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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 presentPermian and CarboniferousPrecambrian – Snowball Earth

…after Snowball Earth

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 heatfrom radioactive decay).

Dense elements (especially iron and nickel) sank to the centre of the Earth, forming the metallic core.

After Earth formation……

Lighter material rose to the surface (forming a thin, weak, crust).

Initial crust was likely too support plate tectonics as we know it today.

Therefore, granitic continental crust must have been present by 4.2 billion years ago.

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) are 4.2 billion years old.

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 lifeInitial 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.

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Ar40 formed by decay of K40, therefore, was derived from the Earth after it formed.

E.g., much more Ar40 compared to other isotopes of Ar in the Earth’s atmosphere.

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).

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The oldest organisms that could produce chlorophyll are cyanobacteria: single celled organisms that lacked an organized nucleus and lived in the oceans.

First cyanobacteriaappeared 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.

By 600 million years ago Oxygen levels had almost reached the modern concentration of about 20% of the Earth’s atmosphere.

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.

The evolution of land plants, resulted in a modest increase in O2.

Permian and Triassic: arid conditions were extensive on land and the abundance of land plants diminished.

O2 levels dropped to below 15%.

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.

Climate versus Weather

Weather: the condition of the atmosphere at a particular point in spaceand 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

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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.

α = albedo, reflectivity of a surface (atmosphere or land).

Surface αFresh 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

The albedo of the atmosphere varies with the amount of cloud cover and the concentration of atmospheric aerosols (solid dust and tiny liquid particles).

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Greenhouse gases: absorb longwave radiation and emit some of it back to the Earth as heat.

Water vapor and CO2are important greenhouse gas, others include:

MethaneNitrous oxideChlorofluorocarbons

http://eosweb.larc.nasa.gov/EDDOCS/images/Erb/components2.gif

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 radiativebalance 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 (an increase in radiation reaching the Earth) causes a net increase in global temperature.

Negative radiative forcing (a decrease in radiation reaching the Earth) 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.

If the amount of incoming radiation is reduced (e.g., an increase in albedo), global temperature drops so that outgoing radiation is reduced to return to a radiative balance.

The current concentration of CO2 in the atmosphere is 370 ppm.

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.

For example: 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.

Over the past 200 years the Earth’s temperature has increased by 0.6 degrees C.

Prior to 1800 temperatures were low (known as the Little Ice Age.

Variation in incoming solar radiation also accounts for some of the global temperature change.

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Why has Earth’s temperature been increasing?

Proportion of Greenhouse Gases (GHG) in the atmosphere.

Carbon dioxide

Methane

Fluorocarbons

Rising GHG concentrations correspond reasonably well with rising temperatures.

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 positive radiative forcing (Earth becomes cooler).

Variation in the amount of solar radiation reaching the top of the atmosphere over two years.

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!).

Sunspots

Average diameter of about 37,000 km and they are regions on the sun’s surface that are cooler than average.

The outer layer of the sun is on average 6,000 degrees Kelvin

Sunspots have an average temperature of about 4,600 Kelvin.

Sunspots form in regions of strong magnetic force intersecting the sun’s surface.

http://earthobservatory.nasa.gov/Library/SORCE/sorce_03.html

Sunspots are darker, cooler regions on the sun.

faculae

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.

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http://earthobservatory.nasa.gov/Library/SORCE/sorce_03.html

Overall, the sun is brighter when there are many sunspots.

Since 1710 AD the number of sunspots has been increasing.

From 1650 to 1750 AD there were almost no sunspots and the global temperature was particularly low.

The pattern of increasing global temperature over the past few hundred years corresponds reasonably well with the rise in the number ofsunspots.

Variation in Solar Radiation accounts for 71% of the recorded variation in temperature.

Variation in Greenhouse Gases (GHG) accounts for 51% 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.

http://www.john-daly.com/forcing/moderr.htm

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.

Long term variation in global temperature

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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 (estimates range up to 20 degrees warmer than today).

Over two different spans of time the Earth was much cooler than any other times 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:

20 degrees warmer than today.

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 longwaveradiation 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.

Why did volcanic activity peak in the Cretaceous?

Studies of the RMS of volcanic lava flows have shown that such flows more commonly have normal magnetic polarity than reverse polarity.

This suggests that volcanic activity is more common when the poles are normal.

The Cretaceous Period was a time of prolonged normal polarity, therefore, there was likely much more volcanic activity.

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The relationship between normal polarity and volcanic activity is likely related to the temperature of the interior of the Earth.

When the internal temperature is high, convection within the outer core is more vigorous…..leading to normal polarity of the magnetic poles (again, the geodynamo).

When the internal temperature is lower, convection slows down and mixed polarity or even long periods of reversed polarity prevail.

Possibly……

The higher internal temperature leads to more frequent and violent volcanic activity (and more atmospheric CO2 which leads to higher global temperature).

Since the end of the Cretaceous polarity has been variable and temperature has declined due to lower CO2 concentrations in the atmosphere in response to a reduction in volcanic activity.

Climatic fluctuation over theTertiary 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

Glaciers and Ice Sheets 24,000,000 km3

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 glaciersKm2

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. Kilimanjiro is expected to disappear between 2010 and 2020 due to global warming as the snowline rises to higher elevations.

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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.

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.

Growth of ice sheets has a dramatic impact on sea level.

18,000 years ago during the peak of the last continental glaciationsea 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.

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.

At the glacial maximum 18,000 years ago the shoreline was up to several hundred kilometres offshore of the modern shoreline.

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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.

Average global temperature today is 15 degrees C.

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.

What caused the Pleistocene continental glaciations?

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 0.6 degrees C (little chance of 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:

EccentricityObliquityPrecession

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

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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.

Slightly influences amount of sunlight in summer and winter in polar regions.

Precession: a wobble of the Earth’s rotational axis.

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.

Cold, dry winters, hot summers.

Glaciers will not develop.

Conducive to development of glaciers.

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 withhigh 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.

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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 PhanerozicEon.

It coincided with a marked decrease in O2concentration 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.

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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 EarthAn 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.

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.

How would such glaciation come to be?

Based on a climatic energy balance. Simplest case:

Net Energy Reaching Earth =

incoming radiation (I) – reflected radiation (α 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.

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Such freezing of the Earth would drastically reduce the amount of water vapor, reducing the greenhouse effect even further, causing even further cooling.

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).

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).

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.

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.

Geological phenomena that can be explained by this hypothesis.

Sedimentary deposits of alternating layers of magnetite and chert.

1. Banded Iron Formations (BIF).

An important source of Iron Ore.

Snowball Earth

Most BIF formed prior to about 2 billion years ago.

Early atmosphere lacked Oxygen.

Iron derived from weathering of rocks accumulated in solution in the oceans.

As early algae evolved, producing O2 in sea water, the iron became oxidized into an insoluble oxide and deposited as magnetite.

Other than the 2 BY old BIFs, the only other such deposits were formed between 600 and 900 million years ago….the time postulated for Snowball Earth.

How does Snowball Earth explain the BIFs?

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Under Snowball Earth the lack of life and exclusion from the atmosphere would starve the oceans of O2

Iron would be much more soluble in the absence of O2 and it would accumulate in solution.

When the glaciers and sea ice melted exposure to the atmosphere and the recovery of life would add O2 to the oceans.

The iron in solution would oxidize and deposit as magnetite and iron oxide to form the BIFs.

2. Carbon-13 in Carbonate rocks

Carbon-13 (C13) makes up 1.0% of all carbon in the atmosphere, 99% is the more common isotope, Carbon-12 (C12).

Organisms preferentially avoid incorporating C13 into tissue and skeletal material.

This increases the amount of C13 in inorganic carbon deposits (deposits not produced by organisms) in comparison to the amount of C12.

Selective uptake of C12

by organisms increases the proportion of C13

deposited with inorganic sediments.

Without organic activity proportionally less C13 is deposited.

Global glaciation would kill off almost all organisms so that organic activity would grind to a halt and C13 would be particularly low.

High levels of C13 indicates abundant organic activity.

Low levels of C13 indicates a lack of organic activity.

Sediment deposited prior to global glaciation is rich in C13.

Sediment deposited after global glaciation is depleted in C13.

Analyses of sedimentary rocks through a “snowball event” (glacial deposits) indicate a marked drop in C13 just before the event.

C13 slowly increases following the event as life becomes re-established in the oceans.

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C13 data suggest that four or five such events took place just before the Cambrian and the rapid diversification of life on Earth.

Paleontologists have often pondered the question of why a rapid diversification of organisms took place about 600 million years ago.