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Charles Lyell and climatic change: speculation and certainty
JAMES
RODGER
FLEMING
Science, Technology and Society Program, Colby College, Waterville,
ME, 04901, USA
Abstract: In the first edition of the Principles of Geology, Charles Lyell announced his theory of
the geographical determination of climate and speculated on possible climatic changes during the
geological and historical past. In light of the subsequent discovery of ice ages, the proliferation
of theories of climatic change, and the great climate debates of his time. Lyell's theory remained
remarkably stable. This paper examines Lyell's appropriation, modification and rejection of the
views of his contemporaries. It provides perspectives on elite and popular ideas of climate and
climatic change from the late eighteenth century to 1875. examines Lyell's position on climatic
change in geological and historical times, and explores in some detail the mutual influences of
Lyell and James Croll, the proponent of an astronomical theory of ice ages.
I have often told (and been told) humorous stories
about climate and human affairs. This one, from
1865, comes from a letter to Lyell from John
Carrick Moore, FRS, field geologist and long-time
member of the Geological Society. Moore writes:
I fear you have not time to read the Reader, I
must call your attention to the last number, in
which there is a true story of a Physician who
warned a Unitarian preacher that he would make
no proselytes in Northern Virginia because the
people all had fair complexions and therefore
were Calvinists. If he wished to preach against
the eternity of punishment, he should go to the
Hill Country. A map of the world is evidently
much wanted to show the influence of Climate
on Creeds, with contour l i n e s - [ B u d d h i s m ]
below the 50 foot level, Calvinists near the Snow
line, and Papists principally on the Volcanic
tufts... (J. C. Moore to C. Lyell, 5 March 1865 in
Lyell papers)
So you see, climate is not only a complicated
issue, it is also a cultural one; even more so for
climatic change.
Apprehending climatic change
In pursuing historical research on climate change, I
have had to ask several crucial questions. How do
people (scientists included) gain awareness and
understanding of phenomena that cover the entire
globe, and that are constantly changing on timescales ranging from geological eras to centuries,
decades, years and seasons? How was this
accomplished by individuals immersed in and surrounded by the phenomena? How were privileged
positions created and defined? The answers are
varied and worthy of extended reflection. In the
absence of means to observe the climate system in
its entirety, (as an astronomer might view a star or
planet) or to experiment on it directly (as a chemist
might view a reaction), how did scientific understanding of it emerge?
One approach, popular in the eighteenth century,
was through appeals to authority-references to
historical literature, first impressions of explorers
or the memory of the elderly. This was the
rhetorical strategy of Enlightenment writers who
wanted to support a particular theory of cultural
development or decline. I will say more about this
shortly.
Another way of approaching the issue was to
collect massive amounts of meteorological data
over large areas and extended time periods in the
hope of deducing climatic patterns and changes.
Individual observers in particular locales dutifully
tended to their journals, and networks of cooperative observers gradually extended the frontiers of
meteorology. Although many of the basic meteorological instruments were invented in the seventeenth century, they were not standardized or
widely distributed until well into the mid-nineteenth century.
During Lyell's lifetime, meteorology emerged as
an organized, if not yet fully disciplined, observational science. Observations were tabulated,
charted, mapped and analysed to provide representative climatic inscriptions. This process profoundly
changed climate discourse and established the
foundations of the science of climatology (Fleming
1990). National weather services were established
in Europe, Russia and the United States in the third
FLEMING,J. R. 1998. Charles Lyell and climatic change: speculation and certainty, hz: BLUNDELL,
D. J. & SCOTT, A. C. (eds) Lyell: the Past is the Key to the Present. Geological Society, London,
Special Publications, 143, 161-169.
161
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J.R. FLEMING
quarter of the century, and by 1872, within Lyell's
lifetime, regular meetings were being held of the
directors of national weather services (Fleming
1997).
A third approach to privileged knowledge was to
establish from first principles what the climate
ought to be and how it ought to change. Joseph
Fourier, John Tyndall and James Croll, to name but
a few, engaged in such speculative and theoretical
practices. These approaches-based on mathematical, physical and astronomical principlestended to be most satisfying to those scientists
working within a particular disciplinary perspective; most only grudgingly admitted other
possible secondary causes of climate change. Lyell,
of course, had his own favourite causal mechanism
which was solidly grounded in geological field
evidence.
In the twentieth century, climatic phenomena
have been rendered three dimensional by the
development of upper-air observations, extended
into the indefinite past by palaeoclimatic
techniques and, finally, globalized in the era of
satellite remote sensing. Many climate scientists
today are working on links between remote sensing
and more sophisticated computer models. They are
hoping, through advances in technology, to provide
new privileged positions. For most scientists the
goal is better understanding of climate: for some it
is also prediction and, ultimately, control. I might
add that an additional strategy for claiming
privileged knowledge is the consensus method, for
example
as currently practised
by the
Intergovernmental Panel on Climate Change (IPCC
1995).
Perceptions of climatic change in the
eighteenth century
Climate-from the Greek term klima, meaning
slope or inclination-was originally thought to
depend only on the height of the Sun above the
horizon, a function of the latitude. A second
tradition, traceable to Aristotle, linked the quality
of the air (and thus the climate) to the vapours and
exhalations of a country. The Hippocratic tradition
further linked climate to health and national
character. Enlightenment ideas linking climate
change and culture were grounded in the work of
the diplomat, historian and critic Abb6 JeanBaptiste Du Bos, perpetual secretary of the French
Academy, who argued that the rise and fall of
creative genius was not due primarily to 'les causes
morales' (education, cultivation, governance), but
was largely attributable to changes in 'les causes
physiques' (the nature of the air, land, soil and
especially, climate). These ideas influenced
Montesquieu's ideas on climate and governance,
David Hume's ideas on recent climate change in the
Americas, and of course, generations of colonial
settlers and revolutionary patriots (Fleming 1998).
As late as 1779 - in other words in the prime of
Hutton's l i f e - the EncyclopdFdie of Diderot and
D'Alembert still defined 'climat" in the ancient
way, geographically, as a 'portion or zone of the
surface of the Earth, enclosed within two circles
parallel to the equator', within which the longest
day of the year on the northern and southern
boundaries differs by some quantity of time, for
example one half hour. The Encyclopd(die
provided a medical definition of climate as well,
understood primarily through the effects of climate
on the health and well-being of the inhabitants of
various climes. It also mentioned Montesquieu's
position on the influence of climate on people's
mores, character and forms of governance (Diderot
& D'Alembert 1751-1765).
With no established science of climatology,
authors such as Du Bos, Montesquieu and Hume
appealed directly to cultural sensibilities and
prejudices, the authority of their positions residing
in their considerable literary skills and the lack of
other evidence to prove them wrong. Collectively,
they generated a powerful vision of the climates of
Europe and America, shaping the course of empire
and the arts: the concerted efforts of innumerable
individuals in turn shaping the climate itself. By the
end of the century, physiocrats had come to the
following general conclusions on climate change,
culture and cultivation:
I. Cultures are determined or at least strongly
shaped by climate.
2. The climate of Europe had moderated since
ancient times.
3. These changes were caused by the gradual
clearing of the forests and by cultivation.
4. The American climate was undergoing rapid and
dramatic changes caused by settlement.
5. The amelioration of the American climate
would make it more fit for European-type
civilization and less suitable for the primitive
native cultures.
This was the dominant popular understanding of
climate at the dawn of the nineteenth century
(Fleming 1998).
LyeU's position
In the first edition of his Principles of Geology
(1830-1832), Charles Lyell announced his theory
of the geographical determination of climate: a
theory that influenced generations to follow,
including G. E Wright (1889), M. Ramsay (19091910), and C. E. R Brooks (1926). He syste-
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CLIMATIC CHANGE
matically rejected catastrophic agents of climatic
change, arguing patiently, systematically and
forcefully, as Martin Rudwick has recently
reminded us, that 'modern causes', acting at their
present intensities, were 'entirely adequate' to
explain the evidence of the past (Rudwick 1990).
Lyell also maintained that geology should remain
independent of cosmogony, just as history had been
divorced from myths of human origins (Bailey
1962). For Lyell, the geographical arrangements of
oceans and continents, currents and winds were
sufficient to explain the immense variety of
climatic zones being revealed by meteorologists,
such as Heinrich Wilhelm Dove, and scientific
travellers, such as Alexander von Humboldt. As
Lyell perceptively noted, the ocean tempered the
climate, 'moderating alike an excess of heat or
cold', while elevated land, extending into the colder
regions of the atmosphere, 'becomes a great
reservoir of ice and snow, arrests, condenses, and
congeals vapour, and communicates its cold to the
adjoining country'. Lyell made additional perceptive comments on the role of particular largescale features such as the African continent- "an
immense furnace' that distributes its heat to Asia
and Europe- and ocean currents such as the Gulf
Stream- which 'maintains an open sea free from
ice in the meridian of East Greenland and
Spitzbergen' (Lyell 1830-1832).
Lyell used his climate theory to demonstrate that
the past history of the Earth was 'one uninterrupted
succession of physical events, governed by the laws
now in operation' (Wilson 1972). Such a position
assumes that geologists know all the laws currently
in operation- a precarious assumption in 1830 or
even today. Nevertheless, if it is accepted, Lyell's
position has important implications. Over immense
geological time, in this view, gradual processes
shaped the distribution of land and sea, which in
turn determined the climates of the world. The
geographical distribution of species, which depends
greatly on the climate and geographical conditions,
was thus shaped by natural laws. In Lyell's terminology, 'transportations of climate' contributed to
'local extermination of species', while other
species better suited to the new conditions eventually took their places (Bailey 1962).
Lyell introduced a substantial amount of
evidence indicating that the climate of the northern
hemisphere was 'formerly hotter'. He included
proofs from analogy derived from extinct quadrupeds; and direct proofs from the organic remains
of the Sicilian and Italian strata, from fossil remains
in Tertiary and Secondary rocks and from the plants
of the coal formation. He argued that the climate of
Siberia and other Arctic regions had been formerly
temperate, but had become subjected to 'extremely
severe winters' due to changes in landforms. These
163
changes were theorized to account for animal
migration and evolutionary changes as animals
adapted to different climates. Lyell concluded:
the remains both of the animal and vegetable
kingdom preserved in strata of different ages,
indicate that there has been a great diminution of
temperature throughout the northern hemisphere,
in the latitudes now occupied by Europe, Asia,
and America. The change has extended to the
Arctic circle, as well as to the temperate zone.
The heat and humidity of the air, and the
uniformity of climate, appear to have been most
remarkable when the oldest strata hitherto
discovered were formed. The approximation to a
climate similar to that now enjoyed in these
latitudes, does not commence till the era of the
formations termed tertiary, and while the
different tertiary rocks were deposited in succession, the temperature seems to have been still
farther lowered, and to have continued to
diminish gradually, even after the appearance of
a great portion of existing species upon the earth.
(Lyell 1830-1832, p. 103).
9
Lyell rejected, however, the notion of a secularly
cooling Earth. Six years earlier, in 1824, Joseph
Fourier had determined that the internal heat of the
Earth had decreased no more than 3/100 of a degree
during the course of recorded history (Fouvier
1824). Instead, Lyell fixed his thoughts on gradual
processes occurring steadily and repeatedly at the
Earth's surface-"on the connection at present
between climate and the distribution of land and
sea; and if we then consider what influence former
fluctuations in the physical geography of the earth
must have had on superficial temperature, we may
perhaps approximate to a true theory' (Lyell
1830-1832, p. 105).
For Lyell, the driving forces of climatic change
were due to continuous changes in the distribution
of land and sea:
When land is massed in equatorial and tropical
latitudes polar climates are mild. The land,
heated to an excess under the equatorial sun,
gives rise to warm currents of air that sweep
north. On the other hand, land massed around the
poles produces the reverse effect. There is no
land at the equator to soak up heat and no warm
winds coming into polar regions.
Lyell challenged his readers to imagine the
Himalaya Mountains, 'with the whole of
Hindostan', sinking down and being replaced by
the Indian Ocean, while an equal extent of
mountainous lands rose up, extending from North
Greenland to the Orkney Islands. He pointed out
that under such altered circumstances "it seems
difficult to exaggerate the amount to which the
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164
J.R. FLEMING
climate of the northern hemisphere would now be
cooled down'. Lyell's imagined refrigeration,
however, did not stop there. Icebergs would find
their way into southern waters, their melting
creating vapour, fogs and clouds that would reduce
solar insolation by half, causing the Earth to cool
further, wrapping large portions of the northern
hemisphere in a 'winding sheet of continental
i c e ' - a phrase of ominous significance for the
organic world. When in the course of geological
time, conditions had reversed and continents again
dominated the equatorial regions, snow would be a
rarity, the Earth's crust would be heated to considerable depths, and springs and surface waters
would run hotter, even in the winter (Lyell
1830-1832).
In pondering the vicissitudes of climate, Lyell
made the following four assumptions, quoted here
from the eleventh (1872) edition of the Principles.
I cite the last edition published in his lifetime
because Lyell's principles remained basically
unchanged from the first edition.
I shall assume, 1st, that the proportion of dry
land to sea continues always the same. 2ndly,
That the column of the land rising above the
level of the sea is a constant quality; and not only
that its mean, but that its extreme height, is liable
only to trifling variations. 3rdly, That on the
whole, and in spite of local changes, both the
mean and extreme depth of the sea are
invariable; and 4thly, That the grouping together
of the land in continents is a necessary part of the
economy of nature. (Lyell 1872, p. 264).
E E Cunningham found it astonishing that 'Lyell
could consider it consistent with his "uniformity"
that in recent times there had been a large rise of sea
level and an even more recent withdrawal of the sea
of similar dimensions' (Cunningham 1990). What
Cunningham found unusual-what he called a
'catastrophic fluctuation' in sea level - Lyell would
surely have explained, in accordance with his four
principles, by a rearrangement of continents and
oceans, and a gradual yet dramatic local (but not
mean global) increase in the depth of the sea.
As described earlier, it was widely held that
humans might have altered the climate of the Old
and New Worlds by clearing the forests and
cultivating the fields. Lyell was dismissive of such
notions, in particular that climate had changed
much for any reason in historical times; he
considered the time period 'insufficient to affect the
leading features of the physical geography of the
globe'. Lyell acknowledged popular perceptions of
the variability of the seasons, but cited recent
analyses of long series of meteorological
observations which indicated the relative constancy
of the mean temperatures of particular locations.
He admitted, however, that in certain locations 'the
labours of man have, by the drainage of lakes and
marshes, and the felling of extensive forests',
caused minor changes in the climate system (Lyell
1830-1832). Waxing speculative (and realizing he
was doing so), Lyell explored the possibility of
future climatic influences caused by the progressive
development of human power, 'or perhaps by some
other new relations, which may hereafter spring up
between the moral and material worlds' (Lyell
1853). He did not speculate, however, on what
these relations were. Nor did he venture an opinion,
for example, on the recent rise of industrial power.
Undoubtedly, many other aspects of Lyell's
climate arguments constituted gross speculation.
For example, as Patrick Boylan reminds us (this
volume), in 1840 Lyell briefly joined forces with
Louis Agassiz and William Buckland in decidedly
non-uniformitarian speculations on glaciation.
Another example comes from Lyell's well known
maps 'showing the position of land and sea which
might produce the extremes of heat and cold in the
climates of the globe' (Fig. 1).
These maps appear in all eleven editions of the
Principles published in Lyell's lifetime. The maps
depict, recognizably, the seven current continents
all bunched up near the equator to represent
'extreme of heat', and then shifted to polar regions
to represent 'extreme of cold'. Of course, there was
no discussion of a possible mechanism to cause
such 'continental drift.' Lyell added a note in the
ninth edition saying, 'These maps are intended to
show that continents and islands having the same
shape and relative dimensions as those now
existing, might be placed so as to occupy either the
equatorial or polar regions' (Lyell 1853, p. 111).
While this exercise resembles nothing more than a
child's map game, the result is spookily familiar in
the contemporary era of plate tectonics.
By 1853 Lyell had examined and rejected the
notion that changing sunspot abundances, as
reported by Schawbe and Sabine, had any influence
on climate (Lyell 1853). In 1861 John Tyndall
began to popularize the results of his experiments
on the absorption of radiant heat by gases. He noted
that changes in the amount of any of the radiatively
active constituents of the a t m o s p h e r e - w a t e r
vapour, carbon dioxide, ozone and hydrocarbonscould have produced 'all the mutations of climate
which the researches of geologists r e v e a l . . , they
constitute true causes, the extent alone of the
operation remaining doubtful' (Tyndall 1861).
Neither Tyndall nor anyone else pursued this
hypothesis, however, until the turn of the century
(Fleming 1998). Lyell's biggest challenge came in
1864, when James Croll introduced his astronomical theory of the glacial epochs. As J. C.
Moore wrote to Lyell in March 1865, 'Who would
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CLIMATIC CHANGE
have thought fifty years ago, after astrology had
gone out of fashion, that the stars were to enlighten
us upon what is going on here.' (Lyell papers).
Lyell, Croll and the glacial epoch
James Croll (1821-1890), proponent of an
astronomical theory of ice ages, was a selfeducated Scotsman who was employed, after 1867,
by the Scottish Geological Survey. The outlines of
his life are well documented in his touching autobiography, a chronicle of poverty, physical suffering and neglect (Irons 1896). In 1864, Croll
published a paper in the Philosophical Magazine
165
'On the physical cause of the change of climate
during geological epochs'. In this paper Croll introduced revolutions in the Earth's orbital elements as
likely periodic and extraterrestrial mechanisms for
initiating multiple glacial epochs.
Inspired by the R~volutions de lamer of Joseph
Adh~mar (1842), and employing the calculations of
Leverrier and Lagrange for the maximum
eccentricity of the Earth's orbit, Croll proposed that
this 'eccentricity was sufficiently great to account
for every extreme of climatic change evidenced by
geology' (Irons 1896). Croll's theory of ice ages
took into account both the precession of the
equinoxes and variations in the shape of the Earth's
Fig. 1. Maps showing the position of land and sea which might produce the extremes of heat and cold in the climates
of the globe. Top: Extreme heat occurs when land masses are concentrated near the equator. Bottom: Extreme cold
occurs when land masses occupy polar regions. (From Lyell 1853, p. 111.)
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166
J.R. FLEMING
orbit. It predicted that one hemisphere or the other
would experience an ice age whenever two conditions occur simultaneously: 'a markedly elongate
orbit, and a winter solstice that occurs far from the
sun' (Imbrie & Imbrie 1979). Croll rejected two
astronomical notions of climate change: that the
Earth had passed through hotter and colder regions
of space and that the Earth's axis had shifted. He
assumed only the well established variations in
orbital excentricity and the obliquity of the ecliptic.
This provided a mechanism for multiple glacial
epochs and alternating cold and warm periods in
each hemisphere. In other words, when the northern
hemisphere was in the grips of an ice age, the
southern hemisphere would be in an interglacial. As
the Earth's orbital elements varied, this situation
would eventually be reversed. This potentially
serious challenge to the geographical theory caused
an uproar among Lyell and his associates.
In 1864, as he was preparing the important tenth
edition of his Principles (1866-1868), Lyell sought
expert advice on how to deal with the new
contender. He asked Sir John Herschel's opinion on
the reliability of Croll's 'facts and reasons', adding:
Of their applicability to Geology I may perhaps
form an independent opinion .... I feel more than
ever convinced that changes in the position of
land & sea have been the principle cause of past
variations in climate, but astronomical causes
must of course have had their influence & the
question is to what extent have they operated?
Lyell also perceptively noted what was to
become a fatal flaw in Croll's t h e o r y - that,
according to the geological record, the glacial
periods of the southern hemisphere coincided with
those of the northern, which would not be the case
if the eccentricity of the orbit were the controlling
factor (Lyell to Herschel, 31 January 1865 in
Herschel
papers).
Herschel
replied
that
astronomical
causes
could provide huge
temperature fluctuations, 'quite enough to account
for any amount of glacier and coal fields' (Herschel
to Lyell, 6 February 1865, copy in Herschel
papers).
Suppose a distribution of land favorable to cold,
suppose an extreme e[xcentricity], and suppose
the aphelion to coincide with the winter first in
one hemisphere and then in the other, and any
amount of glacier you can want is at your
disposal... (Herschel to Lyell, 15 February 1865,
copy in Herschel papers)
Lyell was very serious about this issue and
responded with a 22 page letter to Herschel
explaining why geographical causes had to predominate over astronomical ones. He knew the
enormous influence on climate of varying configur-
ations of land and sea from direct accounts and
observations; the effects of varying eccentricity had
yet to be proven (Lyell to Herschel, 11 February
1865, in Herschel papers). Lyell conducted a
similar correspondence with the Astronomer Royal,
Sir George Biddell Airy, concerning the 'ancient
state of the Earth's orbit' (Airy to Lyell, 27 March
1865 in Lyell papers).
In response to Lyell's queries, J. C. Moore
responded:
The more I think of it, the more I feel puzzled to
understand how Astronomic causes can give us
the conditions required for glaciation. Mr. C[roll]
talks of cold periods, but a winter of -17 ~ F
followed by a summer of +119 ~ F is not what I
should call a cold but an extreme climate ....
[There is nothing on the Globe which approaches
such a state ] ... I cannot believe in these monstrous results, and I think, as I suppose you do,
that a vast extension of land about the S. Pole is
at the bottom. (Moore to Lyell, 20 April 1865 in
Lyell papers).
By 1866 Lyell, on the advice of Herschel and
Airy, had tentatively accepted Croll's theory as a
true, but minor cause of climatic change. He wrote
to Darwin:
... the whole globe must at times have been
superficially cooler. Still, during extreme excentricity the sun would make great efforts to
compensate in perihelion for the chill of a long
winter in aphelion in one hemisphere, and a cool
summer in the other.
Lyell also incorporated into his explanation
aspects of Tyndall's work on radiative transfer,
noting that plants requiring heat and moisture could
be saved from extinction during an ice age 'by the
heat of the earth's surface, which was stored up in
perihelion, being prevented from radiating off
freely into space by a blanket of aqueous vapour
caused by the melting of ice and snow'. Here he
was grasping at straws, aware of new theoretical
problems, but taking from them only the aspects
that reinforced his own preconceptions. Lyell's
letter to Darwin concluded:
But though I am inclined to profit by Croll's
maximum excentricity for the glacial period, I
consider it quite subordinate to geographical
causes or the relative position of land and sea and
abnormal excess of land in polar regions. (Lyell
to Darwin, 1 March 1866, in Darwin & Seward
1903)
By this time Darwin had adopted Croll's
conclusion that 'whenever the northern hemisphere
passes through a cold period the temperature of the
southern hemisphere is actually raised'. He gently
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CLIMATIC CHANGE
teased Lyell, pointing out that he had generally
been a 'good and docile pupil', but he could not
believe 'in change of land and water being more
than a subsidiary agent' of the glacial period
(Darwin to Lyell, 8 March 1866 in Darwin &
Seward 1903). Darwin also agreed with Croll that
the advocates of the iceberg theory (Lyell) had
formed 'too extravagant notions regarding the
potency of floating ice as a striating agent', and that
the 'scored rocks throughout the more level parts of
the United States result from true glacier action'
(Darwin to Croll, 24 November 1868, in Darwin &
Seward 1903). Of course Darwin's old nemesis,
Agassiz, was still actively pursuing his defeat with
the argument that extensive glaciation, in the
equatorial Amazon valley and over the entire
continent of North America, would have prevented
the descent of any terrestrial life form from the
Tertiary period (Darwin to Lyell, 8 September
1866, in Darwin & Seward 1903).
In September 1866, Croll received Lyell's proof
sheets for the climate chapters of the tenth edition
of the Principles (Croll to Lyell, 24 September
1866 in Lyell papers). After seeing Lyell's
summary of his theory, which was fair but noncommittal, Croll wrote back immediately that he
had altered his position considerably in his latest
manuscript. Croll now argued that the glacial epoch
could not possibly have been caused directly by any
change in the eccentricity of the Earth's orbit, but
by the combined physical effects of 'certain
agencies which were brought into operation by
means of the change' (Croll to Lyell, 28 September
1866 in Lyell papers). For example, an early edition
of Herschel's Astronomy pointed out that the
amount of direct heat received by the Earth over the
course of a year is independent of eccentricity.
However, Croll pointed out in letters to both Lyell
and Herschel that the climate would not be so
independent because of the latent heat effects of
snow cover (Croll to Lyell, 23 April 1866 in Lyell
papers). This is just one example of the ways
Croll's rough calculations and constant modifications of his theory to incorporate geographical
feedback served to keep Lyell from dismissing the
astronomical theory altogether. Croll received the
first volume of the tenth edition of the Principles in
November 1866 (Croll to Lyell, 30 November 1866
in Lyell papers). He thanked Lyell for the handsome
gift and for the 'highly complimentary way' in
which his astronomical theory had been treated.
Lyell had agreed with Croll on many points.
Although they still had deep disagreements, Croll
attributed their differences to basic incompatibilities in the approaches of physics and
geology (Croll to Lyell, 12 December 1866 in Lyell
papers).
Compared with previous editions, the tenth and
167
subsequent editions devoted more than twice as
much space to climatic change. While the ninth
edition had had about 58 pages on climate and its
vicissitudes, the tenth edition had 130 pages,
including a new chapter on astronomical influences
with a 37 page section on Croll:
Mr. Croll's suggestion as to the probable effects
of a large excentricity in producing glacial
epochs is fully discussed, and the question is
entertained whether geological dates may be
obtained, by reference to the combined effect of
astronomical and geographical causes. (Lyell
1866-1868, vol. 1, viii).
Lyell had complimentary things to say about
Adh~mar's Rdvolutions de la mer, even though
most contemporaries considered it to be extremely
speculative, not to mention catastrophic in its view
of dramatic oceanic flooding. According to Lyeli,
Adhbmar 'called attention to a vera causa hitherto
neglected' (the precession of the equinoxes) and
reopened the question of historical climate changes,
for example in understanding the advance of the
Swiss glaciers since the thirteenth century. Lyell
added that Croll's primary mechanism - changes in
the eccentricity of the Earth's o r b i t - w a s also a
vera causa and could result in a 20 per cent
reduction or augmentation of the entire heat the
Earth received from the Sun.
Upon this difference of heat Mr. Croll has
founded a theory which attempts to account for
former changes of climate by the tendency which
a maximum excentricity would have to exaggerate the cold in that hemisphere in which winter
occurred in aphelion. (Lyell 1872, p. 277)
Lyell concluded his review of Croll's theory by
pointing out that precession of the equinoxes would
cause the alternate glaciation of only that hemisphere in which winter occurred at aphelion. As a
consequence, Croll had supposed that a vast ice cap
on one side of the Earth 'would so derange the
earth's centre of gravity as to draw the ocean
towards that pole, and cause the submergence of
part of the land'. The depth of the submergence he
supposed was on the order of 500 feet. Lyell
pointed out to Croll that sea level would be lowered
during a glacial epoch, since an enourmous amount
of ocean water would now be deposited as snow on
the ice cap (Croll to Lyell, 6 January 1866 in Lyell
papers). Croll stuck to his theory, which was in fact
derived from Adhemar, but corrected his
calculations to show that sea-level rise (in the
northern hemisphere only) would be 500 feet minus
the amount subtracted to build the continental
glaciers (Croll to Lyell, 16 January 1866 in Lyell
papers).
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168
J.R. FLEMING
Lyell faulted Croll for not paying sufficient
attention to abnormal geographical conditions. He
pointed out that astronomical causes alone could
not account for the storing up of ice when deep
oceans prevailed at both poles. He further noted
that during extreme eccentricity, the minor axis of
the ellipse would be shortened, causing the amount
of heat received from the Sun to exceed its present
value, working against the formation of a glacial
period. He rested his case by reasserting the power
of geographical causes over astronomical causes: 'I
consider the former changes of climate and the
quantity of ice and snow now stored up in polar
latitudes to have been governed chiefly by geographical conditions' (Lyell 1872, p. 284).
Conclusion
During Lyell's lifetime many of the major
mechanisms of climatic change were proposed, if
not yet fully explored: changes in solar output,
changes in the Earth's orbital geometry, changes in
terrestrial geography, and changes in atmospheric
transparency and composition. New climate theories were introduced and new work was done on
heat budgets, spectroscopy and the rising carbon
dioxide content of the atmosphere. Although older
theories of human agency were disproved, new
ideas about industrial impacts were beginning to
circulate. Through such tempestuous theoretical
waters, Lyell kept a steady course, providing his
readers with reasoned arguments why they should
keep faith in a c t u a l i s t - t h a t is mid-nineteenth
century a c t u a l i s t - g e o l o g i c a l processes. Lyell's
ideas on climatic change can be understood only in
the context of the times; but we can understand the
times themselves better by studying Lyell's negotiated responses to new theories of climatic change.
If Lyell eschewed the deplorable speculation of
the geological system builders who had preceded
him, he also engaged, quite systematically, in
disciplined speculation of his own on climate
matters. If, as he pointed out, we are prejudiced by
our limited conception of geological time and our
surficial habitats, so too are we immersed in the
climate itself, heavily dependent on imagination
and the assistance of others if we are ever to
envision a system as vast as the Earth's climate and
to apprehend its secular changes. Lyell followed the
speculations of others, up to a point, but he
tempered his judgements with solid evidence
gathered from the record of the rocks. If being
thoroughly Lyellian meant being scientific in such
matters, then it also meant being cautious and
judgemental. If he was speculative on some issues,
he was very very certain about many others. These
were not bad attributes to possess on a topic as
nebulous as climatic change.
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