Transcript The Earth`s atmosphere and climate.

The Earth’s Atmosphere and Climate
The history of the atmosphere.
What controls Earth’s climate?
How has climate changed in the past?
Cretaceous to the present
Permian and Carboniferous
Precambrian – Snowball Earth
…after Snowball Earth
After Earth formation……
The process of forming the Earth was complete by about 4.5 billion
years ago.
Earth experienced a period of
internal melting (due to initial
high temperatures and heat
from radioactive decay).
Dense elements (especially iron and
nickel) sank to the centre of the
Earth, forming the metallic core.
Lighter material rose to the
surface (forming a thin,
weak, crust).
Initial crust was likely too thin to
support plate tectonics as we
know it today.
As the Earth cooled and differentiation continued the crust became
thicker and continents began to "grow" due to plate tectonics.
First crust likely had a basaltic composition (like modern oceanic
crust) and lacked continents.
At zones of subduction, intrusion of magma into overlying crust
would have caused thickening to form continental crust.
Oldest continental igneous rocks are 4.02 billion years old.
Oldest sedimentary rocks (sandstones) contain 4.2 billion year old
minerals.
Therefore, granitic continental crust must have been present by 4.2
billion years ago.
The continents evolved
over the past 4.2 Billion
years due to plate
tectonics.
Repeated collisions
between early continents
constructed the modern
continents over time.
Earth’s atmosphere and early life
Initial atmosphere may have had a "primitive"
composition, like that of the sun (gases derived from the
initial nebula).
The modern
atmosphere has a
composition that is very
different from that of
the sun.
E.g., much more Ar40 compared to other isotopes of Ar in
the Earth’s atmosphere.
Ar40 formed by decay of K40, therefore, was derived from
the Earth after it formed.
Other Argon isotopes
were removed with the
early atmosphere and the
Argon40 formed by
subsequent
K40 decay.
Solar wind likely drove off the primitive atmosphere early
in Earth’s history when all of the inner planets lost their
initial atmosphere.
The combined composition of the modern atmosphere,
biosphere, hydrosphere and sediments is very similar to
that of
volcanic gas.
The modern atmosphere resulted from "outgassing" of the Earth:
emission of gases during volcanic eruptions.
Over time the Ar40 was formed in the Earth by decay of K40 and
released to the atmosphere with the volcanic eruptions.
Note: there is growing evidence that the bulk of the atmosphere
(including water) was delivered by millions of comets that collided with
Earth during its very early history.
By 4 billion years ago the early atmosphere had the following
composition:
Principle gases:
Carbon dioxide (CO2)
Water vapour (H2O)
Nitrogen(N2)
Minor gases:
Hydrogen (H2)
Hydrogen chloride (HCl)
Sulfur dioxide (SO2)
Note: there was virtually no free oxygen (which makes up about
20% of the modern atmosphere).
Some free Oxygen was produced by dissociation of water in the
upper atmosphere due to ultraviolet radiation:
2H2O + ultraviolet rays = 2H2 +O2
Light H2 is lost to space and heavier O2 remains in the atmosphere.
However, the rate at
which O2 appears to have
increased could not have
been produced by
dissociation alone.
Photosynthesis is likely responsible for more rapid production of
Oxygen:
CO2 + H2O + light (+ chlorophyll) = (CH2O) + O2
Photosynthesis
requires chlorophyll,
a complex chemical
that is produced by
some organisms (e.g.,
plants).
The oldest organisms that could produce chlorophyll are
cyanobacteria: single celled organisms that lacked an organized
nucleus and lived in the oceans.
First cyanobacteria
appeared about 3.5 billion
years ago and were
anaerobic (did not require
free Oxygen).
They are very common in rocks younger than about 2.5 billion years
old.
There is strong correlation
between O2 levels in the
atmosphere and the
development of life, on Earth.
Cyanobacteria began the
process of increasing
atmospheric Oxygen.
As Oxygen levels increased as aerobic organisms developed (that
used free Oxygen) and these, in turn produced even more Oxygen.
Oxygen levels became high
enough to support more
complex life, which in turn,
produced more oxygen.
By 600 million years ago
Oxygen levels had almost
reached the modern
concentration of about 20%
of the Earth’s atmosphere.
The evolution of land plants, resulted in a modest increase in O2.
Variation in O2 levels over the past 500 million years reflect changes
in plant cover on Earth.
Carboniferous: warm,
moist, tropical settings
predominated on land and
land plants thrived.
O2 levels almost doubled.
Permian and Triassic: arid
conditions were extensive
on land and the abundance
of land plants diminished.
O2 levels dropped to below
15%.
Climate versus Weather
Weather: the condition of the atmosphere at a particular point in space
and time.
Weather changes over short periods of time and is described in
terms of:
Air temperature
Air pressure
Humidity
Cloud cover
Precipitation (rain, snow)
Visibility
Wind
Climate: the average weather for a defined region.
Based on long-term, accumulated data derived from weather
observations.
Long term average temperature, humidity, cloud cover, etc.
Climate (our perception of long term average weather) generally
changes relatively slowly as average conditions change due to some
long-term changes in factors that control climate.
Current focus on climate change is on the long term change in global
temperature (Global Warming).
Climate Change in North America?
Predicted summer temperature change.
Predicted winter temperature change.
Climate varies with the long term radiative balance:
The balance between incoming solar radiation and radiation emitted
back to space from Earth.
a = albedo, reflectivity of a surface (atmosphere or land).
The albedo of the atmosphere varies with the amount of cloud cover
and the concentration of atmospheric aerosols (solid dust and tiny
liquid particles).
Surface
a
Fresh snow
0.8-0.95
Old snow
0.42-0.70
Sandy soils
0.25-0.45
Clay soils
0.20-0.35
Peat soils
0.05-0.15
Water surface 0.05-0.08
(albedo)
On average, 30% of incoming solar radiation is reflected back to space.
http://www.physicalgeography.net/fundamentals/7f.html
Greenhouse gases: absorb longwave radiation and emit some of it back
to the Earth as heat.
Water vapor and CO2
are important
greenhouse gas, others
include:
Methane
Nitrous oxide
Chlorofluorocarbons
The role of water vapour as a greenhouse gas is poorly understood.
As earth warms more water vapour will be added to the atmosphere.
Water vapour both increases greenhouse effect AND increases
atmospheric albedo.
Evaporation from the oceans acts to cool the earth.
Water vapour is added by the breakdown of methane.
Greenhouse Gas
Atmospheric lifetime; GWP
CO2 tens of thousands of years; GWP of 1 over all time periods.
Methane 12 ± 3 years; GWP of 72 over 20 years, 25 over 100 years
and 7.6 over 500 years. The decrease in GWP at longer times is
because methane is degraded to water and CO2 through chemical
reactions in the atmosphere.
Nitrous oxide 114 years; GWP of 289 over 20 years, 298 over 100
years and 153 over 500 years.
CFC 100 years; GWP of 11000 over 20 years, 10900 over 100 years
and 5200 over 500 years.
Atmospheric Lifetime: the average time that a molecule of the greenhouse gas will
remain in the atmosphere.
Global warming potential (GWP): measure of how much a given mass of greenhouse
gas is estimated to contribute to global warming. It is a relative scale which compares
the gas in question to that of the same mass of carbon dioxide (whose GWP is by
definition 1). A GWP is calculated over a specific time interval.
Overall, there is a balance between incoming energy and energy
emitted to space; over the long term they are equal.
Climate changes as any component of the system changes.
Especially:
The amount and distribution of incoming energy from the
Sun.
The reflectivity of the Earth (changes in snow cover and atmospheric
aerosols).
The concentration of “greenhouse” gases in the atmosphere.
Changes in any of these factors cause an imbalance in the radiative
balance and the response is a change in climate.
Radiative Forcing is any change in the average net radiation
(incoming minus outgoing) reaching the top of the Earth’s
atmosphere.
Positive radiative forcing (outgoing radiation is less than
incoming) causes a net increase in global temperature.
Negative radiative forcing (outgoing radiation is more than
incoming) causes a net decrease in global temperature.
The temperature of the Earth controls the amount of outgoing
radiation:
As temperature increases so does the amount of outgoing radiation.
As temperature decreases so does the amount of outgoing radiation.
For example:
If the amount of incoming radiation is reduced, global temperature
drops so that outgoing radiation is reduced to return to a radiative
balance.
The amount of radiation sent back to space from Earth can be
reduced by increasing the concentration of greenhouse gases. More
heat remains on Earth, the global temperature increases such that
the amount of radiation from Earth to space increases to return to
the radiative balance.
The current concentration of
CO2 in the atmosphere is 370
ppm.
387
PPM
Northern
Hemisphere
Growing
Season
Short-term variation in climate
In geological terms, “short-term” refers to hundreds to thousands of years.
Variation in greenhouse gases accounts for some variation in global
temperature.
Variation in incoming solar radiation also accounts for some of the global
temperature change.
Over the past 155 years the
Earth’s temperature has increased
by 0.8 degrees C.
Prior to 1800 temperatures were
low (known as the Little Ice Age).
Why has Earth’s temperature been increasing?
Carbon dioxide
Proportion of Greenhouse Gases
(GHG) in the atmosphere.
35%
Methane
88%
Rising GHG concentrations correspond
reasonably well with rising temperatures.
Fluorocarbons
10%
Variation in incoming solar radiation also results in a change in the
radiative balance and global temperature.
Increased radiation: a positive radiative forcing (Earth becomes
warmer).
Decreased radiation: a negative radiative forcing (Earth becomes
cooler).
Variation in the amount of
solar radiation reaching
the top of the atmosphere
over two years.
0.2%
Incoming solar radiation varies with an 11 year cycle.
The number of sunspots also varies with an 11 year cycle (9.5 to 11
years, averaging 10.8 years over the last 150 years).
Sunspot activity has an important impact on the amount of radiation that
reaches the Earth (and it’s the opposite of what you might think!).
The sun rotates with an average period of 27
days; 30 days at its poles and 24 days at its
equator.
Average diameter of sunspots is about 37,000 km and they are regions
on the sun’s surface that are cooler than average (sun’s diameter:
1,390,000 km).
The outer layer of the sun is on average 6,000 degrees Kelvin
Sunspots have an average temperature of about 4,600 degrees Kelvin.
Sunspots form in regions of strong magnetic force intersecting the sun’s
surface.
Sunspots are darker, cooler regions on the sun.
Associated with sunspots are faculae, brighter, hotter regions on the sun.
Overall, the combination of sunspots and faculae result in a net
increase in the average solar radiation.
faculae
http://earthobservatory.nasa.gov/Library/SORCE/sorce_03.html
Overall, the sun is brighter when
there are many sunspots.
http://earthobservatory.nasa.gov/Library/SORCE/sorce_03.html
The most recent sunspot maximum was in 2001 and we have
passed the trough of the cycle and numbers are on the rise
The solar magnetic field reversed in 2001 and will do so again in
2012 (as it always does at the sunspot maximum).
From 1650 to 1700 AD there
were almost no sunspots and
the global temperature was
particularly low.
Since 1700 AD the number of
sunspots has been increasing.
The pattern of increasing global temperature over the past few hundred
years corresponds reasonably well with the rise in the number of
sunspots.
http://www.john-daly.com/forcing/moderr.htm
Variation in Greenhouse Gases (GHG) accounts for 51% of the recorded variation in
temperature.
Variation in Solar Radiation accounts for 71% of the recorded variation in temperature.
Combined Solar Radiation and GHG explain 92% of the variation in temperature.
http://www.john-daly.com/forcing/moderr.htm
57% of the global warming over the last century is attributed to variation in solar
radiation.
43% of the global warming over the last century is attributed to variation GHG.
The relative role of greenhouse gases and solar variability is the topic of
an ongoing debate.
The fact is, the Earth is 18,000 years into a “warming trend” that
began when the glaciers began to retreat northward.
The CO2 levels are certainly much higher than any time
over the past several hundred thousand years, certainly
due to addition CO2 produced by human activity.
Industrial
revolution
Long term variation in global temperature
The geologic record shows a wide range of
variation in global temperature on a
variety of scales.
Radiative forcing due to a variety of
causes has been postulated for many
extreme climatic events.
Climate Change Over Geologic Time
Tertiary and Quaternary periods have
experienced wide fluctuations over
relatively short spans of time with overall
cooling towards the present.
The Quaternary is characterized by
repeated continental glaciation.
The Cretaceous Period was the warmest
time in all of geological history (average
global temperature is estimated to have
been 20 degrees C).
Over two different spans of time the
Earth was much cooler than any other
time in geologic history:
The end of the Carboniferous Period.
The end of the Precambrian Eon.
From the Cretaceous to the present.
Conditions in the Cretaceous:
5 degrees warmer than today (average
global temperature = 20 degrees C).
Ice caps were not present.
Sea level was higher than any
time in geological history.
Due to melted ice caps and rapid sea floor
spreading (twice the modern rate).
Sea levels were so high that over
much of the Cretaceous the
central portion of North
America was occupied by the
“North American Interior
Seaway”.
Why was the Cretaceous so warm?
A period of Greenhouse Earth.
Rapid rates of seafloor spreading suggest high rates of magma
delivery to the Earth’s surface via volcanoes; including large
volumes of CO2 that accumulated in the atmosphere.
Elevated CO2 concentrations caused a positive radiative forcing.
Temperatures rose to the level required for outgoing longwave
radiation to match incoming solar radiation (same level as today).
Volcanic activity peaked in the mid-Cretaceous and diminished to the
present…..resulting in a long period of global cooling.
Climatic fluctuation over the
Tertiary and Quaternary periods.
Through the Tertiary climate varied
from colder to warmer in regular cycles.
Average global temperature was
warmer during the Tertiary Period so
that continental glaciations did not take
place.
By the Pleistocene Epoch average global
temperature had been reduced so that
cold periods resulted in spreading of ice
sheets over the continents on four
different occasions.
Glaciers: more-or-less permanent bodies of ice and compacted snow
that have reached a thick enough accumulation to flow under their
own weight.
Glaciers currently cover 10% of the current Earth’s surface.
The Water Budget
Total Water on Earth
1,360,000,000 km3
Oceans and Seas
1,331,746,800 km3 (98%)
Glaciers and Ice Sheets
24,000,000 km3 (<2%)
Groundwater
4,000,000 km3
Lakes and Reservoirs
155,000 km3
Soil Moisture
83,000 km3
Vapor in the atmosphere
14,000 km3
Rivers
1,200 km3
The distribution of modern glaciers
Glaciers cover 15,800,000 km2 of the Earth’s surface (almost the
area of South America).
Location
Area of glaciers
Km2
Approximately 97% of
glaciers are in regions of high
latitude (near the north and
south poles).
Remaining 3% of glaciers are
at low latitudes but high
altitudes.
The elevation above which glaciers will form (i.e., the snowline)
depends on local climate.
Southern British Columbia: approximately 1500 metres above sea
level.
Central Africa: approximately 5000 metres above sea level.
The glacier ice cap on the top
of Mt. Kilimanjaro is
expected to disappear
between 2010 and 2020 due
to global warming as the
snowline rises to higher
elevations.
Antarctica: snow line is at sea level.
http://users.aber.ac.uk/gwr1/typesoficemasses.htm
18,000 years ago 32% of the land surface and 30% of the ocean
surface were covered by glaciers.
Canadian landscape was
shaped by glaciers.
http://www.csulb.edu/~rodrigue/geog140/lectures/glaciers.html
18,000 years ago 10 million
square kilometres of North
America was covered by a
continental ice sheet up to 3 km
thick.
The centre of greatest ice
thickness was near Hudson
Bay.
Isostatic subsidence depressed
the land surface by
approximately 380 metres.
The region has rebounded by
about 300 metres and continues
to rise at 2 cm/yr.
We currently live in a “glacial period” over which
several continental glaciations have taken place.
Sea level rose and fell over each period of glaciation.
Growth of ice sheets has a dramatic impact on sea level.
18,000 years ago during the peak of the last continental glaciation
sea level was approximately 140 metres lower than today.
Sea level has been
rising since that
time but the rate
has become
progressively
slower as the
continental
glaciers
disappeared.
At the glacial maximum 18,000 years ago the shoreline was up to
several hundred kilometres offshore of the modern shoreline.
If all of the ice sheets and glaciers were to melt sea level would rise
by an additional 80 metres.
Heavily populated areas worldwide would be flooded by a sea level
rise on this scale.
Modern rates of sea level change are known from long term records.
At present sea level is rising by 2 mm per year.
Global warming is expected to cause an increase in the rate of sea
level rise:
18 cm above present by 2030 (5.6 mm/yr, average)
44 cm above present by 2070 (6.1 cm per year average)
Animated Earth with changing sea level.
(http://earth.rice.edu/mtpe/cryo/cryosphere/topics/ice_age/sealevel.mov)
Disintegration of Modern Ice Sheets
Ice sheets are the most extensive glaciers (>50,000 km2).
Flow outward, from the centre of thickest ice.
At the shores of the land mass the ice sheet may flow onto the ocean
to form an extensive, floating ice shelf.
Ice
Shelf
x
y
Greenland: 80% of total land mass is covered by an ice sheet.
Average thickness 1.5 km, 3 km locally.
Antarctica: Ice sheet covers 13.5 million square kilometres.
Exceeds 4 km in thickness.
Bounded by extensive Ice Shelves; e.g., Ross Ice Shelf: total area of
500,000 km2
Larsen Ice Shelf: a relatively small ice shelf that is breaking up.
Regional temperature has
increased by 2.5 degrees C over
the past 50 years.
In February 1995 a major storm
initiated the breakup of Larsen A
Ice Shelf.
The largest iceberg was 70 km
long and 25 km wide.
Hundreds of icebergs 1–2 km in
size were liberated by the
breakup.
Larsen Ice Shelf continues to break up.
March 2002 Larsen B disintegrated.
Over the year following the disintegration of Larsen B the glaciers
that fed the shelf increased their flow rates from 1.7 m/day to 3.1
m/day (a 180% increase).
Implications:
Suggests that global warming may lead to breakup of other shelves.
Such breakup leads to an increase in the rate of flow of glaciers into
the Ocean.
This, in turn, will increase the rate at which sea level rises with global
warming.
There is evidence that just such an event took place in the geologic
past, when average global temperature was a few degrees warmer,
raising sea level by more than 6 metres.
In March, 2003 Iceberg B-15 broke off the Ross Ice Shelf.
One of the largest icebergs ever seen, it was 300 km long and 40 km
wide (11,000 km2).
The Ward Hunt Ice Shelf on the coast of Ellsmere Island (the Arctic’s
largest ice shelf) broke up in 2003 due to climatic warming.
The Point:
With global warming the ice shelves are breaking up.
This is expected to increase the rate at which glacier ice flows into
the world’s oceans.
This will lead to an increase in the rate of sea level rise.
http://pubs.usgs.gov/fs/fs133-99/gl_vol.html
18,000 years ago
Images are from William Haxby at the Lamont-Doherty Earth Observatory
Melting of the Western Antarctic Ice Sheet
Melting of the Eastern and Western Antarctic Ice Sheet
18,000 years ago
Melting of the Western Antarctic Ice Sheet
Melting of the Eastern and Western Antarctic Ice Sheet
18,000 years ago
Melting of the Western Antarctic Ice Sheet
Melting of the Eastern and Western Antarctic Ice Sheet
18,000 years ago
Melting of the Western Antarctic Ice Sheet
Melting of the Eastern and Western Antarctic Ice Sheet
What caused the Pleistocene continental glaciations?
Many periods of continental glaciation have taken place over the
Earth’s history.
4 glacial periods occurred in recent succession, approximately
100,000 years apart.
Between each glacial period is an interglacial period when conditions
are like modern conditions.
Most theories of the continental glaciation involve changes in global
climate.
Average global temperature today is 15 degrees C.
A reduction in temperature by 2 to 4 degrees C may cause the
continental ice sheets to extend across the continents.
Over the past 200 years the Earth’s temperature has increased by
about 1 degrees C (little chance of glaciation).
Past speculation for causes of continental glaciation:
Large volumes of volcanic ash in the upper atmosphere would
increase the global albedo and reflect solar radiation back into space
and cooling the Earth.
Large asteroid impacts throw dust into the atmosphere, reducing
receipt of solar radiation (and result in cooling).
Such events have caused considerable cooling but there is no
correlation between continental glaciations and such events.
These mechanisms cannot explain the 100,000 year cycles of
glaciation over the recent past.
What appears to be the correct answer to the question of the cause of
continental glaciations came from Mulatin Milankovich….
….Milankovich Cycles
Milankovitch conducted a mathematical study of the effect of the
Earth’s orbit on climate.
He identified three components of the Earth’s orbit and rotation that
would affect climate:
Eccentricity
Obliquity
Precession
Eccentricity: variation in the form of the Earth’s orbit about the
sun.
High eccentricity increases differences between seasons, low
eccentricity decreases difference between seasons.
Low eccentricity
High eccentricity
Obliquity: variation in the angle between the axis of rotation of the
Earth and the plane of the orbit about the sun.
Influences the difference
in sunlight reaching the
poles in winter and
summer (high angle,
more summer sunlight,
less winter sunlight).
At present the angle is 23.5 degrees.
Precession: a wobble of the Earth’s rotational axis.
Slightly influences amount
of sunlight in summer and
winter in polar regions.
All three components vary in a cyclical manner.
Duration of each cycle:
Eccentricity: 100,000 years
Obliquity: 40,000 years.
Precession: 26,000 years.
All three cycles are superimposed and cause variation in the
distribution of solar radiation over the seasons.
During parts of the cycle:
Mild winters, abundant snow and cool summers.
Conducive to development of glaciers.
Cold, dry winters, hot summers.
Glaciers will not develop.
The 100,000 year cycle of eccentricity has the greatest effect and
corresponds to continental glaciations.
When eccentricity is low widespread glaciation is most likely (more
uniform seasons)
Seasonal extremes (hot summers and cold winters) associated with
high eccentricity are less likely to see the spread of glaciers.
Once glaciers begin to develop they increase global albedo, reducing
incoming solar radiation and causing further cooling of the Earth.
In the 1970s a means of testing the hypothesis was developed based
on the chemistry of shells produced by organisms.
The concentration of certain isotopes that are incorporated into shell
material was found to be a function of seawater temperature.
The correlation between eccentricity and global temperature was
striking and supported Milankovich’s ideas.
Periods with smaller seasonal differences in temperature cause the
beginning of glacier advance away from the poles to extend across
the continents.
Once the glaciers begin to advance they increase global albedo
(reflecting more incoming solar radiation away from the Earth).
This reduces the average global temperature which, in turn, causes
the further advance of glaciers.
The glacial advance ends when seasonal differences increase and
the glaciers retreat back towards the poles.
Carboniferous/Permian
The Permian Period began with the
lowest temperatures of the Phanerozic
Eon.
It coincided with a marked decrease in O2
concentration following the peak in the
Carboniferous.
The Permian was a time
when all of the major
continents were grouped into
a “supercontinent” called
Pangea.
Much of the land surface was
exposed to a very dry, inland
climate, much like central
Canada or Asia today.
Much of the land surface
extended north and south of
the equator, in climatic zones
that experience desert-like
conditions.
Extensive arid conditions reduced plant life which, in turn, led to a
reduction in atmospheric O2.
With the widespread, arid land mass, continental weathering of
existing rocks took place at a greater rate than in the past.
Weathering of rocks can remove CO2 from the atomosphere:
Carbonic acid is important for several weathering reactions.
E.g., weathering of limestone (made up of the mineral Calcite)
The end result is the loss of CO2 from the atmosphere.
Lower CO2 results in a negative radiative forcing and global
temperature falls.
The subsequent increase in temperature was likely due to increased
volcanic activity that continued through to the Cretaceous.
Snowball Earth
An excellent article is located at Paul Hoffman’s site at:
http://www-eps.harvard.edu/people/faculty/hoffman/snowball_paper.html
Hypothesis: That 600-700 million years ago the Earth was
effectively covered by glacial ice, including 500 to 1500 m
thick sea ice cover.
Evidence: Glacial deposits (e.g., Tillite….the rock form of
Till) that were laid down at low latitudes, apparently at the
equator and near sea level on several continental masses of the
same age.
Tillite: a sedimentary rock that is made up of a wide range of
sizes of materials…ranging from fine mud to huge boulders.
Such deposits are laid down from retreating glaciers.
How do we know the latitude of ancient tillites?
By measuring the magnetic inclination of the weak magnetic
field associated with the deposits.
Near horizontal inclination indicates deposition near the
equator.
There was considerable skepticism that these were glacial
deposits because it seemed impossible for glaciation to
proceed all the way to the equator.
How would such glaciation come to be?
Mikhail Budyko (USSR) undertook climatological modeling
to determine whether or not a condition could exist that would
cause glaciers to extend to sea level at the Equator.
Based on a climatic energy balance. Simplest case:
Net Energy Reaching Earth =
incoming radiation (I) – reflected radiation (a x I)
Important factors:
Albedo: The higher the global albedo the more incoming
radiation is reflected and the less is absorbed to warm the
Earth.
Greenhouse gases: The more greenhouse gases are in the
atmosphere, the more heat is retained (temperatures rise).
Budyko found that as glaciers advance southward from the
poles that global albedo increases, cooling the Earth and
enhancing formation of glaciers.
If sea ice and glaciers come to extend to within 30 degrees
North or South of the Equator then global albedo becomes
high enough so that the glaciers will proceed to cover the
Earth to the equator (forming a Snowball Earth).
Such freezing of the Earth would drastically reduce the
amount of water vapor, reducing the greenhouse effect even
further, causing even further cooling.
What would cause such a glaciation 700 million years ago?
1. Sun’s energy was 6% less than today (less incoming heat).
2. With the evolution of algae CO2 uptake by photosynthesis
may have decreased the amount of CO2 in the atmosphere:
lessening Greenhouse Effect.
But, if this were to ever had taken place, why does the
Earth not continue to be a snowball?
Joe Kirschvink: Found a possible cause of the destruction of
such global glaciers.
Even with a cover of ice plate tectonics will continue to be
active.
Volcanism would continue to add gases to the Earth’s
atmosphere, including CO2.
Over time, the CO2 concentration would increase at a constant
rate.
CO2 would not be lost to weathering and the formation of
carbonate rocks (ice covered continents don’t weather).
To melt the glaciers and sea ice, given the high albedo, would
require 350 times as much CO2 as there is in the current
atmosphere.
It would take 10 million to 40 million years for sufficient CO2
to accumulate to the required levels.
Average global temperatures would rise to almost 50 degrees
Celsius and it would take only a few thousand years for sea ice
to melt….reducing the global albedo.