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GACGeoCanV32No1.qxp 3/18/2005 3:13 PM Page 13
GEOSCIENCE CANADA Volume 32 Number 1 March 2005 13
ARTICLE
hit the atmosphere, potentially generat- hausse de température observée au
ing cloud condensation nuclei (CCN). cours du siècle dernier, soit environ 0,6
Clouds then cool, act as a mirror and °C. D’où la nécessité d’un facteur d’am-
reflect the solar energy back into space. plification. Dans les modèles clima-
The intensity of CRF reaching the earth tiques généraux (GCM), le gros de l’ac-
depends on the intensity of the solar croissement calculé de température est
(and terrestrial) magnetic field that acts dû à « la rétroaction positive de la
as a shield against cosmic rays, and it is vapeur d’eau ». Dans le modèle à
this shield that is, in turn, modulated by moteur solaire, ce pourrait être le flux de
solar activity. rayonnement cosmique (FRC), ce pour-
Cosmic rays, in addition to CCN, rait être l’effet des particules énergiques
also generate the so-called cosmogenic qui en frappant l’atmosphère entraînent
Celestial Climate Driver: A nuclides, such as beryllium-10, carbon- une génération possible de nucléus de
14 and chlorine-36. These can serve as condensation des nuages (NCN). Alors,
Perspective from Four indirect proxies for solar activity and can les nuages se refroidissent et, comme un
Billion Years of the Carbon be measured e.g., in ancient sediments, miroir, réfléchissent l’énergie solaire
trees, and shells. Other proxies, such as dans l’espace. L’intensité du FRC
Cycle oxygen and hydrogen isotopes can atteignant le sol dépend de l’intensité des
reflect past temperatures, carbon iso- champs magnétiques du soleil et de la
topes levels of carbon dioxide, boron Terre, lesquels agissent comme un
Ján Veizer
isotopes the acidity of ancient oceans, bouclier à l’endroit des rayons cos-
Ottawa-Carleton Geoscience Centre, University
etc. Comparison of temperature records miques, le pouvoir de ce bouclier étant à
of Ottawa, Ottawa, K1N 6N5 Canada &
from geological and instrumental son tour modulé par l’activité solaire.
Institut für Geologie, Mineralogie und
archives with the trends for these prox- En plus d’entraîner la formation de
Geophysik, Ruhr-Universität Bochum,
ies may enable us to decide which one NCN, les rayons cosmiques, génèrent
Bochum, Germany: veizer@science.uottawa.ca.
of the two alternatives was, and poten- aussi ce qu’on appelle des nucléides cos-
tially is, primarily responsible for climate mogéniques, comme le béryllium-10, le
SUMMARY
variability. This, in turn, should enable us carbone-14 et le chlore-36. Ces
The standard explanation for vagaries of
to devise appropriate countermeasures nucléides peuvent servir d’indicateurs
our climate, championed by the IPCC
for amelioration of human impact on air indirects de l’activité solaire puisqu’on
(Intergovernmental Panel on Climate
quality and climate. peut en mesurer la teneur dans des sédi-
Change), is that greenhouse gases, par-
ments anciens, des arbres, et des
ticularly carbon dioxide, are its principal
SOMMAIRE coquilles, par exemple. D’autres indica-
driver. Recently, an alternative model
Généralement, les raisons données pour teurs indirects comme les isotopes
that the sun is the principal driver was
expliquer les caprices de notre climat, les d’oxygène et d’hydrogène peuvent
revived by a host of empirical observa-
mêmes que celles avancées par le CICC refléter les températures de jadis, les iso-
tions. Neither atmospheric carbon diox-
(Comité intergouvernemental sur le topes de carbone peuvent refléter les
ide nor solar variability can alone explain
changement climatique), veulent que ce niveaux de dioxyde de carbone, les iso-
the magnitude of the observed tempera-
soient les gaz à effet de serre, partic- topes de bore peuvent refléter l’acidité
ture increase over the last century of
ulièrement le dioxyde de carbone, qui en des anciens océans, etc. La comparaison
about 0.6°C. Therefore, an amplifier is
soient le moteur principal. Récemment, entre des registres de mesures de tem-
required. In the general climate models
une série d’observations empiriques ont pérature directes et d’archives
(GCM), the bulk of the calculated tem-
ravivé l’intérêt pour un autre modèle géologiques, avec les courbes de ten-
perature increase is attributed to “posi-
voulant que ce soit le soleil qui en soit le dance de tels indicateurs indirects peut
tive water vapour feedback”. In the sun-
moteur principal. Mais seuls, ni le nous permettre de décider laquelle de
driven alternative, it may be the cosmic
dioxyde ce carbone ni les variations d’ac- deux options était et continue possible-
ray flux (CRF), energetic particles that
tivité solaire ne permet d’expliquer la ment d’être la cause principale des varia-GACGeoCanV32No1.qxp 3/18/2005 3:13 PM Page 14
14
tions climatiques. On pourrait alors tive is essential, because events on pro- fore have a large impact on the planetary
décider de contre-mesures appropriées gressively shorter time scales are embed- energy balance.
permettant d’atténuer l’impact des activ- ded in, and constrained by, the evolution A growing body of empirical
ités humaines sur la qualité de l’aire et of the background on longer time evidence, such as correlations between
sur le climat. scales. climate records and solar and cosmic ray
activity, or their proxy indicators (e.g.,
INTRODUCTION CELESTIAL CLIMATE DRIVER 10Be, 14C, 36Cl, geomagnetic field inten-
Carbon dioxide, generally believed to be The solar/Cosmic Ray Flux (CRF)/cli- sity, sunspot numbers), increasingly sug-
the most important greenhouse gas and mate hypothesis, although discussed by gests that extraterrestrial phenomena
climate modifier, is today the focus of a the IPCC (Ramaswamy et al., 2001), was may be responsible for at least some cli-
heated political and scientific debate that not considered to be a likely candidate matic variability (Bond et al., 2001;
has polarized scientists, policy makers, for a principal climate driver. This was Kromer et al., 2001; Neff et al., 2001;
and the public. One side maintains that partly because of the lack of a robust Sharma, 2002; Carslaw et al., 2002; Hu
CO2 is the principal driver of climate, physical formulation for cloud conden- et al., 2003; Usoskin et al., 2003; Blaauw
with the Intergovernmental Panel on sation phenomena and partly because it et al., 2004; Solanki et al., 2004). The
Climate Change (IPCC, 2001) projecting was argued that the observed changes in correlations of climate with these prox-
a global mean temperature rise from 1.5 the Total Solar Irradiance (TSI) flux ies are mostly better than those, if any,
to 5.8° C by the year 2100. The other have been insufficient to account for the between the coeval climate and CO2.
side (e.g., Douglass et al., 2004) claims observed ~0.6°C centennial temperature Moreover, inferred and direct observa-
that the role of anthropogenic CO2 on increase. Therefore, an amplifier is tional data of TSI flux yield a record
climate has not been proven, and that required to account for the discrepancy. that can explain 80% of the variance in
there is therefore no need for emissions However, similar problems have arisen the centennial temperature trend
quotas such as those mandated by the also in the greenhouse hypothesis, where (Foukal, 2002). Celestial phenomena may
Kyoto Protocol. the amplifier is implicit (the centennial have been the principal driving factor of
As is usually the case with con- temperature rise in these models is climate variability and global tempera-
tentious matters, the reality likely lies caused by to the “positive water vapour ture even in the recent past.
somewhere in between. So why is this feedback”, not to the CO2 itself) and The sun-climate link could be
issue so polarizing? First, past, natural, where clouds, a potential net negative through a number of potential pathways
variations in the carbon cycle and cli- feedback and the largest source of (Rind, 2002; Carslaw et al., 2002), where
mate are poorly understood. These vari- uncertainty in the models, are only the solar flux is amplified by (1) stratos-
ations must be taken into account as a “parameterized”. Yet, the solar energy pheric chemistry (e.g., ozone) because of
baseline for any superimposed human reflected by the clouds, or the energy of changes in solar UV spectrum, (2) cloud
impact. Second, the climate models are, evaporation/condensation, are both coverage modulated by the galactic CRF,
at best, only an approximation of reality. about 78 Watts per square metre (Wm-2) or (3) a combination of these or other
Since I am a geologist and not a mod- worldwide. For comparison, the energy factors. Considering that statistical evalu-
eller, I will deal mostly with the empiri- input ascribed to “post-industrial” ation of 20th century data shows that
cal record of climate and the carbon anthropogenic CO2 input is ~ 1.5 Wm-2 solar UV radiation may account for only
cycle, contemplating them at time scales and that of incoming solar radiation ~ about 20% of the variance in surface
ranging from billions of years to the 342 Wm-2 (IPCC, 2001). A change in temperature data (Foukal, 2002), alterna-
human life span (Fig. 1). This perspec- cloud cover of a few percent can there- tive (2) is the favoured hypothesis. In
this alternative, an increase in TSI results
not only in an enhanced thermal energy
flux, but also in more intense solar wind
that attenuates the CRF reaching the
Earth (Tinsley and Deen, 1991;
Svensmark and Friis-Christensen, 1997;
Marsh and Svensmark, 2000; Solanki,
2002). This, the so-called heliomagnetic
modulation effect reflects the fact that
the solar magnetic field is proportional
to TSI and it is this magnetic field that
acts as a shield against cosmic rays. The
terrestrial magnetic field acts as a com-
plementary shield, and its impact on
CRF is referred to as geomagnetic mod-
ulation (Beer et al., 2002). The CRF, in
turn, is believed to correlate with the
low altitude cloud cover (Fig. 2). The
postulated causation sequence is there-
Figure 1. Hierarchy of time scales discussed in this article. fore: brighter sun ⇒ enhanced thermalGACGeoCanV32No1.qxp 3/18/2005 3:13 PM Page 15
GEOSCIENCE CANADA Volume 32 Number 1 March 2005 15
Figure 2. Solar irradiance (SI), galactic cosmic ray (CR) flux and low cloud (LC) cover, 1983 – 2001 (adapted from Marsh and
Svensmark, 2003a and Marsh et al., 2005). Note the reversed scale for SI. Some authors (Laut, 2003) argue that the apparent
post-1995 divergence of clouds from celestial trends disqualifies the correlations. However, the discrepancy may arise from a
modified cross-calibration of satellites, following the late 1994 hiatus in polar orbit flights (Marsh and Svensmark, 2003a). A cor-
rection for this drift (thick full line LC’) results in a good agreement for all parameters (see also Pallé et al., 2004b and Usoskin
et al., 2004).
flux + solar wind ⇒ muted CRF ⇒ less effective than the extrinsic ones.
low-level clouds ⇒ lower albedo ⇒ In a nutshell, the intrinsic inter-
warmer climate. Diminished solar activi- stellar intensity of CRF may have con-
ty results in an opposite effect. The trolled the long-term climate variability
CRF/cloud-cover/climate link is also on multimillion-year time scales.
physically feasible because the CRF like- Superimposed on this long-
ly governs the atmospheric ionization frequency/large-amplitude wavelength
rate (Carslaw et al., 2002), and because are smaller oscillations on millennial to
recent theoretical and experimental stud- annual time scales, generated by the vari-
ies relate the CRF to the formation of able solar activity that modulates either
charged aerosols (Harrison and Aplin, the CRF bombarding the Earth, the
2001; Lee et al., 2003), which could planetary atmospheric dynamics, or
serve as cloud condensation nuclei both. Tentatively, I accept this interpre-
(CCN), as was demonstrated independ- tation as a working hypothesis for the
ently by ground based and airborne subsequent discussion, but hasten to
experiments (Eichkorn et al., 2002). acknowledge that the CRF/cloud link-
The CRF reaching the planet has age is still a hotly contested issue.
not only an extrinsic variability reflecting Accepting this scenario as a working
its attenuation by solar wind, but also an hypothesis, how does it withstand scruti-
intrinsic one arising from a variable inter- ny if tested against the hierarchical geo-
stellar environment (Shaviv, 2002a, b). logic record (Fig. 1) of climate and the Figure 3. Idealized reconstruction of
Particularly large CRF variability should carbon cycle? the oldest, 3.5 billion years (Ga) old, fos-
arise from passages of the solar system sils from Western Australia, considered
through the Milky Way’s spiral arms that LIFE, WATER, AND THE CARBON to be blue-green algae (Schopf, 1983).
harbour most of the star formation CYCLE ON BILLION YEAR TIME The biogenic origin of these fossils has
activity. Such passages recur at about SCALES recently become a matter of controversy
143 ± 10 million years (Ma) intervals To understand the role of atmosphere, (Brasier et al., 2002). Nevertheless, stro-
and these variations are expected to be water, and life in climate evolution over matolites and carbon isotope evidence
about an order of magnitude more geologic history, it is essential to study support the great antiquity of life.GACGeoCanV32No1.qxp 3/18/2005 3:13 PM Page 16
16
ancient examples. Yet, we have no ter, the rough proportion of reduced to To resolve this paradox, some
unequivocal samples of ancient waters, oxidized carbon is calculated to be about argue that a massive greenhouse, caused
and the oldest samples of air are in bub- 1:4 (Schidlowski et al., 1975). principally by CO2 (e.g., Kasting, 1993),
bles frozen into Antarctic ice near the Remarkably, when these carbon isotopes must have warmed up the young earth.
time of its formation, reaching back are traced back in geologic history, the Theoretical calculations, set up to coun-
some 420,000 to 800,000 years. The situ- average carbon isotopic composition of teract the lower solar luminosity, yield
ation is somewhat better with the rem- seawater (Fig. 4) and of most of the CO2 atmospheric concentrations up to
nants of life, because mineralized shells kerogen (Hayes et al., 1983) was similar ten thousand times greater than today’s
go back to about 545 million years, the to today. Hence, we get about the same value of 0.035 %. Yet, this is at odds
times known as the Phanerozoic, and 1:4 ratio as far back as 3.5, and possibly with the geologic record. For example, at
morphological evidence of living things, 4, billion years ago. Assuming that the low seawater pH, expected from such
algae and bacteria, and of fossilized stocks of global carbon were conserva- high partial pressures of carbon dioxide
stromatolites, have been found in west- tive, and stated rather boldly, not only (pCO2), ancient limestones should be
ern Australia in rocks as old as 3.5 bil- did we have life as far back as we had enriched in 18O relative to their younger
lion years (Fig. 3). Kerogen, body tissues rocks, but there was as much life then as counterparts, yet the secular trend that
altered by temperature and pressure, has today, albeit in its primitive form. We we observe in the geologic record
been found in still older rocks approach- can conclude, then, that the fundamental (Shields and Veizer, 2002) shows exactly
ing 4 billion years. This is remarkable, features of the carbon cycle were estab- the opposite. Factors more complex
because the oldest rocks ever recovered, lished as early as 4 billion years ago. than a massive CO2 greenhouse would
found near Yellowknife in northwestern What does this mean for the have to be invoked to explain the warm-
Canada, are of about the same age global carbon cycle? The simplest ing of this planet to temperatures that
(Bowring et al., 1989). assumption would be that it might not may have surpassed those of the present
These observations, however, are have been that different from today. Yet, day. A plausible alternative is a change in
only qualitative. If we want to under- such a proposition is difficult to recon- the cloud cover (Rossow et al., 1982)
stand the operation of the carbon cycle cile with the so-called “faint young sun” because clouds can compensate for 50%
and its role in the climate system, it is paradox (Sagan and Mullen, 1972). variations in radiative energy of the sun
necessary to know not only that there Based on our understanding of the evo- (Ou, 2001), bringing forward again the
was life, but also how much of it there lution of stars, the young sun was about role of CRF as the potential solution.
was. In order to establish this, we have 30 percent less luminous than it is today, Considering that young stars of the
to rely on the derivative, or proxy, sig- and became brighter with age. With such same category as our sun would have
nals. In our case, such proxies are iso- low radiative energy from the sun, our been characterized by a stronger solar
topes, particularly of carbon and oxy- planet should have been a frozen ice ball wind that muted the CRF, the resulting
gen. until about 1 billion years (Ga) ago. Yet, reduction in cloudiness may have com-
From the measurements of iso- we know that running water shaped the pensated for the sun’s reduced luminosi-
tope ratios of carbon in modern living surface of the planet as far back as the ty (Shaviv, 2003). Note also that theoreti-
things and of carbon dissolved in seawa- geologic record goes. cal models of Milky Way evolution indi-
cate a diminished star formation rate
between ~ 2 and 1 Ga ago, while the
Paleo- and Neoproterozoic were strong
maxima. This dovetails nicely with the
geologic record (Frakes et al., 1992;
Crowell, 1999), with massive glaciations
at these two maxima and their absence
in the intermediate time interval.
CLIMATE ON MILLION YEAR TIME
SCALES
The record of climate variations during
the Phanerozoic (Fig. 1) shows intervals
of tens of millions of years duration
characterized by predominantly colder or
predominantly warmer episodes, called
icehouses and greenhouses, respectively
(Fig. 5). Superimposed on these are
higher order climate oscillations, such as
the episodic waning and waxing of ice
sheets.
Figure 4. Carbon isotopic composition of proxies for paleo-seawater, ancient lime- In the Phanerozoic, some organ-
stones and calcareous shells (circles) and dolostones (triangles). Adapted from isms secreted their shells as the mineral
Shields and Veizer (2002). calcite (CaCO3), which often preservesGACGeoCanV32No1.qxp 3/18/2005 3:13 PM Page 17
GEOSCIENCE CANADA Volume 32 Number 1 March 2005 17
the original oxygen isotope ratio, and about 200,000 years, the residual geo- regulate the terrestrial cloudiness and
this, in turn, reflects the ambient seawa- magnetic field and the content of 10Be albedo, hence the climate. Having the
ter temperature. Veizer et al. (1999) gen- in sediments correlate antithetically (Fig. estimates of the geomagnetic field inten-
erated a large database of several thou- 8), at least at the 100,000 year frequency. sity and 10Be concentrations enables cal-
sand well-preserved calcitic shells that 10Be is generated by the CRF interacting culation of the intensity of past solar
cover this entire 545 million years time with our atmosphere. Since the solar and irradiance. The latter appears to reflect
span. Such detrended isotope data corre- terrestrial magnetic fields are the shield surprisingly well coeval climate oscilla-
late well with the climatic history of the that modulates the intensity of the CRF tions as recorded at Vostok and in the
planet (cf. Scotese, 2002; Boucot and reaching the Earth, this anti-correlation stacked oxygen isotope record of the
Gray, 2001), with tropical sea surface is to be expected. The CRF, in turn, may oceans (Fig. 9). This points again to the
temperatures fluctuating by perhaps 5 to
9° C between the apexes of icehouse
and greenhouse times, respectively (Fig.
5, top).
The situation is entirely different
for the CO2 scenario. For the
Phanerozoic, the estimates of atmos-
pheric pCO2 levels are not only internal-
ly inconsistent, but they also do not
show any correlation with the paleocli-
mate record (Fig. 5, bottom). In that
case, what could be an alternative driv-
ing force of climate on geological time
scales?
As suggested by theoretical con-
siderations, the “icehouse” episodes and
the oxygen isotope cold intervals should
coincide with times of high cosmic ray
flux, and the “greenhouse” ones with
the low CRF (Fig. 6). This correlation
may explain about 2/3 of the observed
oxygen isotope “temperature” signal
(Shaviv and Veizer, 2003). Thus celestial
phenomena were likely the principal
driver of climate on million year time
scales.
CLIMATE ON MILLENIAL TIME
SCALES
Drilling at Vostok in Antarctica has pro-
duced an outstanding record of climate
and atmospheric composition on millen-
nial to centennial time scales for the last
420,000 years (Figs. 1, 7). The laminae of Figure 5. Phanerozoic climatic indicators and reconstructed pCO2 levels. The curve
ice contain frozen air bubbles, and in in the upper set is the relative paleotemperature trend as calculated from the δ18O
these the amount of CO2 and methane values of calcitic shells (Veizer et al., 2000). The dotted histograms mark the lowest
indeed increases with temperature. Yet, paleolatitude (right-hand vertical axis) at which the ice rafted debris was observed in
new high-resolution studies show that at ancient sediments. The boxes represent cool climate modes (icehouses) and the
times of cold to warm transitions, tem- intervening intervals the warm modes (greenhouses), as established from sedimento-
perature changes come first, leading CO2 logical criteria (Frakes et al., 1992). The bottom set of curves describes the recon-
changes by several centuries (Mudelsee, structed histories of the past pCO2 variations (GEOCARB III) by Berner and
2001; Clarke, 2003; Vakulenko et al., Kothavala (2001), Klimafakten (Berner and Streif, 2000) and Rothman (2002). A
2004). If so, the CO2 levels would be a recent argument by Royer et al. (2004) that the δ18O trend of Veizer et al. (2000)
response to, and not the cause of, the reflects the pH rather than the temperatures of ancient oceans is interesting, but this
change in temperature (climate). CO2 proposition, apart from being rather arbitrary, cannot explain the magnitude of the
may then serve as a temperature amplifi- δ18O trend (Shaviv and Veizer, 2004a; Wallmann, 2004) and is also at odds with the
er, but not as the climate driver. paleoclimatological reconstructions (see Scotese, 2002, Boucot and Gray, 2001, and
If CO2 were not the driver, what Boucot et al, 2004). As for the “critique” of Rahmstorf et al. (2004) see Shaviv and
could the alternative be? For the last 2 Veizer (2004b), http://www.phys.huji.ac.il/~shaviv/ClimateDebate, and de la Fuente
cycles of the Vostok record, spanning Marcos and de la Fuente Marcos (2004).GACGeoCanV32No1.qxp 3/18/2005 3:13 PM Page 18
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previously discussed extrinsic modula-
tion of the CRF by the solar driver.
Additional support for celestial
forcing comes from ocean sediments
and from caves, records that cover the
times of transition from the last glacial
episode into the warmer climates of our
times, that is the time from about 11,500
to some 2,000 years BP. For an Atlantic
drill core taken west of Ireland (Bond et
al., 2001), the incidence of “ice rafted
debris” (IRD), small debris pieces that
fall to the ocean floor from melting ice
floes that drift on the surface, coincides
with the colder climates (Fig. 10). In
addition, the cold times are characterized
by high concentrations of 10Be, as meas-
ured in sediments, and by an “excess” of
14C, as observed in tree rings on land.
Since both 10Be and 14C are products of
the CRF interacting with our atmos-
phere, and because their subsequent
redistribution pathways are entirely dif-
ferent, the only process that can explain
all these positive correlations is an inten-
sified CRF. Still better correlation is
Figure 6. The variations in the cosmic ray flux (Φ) and tropical seawater tempera- present in the cave sediments of Oman
ture variations (∆T) over the Phanerozoic. The upper curves describe the recon- (Fig. 11). As stalagmites grow, they pro-
structed trends for cosmic ray flux (CRF) within their uncertainty band (stippled). duce growth rings similar to those in the
The bottom curves depict the smoothed temperature anomaly (“Geological trees. The oxygen isotope ratio measured
Reconstruction”) based on the δ18O record and the model cosmic ray flux (“Fit”). in these rings is a reflection of climate,
The peaks and valleys represent greenhouse and icehouse episodes as in Fig. 5. Note in this particular case of monsoon pat-
that no polar ice caps were as yet demonstrated for the third (hatched) icehouse. terns. The correlation with 14C, which is
Adapted from Shaviv and Veizer (2003).
Figure 7. Antarctic (Vostok) ice core data for the last 400,000 years. Temperatures (dashed curve) are derived from oxygen and
hydrogen isotopes of ice and CO2 concentrations (dotted curve) were measured in frozen air bubbles. Adapted from Petit et al.
(1999).GACGeoCanV32No1.qxp 3/18/2005 3:14 PM Page 19
GEOSCIENCE CANADA Volume 32 Number 1 March 2005 19
the product of CRF, is excellent. More
recently, these cosmogenic nuclide/cli-
mate correlations were extended up to
2000 years BP and corroborated by
additional records from an Alaskan lake
(Hu et al., 2003), several European and
American speleothems (references in
Niggemann et al., 2003), polar ice
shields (Laj et al., 2000; St-Onge et al.,
2003), deep-sea sediments (Christl et al.,
2003), and northern peat bogs (Blaauw
et al., 2004) - geographic coverage of a
considerable extent.
CLIMATE ON TIME SCALES OF CEN-
TURIES
Let us now look at the record of the last
millennium (Fig. 1), starting with
Greenland, the climate record of the
northern hemisphere. The calculations
based on oxygen isotope values in ice
layers suggest that the temperatures in
the 11th century were similar to those of
today (Fig. 12). This warm interval was
followed by a temperature decline until
the 14th century, then by generally cold
temperatures that lasted until the 19th
century, and finally by a warming in the
20th century. The “Medieval Climatic
Optimum” (MCO) and the “Little Ice
Age” (LIA), were both global phenome-
na (Soon and Baliunas, 2003; McIntire
and McKitrick, 2003), and not, as previ-
ously claimed (Mann et al., 1999), Figure 8. The intensity of the residual terrestrial magnetic field, and the 10Be con-
restricted solely to Greenland or to the tent of marine sediments, for the last 200,000 years. Adapted from Sharma (2002).
North Atlantic. Note that the coeval “ice See also Christl et al. (2003).
bubble CO2” pattern in Greenland and
Antarctic ice caps was essentially flat
(IPCC, 2001), despite these large climatic
oscillations. CO2 begins to rise only at
the termination of the “Little Ice Age”,
toward the end of the 19th century. In
direct contrast to CO2, 14C and 10Be
correlate convincingly with the climate
record (Fig. 13), again arguing for celes-
tial phenomena as the primary climate
driver.
THE DECADAL TO ANNUAL RECORD
OF THE LAST CENTURY
The IPCC (2001) global mean surface
temperature record shows an increase of
about 0.6°C since the termination of the
“Little Ice Age”. The bulk of this rise
happened prior to the early 1940’s, fol-
lowed by a cooling trend until 1976 and Figure 9. Calculated intensity of solar irradiance (dots) during the past 200,000
a resumption of temperature rise subse- years juxtaposed with the normalized δ18O record of the oceans (shading), the δ18O
quently (Fig. 14d). In contrast to temper- being a climate proxy. Adapted from Sharma (2002). In contrast to the CO2/temper-
ature, the rise in atmospheric CO2, most ature correlation (Fig. 7), any potential causative sequence can only be from sun to
likely from the burning of fossil fuels earth, and temporal resolution is therefore not critical.GACGeoCanV32No1.qxp 3/18/2005 3:14 PM Page 20
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plus land-use changes, proceeded in an essentially water-cycle models that gen- the earth. As a final point, the GCMs
exponential fashion. This mismatch rais- erally do not incorporate the active car- predict that the most prominent centen-
es two questions: (1) why the large tem- bon cycle and its dynamics. CO2 is “pre- nial temperature rise should have been
perature rise prior to the early 40’s, when scribed” in most models as a spatially evident in the higher troposphere. Yet,
80% of the cumulative anthropogenic uniform concentration, and inputted in the balloon and satellite data (Fig. 17) do
CO2 input is post-World War II?, and (2) the form of energy (~ 4 Wm-2 for CO2 not show any clear temporal tempera-
why the subsequent three decade long doubling). These models would yield ture trend (IPCC, 2001). Instead, their
cooling despite the rising CO2? In con- outcomes in the same general direction, interannual temperature oscillations cor-
trast to CO2, the temperature trend cor- regardless of the source of this addi- relate clearly with the solar irradiance
relates well with the solar properties, tional energy, be it CO2 or TSI. and CRF, with “no vestiges of the
such as the CRF and TSI (Figs. 14b,c), Moreover, taking into account the anthropogenic signal” (Kärner, 2002).
except perhaps for the last two decades empirical evidence, such as the unprece- All this favours the proposition that
of the 20th century that may or may not dented solar activity during the late 20th celestial phenomena may have been the
be an exception to this pattern. For century (Fig. 13) or the coeval decline in primary climate driver even for the most
these decades, the direct estimates of global albedo (“earthshine”) (Fig. 15), recent past.
TSI flux (Fig. 14c) could not apparently and considering that the 1915-1999 TSI In summary, the above empiri-
explain the entire observed magnitude of trend from the Mt. Wilson and cal observations on all time scales
the temperature rise (Ramaswamy et al., Sacramento Peak Observatories can point to celestial phenomena as the
2001; Solanki, 2002; Solanki et al., 2004; explain 80% of the 11-year smoothed principal driver of climate, with
Foukal et al., 2004) and the discrepancy variance in global temperature (Foukal, greenhouse gases acting only as potential
has to be attributed, therefore, to green- 2002), the celestial cause as a primary amplifiers. If solar activity accounts sta-
house gases, specifically CO2. It is this driver again appears to be a more con- tistically for 80% of the centennial glob-
discrepancy, and the apparent coherency sistent explanation. Additional support al temperature trend, while at the same
of model predictions with observed cli- for such a scenario arises from the time the measured variability in solar
mate trends (Karoly et al., 2003), that apparent relationship between solar cycle energy flux is insufficient to explain its
are the basis for the claim that the and precipitation/biological activity on magnitude, an amplifier that is causally
anthropogenic signal emerges from nat- land (Fig. 16). Terrestrial photosynthe- related to solar energy flux should exist.
ural variability in the 1990’s, with CO2 sis/respiration is the dominant flux for The earlier discussed cloud/CRF link
becoming the “principal climate driver”. atmospheric CO2 on annual to decadal and/or UV related atmospheric dynam-
While this may be the case, note that the time scales and any potential causative ics could be such an amplifier(s). The
General Climate Models (GCMs) are relationship can only be from the sun to existing general climate models may
Figure 10. The borehole record (Bond et al., 2001) of ice rafted debris (IRD), which is a climate proxy, and the coeval record
for CRF proxies, 10Be in sediments and ∆14C in growth rings of trees. Adapted from Kromer et al. (2001).GACGeoCanV32No1.qxp 3/18/2005 3:14 PM Page 21
GEOSCIENCE CANADA Volume 32 Number 1 March 2005 21
therefore “require an improved under-
standing of possible climate sensitivity
to relatively small total irradiance varia-
tions” (Foukal, 2002). I am aware that
some of the discussed trends may have
explanations based on the internal work-
ing of the earth system. For example,
the 14C wiggles can be explained as
changes in ocean circulation efficiency
(ventilation), but this cannot explain the
complementary 10Be patterns. In their
sum, these explanations rely on many, at
times arbitrary, causations and the over-
all structure is thus more complex than
the celestial alternative. When two
hypotheses can equally well explain the
observational data, it is the simpler one
that is to be preferred (Occam’s razor). I
wish to emphasize, nevertheless, that it
is not the intention of this contribution
to discount superimposed geological,
oceanographic, atmospheric and anthro-
pogenic phenomena as contributing fac- Figure 11. The record of δ18O (climate proxy) measured on growth layers of a sta-
tors. Space considerations, however, do lagmite in a cave in Oman juxtaposed with the ∆14C record (CRF proxy) in the
not allow this article to focus on any- growth rings of coeval trees. Adapted from Neff et al. (2001).
thing but the nature of the “primary cli-
mate driver”.
SO WHAT IS THE SEQUENCE?
The review of empirical evidence
strongly suggests that it may be the
celestial phenomena, sun and cosmic
rays, that are the principal climate driver.
While the individual lines of evidence
may have some weak points (but so do
all alternative explanations), overall the
celestial proposition yields a very consis-
tent scenario for all time scales. The
intrinsic CRF flux may have been
responsible for the pronounced climatic
trends on multimillion year time scales,
while the extrinsic modulation by solar
activity and earth dynamo could have
been the major driver for the superim-
posed subdued climate oscillations on
the millennial to annual time scales. This
input drives the water cycle, with water
vapour likely acting as a positive feed-
back and cloud formation as a negative
one (Fig. 18). It also generates the flux
of cosmogenic nuclides, such as 10Be, Figure 12. The temperature change (∆T) and CO2 records of the last millennium
14C and 36Cl. The hydrologic cycle, in from a Greenland ice core (GISP2). Temperature was calculated from the 50 year
turn, provides us with our climate, smoothed record as T(ºC) = 0.6906·δ18O–13.68. The δ18O database is available at
including its temperature component. ftp://ftp.ngdc.noaa.gov/paleo/icecore/greenland/summit/gisp2/isotopes/d18o1yr.t
On land, sunlight, temperature, and con- xt. The detailed structure showing the coincidence of cold intervals with sun activity
comitant availability of water are the minima (W to D; Wolf, Spörer, Maunder, Dalton) may or may not be statistically
dominant controls of biological activity valid because of the noisy nature of the proxy signals, but the overall trend is con-
and thus of the rate of photosynthesis firmed also by the borehole temperature profiles (Dahl-Jensen et al., 1998). Adapted
and respiration. In the oceans, the rise in from Berner and Streif (2000).
temperature results in release of CO2GACGeoCanV32No1.qxp 3/18/2005 3:14 PM Page 22
22
into air. These two processes together ing. Weathering would proceed without
increase the flux of CO2 into the atmos- CO2, albeit with some chemical reac-
phere. If only short time scales are con- tions modified, but not without water,
sidered, such a sequence of events whatever the CO2 levels. For almost any
would be essentially opposite to that of process, and time scale, the water and
the IPCC scenario, which drives the carbon cycles are coupled, but water is
models from the bottom up, by assum- orders of magnitude more abundant.
ing that CO2 is the principal climate The global water cycle is therefore not
driver and that variations in celestial “just there” to react on impulses from
input are of subordinate or negligible the carbon cycle, but is actively shaping
impact. This is not to dismiss CO2 as a it. The tiny carbon cycle is piggybacking
greenhouse gas with no warming effect on the huge water cycle (clouds includ-
at all, but only to point out that CO2 ed), not driving it. In such a perspective,
plays mostly a supporting role in the CO2 can amplify or modulate natural
orchestra of nature that has a celestial climatic trends, but it is not likely to be
conductor and the water cycle as its first their principal “driver”. If so, how are
fiddle. Consider an example that is the global water and carbon cycles cou-
familiar to every geologist, the weather- pled?
ing of rocks. This process is believed to
have been the controlling sink for COUPLING OF THE WATER AND CAR-
atmospheric CO2 on geological time BON CYCLES
scales (Berner, 2003), and indeed it was. The atmosphere today contains ~ 730
Yet, in reality, it is the water that is the PgC (1 PgC = 1015 g of carbon) as CO2
agent of physical and chemical weather- (Fig. 19). Gross primary productivity
Figure 13. Time series of the sunspot numbers (reconstructed from 10Be in ice
cores from Antarctica and Greenland), and of direct observations of sunspot num-
bers since 1610. The record of 14C in tree rings, not reproduced here due to visual
considerations, shows a similar pattern. Note the low 10Be (reciprocal of sunspot
numbers) and 14C during the Medieval Climate Optimum (MCO) and their high val-
ues during the Little Ice Age. Note also the very high solar activity for the latest 60
years, unprecedented for the last 8,000 years of Earth history (Solanki et al., 2004).
W, S, M and D are the sun activity minima as in Fig. 12. O is the Oort minimum.
Modified from Usoskin et al. (2003).
Figure 14. Eleven year average of northern hemispheric temperatures (dotted curve) and (a) solar cycle length (diamonds), (b)
cosmic ray flux from ion chambers (dashed curve) and from the Climax neutron monitor (full curve), (c) solar irradiance (dia-
monds) (all modified after Svensmark, 1998), and (d) atmospheric CO2 concentrations and global temperature trend (modified
from Berner and Streif, 2000). Note that the polarity of CRF is reversed to facilitate comparisons and that the time frame of (a)
to (c) represents only the post-1940 trough in (d).GACGeoCanV32No1.qxp 3/18/2005 3:14 PM Page 23
GEOSCIENCE CANADA Volume 32 Number 1 March 2005 23
(GPP) on land, and the complementary pheric CO2, is about 250:1, much less relation between NPP and precipitation
respiration flux of opposite sign, each than the WUE demand of the dominant is clear-cut (Fig. 20) and water availabili-
account annually for ~ 120 Pg. The plants, suggesting that the terrestrial ty is therefore the first order limiting
air/sea exchange flux, in part biologically ecosystem is in a state of water deficien- factor of ecosystem productivity
mediated, accounts for an additional ~ cy (Lee and Veizer, 2003). (Huxman et al., 2004). Transpiration by
90 Pg per year. Biological processes are The importance of the water ecosystems of cold and temperate
therefore clearly the most important supply for plant productivity is clearly regions recycles about 1/2 to 2/3 of
controls of atmospheric CO2 levels, with evident from the NPP database that is a precipitation into the atmosphere, while
an equivalent of the entire atmospheric collection of worldwide multi-biome for tropical regions the recycling is
CO2 budget absorbed and released by productivities, mostly established by bio- almost wholesale. Thus the former
the biosphere every few years. The ter- logical methods (Fig. 20). The principal appear to have been water starved (Fig.
restrial biosphere thus appears to have driving force of photosynthesis is 20), while the tropical ecosystems with
been the dominant interactive reservoir, unquestionably the energy provided by their efficient water recycling are likely
at least on the annual to decadal time the sun, with the global terrestrial system limited only by the amount of available
scales, with oceans likely taking over on reaching light saturation at about an sunlight, the latter modified within rela-
centennial to millennial time scales. NPP of 1150 ± 100 g carbon per year tively narrow limits, mostly by clouds.
Interannual variations in atmospheric (Fig. 20). If the sun is the driver, what For the global ecosystem, an increase in
CO2 levels mimic the Net Primary might be the limiting variable? Except sunlight, humidity and temperature is a
Productivity (NPP) trends of land locally, CO2 cannot be this limiting fac- precondition for, not a consequence of,
plants, and the simulated NPP, in turn, tor because its concentration is globally CO2 or nitrogen “fertilization”. And
correlates with the amount of precipita- almost uniform, while NPP varies by luckily so, otherwise our tree planting
tion (Nemani et al., 2002, 2003; Huxman orders of magnitude. Temperature, effort to sequester CO2 would only lead
et al., 2004) (Fig. 16). The question because of its quasi anticorrelation with to a continuous massive pumping of
therefore arises: is the terrestrial water the NPP (Fig. 16), is not a viable alterna- water vapour, a potent greenhouse gas,
cycle and NPP driven by atmospheric tive either. In contrast, the positive cor- from the soils to the atmosphere.
CO2 (CO2 fertilization) or is it the other
way around? As a first observation, note
that the “troughs” in precipitation and
NPP coincide with the minima in
sunspot activity (Fig. 16). As already
pointed out, if a causative relationship
exists, it can only be from the sun to the
earth.
During photosynthesis, a plant
has to exhale (transpire) almost one
thousand molecules of water for every
single molecule of CO2 that it absorbs.
This so-called “Water Use Efficiency”
(WUE), is somewhat variable, depending
on the photosynthetic pathway
employed by the plant and on the tem-
poral interval under consideration, but in
any case, it is in the hundreds to one
range (Taiz and Ziegler, 1991; Telmer
and Veizer, 2000). The relationship
between WUE and NPP deserves a
more detailed consideration. In plant
photosynthesis, water loss and CO2
uptake are coupled processes (Nobel,
1999), as both occur through the same Figure 15. Reconstructed annual reflectance anomalies (∆p*) relative to 1999-2001
passages (stomata). The WUE is deter- calibration interval (shaded). The observed anomalies are represented as a thick line.
mined by a complicated operation that In general, ∆p* is a measure of earth albedo, likely cloudiness, by observing the
maximizes CO2 uptake while minimizing "earthshine", the light reflected by Earth's sunlit hemisphere toward the moon and
water loss. Consequently, the regulating then retroflected from the lunar surface. Note that the decline in albedo (cloudiness)
factor for WUE, and the productivity of from 1985 to 2000 is a feature that is consistent with the increase in solar irradiance
plants, could be either the atmospheric TSI (Fig. 13) and implicitly also with a decline in cloud nucleation due to diminished
CO2 concentration or water availability. CRF. Note also that the cloud-driven changes in the Earth's radiation budget (up to
From a global perspective, the amount 10 Wm-2) during the last two decades exceed considerably the forcing that is attrib-
of photosynthetically available soil uted by IPCC (2001) to the entire "industrial", that is post-"Little Ice Age", anthro-
water, relative to the amount of atmos- pogenic greenhouse impact (2.4 Wm-2). Adapted from Pallé et al. (2004a).GACGeoCanV32No1.qxp 3/18/2005 3:14 PM Page 24
24
In order to test the hypothesis of
CO2 “piggybacking” on the water cycle,
several large watersheds were examined,
because there the water balance can be
deconvolved into precipitation, dis-
charge, evaporation, interception and
transpiration fluxes. Knowing the tran-
spiration flux and the requisite WUE, it
is then possible to calculate the photo-
synthetic sequestration capacity for CO2
for a given watershed. Taking the
Mississippi basin (Fig. 20) as an example
(Lee and Veizer, 2003), plant transpira-
tion recycles about 60% of precipitation
back into the atmosphere and the calcu-
lated, water balance-based, annual pho-
tosynthetic sequestration of CO2 by
plants is then 1.16 Pg of carbon. This is
essentially identical to the heterotrophic
soil respiration flux of 1.12 PgC derived
by biological approaches for the same
watershed. Hence, the suggestion that
the carbon cycle is “piggybacking” on
the water cycle is a viable proposition.
Figure 16. 1900-1993 variations in annual averages of air temperature (T - dotted This scenario is supported also by the
line) and precipitation (P - dashed line) for conterminous U.S. together with the sim- satellite data of global productivity for
ulated Net Primary Productivity (NPP - full line) smoothed with a 5-year filter the 1982-1999 period, with “climatic
(adapted from Nemani et al., 2002). The arrows are the years of sunspot minima variability overland exerting a strong
(dampened solar irradiance) from the Royal Observatory of Belgium control over the variations in atmospher-
(http://sidc.oma.be/index.php3). Note that except for 1944, the troughs in precipi- ic CO2” (Nemani et al., 2003). In these
tation and NPP appear to coincide with the sunspot minima. Figure courtesy of two decades the global biomass grew by
Ajaz Karim. 6% (3.4 PgC). Almost one half of the
increase happened, surprisingly, in the
Figure 18. Schematic presentation of
the sequence of events for a model
based on celestial forcing as the princi-
Figure 17. Annual variability in tropospheric temperature, TSI (∆Fs) and CRF. Note pal climate driver. The dashed arrow is a
the reversed scale for CRF. Modified from Marsh and Svensmark (2003b) and Marsh feedback from the biosphere on climate,
et al. (2005). including its anthropogenic component.GACGeoCanV32No1.qxp 3/18/2005 3:14 PM Page 25
GEOSCIENCE CANADA Volume 32 Number 1 March 2005 25
Amazon basin, and was caused by a
decrease in the cloud cover (decline in
CRF?) and to a concomitant 20th centu-
ry increase in solar radiation (Figs. 13,
14, 15). Again, while CO2 may act as an
amplifying greenhouse gas, the actual
atmospheric CO2 concentrations are
controlled in the first instance by the cli-
mate, that is by the sun-driven water
cycle, and not the other way around.
ENVIRONMENTAL IMPLICATIONS
At this stage, two scenarios of potential
human impact on climate appear feasi-
ble: (1) the standard IPCC model that
advocates the leading role of greenhouse
gases, particularly of CO2, and (2) the
alternative model that argues for celestial
phenomena as the principal climate driv-
er. The two scenarios are likely not even
mutually exclusive, but a prioritization
may result in different relative impact.
Models and empirical observations are
both indispensable tools of science, yet
when discrepancies arise, observations Figure 19. Simplified annual carbon cycle. Based on data in Prentice et al. (2001).
should carry greater weight than theory.
If so, the multitude of empirical obser-
vations favours celestial phenomena as
the most important driver of terrestrial
climate on most time scales, but time
will be the final judge. Should the celes-
tial alternative prevail, the chain of rea-
soning for potential human impact may
deviate from that of the standard IPCC
model, because the strongest impact
may be indirect, via the formation of
cloud condensation nuclei (CCN). The
CRF-generated positive and negative
ions combine, within minutes, into elec-
trically neutral aerosols, but only if the
two ions are large enough. The required
size of these “cluster ions” is reached by
addition of atmospheric molecules, par-
ticularly sulphuric acid. Since H2SO4 is
highly hygroscopic, it attracts also water
molecules. In this way, the ~30-100 nm
large CCN required as precursors for
droplets can potentially be generated
(Carslaw et al., 2002; Lee et al., 2003).
Thus, sulphur compounds (and perhaps
dust, soot and secondary particles, which
are formed by condensation of low-
vapour-pressure gases) could play a Figure 20. The Net Primary Productivity vs precipitation for global biomass
major role in this seeding process. In the (GPPD1 Grid Cells NPP Dataset; http://www.daac.ornl.gov; Zheng et al., 2003).
northern hemisphere, the precursor of The "cross" represents the Mississippi watershed. Note that plants are very impor-
sulphuric acid, sulphur dioxide gas, origi- tant not only for the carbon, but also for the water cycle, with almost 2/3 of precipi-
nates mostly from anthropogenic activi- tation (and more in the jungles) recirculated to the atmosphere by plant transpira-
ties, but natural sources, such as volcanic tion. For example, the parcel of air in the Amazon basin is "wetter" on the eastern
eruptions or DMS from marine plank- slopes of the Andes than at its origin in the Atlantic. Adapted from Lee and Veizer
ton, are also substantial. (2003).GACGeoCanV32No1.qxp 3/18/2005 3:14 PM Page 26
26
Although the role of clouds is Natural Sciences and Engineering cycle, fossil fuels and atmospheric com-
not well understood (IPCC, 2001), it Research Council of Canada (NSERC) position: Nature, v. 426, p. 323-326.
appears that the upper tropospheric and the Deutsche Forschungsgemein- Berner, R.A. and Kothavala, Z., 2001, GEO-
clouds warm, while the lower clouds, schaft (DFG). In the last decade, partic- CARB III: A revised model of atmos-
such as those potentially generated by ularly relevant to this article, the research pheric CO2 over Phanerozoic time: Am.
J. Sci., v. 301, p. 182-204.
the above CRF seeded processes, cool was supported by two major sources, the Berner, U. and Streif, H., 2000, Klimafakten,
the climate. In such a scenario, the top research award of the DFG (Leibniz Der Rückblick - Ein Schlüssel für die
impact of pollution, if indeed signifi- Prize endowed with 3 million DM) and Zukunft: Stuttgart: E. Schweizer-
cant, could even potentially result in the support of the Research Chair in bart’sche Verlagsbuchhandlung, Science
global cooling (Carslaw et al., 2002) “Earth System” financed jointly by Publishers.
instead of global warming, similar to the NSERC and the Canadian Institute for Blaauw, M., van Geel, B. and van der Plicht,
IPCC chain of reasoning that is invoked Advanced Research (CIAR). The donors J., 2004, Solar forcing of climate change
as an explanation for the 1940-1976 to CIAR include Noranda and Dr. G.G. during the mid-Holocene: indications
cooling trend (Fig. 14d). In addition, we Hatch, with the sponsorship based on from raised bogs in The Netherlands:
would have to deal not with a global an arms’ length relationship via CIAR The Holocene, v. 14, p. 35-44.
Bond, G., Kromer, B., Beer, J., Muscheler, R.,
issue of atmospheric CO2, but with and NSERC. Evans, M.N., Showers, W., Hoffmann, S.,
large regional phenomena, because it is Personally, this last decade has Lotti-Bond, R., Hajdas, I. and Bonani,
these that control the dispersion of been a trying period because of the G., 2001, Persistent solar influence on
aerosols, sulphur and nitrogen com- years of internal struggle between what North Atlantic climate during the
pounds. We are not yet in a situation I wanted to believe and where the Holocene: Science, v. 294, p. 2130-2136.
where quantitative projections of this empirical record and its logic were lead- Boucot, A.J. and Gray, J., 2001, A critique of
impact on climate can be provided ing me. This article is clearly not a com- Phanerozoic climatic modes involving
(Schwartz, 2004). Indeed, we do not prehensive review of the alternatives, changes in the CO2 content of the
even know if it is at all globally signifi- partly because of space limitations, but atmosphere: Earth Science Reviews, v.
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generated by CO2, or much larger. In was eloquently argued elsewhere (e.g., Boucot, A.J, Xu, C. and Scotese, C.R., 2004,
Phanerozoic climate zones and paleo-
any case, the strategy that emphasizes IPCC, 2001). It is rather a plea for some geography with a consideration of
reduction of human emissions is sound reflection in our clamour for over-sim- atmospheric CO2 levels: Paleont. J., v. 38,
for both the celestial and the CO2 alter- plified beliefs and solutions in the face p. 115-122.
native. Nevertheless, this strategy can be of the climate conundrum. Due to space Bowring, S.A., Williams, I.S. and Compston
pursued in two ways. It can be based on considerations, the article also does not W., 1989, 3.96 Ga gneisses from the
global reduction of CO2, because this explore the potential role that the lethal Slave Province, Northwest Territories,
would result also in collateral reduction CRF may have played in the evolution of Canada: Geology, v. 17, p. 971-975.
of particulates, sulphur and nitrogen life, as a cause of extinctions and/or Brasier, M.D., Green, O.R., Jephcoat, A.P.,
compounds. These are not only poten- mutations. And above all, this article is Kleppe, A.K., van Kranendonk, M.J.,
tial climate drivers, but also pollutants not a discussion of Kyoto, a treaty with Lindsay, J.F., Steele, A. and Grassineau,
N.V., 2002, Questioning the evidence for
and their reduction will improve our air social, economic and political aims, but a Earth’s oldest fossils: Nature, v. 416, p.
quality, regardless of the climate impact scientific treatise of the past climate 76-81.
of otherwise environmentally benign record. Time will rule on its validity, but Carslaw, K.S., Harrison, R.G. and Kirkby, J.,
CO2. At current atmospheric levels, CO2 in the meantime I ask that the discussion 2002, Cosmic rays, clouds and climate:
is in fact an essential commodity for of its merits/demerits be confined to Science, v. 298, p. 1732-1737.
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remedial measures based on the global As a final point, I am indebted Berylium-10 in deep-sea sediments: a
CO2 scenario are also costly. For the to several experts worldwide, covering tracer for Earth’s magnetic field intensity
celestial alternative, the remedial meas- the whole gamut of fields from astro- during the last 200 000 years: Quatern.
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result in a substantial reduction of the to make sure that its statements are sci- (http://www.nature.com/nsu/nsu_pf/03
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a simple prerogative of science, but Hildebrand, helped to set the tone of Their Bearing on Understanding the
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