Odekon M. Encyclopedia of paleoclimatology and ancient environments

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Cross-references

Arid Climates and Indicators

Coastal Environments

Continental Sediments

Marine Biogenic Sediments

Messinian Salinity Crisis

Mineral Indicators of Past Climates

Sedimentary Indicators of Climate Change

Stable Isotope Analysis

EVOLUTION AND CLIMATE CHANGE

Introduction

Darwin’smodel(1859) for evolutionary change was based on a

single branching tree of life, originating with one species at a point

in deep time and concludingwith the spectacular organic variety of

modern ecosystems. Through time, the splitting of species devel-

oped both the biocomplexity and biodiversity of life present today,

during some 4,000 million years of organic evolution. Neverthe-

less the process was not a gradual progression; life evolved at vari-

able rates, commonly punctuated by a series of catastrophic events

of varying magnitudes(Benton, 1995). Patterns,rates and trends in

evolution were governed by a range of biological, chemical and

physical factors; moreover their distributions and affects were

rarely even or random. Climate is the manifestation of long-term,

time-averaged atmospheric change, in simple terms, weather.

The various patterns of climate change exerted a strong influence

on a number of types of evolutionary process. Temperature was

probably the most important climatic factor in marine environ-

ments, whereas in the morecomplex terrestrial milieu, temperature

together with rainfall, solar luminosity and wind strength exerted

their influence on the distribution and evolution of biotas. Climate,

particularly temperature, is thus one of a number of factors includ-

ing also sea-level change and volcanicity that may have been asso-

ciated with both small and larger-scale evolutionary events

(Figure E17). Climatic change has demonstrably driven the migra-

tion of biotas; less clear, however, is its control on microevolution

processes at the species level. On a larger scale, extreme climate

change has forced the restructuring of the planet’s ecosystems

and more rarely caused major extinction events.

Climatic fluctu ations through time

Short-term trends

Many climatic events are short term, occurring within a time

span of 100,000 years (Cronin, 1999). Many Earth surface

processes respond rapidly to climate change, for example the

atmosphere and ocean surface waters can change within days

to a few years whereas the deep water of the ocean basins and

terrestrial vegetation may take centuries to alter; the buildup of

ice sheets and associated sea-level changes, however, occur over

millennia. Changes in precipitation and temperature in the recent

past may have influenced the course of human events and almost

certainly impacted on the direction of hominid evolution during

the late Pliocene and Pleistocene. Many short-term climatic fluc-

tuations have been related to Milankovitch cycles associated

with the eccentricity, obliquity and precession of the Earth’s

orbit, and generally lasted from 20,000 to 400,000 years. These

short-term trends are associated with evolutionary changes at the

speciation level and more local regional changes in the composi-

tion and structure of ecosystems.

Long-term trends

Data available for the late Proterozoic and Phanerozoic suggest

that during the last 900 million years the Earth oscillated

between greenhouse and icehouse conditions at least five times

(Frakes, 1979;Frakesetal.,1992). These megacycles, first

developed by Alfred Fischer, have been compared with patterns

of change in extinctions, sea-level and volcanicity (Figure E18).

Moreover, there may be a correlation between all three of these

EVOLUTION AND CLIMATE CHANGE 325

variables and the assembly and breakup of the supercontinents.

In marine environments, two extreme states occurred: The ice-

house state involved stratified, stable oceans, cool surface waters

between 2 and 25



C and bottom waters ranging from 1 to 2



together with rapidly moving bottom waters, rich in oxygen and

with high productivity in areas of upwelling. On the other hand,

greenhouse oceans were less stable and poorly stratified, with

surface waters ranging in temperature from 12 to 25



Cand

deep-water temperatures between 10 to 15



C. Slow bottom cur-

rents carried little oxygen and productivity was generally low.

Extinctions were associated with the transitions between these

oceanic states. In addition to these major climatic fluctuations,

a series of major extinction events, some associated with extra-

terrestrial causes, clearly prompted major climate change over

several million years. Such events caused major taxonomic

extinctions together with major restructuring of the marine and

terrestrial ecosystems. Generally, greenhouse biotas were most

susceptible to extinction; their species were more specialized

and thus exposed to environmental change.

Biocomplexity

Microevolution

Evolution at the species level is based on the natural selection

of individuals from genetically and morphologically variable

populations. Parts of populations with different samples of

genetic material may become isolated from the parent popula-

tion by physical or ecological barriers, leaving this subset free

to evolve in a different direction, where selection pressures

may also be different. Allopatric speciation based on the foun-

der principle has been widely used to account for evolutionary

change. A further mechanism is genetic drift, whereby small

peripheral isolates of a population may break away from the

parent population to establish new species eventually. More-

over, species evolution may have occurred in two quite differ-

ent ways. Phyletic gradualism implies gradual morphological

change whereas the punctuated equilibrium model involves

rapid, sudden changes in morphology followed by intervals of

no change, or stasis. Microevolution has been demonstrated

Figure E17 The relationships between temperature, volcanism and sea-level change during the last 570 million years (redrawn and replotted from

Frakes et al., 1992).

326 EVOLUTION AND CLIMATE CHANGE

in many fossil lineages (Benton and Pearson, 2001) although

the link between speciation events and climatic change is more

controversial. In general terms, marine plankton show gradual

evolution whereas marine invertebrates and vertebrates display

punctuated equilibria. Moreover, it is probable that narrowly

fluctuating, changing environments hosted persistent gradualis-

tic evolution whereas widely fluctuating environments were

better characterized by morphological stasis (Sheldon, 1996).

This resistance to morphological change has been demonstrated

in a number of lineages such as Ordovician trilobites (Sheldon,

1987) and Pliocene mollusks (Williamson, 1981).

Short-term climatic fluctuations, for example those associated

with Milankovitch cycles, can clearly disrupt and promote the

reassembly of both marine and terrestrial communities (Bennett,

1997) and in some cases drive local extinctions and radiations,

for example in the conodont and graptolite faunas of the Silurian

(Jeppson, 1990). Regarding possible speciation, at least two

outcomes are possible: Widespread evolutionary stasis or the

development of isolated populations forming the focus of gene-

tic and morphological divergence (Sepkoski, 1998). The Plio-

Pleistocene deposits of East Africa have provided some of the

best constrained studies of the relationship of climatic change

to evolution. Datasets comprising a variety of mammals includ-

ing hominoids have formed the focus for microevolutionary

studies (Vrba, 1993). Cooler climates in the early Neogene

prompted biotic turnovers in the evolution of animals with lar-

ger bodies and either relatively smaller or larger body parts

(Vrba, 1996). The relative enlargement of the hominoid brain

exemplifies the latter. Some similar trends are apparent in Jur-

assic and Cretaceous marine environments. Deeper-water pela-

gic faunas are more diverse, such that communities that are

more complex and have larger predators occur in oceans with

raised temperatures.

Macroevolution

Macroevolution involves the study of larger scale evolutionary

processes, usually involving major and rapid morphological

change in a number of contemporary organisms. Generally,

the appearance and disappearance of higher taxonomic groups

during adaptive radiations or mass extinctions signal macroevolu-

tionary change. Adaptive radiations are usually associated with

the development of a successful morphological innovation and

the exploitation of new ecospace. Such major evolutionary

changes depend on many factors, both intrinsic and extrinsic,

and are difficult to relate directly to climate. Many adaptive radia-

tions, however, occur following major extinction events and

take advantage of a range of vacant niches. Nevertheless, many,

such as the diversification of skeletal organisms, reef-building

organisms and the first predators, are clustered at the Precam-

brian-Cambrian boundary (Benton and Harper, 1997), associated

with increasingly warm climates and higher sea levels. On land,

the radiation of early terrestrial tetrapods in the early Carbonifer-

ous together with the diversification of large flying insects in the

first extensive forests during the late Paleozoic, in cooler climates

and more exposed land areas, have also been correlated with

higher levels of atmospheric oxygen (Berner et al., 2000).

Figure E18 Alternations between icehouse and greenhouse states: the so-called “Fischer cycles.” The relationships between climate, extinctions,

sea-level and volcanism are also indicated (adapted and redrawn from Fischer in Berggren and van Couvering, 1982).

EVOLUTION AND CLIMATE CHANGE 327

Megaevolution

The appearance of entire new biotas and commonly grades of

organization, for example the origin of life itself, the develop-

ment of photosynthesis and the appearance of the metazoans

rank, together with the origin and diversification of skeletal

organisms, are megaevolutionary events (Benton and Harper,

1997). Data are as yet too imprecise to relate these events to

climate change. However, there are a few indications. The first

two events have been associated with a stable Archean crust

and relatively cooler climates, favorable for carbon-based life

to evolve. Metazoans appeared and diversified after the decay

of the near global ice sheets of Snowball Earth whereas skeletal

organisms radiated during the greenhouse climates of the early

Cambrian.

Biodiversity

The fossil record holds a unique database of the diversification

of life on land and in the sea (Harper and Benton, 2001). Range

data of animals and plants through time form the basis of the

study of biodiversity (Conway Morris, 1999). Qualitative and

quantitative analyses have revealed that time trends are not ran-

dom; biodiversity dynamics and the appearance of evolutionary

innovations suggest that a nonrandom group of both intrinsic

and extrinsic processes controlled changes in biodiversity

(Jablonski, 1999). Climate is one of a number of important fac-

tors that mediates the planet’s biodiversity through time.

Extinctions

During the Phanerozoic, animals and plants experienced a

steady turnover in species. Clearly, climate change was one

of a range of causes for background extinctions, particularly

when habitats were destroyed and the tolerance range of indivi-

dual species was exceeded. In addition, five major extinctions

and a number of more minor events have been recognized

in the fossil record, based primarily on paleontological data

(Hallam and Wignall, 1997). All five, the end-Ordovician, late

Devonian, end-Permian, end-Triassic and end-Cretaceous have

been associated with major climatic changes linked to a range

of terrestrial and extra-terrestrial causes (Brenchley and Harper,

1998). The end-Ordovician event was marked by a rapid tran-

sition from greenhouse to icehouse conditions associated with

an increase in polar ice, a cooling of oceanic surface water

and cycling of carbon within a changing oceanic system; most

clades and ecological groups were affected and up to 85% of

all species probably disappeared. The late Devonian event

was less severe with the loss of about 80% of species during

a phase of climatic cooling associated with sea-level change

and anoxia. The end-Permian extinction was the most marked

with about 95% of all species affected. All clades and ecologi-

cal groups were devastated during a range of climatic changes,

linked to a runaway greenhouse (Benton and Twitchett, 2003).

The end-Triassic event witnessed the demise of about 75% of

species, particularly the terrestrial vertebrates, during an event

dominated by climate and sea-level changes together with

anoxia. The end-Cretaceous event was near instantaneous with

the disappearance of about 75% of species across a number of

clades, including the ammonites and dinosaurs but particularly

the plankton. Major climatic change has been linked to a bolide

impact. Two more minor events, the Turonian-Cenomanian

(loss of 50% of species) and the Eocene-Oligocene (loss of

35% of species) events have been linked to climatic changes

associated with bolide impact or comet showers, respectively.

Radiations

Some intervals of apparent adaptive and rapid speciation can be

related to climate change. Most profound was the emergence of

the soft-bodied Ediacara fauna following the reversal of Snow-

ball Earth. These remarkably diverse animals occupied a range

of marine environments and ecological niches during the war-

mer climates of the late Proterozoic. The Cambrian explosion,

marked by the rapid diversification of skeletal organisms with

a spectrum of new body plans within the early part of the per-

iod, may have been associated with a continued rise in tempera-

ture, intense biological production and high rates of phosphate

deposition coincident with changing patterns of oceanic circula-

tion. The Ordovician radiation nevertheless marked the most

significant diversification in numbers of marine organisms.

The firm establishment of greenhouse conditions is correlated

with widespread carbonate platforms and relatively high sea

levels. This radiation set the agenda for Paleozoic marine life

or perhaps marine biotas throughout the entire Phanerozoic

(Alroy et al., 2001). The progressive diversification of marine

life within the Modern evolutionary fauna following the end-

Permian extinction event is again associated with persistent

rises in temperature and sea level within a prolonged green-

house phase.

On land some broad patterns are recognizable; for example the

radiation of the pteridophyte plants together with the amphibians

and mammal-like reptiles occurred during the cooler climates

of the Carboniferous and Permian whereas the large reptiles, for

example the dinosaurs, diversified during the warmer climates

of the Jurassic and Cretaceous, together with the gymnosperm

plants. Following the end-Cretaceous extinction event, angios-

perm plants together with the mammals continued to diversify

against a background of declining temperature.

Biotic change

In addition to the development of biocomplexity and biodiver-

sity, a number of patterns of long-term change in the Earth’s

paleobiotic ecosystems can be detected (Figure E19a–c). Such

large-scale ecologic changes are more closely related to major

swings in climate associated with extinction events. Neverthe-

less, in many cases the severity of the ecologic crisis was

decoupled from changes in biodiversity (Droser et al., 2000)

suggesting that the environmental controls on both processes

were different.

Precambrian

The Precambrian covers some 80% of geological time but

organic remains are relatively few and many are controversial

(Brasier et al., 2004). The era includes a series of major biolo-

gical events, for example the origin of life and photosynthesis

and the appearance of multicellular organisms together with

the rise of the earliest metazoans and the evolution of the first

skeletal animals. The origin of life (Rosing, 1999) and photo-

synthesis (Rosing and Frei, 2003) may have marked the estab-

lishment of more stable but warm climates on Earth following

the development of continental crust together with marine

basins and coincident with the cessation of meteorite showers.

The role of climate during much of the later Archean and Pro-

terozoic is unclear, while the development of an oxygenated

atmosphere and hydrosphere favored the radiation of aerobic

multicellular organisms (Knoll, 2003). Anaerobic environments

and their organisms were marginalized. Moreover, the greater

availability of atmospheric oxygen around 2.5–2.3 Ga may

328 EVOLUTION AND CLIMATE CHANGE

Figure E19 a. The marine evolutionary faunas together with the five big extinction events indicated by arrows, b. The nonmarine evolutionary

floras, c. The nonmarine evolutionary tetrapod faunas and, d. Origins for some of the main mega and macroevolutionary events (replotted from

various sources including Benton and Harper, 1997).

EVOLUTION AND CLIMATE CHANGE 329

have reduced concentrations of methane, possibly driving the

Huronian glaciation (Kasting et al., 2001). Nevertheless, major

climate changes at the end of the Proterozoic virtually annihi-

lated life on the planet; subsequent climatic amelioration may

have strongly influenced the direction of metazoan life. The

Snowball Earth hypothesis (Hoffman et al., 1998) envisages a

runaway extreme icehouse state, with ice sheets extending from

pole to pole and the oceans covered by sea ice. Marine organ-

isms were thus restricted to refugia as the majority of marine

habitats were destroyed. Earth emerged into a subsequent

greenhouse state, populated initially by mutations generated

during periods of extreme environmental stress. The first signs

of relatively abundant metazoan life, the soft-bodied Ediacara

biota, occurred globally above the late Proterozoic glacial

deposits associated with Snowball Earth. They form the basis

for the Ediacara Fauna (Figure E19a).

Phanerozoic

Phanerozoic life can be organized into a number of major

evolutionary faunas and floras (Figure E19a). In the sea, the

Cambrian evolutionary fauna was characterized by loosely-

structured communities with morphologically varied species

dominated by trilobites and primitive echinoderms and mol-

luscs. It followed sequentially the appearance of the Tommo-

tian (or Small Shelly) Fauna, dominated by small skeletal

organisms. These first skeletal-dominated evolutionary faunas

developed during the warmer climates of the early Cambrian.

The Paleozoic evolutionary fauna (Ordovician-Permian) was

dominated by abundant suspension feeders such as the brachio-

pods, bryozoans and corals, whereas the Modern evolutionary

fauna (Triassic-Recent) consisted predominantly of detritus fee-

ders with a substantial bioturbation of the seabed, together with

efficient predators that participated in an arms race with their

prey. The sharp transition between the Paleozoic and Modern

evolutionary faunas is marked by the end-Permian extinction

event (Benton and Twitchett, 2003), when over 95% of all spe-

cies disappeared.

Similar models have been developed for terrestrial biotas

(Figure E19b and E19c). The sequential appearance of evolu-

tionary faunas dominated by the amphibians and mammal-like

reptiles, the dinosaurs and pterosaurs and finally the modern

groups is partly determined by extinction events: the end-Triassic

event marked the transition between the first two groups and

the end-Cretaceous event witnessed the demise of the dinosaurs

and the rise of the mammals. The floral history of the planet

was most affected by the end-Permian event (subsequent rise

of the gymnosperms) and the end-Cretaceous event (subse-

quent dominance of the angiosperms).

Biological feedbacks

The relationship between climate and life suggests the exis-

tence of a feedback between the two over time. Today and dur-

ing the recent past much discussion has been focused on the

anthropogenic influences on our climate as we move again into

a greenhouse phase. The industrial revolution set the agenda

for the sustained use of fossil fuels and the persistent genera-

tion of greenhouse gases. Climatic warming will have a greater

impact on the biotas of the cold-temperate and polar regions,

with a pole-ward shift in climate on the order of 100 km per

century (Wilson, 1992). Nevertheless, a number of models for

long-term climatic change have also involved the role of feed-

backs from biological organisms (Figure E20). Most marked

are changes during the Precambrian, predicted by the Gaia

hypothesis (Lovelock, 2000). The diversification of photo-

synthesizers, together with consumers from the early Protero-

zoic onwards, hiked oxygen levels concomitant with declines

in greenhouse gases. Such models promote the vital effects of

life as a stabilizing influence on the planet’s climate, reducing

the otherwise steady rise in the Earth’s surface temperatures.

In the same way, the extensive coal swamps and forests of

the later Paleozoic may also have contributed to an interval

of cooler climate, as diversifying land plants mediated atmo-

spheric oxygen levels, anticipating the importance of modern

rainforests as a climatic buffer.

David A. T. Harper

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424pp.

Cross-references

Astronomical Theory of Climate Change

Bolide Impacts and Climate

Greenhouse (warm) Climates

Human Evolution and Climate Change

Icehouse (cold) Climates

Snowball Earth Hypothesis

Volcanic Eruptions and Climate Change

EVOLUTION AND CLIMATE CHANGE 331

FAINT YOUNG SUN PARADOX

Standard models of solar evolution indicate that the intensity of

solar radiation should have increased by about 25–30% over

the 4.6 billion year duration of geologic time (Gilliland,

1989). The phrase “faint young Sun” was coined by Sagan

and Mullen (1972) to describe the implications of these stan-

dard models for Earth’s climate, because early in the Sun’s life-

time, the radiation reaching a planet like Earth today (with both

similar orbit and amount of greenhouse forcing) would not

have been sufficient to prevent the planet’s average tempera-

ture from dropping below the freezing point of water. Sagan

and Mullen evaluated the implications of the faint young Sun

paradox in the context of geologic evidence from early in

Earth’s history that rules out a long-lived frozen state. They

argued that a greater amount of greenhouse gas forcing was

required to offset the lower solar luminosity of the young

Sun. The faint young Sun hypothesis is still being evaluated

today and many hypotheses reconcile the reduced solar lumin-

osity with geologic evidence, using refinements of the green-

house gas arguments of Sagan and Mullen (1972), but some

other hypotheses raise the possibility of variations of Earth’s

orbital properties and even call into question the standard mod-

els of solar evolution.

A significant part of the Sun ’ s evolution is thought to occur

along the main sequence, a field defined by observations of

many stars and represented on a Herzsprung-Russel diagram

of luminosity versus temperature. Like other stars, the Sun

derives its energy from nuclear fusion in its core and radiates

most of this energy from its surface, the solar disc, as continu-

ous blackbody radiation with a maximum in the visible region

and tails that extend to the infrared and microwave and also

into the ultraviolet and X-ray regions of the spectrum. The

amount and spectral characteristics of this blackbody radiation

can be described by one principal equation, the Planck func-

tion, and two derivative laws, the Stephan Boltzman Law and

Wien’s Law. The Planck function describes the spectral charac-

teristics of blackbody radiation (B

), and takes the form:

2hc

p e

hc=lkT

 1ðÞ

where c is the speed of light, l is the wavelength, k is Boltzman

constant, T is temperature, and h is Planck’ s constant. The Ste-

phan Boltzman Law describes the total energy emitted by a

blackbody and is given by:

E ¼ 4pR

where s is the Stephan-Boltzman constant (5.67  10

8

2

4

) and 4pR

is the surface area. At its present size and aver-

age surface temperature of 5,785 K, the Sun emits a total black-

body energy of approximately 3.8  10

W. The standard solar

model indicates that the size of the Sun has increased with

time, resulting in an increase in solar radiation (brightening

of the Sun) over the course of its 4.6 billion year existence

(Gilliland, 1989).

Wien’s Law describes the wavelength of the maximum of

Planck’s function as

max



2898mm K

where l is given in µm. Wien’s Law places the peak of radia-

tion in the visible region and this is essential for the Earth’s

greenhouse effect.

The underlying principle of the greenhouse effect requires a

planetary energy balance (e.g., E

incident

= E

radiated to space

), and

an atmosphere that transmits incoming radiation, but absorbs

energy radiated from the planet’s surface. Greenhouse gases

in the Earth’s atmosphere are transparent to visible radiation,

transmitting a high dose of incident solar radiation through

the atmosphere to warm the Earth’s surface, but these same

gases absorb and reradiate the infrared radiation emitted from

the surface. The energy recycled (absorbed and reradiated) by

greenhouse gases in the atmosphere contributes a finite amount

of surface warming beyond that provided by incoming solar

radiation. In today’s atmosphere, the greenhouse effect contri-

butes approximately 33 K of warming, raising the average sur-

face temperature to 298 K.

The warming provided by the present-day greenhouse effect

is essential for keeping the average surface temperature of

Earth above the freezing point of water, but as recognized by

Sagan and Mullen (1972), this amount of warming would not

have been sufficient to have kept the surface temperatures

above freezing if solar radiation had been lower by more

than ~10%. Furthermore, the standard solar model points to

as much as 25–30% lower solar luminosity 4.6 billion years

ago than today. Moreover, for the vast majority of geologic

time, this standard solar model predicts that incident solar

radiation would have been less than 90% of present-day values.

It was also recognized that if the mean surface temperature of

Earth fell below the freezing point of water, the oceans would

have frozen, leading to a negative feedback involving planetary

albedo and planetary ice. The amount of radiation available to

warm the climate would thus have decreased as ice cover

expanded, because more radiation would have been reflected

back to space rather than absorbed by the surface and trans-

formed into heat.

Geologic evidence, however, does not point to a long-lived

frozen state for early Earth. Rather, the presence of sedimentary

rocks suggests the existence of liquid water throughout much

of geologic history (Cloud, 1972). In addition, chemical and

isotopic characteristics of 4.3–4.4 billion year old zircon grains

from the Jack Hills of Australia point to the presence of liquid

water within a few million years of the formation of Earth

(Mojzsis et al., 2001; Wilde et al., 2001). Sagan and Mullen

(1972) argued that the faint young Sun paradox could be recon-

ciled with the geological observations if the Earth’s early

greenhouse effect provided enough additional warming of the

planet (beyond that of the present greenhouse effect) to offset

the lower luminosity of a faint young Sun.

Central to the hypothesis of a more efficient greenhouse is

the identification of the gas that is responsible for the enhanced

greenhouse forcing. Three gases (NH

,CO

, and CH

) have

been suggested as the principal cause of the enhanced green-

house effect during the early parts of Earth’s history. Sagan

and Mullen (1972) and Sagan and Chyba (1997) argued that

was the gas responsible for the enhanced greenhouse.

Debate about the suitability of NH

as an early Earth green-

house gas has focused on its lifetime in the atmosphere. While

Owen et al. (1979) argued that high UV fluxes in a low O

atmosphere would destabilize NH

, making CO

or even CH

which are stable in high-UV environments more likely candi-

dates, Sagan and Chyba (1997 ) argued that a high altitude

organic haze, similar to that observed on Saturn’s moon Titan,

could provide a sufficient UV shield to stabilize NH

. Most

workers agree that CO

played a significant role as a green-

house gas throughout Earth’s history, and the debate revolves

around its sources and sinks and whether it was the principal

greenhouse gas, or whether a second or third greenhouse gas

would have been required to sustain the greenhouse forcing

necessary to offset the effects of a faint young Sun (Walker

et al., 1981; Kasting, 1993; Kasting and Catling, 2003). Using

arguments structured around mineral equilibrium and stability

of siderite (an iron carbonate) and greenalite (an iron-bearing

phyllosilicate mineral) in the 2.7 billion year old Mount Roe

paleosol, Rye et al. (1995) placed limits on the amount of car-

bon dioxide in the atmosphere that would have been too low to

have kept Earth’s surface above freezing, and called instead for

a second greenhouse gas in the early atmosphere, such as CH

Pavlov et al. (2000) used an energy balance model to demon-

strate that a combination of CO

and CH

would have sufficed

to offset the effects of the faint young Sun. It is also thought

that such a greenhouse gas mixture might carry other conse-

quences for the Earth’s early atmosphere and this has been

demonstrated for methane by Catling et al. (2001) who

explored its sources and its implications for the oxidation state

of the early Earth’s atmosphere.

Other hypotheses have been proposed to reconcile the geo-

logical evidence with the faint young Sun paradox. These

include the hypothesis that the standard solar model and its

implications about main sequence evolution are not relevant

for the Sun. This alternate hypothesis centers around a more

massive and more luminous young Sun than that predicted by

the standard model and argues that a more massive (larger)

Sun would radiate more rather than less energy (Graedel et al.,

1991). A second hypothesis calls on changes in Earth’s obli-

quity (tilt of the rotational axis), which if it remained at high

values throughout much of Earth’s history could have sustained

liquid water by keeping parts of the Earth facing the Sun over

longer periods of time, which would have led to hotter sum-

mers (Jenkins, 2000). This hypothesis is similar to that envi-

sioned by Milankovich for the orbital forcing of glacial and

interglacial cycles. These two hypotheses remain to be tested,

leaving the most widely held hypothesis – that of enhanced

greenhouse gas forcing early in Earth’s history – as the pre-

ferred one. Among the fundamental issues in identifying the

implications of greenhouse gas forcing and climate through

deep time is the characterization of the sources, sinks, and sta-

bility (feedbacks) of greenhouse gases in models of early Earth

atmospheres, such that climate was stabilized over the first

3 billion years of geologic history.

James Farquhar

Bibliography

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hydrogen escape, and the irreversible oxidation of early Earth. Science,

293, 839–843.

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537–548.

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low-altitude Proterozoic Glaciation. J. Geophys. Res. Atmos., 105,

7357–7370.

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Rev. Astron. Astrophys., 41, 429–463.

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greenhouse

to compensate for reduced solar luminosity on early Earth. Nature, 277,

640–642.

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2000. Greenhouse warming by CH

in the atmosphere of early Earth.

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dioxide concentrations before 2.2 billion years ago. Nature, 378,

603–605.

Sagan, C., Chyba, C., 1997. The early faint Sun paradox: Organic shielding

of ultraviolet-labile greenhouse gases. Science, 276, 1217–1221.

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spheres and surface temperatures. Science, 177(4043), 52–56.

334 FAINT YOUNG SUN PARADOX

Walker, J.C.G., Hays, P.B., and Kasting, J.F., 1981. A negative feedback

mechanism for the long-term stabilization of Earth’s surface tempera-

ture. J. Geophys. Res. Oceans Atmos., 86, 9776–9782.

Wilde, S.A., Valley, J.W., Peck, W.H., and Graham C.M., 2001. Evidence

from detrital zircons for the existence of continental crust and oceans

on the Earth 4.4 Gyr ago. Nature, 409(6817), 175–178.

Cross-references

Albedo Feedbacks

Archean Environments

Atmospheric Evolution, Earth

Climate Forcing

Greenhouse Effect and G reenh ouse Gases (Encyclopedia of World

Climatology)

Mineral Indicators of Past Climates

Obliquity

FLOOD BASALTS: CLIMATIC IMPLICATIONS

Introduction

Flood basalts represent large-volume, predominantly effusive,

eruptions of magma onto the Earth’s surface. The volume of the

largest flows exceeds 1,000 km

, some two orders of magnitude

greater than the historic basalt flows found on Iceland and Hawaii.

Magma contains dissolved volatiles that exsolve as the magma

depressurizes and may be released into the atmosphere during

and after the eruption. It is theseaerosols, a cocktail including dust,

O, CO

,SO

, HF, and HCl, that have been implicated in cli-

matic change. This has led some authors to propose causative links

between flood basalt volcanism, climate change, and mass extinc-

tions (McLean, 1985; Courtillot, 1999; W ignall, 2001). They sug-

gest that dust and volatiles injected into the atmosphere lead to

successive, short-term changes in atmospheric composition and

temperature, leading ultimately to collapse of global ecosystems.

The causative links between large-scale flood basalt volcanism

and mass extinctions are, however, poorly defined.

Flood basalts may be erupted onto the Earth’s continental crust,

in which case they are called continental flood basalts (CFB).

Most continental flood basalts are erupted subaerially, and any

gases that are released enter directly into the atmosphere. Alterna-

tively, flood basalts may erupt in the ocean basins (where they

build large oceanic plateaus). In this environment, the eruption

may be submarine, in which case the volatiles initially dissolve

in the ocean. Both continental flood basalt provinces and ocean

plateaus comprise the so-called large igneous provinces (LIPs)

(Coffin and Eldholm, 1994). Most large igneous provinces – and

hence flood basalts – originate from mantle plumes, regions of

hot mantle ascending from the deep Earth interior.

Volcanism and climate forcing

It is widely recognized that explosive volcanic eruptions can

perturb the Earth’s climate. The eruption of Pinatubo in 1991

resulted in a small but significant cooling at the Earth’s surface,

and the much larger eruption of Tambora in 1815 caused even

greater cooling (Oppenheimer, 2002). Climate-forcing agents

include gases and aerosols, and volcanic eruptions (especially

basaltic ones) often produce considerable masses of CO

and

. The residence times of sulfate and dust aerosols in the

lower atmosphere (troposphere) are short, usually of the order

of a few days or weeks. The greatest effects on climate are caused

by injection above the tropopause and into the stratosphere,

where the residence times are greater (on the order of 2 years,

the turn-over time of the stratosphere). Injection of material

above the tropopause adds to the Junge Layer (or Stratospheric

Aerosol Layer), a sulfate-bearing layer in the lower stratosphere.

This is turn leads to heating (through absorption of incident

radiation) of the stratosphere, and concomitant cooling of parts

of the troposphere. For example, the radiative forcing 1 year after

the Pinatubo eruption (when ~20 Mt of SO

was injected in to

the stratosphere), was about 4Wm

2

(IPCC, 2001).

Lakagigar, Iceland, 1783

While explosive volcanic eruptions are usually considered as

transitory events for the purposes of climate modeling, effusive

flood basalt events are different, because the source may be

active for several months, years or even decades. The Lakagigar

(“Laki”) eruption in SE Iceland in 1783 is the best documented

and most recent basalt eruption, and offers an analog of much lar-

ger eruptions in the geological past. Approximately 15 km

basalt was erupted over a period of about 8 months (the bulk

being erupted in a series of short explosive pulses during the first

few months), causing local environmental devastation (most

notably the Haze Famine), and cooling of the Northern Hemi-

sphere climate (Thordarson and Self, 2004). Three aspects of

the Laki eruption are important here: firstly, compared with

explosive eruptions such as Pinatubo and Tambora, the eruption

was prolonged, facilitating aerosol formation over several

months. Secondly, some episodes of the eruption were suffi-

ciently vigorous to enable injection of material into the lower

stratosphere, thus producing hemispheric, as well as local, eff-

ects. And thirdly, the aerosols were sulfur-rich, a characteristic

of iron-rich basaltic magmas.

Most basaltic eruptions are effusive in nature, producing

lava fountains and flows. They contrast with eruption of more

silicic magmas, which tend to be explosively ejected, forming

tall eruption columns and widely dispersed pyroclastic fall-

out. The Laki eruption was, however, moderately violent dur-

ing its early, peak stages, with Strombolian or even sub-Plinian

episodes, and eruption columns that may have reached altitudes

in excess of 13 km (Thordarson and Self, 2004). Most of the

time, however, venting was effusive. Volatile dispersal was

therefore both local, with low-level emissions from the vent

and lava flow, and more widespread, with injection into the

tropopause and lowermost stratosphere.

The volatile budget of the Laki eruption has been extensively

investigated (Thordarson et al., 1996). Pre-eruption concentra-

tions of volatile species were determined from glass inclusions

and these were used to estimate the total volatile release from

the erupted magma. During the eruption period, ~122 Mt of

(approximately the current global annual anthropogenic out-

put) was released into the atmosphere, together with a mixture of

F (7 Mt), Cl (~15 Mt) and H

O (~235 Mt) (Thordarson et al.,

1996). Most of these volatiles were released during the first three

months of the eruption. It is estimated that approximately 80%

of the SO

mass (~100 Mt) was injected into the lower strato-

sphere, the bulk of which was probably rapidly converted to

sulfuric acid aerosols (equivalent to ~190 Mt) (Thordarson and

Self, 2004). The remainder (~20 Mt SO

, equivalent to ~40 Mt

of H

), was emitted from the cooling lava flow and formed

a local, low-altitude, toxic haze.

The injection of sulfuric acid into the lower stratosphere

and polar jet stream is thought to have been responsible for

the atmospheric haze observed over large parts of Europe

FLOOD BASALTS: CLIMATIC IMPLICATIONS 335