Henryk SVENSMARK: Cosmoclimatology, a new perspective

7/28/2019 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.




A&G•February2007•Vol.48 1.19

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

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l o w

c l o u d a m o u n t ( % )

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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

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year

c o s m i c r a y s ( % )

high

middle

low





1.5

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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.





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 ( % )

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a e r o s o l s ( c m

– 3 )

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( 1 0 7 c

m – 3 )

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t (hours)

a e r o s o l s ( c m – 3 )

ion density (cm–3)

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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.





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.





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)





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)




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.

Reerences

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1999Nature939437–439.Marcos R de la F and Marcos C de la F2004New

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Mitchel J H Bet al.2001Chapter12pp695–735in

Houghtonet al.2001.

Raymo M E 2006 Science 313492–495.

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Shackleton N J 2000 Science 2891897–1902.Shaviv N2002Phys. Rev. Let.89051102.

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Shaviv N and Veizer J2003GSA Today 13 (July)4–10.

Solanki S K and Fligge M1999Geophys. Res. Lett.262465.

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reportdownloadableathttp://www.geosystems.orgSvensmark H1998Phys. Rev. Let.815027–5030.

Svensmark H.2003http://arxiv.org/abs/physics/

0311087

Svensmark H2006a Astron. Nachrichten327866–870.

Svensmark H2006b Astron. Nachrichten327871–875.

Svensmark H2007eprint(inpreparation).

Svensmark H and Calder N2007The Chilling Stars: A

New Theory o Climate Change(IconBooks,Thriplow,Cambridge).

Svensmark H and Friis-Christensen E1997 J. Atmos. &

Solar-Terrest. Phys. 591225–1232.Svensmark H and Svensmark J2007eprint(inpreparation).

Svensmark Het al.2007Proc. Roy. Soc. A(doi:10.1098/

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Wagner Get al.2001 J. Geophys. Res.1063381–3388.

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)


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Henryk SVENSMARK: Cosmoclimatology, a new perspective