Download What causes Earth`s climate and climate change?

What causes Earth’s climate and climate change?
The Atmosphere
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Earth’s atmosphere extends up to 500 km above ground
 80% of air is concentrated in the first 16 km
(troposphere)
Weight of air
 10,335 kg air m-2 land (or about 1 kg cm-2)
Composition of air
 78.1% N2; 20.9% O2; 0.93% Ar (argon)
 the rest, 0.07%, is water vapour, CO2 (carbon
dioxide), CH4 (methane), O3 (ozone), and NO2
(nitrous oxide)
 CO2 = 389 ppm (parts per million, by volume) or
0.0389% only
Air also contains suspended liquid water and solid
particles (aerosols, typically 0.001 mm in diameter)
Structure of
atmosphere is
stratified according to
temperature
Mount Everest (8,848 m)
http://www.topnews.in/sports/teams/climbing
E.A. Mathez, 2009, Climate Change: The Science of Global Warming and
Our Energy Future, Columbia University Press. Source: Hartmann, 1994
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Troposphere (lowest level, nearest to ground)
 up to (16 or 18) km: high in the tropics (equator),
remains high then starts to fall at 40 N and S until
about 8-10 km in the polar regions
 80% of air is concentrated here and this layer is
responsible for weather and contains the
greenhouses gases
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Stratosphere
 up to 50 km
 contains mostly ozone which absorbs the deadly UV
rays
 without UV absorption, life would not exist on Earth’s
surface
 life would only exist several meters below surface
 only when ozone layer was developed, did life
begin on Earth 400 mil. years ago
Mesosphere
 up to 85 km
Thermosphere
 up to 500 km
 very little air
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Troposphere
 Incoming solar radiation
 50% absorbed by surface (ocean and ground)
 the rest is re-emitted as thermal energy and heats
overlying air; thus, troposphere heated from
bottom
 temperature decreases with increasing elevation
 decreasing by a mean of 6.5 C per km upward
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Troposphere has vigorous convection
 warm, low density air near ground rises
 cold, higher density air sinks to replace warm air
 causes weather due to these thermal motions
 but sometimes large mass of warm air rises and traps
colder air below
 convection inhibited, trapping air pollutants in cities
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Stratosphere
 tropopause (about -70 C), but in stratosphere, air
warms with increasing altitude because of ozone
layer. Ozone absorbs UV; thus, heating air
Mesosphere
 air temperature falls again with increasing altitude
Thermosphere
 air temperature rises with increasing altitude because
geomagnetic field interacts and absorbs some of the
solar wind (high-speed streams of charged protons
and electrons from the Sun; 400 km/s)
 during strong solar activity, temperatures may reach
1500 C, but we would not feel hot up there because
of little atmosphere (gases)
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Little troposphere-stratosphere interaction
 mixing of the two layers is slow
 stratosphere is a stable layer
 danger if pollutants from ground reaches stratosphere
 pollutants tend to stay in the atmosphere for a long
time and are quickly transported around the globe
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The stratified layers of the atmosphere is essential to life
on Earth
Cold tropopause traps water below because the cold
causes condensation to ice particles in the troposphere
 if water enters into the upper layers, solar radiation
would disassociate water molecule into H2 and O2
 H2, being a very light gas, would not be held by
gravity and be lost into space
 eventually increasingly more water would be lost,
leaving Earth dehydrated
 no life on Earth
 this may have happened in our sister planet, Mars
Earth rotates counter clockwise (eastward), looking down on
North Hemisphere, but rotates clockwise, looking up on
Southern Hemisphere
http://sealevel.jpl.nasa.gov/overview/climate-earth.htm
Coriolis Effect
A < B rotational speed
The Coriolis Effect causes things
moving toward the poles
to lead the earth's rotation
because they are headed into
regions where the earth's
rotational speed is slower. They
are deflected to the east.
The Coriolis Effect causes things
moving toward the equator
to lag the earth's rotation because
they are headed into regions
where the earth's rotational speed
is faster. They are deflected to
the west.
Idealized surface and global wind patterns
E.A. Mathez, 2009, Climate Change: The Science of Global
Warming and Our Energy Future, Columbia University Press.
http://www.answers.com/topic/atmospheric-cell
Global Atmospheric Circulation
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Solar heating imbalance means warm equatorial air flow
towards the poles, and cold air from poles toward
equator, creating a huge convection
Hadley cell
 Warm air rises at the equator and flow downward to
create two high pressure zones at 30 N and 30 S
 Some flow poleward, but most flows downward back
to the equator, completing the Hadley cell
Ferrel cell
 The Coriolis Effect breaks up Hadley cell into a
second, mid-latitude circulation patterns, called Ferrel
cell, which occur between 30  and 60 N and S
 less stable and give rise to a series of eddies flowing
from west to east
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Polar cell
 occurs near the poles
 cold, dry air descends to form high-pressure vortices
 the downward air flows toward equator, but directed
toward the west by Coriolis forces called polar
easterlies
 polar easterlies meet the Ferrel cell along the polar
front, creating a zone of unstable and severe weather
Polar jet stream
 high-speed wind, hundreds of kilometers wide and
several kilometers thick
 occurs at the polar front at about 10 km altitude
 undulates back and forth, and determines the weather
between 45  and 60 N and S
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Subtropical jet stream occurs between 30  and 40 N
and S, at 12 km altitude
 corresponds where the tropopause suddenly plunges
to a lower elevation
 wind speeds up to 540 km h-1 but has less influence
on weather in the lower latitudes than the polar jet
stream at the higher latitudes
E.A. Mathez, 2009, Climate Change: The Science of Global Warming and
Our Energy Future, Columbia University Press. Source: Hartmann, 1994
Energy Budget (radioactive and non-radioactive transfers)
http://asd-www.larc.nasa.gov/erbe/components2.gif
Solar radiation spectrum
Longer the wavelength, lesser the energy:
PAR (photosynthetically active radiation, 400-700 nm, same as visible light)
UV too high energy for plants
NIR too low energy for plants
Albedo of various surfaces
Mean Earth’s surface albedo =
30%
E.A. Mathez, 2009, Climate Change: The Science of
Global Warming and Our Energy Future, Columbia
University Press. Source: Hartmann, 1994
Surface
Typical Albedo (%)
Deep water, low wind, low latitude
7
Deep water, high wind, high latitude
12
Moist dark soil
10
Moist gray soil
15
Dry soil, desert
30
Wet sand
25
Dry light sand
35
Asphalt pavement
7
Concrete pavement
20
Short green vegetation
17
Dry vegetation
25
Coniferous forest
12
Deciduous forest
17
Forest with snow cover
25
Sea ice, no snow cover
30
Old, melting snow
50
Dry, cold snow
70
Fresh, dry snow
80
Greenhouse Effect
Happens only in
the troposphere
GHGs transparent
to incoming solar
radiation but
opaque to
outgoing solar
radiation; thus,
entrapping heat
www.eecs.umich.edu/mathscience/funexperiments/agesubject/lessons/images/diagrampa
A passing electromagnetic radiation
can "excite" a molecule, causing it to
vibrate.
Vibration modes of CO2:
Mode (a) is symmetric and results in
no net displacement of the
molecule's "center of charge", and is
therefore not associated with the
absorption of radiation.
Modes (b) and (c) not symmetrical
and cause electrons not to be shared
equally (thus, net charge) and these
modes can absorb radiation,
transferring the energy into to the
molecule.
GHG gases are
molecules with 3 or
more atoms
N2 and O2 are not
GHGs
Image courtesy Martin C. Doege
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GHGs like CO2 and ozone (O3) are natural gases and
are essential to life on Earth
 they help keep the Earth warm for life
 without GHG, Earth mean temperature would be -18
C, instead of 15 C
 GHG help to keep water in stable liquid form for life
The problem today with GHG is too much of GHG is
added into the atmosphere by human activities
(anthropogenic)
 too much added warming caused by human activities
 like a person wearing too many thick clothes (heat
cannot escape)
A simple model with a non-absorbing atmosphere
outgoing
from Earth
incoming
from Sun
Earth
Total solar irradiance outside Earth (solar constant) is Fs = 1370 W m-2
The solar beams are parallel to Earth, so the power intercepted by Earth is
contained in a tube of cross sectional area a2, where a is the Earth’s radius.
The total solar energy received by Earth per unit time is thus Fsa2.
Earth’s mean albedo, A, is 0.3, so total solar energy received, after reflection, is
(1-A)Fsa2.
If the Earth is a blackbody with uniform temperature, T, then by Stefan-Boltzman
law, the power emiited per unit area is T4, where  is the Stefan-Boltzman constant
(5.6703 x 10-8 W m2 K4).
On Earth, the power radiated is over all of Earth’s surface, where the total surface
area is 4a2. So, the total power emitted from all of Earth’s surface area is 4a2T4.
Assuming Earth is in thermal equilibrium, the incoming radiation must equal outgoing
radiation to give
(1-A)Fsa2 = 4a2T4
Substituting the values for A and Fs, T is about 255 K.
But Earth’s measured surface temperature is about 288 K, higher than that
calculated. Why? This is because this model does not include any greenhouse
effect.
A simple model of greenhouse effect
F0 F0 
Ta
1
1  A Fs
4
Fa
lwFg
ATMOSPHERE
Fg Fg   Tg4
swF0
Fa
Tg
GROUND
Area of a2 has an irradiance of Fs, so the total surface area of Earth, 4a2, has an (lower)
irradiance of F0, which is determined by
 a2
F0 
1  A Fs  240 W m-2
2 
4 a
Of F0, swF0 is absorbed by the ground, and the remainder (1- sw)F0 is absorbed by the
atmosphere.
The ground emits Fg, where lwFg escapes the atmosphere, while the remainder, (1-lw)Fg, is
absorbed by the atmosphere.
Above the atmosphere,
F0  Fa  lw Fg
Below atmosphere and ground, Fg  Fa   sw F0
Eliminating Fa, we obtain
Fg   Tg4  F0
1   sw
1  lw
Taking rough estimates for sw and lw as 0.9 (strong transmittance, weak absorption) and
0.2 (weak transmittance, strong absorption), respectively, we obtain Tg (ground temperature)
as 286 K (nearly the same as the mean measured value of 288 K) and Ta (atmosphere
temperature) as 245 K.
Recall: GHG has strong transmittance (high sw) for incoming solar radiation but strong
absorption (low lw) for outgoing radiation
GHG
(Greenhouse gas)
Water vapour (H2O)
Contribution
to
greenhouse
warming (%)
GWP
(Global
warming
potential
for 100
years)
Radiative
forcing (RF)
(W m-2)
55-70
Lifetime
(years)
9 days
Carbon dioxide (CO2)
25
1
1.66
50-200 days
Chlorofluorocarbons
(CFC)
11
10,900
0.34
100
Methane (CH4)
5
25
0.48
12
Nitrous oxide (NO2)
2
298
0.16
114
Ozone (O3)
1
0.35
GWP – measure of GHG potency in relation to CO2
RF – measure of warming/cooling caused by GHG
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GWPs are calculated as the ratio of the radiative forcing
that would result from the emissions of one kilogram of a
greenhouse gas to that from the emission of one
kilogram of carbon dioxide over a period of time (usually
100 years)
Radiative forcing (RF) is the additional radiative power
that the gas is sending back to the ground
 values based on a base period (taken at year 1750,
which is the start of the Industrial period)
 positive RF means increased heating (causes
warming)
 negative RF means less heating (causes cooling)
1.66
0.34
0.16
0.48
1.6
0.35
0.07
0.01
0.1
0.12
-0.05
-0.2
-0.5
-0.7
IPCC, 2007
Contributions of different greenhouse gases
to radiative forcing
E.A. Mathez, 2009, Climate Change: The Science of Global Warming and Our
Energy Future, Columbia University Press. Data from Forster et al., 2007
+1.7 ppm/year
+0.6 ppb/year
CO2 and NO2 continue to increase. Methane (CH4) and CFCs are levelling off.
http://www.greencarcongress.com/2006/05/noaa_reports_st.html
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Other GHGs
 Halocarbons (hydrofluorocarbons, perfluorocarbons,
and sulfur hexafluoride)
 from semiconductor industry
 Methyl bromide
 from fumigant for insect control in agriculture
 Nitrogen trifluoride (NF3)
 from production of flat LCD and plasma TVs
Black soot
 caused by incomplete burning of fossil fuels
 black particles (aerosols)
 will absorb heat (causing warming)
Model estimates of the direct effects of aerosols on
radiative forcing
Radiative forcing
Aerosol
(watts per square meter)
Sulfate (sulfuric acid via sulfur dioxide from fossil-fuel burning)
-0.4 ± 0.20
Organic carbon (from fossil-fuel and biomass burning)
-0.05 ± 0.05
Black carbon (from incomplete fossil-fuel combustion)
+0.20 ± 0.15
Smoke (compounds from forest fires)
+0.03 ± 0.12
Nitrates (ammonium, from ammonia and NOx emissions)
-0.10 ± 0.10
Anthropogenic dust (agriculture, cement production, drying soils)
-0.1 ± 0.20
All aerosols combined, direct effects
-0.50 ± 0.40
Increase in cloud albedo due to aerosols
-0.3 to -1.8
E.A. Mathez, 2009, Climate Change: The Science of Global Warming and
Our Energy Future, Columbia University Press. Source: Forster et al., 2007
Scanning electron microscope images of
various aerosols
E.A. Mathez, 2009, Climate Change: The Science of Global Warming and Our
Energy Future, Columbia University Press. Photographs by V. Martins, NASA
Global distribution of aerosols (2006)
E.A. Mathez, 2009, Climate Change: The Science of Global Warming and Our
Energy Future, Columbia University Press. Source: Reto Stöckli, NASA
The Keeling Curve (CO2 levels from 1958 onward)
May
Oct
The annual fluctuation in CO2 is caused by seasonal variations in CO2
uptake by land vegetation. Since more vegetation are concentrated in the
Northern Hemisphere, more CO2 is removed from the atmosphere during
Northern summer than Southern summer.
http://en.wikipedia.org/wiki/File:Mauna_Loa_Carbon_Dioxide-en.svg
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CO2 levels
 1750 (start of Industrialization) = 275 ppm
 today (2010) = 389 ppm
 increases about 1.7 ppm per year
 present CO2 levels are the highest in the last 400,000
years, based on ice core studies
Since 1865, global mean surface temperature has
increased from 14.4 to 15.4 C, the warmest in the last
1000 years
temperature anomaly
CO2
World development report 2010: Development and Climate Change, The World Bank, Washington
D.C., 2010
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Energy balance is imbalance by as much as +0.25 to
0.75 Wm-2
-2
 mean: +0.5 Wm
 90% of it is absorbed by the oceans
 “warming in the pipeline”
 even if there is no CO2 emissions today, the world
will still warm by up to 0.6 C because the oceans
will slowly release the heat into the atmosphere
 0.75  0.25 C per Wm-2 forcing
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Ought to reduce CO2 levels back to 350 ppm, not 450
ppm, to stabilize the energy balance (bring it back to
balance)
Since start of Industrial Revolution = 275 ppm CO2 and
in 2008, about 385 ppm CO2, and solar forcing by CO2 =
1.66 Wm-2
-2
 Thus, (385-275)/1.66 = 66 ppm CO2 per Wm forcing
Energy imbalance is 0.5 Wm-2, so to remove this
imbalance, the level of CO2 must be reduce to
 0.5 x 66 = 33 ppm CO2 must be removed to restore
energy balance
 so, 385 - 33 = 352 ppm (about 350 ppm) is the level
of CO2 to achieve
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Little doubt that human activities have caused global
warming
 90% certainty according to IPCC 2007 report
The question is how much global warming will occur
 still uncertain because of feedback mechanisms
 where climate component A influences component
B which, in turn, B influences A
Feedback mechanisms on global warming
Links between various elements of the climate
system, illustrating how changes in any one can
interact with others to produce positive or negative
feedback effects
Climate change : the science, impacts and solutions by A. Barrie Pittock, CSIRO, 2009
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Examples of feedback:
 more CO2, higher temperature, which causes higher
evaporation of water into the atmosphere (water is a
GHG and more water vapour would amplify the
greenhouse warming) => effect of CO2 is amplified
 increase in water vapor changes the vertical
temperature distribution of the atmosphere
 close to ground, little effect by vapour because air
already concentrated with vapour
 but higher up (8 km), air is much drier, and the
addition of water vapour would amplify the
greenhouse warming
 more vapour also means more clouds
 clouds trap heat (causing heating), but they also
reflect solar radiation (causing cooling)
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industrial pollution of air may cause cooling by
reflecting solar radiation back into space
 pollutant aerosols such as sulfates
volcanic activity can also reduce warming by sulfur
and ash emissions which reflect heat from solar
radiation
 Mt. Krakatoa, Indonesia, erupted on 1883 reduced
world temperature by 1.2 C for several years
 Mt. Pinatubo, Philippines, erupted on June 15,
1991 reduced world mean surface temperature by
1 C for two years
 Mt. El Chichon, Mexico, erupted on 28 March 1982
reduced world temperature by 0.3-0.5 C
 Mt. St. Helens, USA, erupted on May 18, 1980
reduced world temperature by 0.1 C
http://wattsupwiththat.com/2009/01/13/how-did-the-el-chicon-and-pinatubo-volcanic-eruptions-affect-global-temperature/
Stratospheric aerosol
before and on several
occasions after the
eruption of Mount
Pinatubo, June 15,
1991
E.A. Mathez, 2009, Climate Change: The Science of Global Warming
and Our Energy Future, Columbia University Press. Source: NASA
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Effect of clouds on warming
 Earth’s average albedo (including clouds) is 0.3; that
is, 30% solar radiation is reflected
 clouds reflect incoming solar radiation (cooling)
 but clouds absorb emitted radiation from surface
below (warming)
 absorbed heat is reemitted into space or back to
the surface
 clouds act as insulator for surface
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low, thick cumulus clouds reflect more incoming
radiation than they absorb (cooling)
high, thin cirrus clouds transmit incoming radiation but
trap some outgoing radiation (warming)
overall: net effect of clouds on global warming is
cooling, but much uncertainty still persist on cloud
feedback mechanisms
Cumulus
Cirrus
Cumulus cloud photo (left) by Carlye Calvin. Cirrus clouds photo (right) by Caspar Ammann.