By: Nir J. Shaviv
Article originally appeared in PhysicaPlus.
Sir William Herschel was the first to seriously consider the sun as a source of
climate variations, already two centuries ago. He noted a correlation between
the price of wheat, which he presumed to be a climate proxy, and the sunspot
activity:
“The
result of this review of the foregoing five periods is, that, from the price of
wheat, it seems probable that some temporary scarcity or defect of vegetation
has generally taken place, when the sun has been without those appearances
which we surmise to be symptoms of a copious emission of light and heat.”
~ Sir William Herschel, Phil. Trans. Roy. Soc. London, 91,
265 (1801)
Herschel presumed that this link arises from variation in the luminosity of the
sun. Today, various solar activity and climate variations are indeed known to
have a notable correlation on various time scales. The best example is perhaps
the one depicted in fig. 1, on a centennial to millennial time scale between
solar activity and the tropical climate of the Indian ocean (Neff
et al. 2001). Another example of a beautiful correlation exists on a
somewhat longer time scale, between solar activity and the northern atlantic
climate (Bond
et al. 2001). Nevertheless, the relatively small luminosity
variations of the sun are most likely insufficient to explain this or other
links. Thus, an amplifier of solar activity is probably required to explain
these observed correlations.
Figure 1: The correlation between
solar activity—as mirrored in the 14C flux, and a climate
sensitivity variable, the 18O/16O isotope ratio from
stalagmites in a cave in Oman, on a centennial to millennial time scale. The 14C
is reconstructed from tree rings. It is a proxy of solar activity since a more
active sun has a stronger solar wind which reduces the flux of cosmic rays
reaching Earth from outside the solar system. A reduced cosmic ray flux, will
in turn reduce the spallation of nitrogen and oxygen and with it the formation
of 14C. On the other hand, 18O/16O reflects
the temperature of the Indian ocean—the source of the water that formed the
stalagmites. (Graph from Neff
et al., 2001, Copywrite by Nature,
used with permission)
Several amplifiers were suggested. For example, UV radiation is all absorbed in
the stratosphere, such that notable stratospheric changes arise with changes to
the non-thermal radiation emitted by the sun. In fact, Joanna Heigh of Imperial
College in London, suggested that through dynamic coupling with the
troposphere, via the Hadley circulation (in which moist air ascends in the
tropic and descends as dry air at a latitude of about 30°) the solar signal at
the surface can be amplified. Here we are interested in what appears to be a
much more indirect link between solar activity and climate.
In 1959, the late Edward Ney of the U. of Minnesota suggested that any climatic
sensitivity to the density of tropospheric ions would immediately link solar
activity to climate. This is because the solar wind modulates the flux of high
energy particles coming from outside the solar system. These particles, the
cosmic rays, are the dominant source of ionization in the troposphere. More
specifically, a more active sun accelerates a stronger solar wind, which in
turn implies that as cosmic rays diffuse from the outskirts of the solar system
to its center, they lose more energy. Consequently, a lower tropospheric
ionization rate results. Over the 11-yr solar cycle and the long term variations
in solar activity, these variations correspond to typically a 10% change in
this ionization rate. It now appears that there is a climatic variable
sensitive to the amount of tropospheric ionization—Clouds.
Figure 2: The cosmic ray link
between solar activity and the terrestrial climate. The changing solar activity
is responsible for a varying solar wind strength. A stronger wind will reduce
the flux of cosmic ray reaching Earth, since a larger amount of energy is lost
as they propagate up the solar wind. The cosmic rays themselves come from
outside the solar system (cosmic rays with energies below the "knee"
at 1015eV, are most likely accelerated by supernova remnants). Since
cosmic rays dominate the tropospheric ionization, an increased solar activity
will translate into a reduced ionization, and empirically (as shown below),
also to a reduced low altitude cloud cover. Since low altitude clouds have a
net cooling effect (their "whiteness" is more important than their
"blanket" effect), increased solar activity implies a warmer climate.
Intrinsic cosmic ray flux variations will have a similar effect, one however,
which is unrelated to solar activity variations.
Clouds have been observed from space since the beginning of the 1980's. By the
mid 1990's, enough cloud data accumulated to provide empirical evidence for a
solar/cloud-cover link. Without the satellite data, it hard or probably
impossible to get statistically meaningful results because of the large
systematic errors plaguing ground based observations. Using the satellite data,
Henrik Svensmark of the Danish National Space Center in Copenhagen has shown
that cloud cover varies in sync with the variable cosmic ray flux reaching the
Earth. Over the relevant time scale, the largest variations arise from the
11-yr solar cycle, and indeed, this cloud cover seemed to follow the cycle and
a half of cosmic ray flux modulation. Later, Henrik Svensmark and his colleague
Nigel Marsh, have shown that the correlation is primarily with low altitude
cloud cover. This can be seen in fig. 3.
Figure 3: The correlation between
cosmic ray flux (orange) as measured in Neutron count monitors in low magnetic
latitudes, and the low altitude cloud cover (blue) using ISCCP satellite data
set, following Marsh
& Svensmark, 2003.
The solar-activity – cosmic-ray-flux – cloud-cover correlation is quite
apparent. It was in fact sought for by Henrik Svensmrk, based on theoretical
considerations. However, by itself it cannot be used to prove the cosmic ray
climate connection. The reason is that we cannot exclude the possibility that
solar activity modulates the cosmic ray flux and independently climate, without
any casual link between the latter two. There is however separate proof that a
casual link exists between cosmic rays and climate, and independently that
cosmic rays left a fingerprint in the observed cloud cover variations.
To begin with, climate variations appear to arise also from intrinsic cosmic
ray flux variations, namely, from variations that have nothing to do with solar
activity modulations. This removes any doubt that the observed solar activity
cloud cover correlations are coincidental or without an actual causal connection.
That is to say, it removes the possibility that solar activity modulates the
cosmic ray flux and independently the climate, such that we think that
the cosmic rays and climate are related, where in fact they are not.
Specifically, cosmic ray flux variations also arise from the varying
environment around the solar system, as it journeys around the Milky Way. These
variations appear to have left a paleoclimatic imprint in the geological
records.
Cosmic Rays, at least at energies lower than 1015eV, are accelerated
by supernova remnants. In our galaxy, most supernovae are the result of the
death of massive stars. In spiral galaxies like our own, most of the star
formation takes place in the spiral arms. These are waves which revolve around
the galaxy at a speed different than the stars. Each time the wave passes (or
is passed through), interstellar gas is shocked and forms new stars. Massive
stars that end their lives with a supernova explosion, live a relatively short
life of at most 30 million years, thus, they die not far form the spiral arms
where they were born. As a consequence, most cosmic rays are accelerated in the
vicinity of spiral arms. The solar system, however, has a much longer life span
such that it periodically crosses the spiral arms of the Milky Way. Each time
it does so, it should witness an elevated level of cosmic rays. In fact, the
cosmic ray flux variations arising from our galactic journey are ten times
larger than the cosmic ray flux variations due to solar activity modulations, at
the energies responsible for the tropospheric ionization (of order 10 GeV). If
the latter is responsible for a 1°K effect, spiral arm passages should be
responsible for a 10°K effect—more than enough to change the state of earth
from a hothouse, with temperate climates extending to the polar regions, to an
icehouse, with ice-caps on its poles, as Earth is today. In fact, it is
expected to be the most dominant climate driver on the 108 to 109
yr time scale.
It was shown by the author (Shaviv
2002, 2003),
that these intrinsic variation in the cosmic ray flux are clearly evident in
the geological paleoclimate data. To within the determinations of the period
and phase of the spiral-arm climate connection, the astronomical determinations
of the relative velocity agree with the geological sedimentation record for
when Earth was in a hothouse or icehouse conditions. Moreover, it was found
that the cosmic ray flux can be independently reconstructed using the so called
"exposure ages" of Iron meteorites. The signal, was found to agree
with the astronomical predictions on one hand, and correlate well with the
sedimentation record, all having a ~145 Myr period.
Figure 4: An Iron meteorite. A large
sample of these meteorites can be used to reconstruct the past cosmic ray flux
variations. The reconstructed signal reveals a 145 Myr periodicity. The one in
the picture is part of the Sikhote Alin meteorite that fell over Siberia in the
middle of the 20th century. The cosmic-ray exposure age of the
meteorite implies that it broke off its parent body about 300 Million years
ago.
In a later analysis, with Ján Veizer of the University of Ottawa and the Ruhr
University of Bochum, it was found that the cosmic ray flux reconstruction
agrees with a quantitative reconstruction of the tropical temperature (Shaviv
& Veizer, 2003). In fact, the correlation is so well, it was
shown that cosmic ray flux variations explain about two thirds of the variance
in the reconstructed temperature signal. Thus, cosmic rays undoubtedly affect
climate, and on geological time scales are the most dominant climate driver.
Figure 5: Correlation between the
cosmic ray flux reconstruction (based on the exposure ages of Iron meteorites)
and the geochemically reconstructed tropical temperature. The comparison
between the two reconstructions reveals the dominant role of cosmic rays and
the galactic "geography" as a climate driver over geological time
scales. (Shaviv
& Vezier 2003)
Figure 6: A summary of the 4
different signals revealing the cosmic ray flux climate link over geological time
scales. Plotted are the period and phase (of expected peak coldness) of two
extraterrestrial signals (astronomical determinations of the spiral arm pattern
speed and cosmic ray flux reconstruction using Iron meteorites) and two
paleoclimate reconstruction (based on sedimentation and geochemical records).
All four signals are consistent with each other, demonstrating the robustness
of the link. If any data set is excluded, a link should still exist.
Recently, it was also shown by Ilya Usoskin of the University of Oulu, Nigel
Marsh of the Danish Space Research Center and their colleagues, that the
variations in the amount of low altitude cloud cover follow the expectations
from a cosmic-ray/cloud cover link (Usoskin
et al., 2004). Specifically, it was found that the relative change
in the low altitude cloud cover is proportional to the relative change in the
solar-cycle induced atmospheric ionization at the given geomagnetic latitudes
and at the altitude of low clouds (up to about 3 kms). Namely, at higher
latitudes were the the ionization variations are about twice as large as those
of low latitudes, the low altitude cloud variations are roughly twice as large
as well.
Thus, it now appears that empirical evidence for a cosmic-ray/cloud-cover link
is abundant. However, is there a physical mechanism to explain it? The answer
is that although there are indications for how the link may arise, no firm
scenario, at least one which is based on solid experimental results, is yet
present.
Although above 100% saturation, the preferred phase of water is liquid, it will
not be able to condense unless it has a surface to do so on. Thus, to form
cloud droplets the air must have cloud condensation nuclei—small dust
particles or aerosols upon which the water can condense. By changing the
number density of these particles, the properties of the clouds can be varied,
with more cloud condensation nuclei, the cloud droplets are more numerous but
smaller, this tends to make whiter and longer living clouds. This effect was
seen down stream of smoke stacks, down stream of cities, and in the oceans in
the form of ship tracks in the marine cloud layer.
The suggested hypothesis, is that in regions devoid of dust (e.g., over the
large ocean basins), the formation of cloud condensation nuclei takes place
from the growth of small aerosol clusters, and that the formation of the latter
is governed by the availability of charge, such that charged aerosol clusters
are more stable and can grow while neutral clusters can more easily break
apart. Several experimental results tend to support this hypothesis, but not
yet prove it. For example, the group of Frank Arnold at the university of
Heidelberg collected air in airborne missions and found that, as expected,
charge clusters play an important role in the formation of small condensation
nuclei. It is yet to be seen that the small condensation nuclei grow through
accretion and not through scavenging by larger objects. If the former process
is dominant, charge and therefore cosmic ray ionization would play an important
role in the formation of cloud condensation nuclei.
One of the promising prospects for proving the "missing link", is the
SKY experiment being conducted in the Danish National Space Center,
where a real "cloud chamber" mimics the conditions in the atmosphere.
This includes, for example, varying levels of background ionization and aerosols
levels (sulpheric acid in particular). Within a few months, the experiment will
hopefully shed light on the physical mechanics responsible for the apparent
link between cloud cover and therefore climate in general, to cosmic rays, and
through the solar wind, also to solar activity. [Added Note (4 Oct. 2006): The
experimental results indeed confirm a link]
Figure 7: The Danish National Space
Center SKY reaction chamber experiment. The experiment was built with
the goal of pinning down the microphysics behind the cosmic ray/cloud cover
link found through various empirical correlations. From left to right: Nigel
Marsh, Jan Veizer, Henrik Svensmark. Behind the camera: the author.
The implications of this link are far reaching. Not only
does it imply that on various time scales were solar activity variations or
changes in the galactic environment prominent, if not the dominent climate
drivers, it offers an explanation to at least some of the climate variability
witnessed over the past century and millennium. In particular, not all of the
20th century global warming should be attributed to anthropogenic
sources, since increased solar activity explains through this link more than
half of the warming.
More information can be found at:
1.
A
general article on the
cosmic ray climate link over geological time scales.
2.
Henrik
Svensmark's web site, including various publications on the
cosmic-ray/cloud link.
3.
The
awaited results of the Danish SKY cloud experiment will be reported on their
website within several months.
Notes and References:
* On solar activity /climate correlation:
1.
For
the first suggestion that solar variability may be affecting climate, see:
William Herschel, "Observations tending to investigate the nature of
our sun, in order to find causes or symptoms of its variable emission of light
and heat", Phil.
Trans. Roy. Soc. London, 91, 265 (1801). Note that Herschel
suspected that it is variations in the total output which may be affecting the
climate (and with it the price of wheat).
2.
Perhaps
the most beautiful correlation between a solar activity and climate proxies can
be found in the work of U. Neff et al., "Strong coherence between solar
variability and the monsoon in Oman between 9 and 6 kyr ago", Nature
411, 290 (2001).
3.
Another
beautiful correlation between solar activity and climate can be seen in the
work of G. Bond et al., "Persistent Solar Influence on North Atlantic
Climate During the Holocene", Science,
294, 2130-2136, (2001).
* On cosmic ray and cloud cover correlation:
1.
The
paper by Henrik Svensmark, reports the correlation between cosmic ray flux
variations and cloud cover changes: H. Svensmark, "Influence of Cosmic
Rays on Earth's Climate", Physical
Review Letters 81, 5027 (1998).
2.
The
specific correlation with low altitude cloud cover is discussed in N. Marsh and
H. Svensmark, "Low Cloud Properties Influenced by Cosmic Rays",
Physical
Review Letters 85, 5004 (2000).
3.
Further
analysis including the relative role of CRF variations vs. el-niño can be
found in: N. Marsh and H. Svensmark, "Galactic cosmic ray and El
Niño-Southern Oscillation trends in International Satellite Cloud Climatology
Project D2 low-cloud properties", J.
of Geophys. Res., 108(D6), 6 (2003).
4.
The
analysis showing the geographic signature of the cosmic ray flux variations in
the low altitude cloud cover variations can be found it: I. Usoskin et al., "Latitudinal
dependence of low cloud amount on cosmic ray induced ionization", Geophysical
Research Letters 31, L16109 (2004).
* On cosmic ray climate correlations on Geological time
scales:
1.
The
suggestion that cosmic ray flux variations spiral arm passages could give rise
to ice-age epochs is found at: N. Shaviv, "Cosmic Ray Diffusion from
the Galactic Spiral Arms, Iron Meteorites, and a Possible Climatic
Connection", Physical
Review Letters 89, 051102, (2002).
2.
A
highly detailed analysis, including the cosmic ray reconstruction using iron
meteorites is found in: N. Shaviv, "The spiral structure of the Milky
Way, cosmic rays, and ice age epochs on Earth", New
Astronomy 8, 39 (2003).
3.
The
analysis of Shaviv & Veizer demonstrates the primary importance of comic
ray flux variations over geological time scales, and with it, place a limit on
climate sensitivity: N. Shaviv & J. Veizer, "A Celestial driver of
Phanerozoic Climate?", GSA
Today 13, No. 7, 4, 2003.