C. R. Geoscience 340 (2008) 441–450
http://france.elsevier.com/direct/CRAS2A/

External Geophysics, Climate and Environment

Cosmic rays and climate of the Earth: Possible connection
Ilya G. Usoskin a,*, Gennady A. Kovaltsov baSodankylä Geophysical Observatory, Oulu unit, POB 3000, University of Oulu, 90014 Oulu, Finlandb
Ioffe Physical-Technical Institute, 26, Polytekhnicheskaya, St Petersburg 194021, Russia
Received 15 June 2007; accepted after revision 30 October 2007
Available online 31 December 2007
Presented by Vincent Courtillot

Abstract
Despite much evidence relating climatic changes on Earth to solar variability, a physical mechanism responsible for this is still
poorly known. A possible link connecting solar activity and climate variations is related to cosmic rays and the physical-chemicalchanges they produce in the atmosphere. Here we review experimental evidence and theoretical grounds for this relation. The
cosmic ray–climate link seems to be a plausible climate driver which effectively operates on different time scales, but its exact
mechanism and relative importance still remain open questions. To cite this article: I.G. Usoskin, G.A. Kovaltsov, C. R. Geoscience
340 (2008).
# 2007 Académie des sciences. Published by Elsevier Masson SAS. All rights reserved.
Résumé
Rayons cosmiques et climat de la Terre : connexion possible. En dépit de nombreuses preuves d’une relation entre leschangements de climat sur la Terre et la variabilité solaire, un mécanisme physique pouvant l’expliquer est encore mal connu. Uneliaison entre l’activité solaire et les variations climatiques peut être recherchée du côté des rayons cosmiques et des changements
physicochimiques qu’ils induisent dans l’atmosphère. Dans cet article, les auteurs présentent les preuves expérimentales et les
fondements théoriques de cette relation. La liaison rayons cosmiques–climat semble être un élément moteur possible qui agit, enfait, à différentes échelles de temps, mais son mécanisme exact et son importance relative demeurent encore des questions ouvertes.
Pour citer cet article : I.G. Usoskin, G.A. Kovaltsov, C. R. Geoscience 340 (2008).
# 2007 Académie des sciences. Published by Elsevier Masson SAS. All rights reserved.
Keywords: Cosmic rays’ changes; Terrestrial climate changes; Cosmic rays–climate link; Experimental data; ModellingMots-clés : Variation du rayonnement cosmique ; Changements climatiques terrestres ; Liaison rayonnement cosmique–climat ; Données
expérimentales ; Modélisation

1. Introduction* Corresponding author.
E-mail address: ilya.usoskin@oulu.fi (I.G. Usoskin).

The Sun ultimately defines the climate on Earth,
supplying the planet with energy via radiation, which isreceived by the terrestrial system, but the role of thesolar variability in climate variations is far from clear.
Variability of the total solar irradiance (TSI) measured

1631-0713/$ – see front matter # 2007 Académie des sciences. Published by Elsevier Masson SAS. All rights reserved.
doi:10.1016/j.crte.2007.11.001

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I.G. Usoskin, G.A. Kovaltsov / C. R. Geoscience 340 (2008) 441–450

during the last decades is known to be too small to
explain the observed climate variations [24,25,27,45].
On the other hand, there are other ways that solar
variability may affect climate (see [34] for a review),
e.g., an unknown long-term trend in TSI [68,95], aterrestrial amplifier of the spectral irradiance variations[33,66], or an indirect mechanism also driven by thesolar activity. The latter can be realized by cosmic rays
(CR) via the ionization effect in the atmosphere.
Cosmic-ray-induced ionization (CRII) is the principle
source of the ionization of the low and middle
atmosphere and can slightly modulate cloud formation[20,52,71,73,81]. Even a small change in the cloud
cover modifies the transparency/absorption/reflectanceof the atmosphere and affects the amount of absorbed
solar radiation, even with no changes in the solar
irradiance. Since the flux of CR is modulated by thesolar magnetic activity, this provides a link between
solar variability and climate [47]. It is noteworthy that
the CR flux on Earth is modulated not only by solar
activity but also by the slowly changing geomagnetic
field which does not allow the least energetic but mostabundant CR particles to impinge on the Earth. Since
the two CR modulation mechanisms are independentand act on different time scales, this gives anopportunity to study the CR effect on Earth. Here
we present an overview of recent experimental
evidence and theoretical considerations of the CR–
climate link.
2. Cosmic ray variability
The CR intensity (energy-integrated flux measured
by a certain device) is commonly measured by groundbased neutron monitors (NMs) which are effectively
sensitive to CR with energy of a few tens of GeV [1].
The world network of NMs routinely measures the CR
intensity since 1951. A usual proxy for CR intensity for
longer time scales is the abundance (converted into the
production rate in the atmosphere) of cosmogenic
isotopes 14 C and 10 Be in independently dated terrestrial
archives such as tree rings, ice cores or marine
sediments. While the effective energy of CR for the
production of these isotopes is somewhat lower thanthat for NM, cosmogenic isotope data can still beroughly considered as an extension of the NM record in
the past [6,61]. Variability of CR on different timescales is shown in Fig. 1. Panel a) depicts a ground level
enhancement (GLE) due to a strong flux of solar
energetic particles associated with the extreme solar
flare of 20/01/2005. Such severe GLEs, when the CR
intensity increases by a factor of 3–10 within an hour,

occur 1–2 times per solar cycle. Panel b) shows a
Forbush decrease caused by the passage of aninterplanetary shock, when the CR intensity issuppressed by 10–20% for several days. There is adozen such decreases per solar cycle. Solar modulationof CR on a decadal time scale is shown in panel c). One
can also see a GLE in 1989 and a number of Forbush
decreases. The CR flux, as measured by NMs, varies by
about 25% in anti-phase with the solar activity, i.e. it ismaximum during a solar minimum. The CR changes
due to long-term solar variability are shown in panel d)
and depict the 11-yr cycle superimposed on a long-term
trend of  15% since the Maunder minimum in the 17th
century. The multi-millennium CR variability, quantified in the production rate of radiocarbon 14 C
cosmogenic isotope [79], is shown in panel e) for the
Holocene (the 11 000-yr long period characterized by apretty stable warm climate). The fast variability iscaused by the solar activity, while the slow changing CR
level is mostly related to the changing geomagnetic
shielding. Panel f) depicts smooth variations of CR,
measured as the 10 Be concentration in deep marine
sediments [26], that are caused by geomagnetic
changes. Panel g) shows hypothetical changes in the
CR flux (normalized to the modern flux F0 ) due to thechanging galactic environment of the solar system, assuggested by [64].
Thus, the CR flux varies on all time scales, fromsporadic changes due to solar eruptive and transientphenomena on minutes-to-days time scale, to variations
due to geomagnetic or even galactic changes on kyr and
Myr scale. In subsequent sections, we will compare
these CR changes with the known climatic reconstruction on different time scales and discuss their theoreticalgrounds.
3. Climate variability and its relation to cosmic
rays
3.1. Daily/weekly time scales
In addition to regular daily variations (at the level ofabout 1%) of the CR flux due to the local CR anisotropy,
strong transient phenomena sometimes occur (Figs. 1(a)and (b)) that either enhance or reduce the CR flux forhours – days. These phenomena are sporadic and their
net contribution is tiny [85], but on the other hand, theirrare occurrence and strong momentary effect give agood opportunity for case studies of solar–terrestrial
relations. Many statistical studies have been performed,
looking for a relation between such sporadic CR
variations and the atmospheric/climatic parameters. For

I.G. Usoskin, G.A. Kovaltsov / C. R. Geoscience 340 (2008) 441–450

443

Fig. 1. Cosmic ray variability on different time scales: (a) Event of solar cosmic rays (GLE of 20/01/2005 measured by Oulu NM); (b) Forbush
decrease (the event of Sept. 2005 measured by Oulu NM); (c) Solar cycle modulation of cosmic rays as recorded by Oulu NM; (d) Centennial
variability (reconstruction of a polar NM count rate [82]); (e) Multi-millennium variability (atmospheric production rate of 14 C computed for the
Holocene in [79]); (f) Long-term variability (normalized 10 Be production measured in deep marine sediments [26]); (g) Geological scales(normalized CR flux F proposed in [64]). Note the different time scales.
Variabilité du rayonnement cosmique à différentes échelles de temps. (a) Événement GLE du 20 janvier 2005 du rayonnement cosmiquesolaire, mesuré par Oulu NM ; (b) diminution Forbush (événement de septembre 2005 mesuré par Oulu NM) ; (c) modulation du cycle solaire
du rayonnement cosmique enregistré par Oulu NM ; (d) variabilité centenaire (reconstitution du taux de comptage polaire NM [82] ;(e) variabilité multimillénaire (taux de production atmosphérique de 14C estimé pour l’Holocène [79] ; (f) variabilité à long terme (productionde 10Be normalisé, mesurée dans les sédiments marins profonds [26]) ; (g) échelles géologiques (flux F normalisé CR [64] ; noter les différenteséchelles de temps).

example, it was suggested that a small reduction of themean cloud cover at high latitudes ( > 60 N) [58], as
well as changes in the atmospheric transparency at highlatitudes ( > 55 N) [60], could be associated with
Forbush decreases. Another superposed epoch analysis[40,41] of ISCCP data suggests that the cloud cover isreduced at all latitudes over regions with relatively
higher cloud cover and probably increased over ocean
surfaces at middle and lower latitudes in regions withthinner cloud, in association with CR flux reduction.
However, a recent thorough analysis [10] has not shown
any statistically significant changes in the ISCCP-based
cloud cover in correlation with six largest Forbush
decreases. Also, formation of an aerosol layer, observedin stratosphere, has been associated with some strongsolar particle events [50,51,67]. On the other hand, assuggested in [75,77], CR may primarily affect vorticity
in polar/subpolar regions during the cold season,
leading either to a reduction [77] after Forbush
decreases or to an increase [89] during/after solar
particle events. Also, a lowering of the troposphericpressure layers has been reported in association withsolar energetic particle events in North Atlantic [90].
In summary, although a global one-to-one relation
between the cloud cover and CR variations is notconfirmed [10] on the daily scale, much evidence
implies that CR changes may affect the process of
cyclogenesis through changing transparency and pressure, particularly in the North Atlantic during coldseasons.

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I.G. Usoskin, G.A. Kovaltsov / C. R. Geoscience 340 (2008) 441–450

3.2. Interannual variability
The 11-year solar cycle is the most pronounced
variation of the CR flux on the time scale of decades,
when there exist reliable satellite-based data on cloud
cover. Accordingly, the CR–climate relations are mostly
studied on the interannual time scale. A reported
decadal cycle in the global coverage of low clouds was
attributed to the CR flux variations [71,73]. This result
initiated a dispute in the literature covering botharguments, pro [11,39,46–48] and con [30,43,44,70,78].
However, as mentioned by Gierens and Ponater [30], theglobally averaged cloud data hardly shows a consistentcorrelation with the CR flux. On the other hand, a highly
significant persistent correlation between the low cloud
cover and cosmic rays was found in some geographicalregions during the last 22 years [56,83,92]. Note,however, that low clouds can be partly masked by highclouds in the satellite data set, leading to spurious
correlations in some regions [55,86]. Other mechanisms
may also affect the cloud formation, especially at higher
altitudes, e.g., via the global current system [74] or UV
heating [33], which work in anti-phase with CR

variations, as well as volcanos or ENSO variability.
A recent thorough study [92,93] explored all these
possibilities and has shown that a statistically significant
direct correlation between low clouds and CR exists in a
few climate-defining regions in mid-high latitudes (seeFig. 2), the largest being the North Atlantic+Europe andthe South Atlantic. This pattern roughly corresponds tothe model result [38] which predicts that the strongest
response of the aerosol concentration to changes in
CRII is expected at mid-to-high latitudes over oceans.
An example of the temporal variations of low clouds
and CRII in Europe is shown in Fig. 3, depicting the
striking similarity between the two quantities. However,almost no correlation is observed outside these regions.
In summary, the link between low clouds and CR is
statistically significant on the interannual time scale insome limited geographical regions since 1984. Use of
global or even zonally averaged data may be misleading.
3.3. Centennial to millennial time scales
A detailed study [54] of a possible link between CR
and cloudiness was performed by using the results ofFig. 2. Geographical distribution of the partial correlation between low cloud amount [36] and solar indices, CRII and UV-index for 1984–2004 (see[92] for details). Only areas with statistically significant (at the 90 % level) correlation are hatched. The horizontal and vertical hatching depict areas
where the correlation with UVI and CRII dominates, respectively (red/blue colour stands for positive/negative correlation). Inclined hatchingindicates that the observed correlation may be spuriously induced by other types of clouds (middle or high clouds).
Distribution géographique de la corrélation partielle entre proportion de nuages bas [36] et indices solaires, CRII et UV pour la période 1984–2004(voir [92] pour les détails). Seules les zones à corrélation statistiquement significative (au niveau 90%) sont hachurées. Les hachures horizontales etverticales correspondent aux zones pour lesquelles la corrélation avec UVI et CRII est dominante (respectivement rouge/bleu pour la corrélation
positive/négative). Les hachures obliques indiquent que la corrélation observée peut être faussement induite par d’autres types de nuages (nuages demoyenne ou de haute altitude).

I.G. Usoskin, G.A. Kovaltsov / C. R. Geoscience 340 (2008) 441–450

445

Fig. 3. Variability of the low cloud amount [36] (upper curves, left axis) and cosmic ray-induced ionization at about 3-km altitude [80] (lower curve,
right axis), averaged over the West European region (30–70 N, 10 W–15 E). The grey curve depicts the annual mean of the cloud data.
Variabilité de la quantité de nuages bas [36] (courbes du haut, axe de gauche) et du rayonnement cosmique induit par ionisation à environ 3 km
d’altitude [80] (courbe du bas, axe de droite), moyennée sur l’Europe occidentale (30–708 N, 108 W–158E). La courbe grise correspond à lamoyenne annuelle des données sur les nuages.

sunshine observations during the 20th century.
Although the data is not easy to interpret and analyze,
it suggests that a link between the total cloud cover and
CR is unlikely. On the other hand, the data is in generalagreement with the hypothesis of a link between low
clouds and CR.
Numerous studies (see reviews [16,81,91]) confirm arelation between solar/CR activity and different indicesof climate behaviour (e.g., d18 O or drift ice debris [8])during the Holocene.
For example, there is an apparent agreement between
the grand minima of reduced solar activity and cold/wet
climate episodes (see [91] for review). Fig. 4 confronts aphysics-based reconstruction of the solar activity fromthe 14 C data [69,87] with periods of identified sudden
climatic shifts to cold/wet conditions in Europe during
the last 7000 years [7,49]. One can see that they tend to
appear during the grand minima periods. We note that 12
out of 14 climate shifts after 5000 BC occurred during
grand minima of solar activity identified by Usoskin et al.
[87], giving a 86% hit rate. On the other hand, only 3 (ca.
4200 BC, 700 AD and 1050 AD) out of 15 grand minima
identified for the same period are not accompanied byclimate shifts (80% hit rate). This also suggests, with highsignificance, a close relation between shifts of the climate
type and solar activity (cosmic rays), at least in theEuropean region. Although such studies cannot distinguish whether the primary effect is via CR or solar
irradiation (e.g., [17]), an analysis of the geomagnetic

field variation may help in disentangling the mechanisms(see a subsequent section).
In summary, there is a set of evidence that the solar
variability affects the climate changes on centennial–
millennial time scales, but it is hard to distinguish therole of cosmic rays, and the exact mechanisms need tobe resolved.
3.4. Effect of the geomagnetic field
Although CR and solar activity variations are closely
related, the former are also affected by another factor,
the geomagnetic field, which changes independently of
the solar variability, and with a different characteristic
time scale. Therefore, a simultaneous study of climate,solar and geomagnetic proxies would help in disentangling the CR and solar irradiance effects on climate.
A detailed study [84] has revealed a weak but
persistent correlation between the Northern Hemispheretemperature and the geomagnetic field intensity during
the last millennium, implying that CR play a role inclimate variations, although this correlation is notstrong [94]. It is suggested that centennial climaticchanges could be triggered by enhanced secularvariation of the geomagnetic field [5,18], in particular
by archeeomagnetic jerks [28,29]. It was found that on
the multi-millennial time scale, periods of geomagnetic
field reversal roughly correspond to cold episodes of thepalaeoclimatic reconstructions [12–14,21,96].

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I.G. Usoskin, G.A. Kovaltsov / C. R. Geoscience 340 (2008) 441–450

Fig. 4. Sunspot number reconstruction (line – [87]) as well as known climate shifts to cold/wet conditions (dots – [7,49,88]) for the last 6500 years.
Reconstitution du nombre de taches solaires (ligne – [87]) ainsi que des dérives climatiques connues, dans les conditions de climat froid/humide(points – [7,49,88]) pour les 6500 dernières années.

It is important to note that not only the changes of thegeomagnetic dipole strength affect CRII, but also fastmigration of the geomagnetic axis can lead to dramaticchanges in different regions [42]. Accordingly, studying
regional data may shed new light on the possible
mechanisms.
In summary, the relation between the geomagnetic
field changes and climatic variations provides anevidence favouring a possible CR influence on climate.

flux impinging on the Earth also in synchronization with
the CR effect due to spiral arm crossing. Moreover, thetype of climate, seismic activity and physical-chemical
properties of the atmosphere were essentially different
on the geological time scale, and it is hardly possible todistinguish the role of indirect mechanisms.
In summary, although there is an indication of theclimate changes synchronously with the putative CR
flux on Myr time scales, these results remain
speculative.

3.5. Mega-year time scales
4. Possible mechanisms
On the geological time scale (longer than a million
years), CR variations are assumed to be determined
either by the geomagnetic field changes or by thechanging local galactic environment. For example, if
the Solar System enters a galactic spiral arm during its
evolution, a greatly enhanced flux of CR would be
impinging on Earth [9,61]. Although the path of the
Solar System relative to the Milky Way spiral arms ispoorly known, CR variability on the geologal time scale(see Fig. 1g)) can be guessed [63–65] to depict a
similarity with palaeoclimatic reconstructions [62,63].
This result has been both disputed [59] and supported[31] by other researchers. Note that, even if thecorrelation itself was real, its interpretation is notstraightforward since it is based on an assumption thatthe climate and its drivers remain unchanged throughout
millions of years. Also, there are concurrent effects
which may lead to similar relations. E.g., a large amountof galactic dust would be loaded into the Earth’s
atmosphere in galactic spiral arms, leading to enhanced
opaqueness of the atmosphere and thus to a cool climate[57], in concurrence with the CR effect. Interestingly,
the rate of geomagnetic field reversals also varies on thegeological time scale [23,96] quite synchronously withthe climatic variations. This itself modulates the CR

Although the amount of energy deposited byenergetic CR in the atmosphere is very small (about
10 5 W/m2), they affect, via their ionizing effect, thechemical-physical conditions of the atmosphere. This,in turn, can influence the ability of the terrestrial system
to absorb/trap/reflect solar radiation through, e.g., thecloud cover [52,71]. Therefore, with a negligible
amount of its own energy, CR may greatly modify
the amount of solar radiation received by the planet, andchange its radiation budget, thus being a very effective
trigger/amplifier of the solar variability. Note thatclouds, whose formation can be supposedly modulated
by CR, lead to both the trapping of outgoing long-wave
radiation and reflection of the incoming solar radiation.
However, the net effect of these concurrent processes iscooling (i.e. more clouds correspond to a cooler
climate).
Essential progress has been achieved recently in thedevelopment of a precise physical model of the CRinduced nucleonic-electromagnetic cascade in theatmosphere, leading to a full understanding of theionizing effect of cosmic rays [19,53,80,83]. However, adetailed mechanism of how CRII can affect cloudformation is still not well understood, and two main

I.G. Usoskin, G.A. Kovaltsov / C. R. Geoscience 340 (2008) 441–450

hypotheses are discussed in the literature (see, e.g.,reviews in [35,72,76]).
One hypothesis is the most ‘apparent’ and exploits
the ionizing effect of CR [4,20,38,46,97,98]. It assumes
that ions produced by CR rapidly interact withmolecules in the atmosphere and are converted into
complex cluster ions, which may grow by ion–ion
recombination or ion–aerosol attachment and thus
affect the number of aerosols acting as cloud
condensation nuclei. A simple analogy for thismechanism is a cloud chamber (also known as the
Wilson chamber), where charged particles leave visible
tracks of droplets condensated along their paths.
However, this analogy is not entirely valid, as thecloud chamber uses heavily supersaturated vapour andin this sense is different from the real atmosphericconditions, where vapour is not (or is marginally)saturated and there are other admixtures, like aerosols,
also affecting the condensation process. The presence ofaerosols, especially gaseous H2SO4, in the atmosphereis important to allow new particle growth under CRII
[4,37]. The role of ions as nucleation agents was firstestablished for the stratosphere and mesopause [2,3]and later [4,22] confirmed for the cold upper troposphere. However, this mechanism can work in different
ways, depending on the ambient conditions [37,38].
This may lead to the geographical pattern of theobserved CR-cloud relation (see Section 3.2).
An alternative hypothesis [74,75] is based on theassumption that cloud formation is affected by theatmospheric electric field via precipitation andice formation in super-cooled water. On one hand,the electric field is influenced by CR since CRII controls
the atmospheric conductivity. On the other hand, thesame processes which modulate CR (interplanetary
magnetic field, solar wind, interplanetary shocks, etc.)
also greatly affect the planet’s global current system andthus the electric fields. Cosmic rays only play a partial
role in this mechanism and work in concurrence withthe direct solar activity.
There is also a possibility that solar variability may
affect cloud formation even without the direct influenceof CR, i.e. via circulation changes due to stratosphericheating caused by the ozone absorption of solar UV
radiation [32,33]. In particular, such changes may leadto changes in winter circulation patterns that affectmiddle-latitude storm tracks.
All these mechanisms can work in different
conditions (e.g., at different altitudes or geographicalregions – see, e.g., [37,92]) or in concurrence, and it isdifficult to distinguish between them. Unfortunately,
none of these possible mechanisms has a solid support447

of a quantitative physical model. Despite the significant
progress in modelling the aerosol nucleation [15,38],
the predictions of current atmospheric nucleation
models are quite uncertain, since the link between
micro- and macro-physics is still missing. On the other
hand, all experimental evidences, as described above,
are based on statistical, often marginally significant,studies, lacking for a clear case study.
A new step has been made recently in modelling thechemistry of the lower atmosphere in laboratory
conditions – the SKY experiment conducted at theDanish National Space Center [72] (an enlarged andimproved version of this experiment is under construction at CERN [11].) The result of this experiment (Fig. 5)
indicates that enhanced ionization of air in realistic
conditions facilitates the formation of stable, ultra-small
clusters of sulphuric acid and water molecules. This
result is preliminary and waits to be confirmed. Even
though further steps from micro- (molecular clusters) to
macro- (cloud condensation nuclei) physics remainunresolved, it provides the first laboratory evidence thationization can affect physical-chemical properties of theatmosphere. This complements the existing statistical
relations and theoretical models linking ultra-fineaerosols and CRII [4,15,37,51] and confirms experimentally the first step of CRII-related mechanism linking CR
to the cloud formation.

Fig. 5. The results of SKY experiment [72] – a relation between ion
density and aerosol nucleation. Asterisks depict the experimental data,
while the line corresponds to a best-fit linear relation.
Résultats de l’expérience SKY [72] – une relation entre densité ioniqueet nucléation d’aérosols. Les astérisques correspondent aux données
expérimentales, tandis que la ligne correspond à la relation linéaire laplus appropriée.

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5. Conclusions
We have reviewed the experimental evidence andtheoretical models relating cosmic ray variations to theterrestrial climate changes.
On a short time scale of a few days, there exists muchevidence that CR changes may affect the process of
cyclogenesis via the changing transparency andpressure, particularly in the North Atlantic during coldseasons. Although each individual piece of evidence isbarely significant, in aggregate, they suggest that therelation can be real.
A link between low clouds and CR appears
statistically significant on the interannual time scale
since 1984 in limited geographical regions, the largest
being North Atlantic + Europe and South Atlantic. We
note that many reconstructions of the past climate arebased on European data, where the CR–cloud relation is
the most pronounced. Extension of this relation to theglobal scale may be misleading.
A relation between the geomagnetic field changes
and climatic variations provides evidence favouring thepossible CR influence on climate. A study of regional
climate variations in relation to the geomagnetic dipole
axis migration over the last millennium is also
promising.
There is an indication of the climate changes
synchronously with the CR flux on Myr time scales,
but this result is not straightforward to interpret. Large
uncertainties make it only indicative.
Essential progress has been recently achieved intheoretical modelling of both ionizing effect of CR andphysical mechanisms relating CRII to cloud variations,but the link between micro- and macro-physics is still
missing. A new experimental evidence, obtained by theSKYexperiment team, confirm that enhanced ionization
notably facilitates the production of small ion clusters inrealistic atmospheric conditions.
In conclusion, a CR–climate link seems to be aplausible climate driver, as supported by the bulk ofstatistical studies and existing theoretical models.
However, further studies, in particular a clear casestudy as well as improved model development, areforeseen to improve our understanding of the link
between cosmic rays and the climate on Earth.
Acknowledgements
Support from the Academy of Finland and theFinnish Academy of Science and Letters Vilho, Yrjö
and Kalle Väisälä Foundation is gratefully acknowledged. GAK acknowledges also the Program of

Presidium RAS N16-3-5.4. E. Usoskina is acknowledged for suggestions on improving the manuscript
style.

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