnetic eld, the cosmic-ray infux increased
dramatically. In a Greenland ice core, the counts
o beryllium-10 and chlorine-36 atoms produced
by cosmic rays went up by more than 50% –and
no cooling ensued. The result was compelling
because the climate indicators – oxygen-18 and
methane abundances – came rom the same lay-
ers o ice as the radionuclides.
This clear example o the climate ailing to ol-
low the cosmic-ray variations was challenging.
No quantitative answer was orthcoming untilrecent calculations traced the origin o the pen-
etrating muons that are responsible or most o
the ionization o the air at low altitudes (Sven-
smark and Svensmark 2007). Then a clear and
consistent picture (gure 7) emerged rom the
CORSIKA program developed or the Karlsruhe
Shower Core and Array Detector o Forschung-
szentrum Karlsruhe, at progressively higher ener-
gies o the incoming primary cosmic rays.
Most o the penetrating muons come rom rela-
tively rare primaries o such high energy that they
are indierent even to the solar magnetic eld.
Primaries o low enough energy to be repelled
by the geomagnetic eld account or only 3% o
the low-altitude muons. So it is unsurprising that
the near-disappearance o the geomagnetic eld,
whether in Laschamp-type events or ull revers-
als, should have little eect on climate compared
with changes due to solar modulation.
On the other hand, radionuclides are mainly
produced higher in the atmosphere, by cosmic
rays o lower energy that are more susceptible
to variations in the geomagnetic eld. Although
they remain invaluable or registering cosmic-ray
changes due to solar variability, as in gure 5 orexample, radionuclides can no longer be taken as
inallible guides to climatically eective cosmic
radiation, when either the geomagnetic or the
galactic environment changes.
Evidence in summary
This article so ar has summarized the evidence
or the climatic role o cosmic rays, which under-
pins cosmoclimatology:
Observations o variations o low cloud cover
correlated with cosmic-ray variations;
Experimental evidence or the microphysical
mechanism whereby cosmic rays accelerate theproduction o cloud condensation nuclei;
The Antarctic climate anomaly as a symptom
o active orcing o climate by clouds;
Quasi-periodic climate variations over thou-
sands o years that match the variations in radio-
nuclide production by cosmic rays;
Calculations that remove an apparent diculty
associated with geomagnetic eld variations.
From this secure base, we can broaden the
horizons o space and time to consider the rel-
evance o cosmic rays to climate change since
the Earth was young. The climatic eects o
the Sun’s quasi-cyclical variations on millennial
timescales are seen throughout the Phanerozoic
(Elrick and Hinnov 2006). But more emphatic
changes in climate become apparent on longer
timescales when the galactic environment o
the solar system changes and the variations in
the cosmic-ray fux are an order o magnitude
greater than those due to the Sun.
What drove the big alternations?
Large, slow swings, to and ro between ice-ree
and glaciated climates, are evident in the geologi-
cal record o the past 550 million years. Eorts
went into using the greenhouse-warming para-
digm to try to account or these changes, but thepattern was wrong. There were our alternations
between hothouse and icehouse conditions dur-
ing the Phanerozoic, while reconstructions o atmospheric carbon dioxide show just two major
peaks (Cambrian-Devonian and Mesozoic) and
troughs (Carbonierous-Permian and Cenozoic).
A more persuasive explanation comes rom cos-
moclimatology, which attributes the icehouseepisodes to our encounters with spiral arms o
the Milky Way galaxy, where explosive blue stars
and cosmic rays are more concentrated.
Nir Shaviv, an astrophysicist at the Hebrew
University in Jerusalem, pioneered this inter-pretation several years ago (Shaviv 2002). The
relative motion o the spiral arm pattern with
respect to the solar orbit around the galactic
centre was uncertain, but Shaviv ound that
reasonable assumptions gave a good t with
the climatic record, in cycles o ~140 millionyears. He ound independent evidence linking
the icehouse episodes with high cosmic radiation
in a ~140 million-year cycle o clustering o the
apparent exposure ages o iron meteorites by
cosmogenic potassium isotope ratios (41K/ 40K).
Later, Shaviv joined orces with a geologist, Jan
Veizer o the Ruhr University and the University
o Ottawa, to rene the analysis using a large
database on tropical sea-surace temperatures,
as seen in gure 8 (Shaviv and Veizer 2003). The
matches between spiral-arm encounters and ice-
house episodes are as ollows:
Perseus Arm: Ordovician to Silurian Periods;
Norma Arm: Carbonierous;
Period Scutum-Crux Arm: Jurassic to Early
Cretaceous Periods;
Sagittarius-Carina Arm: Miocene Epoch, lead-
ing almost immediately (in geological terms) to
Orion Spur: Pliocene to Pleistocene Epochs.
The Jurassic to Early Cretaceous icehouse is a
matter o special interest. Until recently, geolo-gists considered the Mesozoic Era to have been
warm throughout, so when Shaviv rst saw that
his analysis needed that icehouse, he was disap-
pointed. Then he was reassured by recent reports
o signs o ice-rating, just as required. The rst
clear evidence or glaciers ~140 million years
ago (Australia, Early Cretaceous) was published
in 2003. That a story should become better as
the data improve is characteristic o a successul
paradigm. For the greenhouse theory o climate
change, on the other hand, the Mesozoic glacia-
tion was bad news, because carbon dioxide con-
centrations in the atmosphere were high at thetime. The comparative mildness o the Mesozoic
icehouse may have been due to the carbon dioxide
(Royer et al. 2004) or perhaps to a relatively quick
crossing o the Scutum-Crux Arm.
8
4
0
–4 s e a - s u r f a c e t e m p e r a t u r e a n o m a l y ( d e g . )
500 400 300 200 100 0
time (million years BP)
0.0
0.5
1.0
1.5
r e l a t i v e c o s m i c - r a y f l u x
P e r s e u s s p i r a l a
r m
N o r m a s p i r a l a r m
S c u t u m -
C r u x s p
i r a l a r m
S a g - C a r s p i r a l a
r m
i c e h o u s e
i c e h o u s e
i c e h o u s e
i c e h o u s e
h o t h o u s e
h o t h o u s e
h o t h o u s e
h o t h o u s e
8: Four switchesrom warm“hothouse” tocold “icehouse”conditions duringthe Phanerozoicare shown invariations o
several degreesK in tropicalsea-suracetemperatures(red curve). Theycorrespond withour encounterswith spiral armso the Milky Wayand the resultingincreases in thecosmic-ray fux(blue curve, scaleinverted). (AterShaviv and Veizer
2003)
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SvenSmark:CosmoClimatology
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Turn the reasoning around, and geophysical
data can help to rene the astronomical descrip-
tion o the galaxy. When the ossil organisms
in carbonate rocks were alive in the near-sur-
ace water they were, in eect, cosmic-ray
telescopes, registering the fux changes as theinverse o the sea-temperature changes logged
by oxygen-18 counts. Besides the eects o spi-
ral-arm encounters, the oxygen-18 record shows
higher-requency changes in temperature associ-
ated with the Sun’s oscillations about the galactic
mid-plane, and the temperatures set tight limits
on the dynamics (Svensmark 2006a).
The coolest phase every 34 Myr or so corre-
sponds with a crossing o the mid-plane, where
cosmic rays are locally most intense, and it is
well dated by geologists. The concentrations o
mass near the mid-plane aect the Sun’s oscilla-
tions, and they are dierent inside and outside
the spiral arms. Figure 9 gives an overview o
the Milky Way and the Sun’s orbit or the past
200 Myr. Only one combination o key num-
bers describing the galactic environment givescorrect cross-plane motions o the Sun needed
to match the climate changes recorded by the
ossil organisms during that period. The results
o the analysis, seen in table 1, all inside a wide
range o previous suggestions rom astronomical
data, but narrow them down decisively rom a
geophysical point o view.
Why did the Earth reeze over?
The discovery o widespread glaciation in the trop-
ics during the Proterozoic, in the Snowball Earth
episodes around 2300 million and 700 million
years ago, set a conundrum or traditional climatetheory. To be explained is not only what might
have caused such events, but why they occurred
just when they did, when the Earth was 50% and
15% o its present age. And why was there a long
warm interval between them with no icy inter-
ludes like those seen in the past 550 million years?
Again, cosmoclimatology provides a cosmic time-
rame that promises to explain it all.
Increases in the rate o star ormation in the
Milky Way galaxy, associated with close encoun-
ters with the Magellanic Clouds, must haveaected the cosmic-ray fux to the Earth, because
o the increased number o supernovae – as seen,
or example, in starburst galaxies. As or the cli-
matic consequences, Nir Shaviv in Jerusalem
pointed out that the Early Proterozoic Snowball
Earth episode coincided with the highest star-or-
mation rate in the Milky Way since the Earth was
ormed, in a mini-starburst 2400–2000 million
years ago (Shaviv 2003a).
In data rom Rocha-Pinto et al. (2000), Shaviv
noted star ormation at twice the present rate
during the Early Proterozoic, ollowed by a
billion-year lull when the rate dropped to hal
o what it is now. That can explain the long ice-
ree interval in the Proterozoic, when visits to
the spiral arms ailed to deliver enough cosmicrays to create icehouse conditions. Although the
Rocha-Pinto data showed a moderate restora-
tion o the star-ormation rate in time or thelater Snowball Earth events, more striking results
came rom Marcos and Marcos (2004). These
authors noted a peak around 750 million years
ago as the highest rate o star ormation during
the past 2 billion years and remarked on its apt-
ness or the Snowball Earth scenario.
Going back 4000 million years, to when theSun and Earth were young, the puzzle is why the
Earth was not rozen then. The Sun’s luminosity
was, according to the standard model o solarevolution, less than 75% o its present value, the
Earth’s mean surace temperature should have
been 25 K cooler than now. Yet there is mineral
evidence or liquid water 4400 million years ago,
and the oldest remains o lie, 3800 million years
old, are ound in sediments rom an ancient sea.
This paradox o the aint young Sun has been
discussed or more than 30 years.
However, revised solar models suggest a near-
absence o cosmic rays (Shaviv 2003b, Sven-
smark 2003). With a ar more vigorous solar
wind, the young Sun would have reduced the
infux to such a small raction o the present rate
that, in the cosmoclimatological interpretation,
the Earth would not have had much cloud cover.
That would compensate or much o the weak-
ness o the solar irradiance, and so reduce the
contribution required rom greenhouse gases.
A surprising by-product o this line o enquiryis a new perspective on the changing ortunes o
lie over 3.5 billion years (Svensmark 2006b).By combining calculations about the changing
ability o the Sun to repel cosmic rays with data
on the changing star-ormation rate, one canreconstruct the resulting cosmic-ray fux. Figure
10 compares the reckonings with data rom an
entirely dierent source, concerning variability
in the overall productivity o the biosphere,gauged by the proportion o carbon-13 in car-
bonate rocks. The biggest fuctuations in prod-
uctivity between boom and bust coincided with
the highest cosmic-ray rates. Conversely, duringthe billion years when star ormation was slow
and cosmic rays were less intense, the biosphere
was almost unchanging in its productivity. This
reveals a link more subtle than any straight-
φ1
–15 –10 –5 0 5 10 15Kpc
K p c
Perseus
S g r - C a r i n a
Scutum-Crux
Norma
SunOrionspur
15
10
5
0
–5
–10
–15
φ
φ2
1: What does theclimate tell us aboutthe Milky Way?
Mass density
ρlocal(now) 0.115±0.1M⊙pc–3
ρ– 0.145±0.1M⊙pc–3
ρarm/ρinterarm ≈1.5–1.8
Timing o spiral-arm crossings
Sgr-Car 34±6MyrScrutum-Crux142±8Myr
Pattern speed
ΩP 13.6±1.4kms–1kpc–1φ1 25±10°φ2 100±10°
Properties o the Milky Way derived romvariation in Earth’s temperature during thelast 200 million years (Svensmark 2006a).Due to the solar system’s position within theMilky Way it is dicult to obtain undamentaldynamical parameters by astronomicalobservations. The arm/inter-arm ratio citedhere is in the range seen in other spiralgalaxies. The pattern speed is the angularvelocity o the spiral arms.
9: The Sun’s motion relativeto the galaxy’s spiral armsover the past 200 millionyears is dened by changesin the angle φ. The mostrecent spiral-arm crossingsφ1 andφ2 were at ~100° and~25°. Climatic data show
rhythmic cooling o the Earthwhenever the Sun crossedthe galactic midplane,where cosmic rays arelocally most intense. Fromthese geophysical data, theastrophysical data shownin table 1 can be inerred.(Svensmark 2006a)
7/28/2019 Henryk SVENSMARK: Cosmoclimatology, a new perspective
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1.24 A&G•February2007•Vol.48
orward idea o, say, a warm climate being
lie-riendly or a cold climate deadly. It may be
related to the better recycling o trace elements in
cold conditions (see Svensmark 2006b).
What remains or investigation?
The past 10 years have seen the reconnaissance
o a new area o research by a small numbero investigators. The multidisciplinary natureo cosmoclimatology is both a challenge and an
opportunity or many lines o inquiry. The inter-
action between dierent branches o science is
no mere exchange o text-book inormation, but
takes place at the cutting edge o discovery. An
example comes rom the astrophysics o Gould’s
Belt, the tumultuous region o the galaxy into
which the solar system has wandered.
The possibility that cosmic rays rom a nearby
supernova provoked the onset o northern glaci-
ation 2.75 million years ago was mooted by Knie
et al. 2004. This is o special interest because o the replacement o some Arican orests by grass-
land and the emergence o human beings. The
earliest known stone tools date rom 2.6 million
years ago. Whether or not the particular event
cited by Knie et al. was responsible, gamma-ray
astronomers are alert to the need to identiy
supernova events within Gould’s Belt during the
past ew million years i climate change on that
timescale is to be ully understood.
Better knowledge o the spiral arms and star-
ormation history o the galaxy should clariy
the climate connection over longer spans o time,
and ESA’s Gaia mission can be expected to make
a big stride orward. At the same time, the onus
alls on Earth scientists to improve knowledge
o climate history beore 200 million years ago
with an on-shore drilling programme to match
the scope and success o ocean-foor drilling
(Soreghan et al. 2005). These very dierent
ventures in astronomy and geophysics should
converge, and also be o great interest to palae-
ontologists and evolutionary biologists.
The Milankovitch changes in the Earth’s atti-
tude and orbit show up persistently in the oxy-
gen-18 record o recent ice ages, notably at highlatitudes. They pose conundrums or both the
greenhouse and cosmic-ray theories o climate
change. Large rises in temperature within theglacial periods, related to cosmic-ray decreases,
do not melt the main ice sheets. Terminations
leading to interglacial conditions seem to need
an insolation trigger, whether rom obliquity(40-kyr cycle) in the early Pleistocene or ellip-
ticity (100-kyr cycle) in the late Pleistocene. To
account or high rates o deglaciation associated
with particular insolation requencies, ampli-
ying mechanisms on oer include surges in
atmospheric carbon dioxide (Shackleton 2000)and changes in ice-sheet geography (Raymo et al.
2006). In Copenhagen we hope to use simple cli-
mate models o the glacial cycles to test whether
geographical actors, including ice-sheet extent
and sea-level changes, may account or variable
sensitivity to, or resonance with, climate orcing
by orbital changes, clouds or greenhouse gases.
The physics o the Sun and the heliosphere runs
through the story on all timescales rom the early
Earth to the present day. Whatever the verdict
may be about the relative importance o cosmic
rays and greenhouse gases in current and uture
climate change, there is an obvious need to pre-
dict uture solar behaviour better, by clearer
observations o the magnetic eld at the Sun’s
poles. There are already strong hints that taking
cosmic rays into account should help to improve
the annual orecasts o the Asian monsoon.
The complete checklist o uture research con-
cerning cosmic rays and climate ranges rom
more thorough investigation o aerosol chemistry
(as promised by CLOUD) to the implications or
astrobiology and the search or alien lie. Besides
the traditional “Goldilocks” comort zone set by
stellar irradiance, it now seems clear that stel-lar winds and magnetism are crucial actors in
the origin and viability o lie on wet Earth-like
planets. So are the ever-changing galactic envi-
ronments and star-ormation rates. The tropical
glaciers o Snowball Earth tell us that survival
was a close-run thing, even here.
Henrik Svensmark, Director, Center for Sun-Climate
Research in the Danish National Space Center,
Copenhagen, Denmark; [email protected].
I thank Nigel Calder FRAS for suggesting this article
and assisting in its preparation.
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bacteria
Eukarya
Proterozoic
glacial gap
4 3 2 1 0time (billion years BP)
relativechan
geincosmic-rayfl
ux
Metazoa
glaciations
life
forms
e a
r l y b
o m b
a r d m
e n t
s i z e o f f l u c t u a t i o n s i n b i o m a s s
1.0
2.0
4.0
2.0
6.0
0.00.0
10: When lie beganabout 3.8 billion yearsago, the cosmic-rayfux (blue curve) wasvery low, because othe vigour o the solarwind. Complex lie orms(single-cell eukarya
and multi-cell metazoa)rose to success duringglobal glaciations,which coincided withhigh cosmic rayslinked to high star-ormation rates. Thered curve shows thesize o variations inthe productivity o thebiosphere, which wasmost erratic when thecosmic-ray fux wasgreatest. (Svensmark2006b and reerences
therein)
SvenSmark:CosmoClimatology