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CE3A8 SMJ Geology for Engineers
Climate
Effective temperature of the Earth This would be the surface temperature if Earth were a sterile
planet with no atmosphere, oceans or biosphere. The Sun appears to us almost as a black body. The
spectrum and total energy E of electromagnetic radiation emitted by a black body is a function of its
temperature T according to
E = σT 4
(1)
where σ = 5.67 × 10−8 Wm−2 K−4 is the Stefan-Bolzman constant. Viewed from the Sun, Earth is a flat
disk of radius r and illuminated area πr2 . The solar energy absorbed is thus (1 − a)Sπr2 where a is the
albedo, or amount of energy reflected from the Earth back into space (∼ 30% at present) and S is the
solar constant (1.38 kW m−2 at Earth’s astronomical position at present). For equilibrium between the
absorbed solar radiation and emitted radiation from the whole Earth’s surface of area 4πr2 , the effective
temperature will be
0.25
(1 − a)S
(2)
TE =
4σ
So the effective temperature of the hypothetical airless planet in Earth’s position is −18◦ C.
Atmosphere Earth’s effective temperature is much colder than its average surface temperature of 16◦ C.
Most models of evolution of the Sun and solar systems in general imply that the Sun has brightened over
time. When Earth was young, the Sun was roughly three quarters as bright as the modern Sun. Yet we
know from the geological record of that oceans were present at least by 4.04 billion years ago, the age of
the oldest zircon crystal. Thus from very early in Earth’s history the atmosphere has acted as a warm
blanket, increasing the surface temperature so that liquid water is stable. Furthermore, the atmosphere
must have adjusted through Earth’s history to counterbalance the increasing warmth of the Sun and to
maintain the surface temperature within a relatively narrow band.
Gaseous constituent
Nitrogen
Oxygen
Argon
Water vapour
Carbon dioxide
Methane
Nitrous oxide
Ozone
Molecular form
N2
O2
Ar
H2 O
CO2
CH4
N2 O
O3
Proportion (%)
78.1
20.9
0.93
variable 0.1–1
0.0355
0.000172
0.000031
variable ∼ 0.000005
Composition of Earth’s atmosphere.
Earth’s atmosphere contains oxygen but also gases such as methane and ammonia which should not
coexist with oxygen (they should combust). Biological activity maintains the proportions of these gases,
so Earth’s atmosphere signals life. Emission spectra of other planets tell us that life as we know it does
not exist on any planet observed to date.
Greenhouse effect The greenhouse effect arises because of the particular way in which some of the gases
in the atmosphere absorb radiation. The spectrum of radiation emitted by a black body depends on its
temperature. The sun is hot (∼ 6000 K) so incoming solar radiation has a relatively short wavelength
(∼ 0.4 nm; blue-green visible light). Most of the atmosphere is transparent to this short radiation, so
much of the incoming energy is either reflected within the atmosphere or reaches the surface. Ozone is
the most significant atmospheric gas that absorbs energetic short-wavelength radiation. This energetic
light is harmful to most organisms, so the ozone filter protects life on Earth below. In order to maintain
an energy balance, Earth must return energy to space. If it did not it would heat up. The Earth is
much cooler than the Sun, so it emits energy at longer wavelengths in the infrared part to the spectrum.
Many atmospheric gases absorb energy at infrared wavelengths and the act of absorption makes the gas
warm up: these are the greenhouse gases. Water vapour and CO2 are the major greenhouse gases. The
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CE3A8 SMJ Geology for Engineers
contribution to the overall greenhouse effect depends on the amount and absorption potential of each
gas. For example CH4 is 20 times as effective and CFC-12 is 10,000 times as effective at absorbing energy
than CO2 but there are less of these gases in the atmosphere. Contribution percentages are not strictly
additive because the absorption ranges of different gases overlap.
Gas
Water vapour (H2 O)
Carbon dioxide (CO2 )
Methane (CH4 )
Nitrous oxide (N2 O)
Chlorofluorocarbons (CFCs)
Ozone (O3 )
Sulphur dioxide (SO2 )
other oxides of Nitrogen
Carbon Monoxide (CO)
etc.
Basic absorption
wavelengths (µm)
2.66, 2.74, 6.27
4.26, 7.52, 14.99
3.43, 6.85, 7.27
4.50, 7.78, 16.98
9.52, 7.78, 15.4
Contribution
55–70%
25%
5%
2%
1%
< 1% each
Greenhouse effect on Earth.
Water vapour Water vapour is the most significant greenhouse gas on Earth, partly because it is
relatively abundant. It tends to form clouds, which have two two important effects. The greenhouse
properties of water vapour cause a blanketing effect which acts to warm the Earth. The greenhouse effect
of clouds is many times the effect that would result by doubling the atmospheric CO2 concentration.
Clouds are white, so they reflect incoming solar radiation thereby cooling the Earth. The cooling effect
of clouds is about 1.5 times their greenhouse effect. The net overall effect is that clouds cool the planet
by 10◦ –15◦ C.
Venus: Runaway greenhouse effect Venus, Earth, and Mars are approximately at the same distance
from the Sun. This implies they formed out of the same primordial dust and had approximately the same
initial temperatures 4.5 billion years ago. The similarity in density shows that the rocky parts of the
planets are indeed similar and the early atmospheres were presumably similar too. However the present
atmospheres of Earth and Venus are completely different. There is no liquid water on Venus and the water
vapour in its atmosphere would, if condensed, form a layer only 20 cm deep. The relative abundances of
hydrogen isotopes on Venus and Earth are consistent with the idea that they did indeed have the same
amount of water originally (see e.g. www.astronomynotes.com/solarsys/s9.htm). Furthermore, Earth
and Venus have similar amounts of carbon dioxide, although Venus’ CO2 is all in the atmosphere while
Earth’s CO2 is mostly locked up in rocks (limestones, coal, etc.) and biomass. Because of the atmospheric
CO2 , air pressure at Venus’ surface is 90 times the Earth’s surface atmospheric pressure. Venus’ low
effective temperature suggests that it was initially much cooler and it owes its present temperature to a
runaway greenhouse effect. Because Venus was slightly closer to the Sun than the Earth, its water never
liquefied and remained in the atmosphere to start the greenhouse heating. All H2 O and CO2 was baked
out of the rocks and enhanced greenhouse heating. UV radiation caused dissociation of water vapour
high in the atmosphere and the resulting hydrogen was lost to space.
Mercury
Venus
Earth
Mars
Surface
pressure of
atmosphere
(kg/cm2 )
0
90
1.03
0.007
Distance
from
Sun
(106 km)
58
108
150
228
Effective
temperature
(◦ C)
+160
−40
−18
−61
Greenhouse
warming
(◦ C)
0
+510
+33
+6
Actual
surface
temperature
(◦ C)
+160
+470
+15
−55
Mass
relative
to Earth
0.0543
0.8137
1
0.1077
Density
(Mg m3 )
5.59
5.12
5.515
4.1
Mercury has no atmosphere.
Venus atmosphere: 96% CO2 , 4% N2 , trace H2 O, CO2
Earth atmosphere: 78.1% N2 , 20.9% O2 , 0.93% Ar, 0.1–1% H2 O, trace CO2 , CH4
Uncompressed
density
(Mg m3 )
5.2
4.0
4.0
3.7
Probable
Fe
content
(%)
∼57
∼30
∼30
∼26
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Mars atmosphere: 95% CO2 , 3% N2 , 1.6% Ar, 0.13% O2
Venus has a lower effective temperature than Earth because of its high albedo (∼ 75%).
Uncompressed density is an attempt to estimate the mean density if the entire planetary substance were
at zero pressure and 25◦ C (from J.A. Wood ‘Meteorites and the Origin of Planets’).
Distribution of heat In the tropics and subtropics, solar heating exceeds outgoing radiation, giving a
net warming. Near the poles the opposite is true. Together, the atmosphere and ocean carry excess heat
from equator to poles to balance the planetary heat budget. About 60% of this energy transfer occurs in
the air, the rest in the oceans.
Atmosphere Hot air rises over the equator and travels toward pole but is deflected by Earth’s rotation.
Deflection interferes with hemispheric circulation and it breaks it down into 3 cells, each covering 30◦
latitude, termed the Hadley, Ferrell and polar cells (imagine a series of gears driven by air rising at
the equator). Earth’s actual atmospheric circulation is further modified by the presence of landmasses,
particularly in the northern hemisphere. Continents present a physical barrier to air circulation and
generate orographic effects such as rain shadows. There is also a thermal effect; location of high and low
pressure systems varies with season as land heats and cools more than the sea (e.g. monsoon).
Oceans Ocean currents are partly driven directly by winds in the top 100 m. Deeper currents are driven
by variations in temperature and salinity which cause density variations (thermohaline circulation). The
most familiar of the surface currents is the Gulf Stream which carries heat northward in the North
Atlantic. In the modern ocean, waters dense enough to sink to the bottom form in only two regions, the
Southern Ocean and the northern North Atlantic. North Atlantic Deep Water is a current of deep salty
cold water which forms north of Iceland, flows south through the Atlantic, then south of Africa, across
the southern Indian Ocean, south of Australia and finally north through the Pacific toward Alaska. It
has been named the Atlantic Conveyor; in effect it is the cold-water return on the global climate system.
One of the forces driving the Atlantic Conveyor is the salt content of the North Atlantic Ocean. The
Atlantic is a relatively salty ocean because some of the water evaporated from it is lost as rain over
the Pacific or Indian oceans. Salty water is relatively dense and more likely to form a bottom current.
The Atlantic Conveyor current is very sensitive to changes in weather: more transport of water vapour
(e.g. via hurricanes) across Central America and Eurasia would make the Atlantic saltier and intensify
the current; less transport would switch off the current. In the ice ages the current seems to have been
switched off and consequently the transport of heat from equator to poles must have been much less
efficient.
Climate change
Causes
1. Change in insolation
(a) Shift in Earth’s orbit round Sun: see Milanković Theory
(b) Variation in Sun’s radiation: Sun has brightened since Earth formed
2. Alter atmospheric circulation: continental drift controls positions of continents and mountain belts
3. Alter oceanic circulation: continental drift controls ocean connectivity (see
http://oceanusmag.whoi.edu/v42n2/haug.html for examples).
4. Alter atmospheric composition
(a) Natural variations
(b) Volcanic eruptions
(c) Anthropogenic emissions
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It is important to note that there are complex feedback mechanisms between atmospheric and oceanic
circulation and composition.
Measuring climate change Sediment layers on the deep ocean floor represent one of the most important records of climate change over the past 200 million years. Sediment cores are collected by the
International Ocean Drilling Program and its predecessors. Only a few examples of the vast amount of
information contained in these cores are listed below.
Sediments Current speed at the seabed can be inferred from grain size of silt-grade material. Clay mineralogy indicates importance of mechanical vs chemical weathering on land. Dust size and distribution
indicates aridity, wind direction and speed. Ice rafted debris indicative of glacial conditions.
Micro-fauna and flora The layers in each core are dated using characteristic species of fossil plankton
and pollen they contain. Abundance of biogenic material indicates past biological productivity. Temperature and salinity conditions inferred from the assemblage of fauna and flora in each layer. Foraminifera
(forams) are a particularly important fossil group. Morphology and size of certain species influenced by
water temperature.
Geochemical Proxies Oxygen isotopes provide a combined record of temperature and ice volume. Typically, the ratio between 16 O (99.76% of total O) and 18 O (0.2%) in calcite of foram shells is measured
and reported using the delta notation:
#
" 18 16 O/ O sample − 18 O/16 O standard
18
103
δ O=
(18 O/16 O)standard
Evaporation effect: 16 O is evaporated preferentially because it is lighter. Temperature effect: 16 O has
greater vibrational energy so it incorporates less readily into crystal structures; this difference is more
pronounced at lower temperatures. During glacial periods, water is evaporated from the ocean and stored
in ice sheets and 16 O is preferentially removed leaving ocean enriched in 18 O, so δ 18 O of seawater and
foram shells increases. Furthermore, the seawater is cooler, so less 16 O goes into calcite crystals and δ 18 O
of seawater and shells further increases.
Milanković cyclicity Milanković cycles is the name given to the collective effect of changes in the
eccentricity, axial tilt (i.e. obliquity) and precession of the Earth’s orbit on the climate, resulting in
100,000 year ice age cycles of the Quaternary glaciation over the last few million years. They are named
for Milutin Milanković (1879–1958), a Serbian geophysicist.
See http://www.sciencedaily.com/encyclopedia/milutin milankovic and links therein for further
details.
Information sources
Much of the information in the lectures and this handout was drawn from the following sources:
G.R. Bigg ‘The Oceans and Climate’ Cambridge University Press;
E.G. Nisbet ‘Leaving Eden: To protect and manage the planet’ Cambridge University Press.
The remainder of material was from the web.