Evolution of the Atmosphere

10/04/06

The evolution of the atmosphere can be divided into four separate stages:

Origin
Chemical/pre-biological
Microbial era
Biological era

The composition of the present atmosphere however required the formation of oxygen to sufficient levels to sustain life, and required life to create the sufficient levels of oxygen. This era of evolution of the atmosphere is called the "Biological Era."

The Biological Era - The Formation of Atmospheric Oxygen

The biological era was marked by the simultaneous decrease in atmospheric carbon dioxide (CO2) and the increase in oxygen (O2) due to life processes. We need to understand how photosynthesis could have led to maintenance of the ~20% present-day level of O2.

The build up of oxygen had three major consequences.

Firstly, Eukaryotic metabolism could only have begun once the level of oxygen had built up to about 0.2%, or ~1% of its present abundance. This must have occurred by ~2 billion years ago, according to the fossil record. Thus, the eukaryotes came about as a consequence of the long, steady, but less efficient earlier photosynthesis carried out by Prokaryotes.

Oxygen through photolysis

Figure 1. Photolysis of water vapor and carbon dioxide produce hydroxyl and atomic oxygen, respectively, that, in turn, produce oxygen in small concentrations. This process produced oxygen for the early atmosphere before photosynthesis became dominant.

Oxygen increased in stages, first through photolysis (Figure 1) of water vapor and carbon dioxide by ultraviolet energy and, possibly, lightning:

H2O -> H + OH

produces a hydroxyl radiacal (OH) and

CO2 -> CO+ O

produces an atomic oxygen (O). The OH is very reactive and combines with the O

O + OH -> O2 + H

The hydrogen atoms formed in these reactions are light and some small fraction excape to space allowing the O2 to build to a very low concentration, probably yielded only about 1% of the oxygen available today.

Secondly, once sufficient oxygen had accumulated in the stratosphere, it was acted on by sunlight to form ozone, which allowed colonization of the land. The first evidence for vascular plant colonization of the land dates back to ~400 million years ago.

Thirdly, the availability of oxygen enabled a diversification of metabolic pathways, leading to a great increase in efficiency. The bulk of the oxygen formed once life began on the planet, principally through the process of photosynthesis:

6CO2 + 6H2O <--> C6H12O6 + 6O2

where carbon dioxide and water vapor, in the presence of light, produce organics and oxygen. The reaction can go either way as in the case of respiration or decay the organic matter takes up oxygen to form carbon dioxide and water vapor.

Life started to have a major impact on the environment once photosynthetic organisms evolved. These organisms fed off atmospheric carbon dioxide and converted much of it into marine sediments consisting of the innumerable shells and decomposed remnants of sea creatures.

Cumulative history of O2 by photosynthesis through geologic time.

While photosynthetic life reduced the carbon dioxide content of the atmosphere, it also started to produce oxygen. The oxygen did not build up in the atmosphere for a long time, since it was absorbed by rocks that could be easily oxidized (rusted). To this day, most of the oxygen produced over time is locked up in the ancient "banded rock" and "red bed" rock formations found in ancient sedimentary rock. It was not until ~1 billion years ago that the reservoirs of oxidizable rock became saturated and the free oxygen stayed in the air.  The figure illustrates a possible scenario.

We have briefly mentioned the difference between reducing (electron-rich) and oxidizing (electron hungry) substances. Oxygen is the most important example of the latter type of substance that led to the term oxidation for the process of transferring electrons from reducing to oxidizing materials. This consideration is important for our discussion of atmospheric evolution, since the oxygen produced by early photosynthesis must have readily combined with any available reducing substance. It did not have far to look!

We have been able to outline the steps in the long drawn out process of producing present-day levels of oxygen in the atmosphere. We refer here to the geological evidence.

Banded Iron Formations

When the oceans first formed, the waters must have dissolved enormous quantities of reducing iron ions, such as Fe2+. These ferrous ions were the consequences of millions of years of rock weathering in an anaerobic (oxygen-free) environment. The first oxygen produced in the oceans by the early prokaryotic cells would have quickly been taken up in oxidizing reactions with dissolved iron. This oceanic oxidization reaction produces Ferric oxide Fe2O3 that would have deposited in ocean floor sediments. The earliest evidence of this process dates back to the Banded Iron Formations, which reach a peak occurrence in metamorphosed sedimentary rock at least 3.5 billion years old. Most of the major economic deposits of iron ore are from Banded Iron formations. These formations, were created as sediments in ancient oceans and are found in rocks in the range 2 - 3.5 billion years old. Very few banded iron formations have been found with more recent dates, suggesting that the continued production of oxygen had finally exhausted the capability of the dissolved iron ions reservoir. At this point another process started to take up the available oxygen.

Red Beds

Once the ocean reservoir had been exhausted, the newly created oxygen found another large reservoir - reduced minerals available on the barren land. Oxidization of reduced minerals, such as pyrite FeS2 , exposed on land would transfer oxidized substances to rivers and out to the oceans via river flow. Deposits of Fe2O3 that are found in alternating layers with other sediments of land origin are known as Red Beds, and are found to date from 2.0 billion years ago. The earliest occurrence of red beds is roughly simultaneous with the disappearance of the banded iron formation, further evidence that the oceans were cleared of reduced metals before O2 began to diffuse into the atmosphere.

When the red bed reservoir became exhausted too (although it is continually being regenerated through weathering) and oxygen finally started to accumulate in the atmosphere itself. This signal event initiated eukaryotic cell development, land colonization, and species diversification.  The oxygen built up to today's value only after the colonization of land by green plants, leading to efficient and ubiquitous photosynthesis. The current level of 20% seems stable.

The Oxygen Concentration Problem.

Why does present-day oxygen sit at 20%? This is not a trivial question since significantly lower or higher levels would be damaging to life. If we had < 15% oxygen, fires would not burn, yet at > 25% oxygen, even wet organic matter would burn freely.

The Early Ultraviolet Problem

The genetic materials of cells (DNA) is highly susceptible to damage by ultraviolet light at wavelengths near 0.25 µm. It is estimated that typical contemporary microorganisms would be killed in a matter of seconds if exposed to the full intensity of solar radiation at these wavelength. Today, of course, such organisms are protected by the atmospheric ozone layer that effectively absorbs light at these short wavelengths, but what happened in the early Earth prior to the significant production of atmospheric oxygen? There is no problem for the original non-photosynthetic microorganisms that could quite happily have lived in the deep ocean and in muds, well hidden from sunlight. But for the early photosynthetic prokaryotes, it must have been a matter of life and death.

It is a classical "chicken and egg" problem. In order to become photosynthetic, early microorganisms must have had access to sunlight, yet they must have also had protection against the UV radiation. The oceans only provide limited protection. Since water does not absorb very strongly in the ultraviolet a depth of several tens of meters is needed for full UV protection. Perhaps the organisms used a protective layer of the dead bodies of their brethren. Perhaps this is the origin of the stromatolites - algal mats that would have provided adequate protection for those organisms buried a few millimeters in. Perhaps the early organisms had a protective UV-absorbing case made up of disposable DNA - there is some intriguing evidence of unused modern elaborate repair mechanisms that allow certain cells to repair moderate UV damage to their DNA. However it was accomplished, we know that natural selection worked in favor of the photosynthetic microorganisms, leading to further diversification.

Fluctuations in Oxygen

The history of macroscopic life on Earth is divided into three great eras: the Paleozoic, Mesozoic and Cenozoic. Each era is then divided into periods. The latter half of the Paleozoic era, includes the Devonian period, which ended about 360 million years ago, the Carboniferous period, which ended about 280 million years ago, and the Permian period, which ended about 250 million years ago.

According to recently developed geochemical models, oxygen levels are believed to have climbed to a maximum of 35 percent and then dropped to a low of 15 percent during a 120-million-year period that ended in a mass extinction at the end of the Permian. Such a jump in oxygen would have had dramatic biological consequences by enhancing diffusion-dependent processes such as respiration, allowing insects such as dragonflies, centipedes, scorpions and spiders to grow to very large sizes. Fossil records indicate, for example, that one species of dragonfly had a wing span of 2 1/2 feet.

Geochemical models indicate that near the close of the Paleozoic era, during the Permian period, global atmospheric oxygen levels dropped to about 15 percent, lower that the current atmospheric level of 21 percent. The Permian period is marked by one of the greatest extinctions of both land and aquatic animals, including the giant dragonflies. But it is not believed that the drop in oxygen played a significant role in causing the extinction. Some creatures that became specially adapted to living in an oxygen-rich environment, such as the large flying insects and other giant arthropods, however, may have been unable to survive when the oxygen atmosphere underwent dramatic change.

Structure of the Atmosphere

In large measure, the atmosphere has evolved in response to and controlled by life processes. It continues to change as a consequence of human activities, but at a rate that is far in excess of the rate of previous evolutionary change. The atmosphere controls the climate and ultimately determines the quality of life on Earth. We will begin our discussion with a brief review of the composition and structure of the present-day atmosphere. Then we will discuss the major events in the evolution of the atmosphere that led to its current state. We will discuss some important tools along the way that will prove useful in many settings.

As discussed earlier, the ground heats up due to the absorption of visible light from the Sun. The warm ground, in turn, heats the atmosphere via the processes of conduction, convection (turbulence) and infrared radiation. As we move upwards from the ground, we might expect temperature to drop off according to the R-squared law. This happens (more or less) for a while, but the declining thermal structure reverses at the tropopause and increases to a new maximum at the stratopause. In the mesosphere, the temperature drops to the lowest values seen anywhere in the atmosphere. Above the mesosphere, the temperature rises again in the thermosphere. Eventually, the temperature reaches a maximum value at very high altitudes (see Figure above).

Thermal structure of the atmosphere from 0 to 1000 km. The warmer regions are heated by different parts of the Sun's radiative output.

The reason for the strange-looking temperature profile is quite simple. Regions of high temperature are heated by different portions of the solar radiative output.

The stratosphere is heated by the absorption of ultraviolet (UV) light by ozone.

The thermosphere is heated by the absorption of extreme ultraviolet (EUV) light by other atmospheric constituents (primarily molecular and atomic oxygen and molecular nitrogen).

In this course we are mostly concerned with the troposphere--the region where we live--and its variations. The stratosphere, however, also plays a major role in global change and evolution, as we will soon see. Although some scientists think the mesosphere and thermosphere might also play key roles in the story of global change, the question is still not resolved.

We have so far only considered the vertical variation of temperature. Other atmospheric variables also vary with altitude. Since the atmosphere is a gaseous envelope, it is compressible. This means that density and pressure both decrease exponentially with altitude. The lowest regions are weighed down by the mass of the overlying atmosphere, becoming compressed and therefore more dense. Jet aircraft flying in the low-density stratosphere have to pressurize their hulls due to the compressibility of the atmosphere. Figure 3 shows the variation of density and pressure with altitude.

The temperature profile shown in Figures 1 and 2 plays a significant role in controlling atmospheric turbulence. We all know that the troposphere is a turbulent place to live: we experience wind gusts, cloud formation and severe weather. The fact that temperature drops with altitude in the troposphere leads to atmospheric instability. In the stratosphere, on the other hand, the temperatures rise with altitude, leading to a very stable region. It is for this very reason that we are able to drink cups of coffee in jet aircraft. One of the many gifts showered on us by ozone is the ability to fly commercial aircraft in relative comfort!


Composition of the Present Atmosphere

The overall composition of the earth's atmosphere is summarized below along with a comparison to the atmospheres on Venus and Mars - our closest neighbors.

  VENUS EARTH MARS
SURFACE PRESSURE 100,000 mb 1,000 mb 6 mb
  COMPOSITION
CO2 >98% 0.03% 96%
N2 1% 78% 2.5%
Ar 1% 1% 1.5%
O2 0.0% 21% 2.5%
H2O 0.0% 0.1% 0-0.1%
 
(more on Mars)

(more on Earth)

(more on Mars)

The variations in concentration from the Earth to Mars and Venus result from the different processes that influenced the development of each atmosphere. While Venus is too warm and Mars is too cold for liquid water the Earth is at just such a distance from the Sun that water was able to form in all three phases, gaseous, liquid and solid. Through condensation the water vapor in our atmosphere was removed over time to form the oceans. Additionally, because carbon dioxide is slightly soluble in water it too was removed slowly from the atmosphere leaving the relatively scarce but unreactive nitrogen to build up to the 78% is holds today.

Current Composition

The concentrations of gases in the earth atmosphere is now known to be (ignoring water vapor, which varies between near zero to a few percent):

CONSTITUENT CHEMICAL SYMBOL MOLE PERCENT
Nitrogen N2 78.084
Oxygen O2   20.947
Argon Ar   0.934
Carbon Dioxide CO2   0.035
Neon Ne   0.00182
Helium He   0.00052
Methane CH4   0.00017
Krypton Kr   0.00011
Hydrogen H2   0.00005
Nitrous Oxide N2O   0.00003
Xenon Xe   0.00001
Ozone O3   trace to 0.00080

The unit of percentage listed here are for comparison sake. For most atmospheric studies the concentration is expressed as parts per million (by volume). That is, in a million units of air how may units would be that species. Carbon dioxide has a concentration of about 350 ppm in the atmosphere (i.e. 0.000350 of the atmosphere or 0.0350 percent).

Global Warming Potential

The Global Warming Potential (GWP) of a greenhouse gas is the ratio of global warming, or radiative forcing both direct and indirect from one unit mass of a greenhouse gas to that of one unit mass of carbon dioxide over a period of time. Hence this is a measure of the potential for global warming per unit mass relative to carbon dioxide.
Global Warming Potentials are presented for an expanded set of gases. GWPs are a measure of the relative radiative effect of a given substance compared to CO2, integrated over a chosen time horizon. New categories of gases include fluorinated organic molecules, many of which are ethers that are proposed as halocarbon substitutes. Some of the GWPs have larger uncertainties than that of others, particularly for those gases where detailed laboratory data on lifetimes are not yet available.

The direct GWPs have been calculated relative to CO2 using an improved calculation of the CO2 radiative forcing, the SAR response function for a CO2 pulse, and new values for the radiative forcing and lifetimes for a number of halocarbons. Indirect GWPs, resulting from indirect radiative forcing effects, are also estimated for some new gases, including carbon monoxide. The direct GWPs for those species whose lifetimes are well characterized are estimated to be accurate within 35%, but the indirect GWPs are less certain.
 
Direct Global Warming Potentials (GWPs) relative to carbon dioxide (for gases for which the lifetimes have been adequately characterized). GWPs are an index for estimating relative global warming contribution due to atmospheric emission of a kg of a particular greenhouse gas compared to emission of a kg of carbon dioxide. GWPs calculated for different time horizons show the effects of atmospheric lifetimes of the different gases.
    Lifetime Global Warming Potential
    (years) (Time Horizon in Years)
 GAS     20 yrs 100 yrs 500 yrs
Carbon Dioxide CO2
 
1
1
1
Methane CH4
12.0
62
23
7
Nitrous Oxide N2O
114
275
296
156
Chlorofluorocarbons
 
 
 
 
CFC-11  
55
4500
3400
1400
CFC-12  
116
7100
7100
4100
CFC-115  
550
5500
7000
8500
Hydrofluorocarbons
 
 
 
 
HFC-23 CHF3
260
9400
12000
10000
HFC-32 CH2F2
5
1800
550
170
HFC-41 CH3F
2.6
330
97
30
HFC-125 CHF2CF3
29
5900
3400
1100
HFC-134 CHF2CHF2
9.6
3200
1100
330
HFC-134a CH2FCF3
13.8
3300
1300
400
HFC-143 CHF2CH2F
3.4
1100
330
100
HFC-143a CF3CH3
52
5500
4300
1600
HFC-152 CH2FCH2F
0.5
140
43
13
HFC-152a CH3CHF2
1.4
410
120
37
HFC-161 CH3CH2F
0.3
40
12
4
HFC-227ea CF3CHFCF3
33
5600
3500
1100
HFC-236cb CH2FCF2CF3
13.2
3300
1300
390
HFC-236ea CHF2CHFCF3
10
3600
1200
390
HFC-236fa CF3CH2CF3
220
7500
9400
7100
HFC-245ca CH2FCF2CHF2
5.9
2100
640
200
HFC-245fa CHF2CH2CF3
7.2
3000
950
300
HFC-365mfc CF3CH2CF2CH3
9.9
2600
890
280
HFC-43-10mee CF3CHFCHFCF2CF3
15
3700
1500
470
Fully fluorinated species
 
 
 
 
SF6  
3200
15100
22200
32400
CF4  
50000
3900
5700
8900
C2F6  
10000
8000
11900
18000
C3F8  
2600
5900
8600
12400
C4F10  
2600
5900
8600
12400
c-C4F8  
3200
6800
10000
14500
C5F12  
4100
6000
8900
13200
C6F14  
3200
6100
9000
13200
Ethers and Halogenated Ethers
 
 
 
 
CH3OCH3  
0.015
1
1
<<1
HFE-125 CF3OCHF2
150
12900
14900
9200
HFE-134 CHF2OCHF2
26.2
10500
6100
2000
HFE-143a CH3OCF3
4.4
2500
750
230
HCFE-235da2 CF3CHClOCHF2
2.6
1100
340
110
HFE-245fa2 CF3CH2OCHF2
4.4
1900
570
180
HFE-254cb2 CHF2CF2OCH3
0.22
99
30
9
HFE-7100 C4F9OCH3
5
1300
390
120
HFE-7200 C4F9OC2H5
0.77
190
55
17
H-Galden 1040x CHF2OCF2OC2F4OCHF2
6.3
5900
1800
560
HG-10 CHF2OCF2OCHF2
12.1
7500
2700
850
HG-01 CHF2OCF2CF2OCHF2
6.2
4700
1500
450

Explore the absorption of individual atmospheric gases and the atmosphere as a whole.

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The Atmosphere in Motion

One of the more important factors for climate is the global wind system. Winds are driven into motion by forces on the air. There is a system of prevailing winds whose purpose it is to transfer the excess energy received at low latitudes to high latitudes. If the earth did not rotate and did not have any continental land masses, then the wind system would be rather simple.

Figure 6. Wind patterns of the world for (A) a hypothetical world with no rotation,
(B) the world with rotation, and (C) the resulting bands of general circulation.

The excess heat received in the equatorial region would cause the air to rise and blow away towards higher latitudes. In order for air to be conserved, the outward motion at high latitudes near the equator has to be balanced by inward low altitude winds. This system is a huge twin-cell circulation pattern (one cell in each atmosphere). These idealized cells are called Hadley Cells.

Because the earth rotates and has continental land masses, the actual prevailing winds do not directly blow from pole to equator as in but rather curve around and form a multicellular circulation pattern. The curving form the initial direction of the winds is called the "Coriolis effect" and is due to rotation. The curvature is so great as to split up each Hadley cell into three smaller cells.

The highest temperatures occur in the subtropical deserts, e.g., the African Sahara. The lowest mean temperatures occur in Antarctica, where the Sun is either below the horizon or too low in the sky to effectively warm the surface.

It is interesting to note here an important feedback process that can occur at high latitudes. In very cold regions, such as Antarctica, water can only exist as ice and snow. Both these forms of water are very good reflectors of visible light (~80% of visible light gets reflected) and therefore the production of ice and snow tends to depress temperatures by reflecting much of the potentially warming sunlight back to space! This is a positive feedback mechanism - cold temperatures lead to ice - leads to colder temperatures - leads to more ice, etc.


Natural Climate Change

We believe that the temperature of the earth has varied wildly over the evolution of the earth. Figure 1 shows an estimate of temperature changes as complied by Scotese. So how can it be that the climate has changed so over the ages and what processes could lead to these changes?

Figure 1. Estimated changes in global temperature.

The processes for changing climate naturally include:

  1. Plate reorganization
  2. CO2 and Volcanic eruptions
  3. Solar Variations, and
  4. Orbital Variations

Plate Tectonics

The movement of the continents has obviously influenced the climate at specific locations (Figure 2), but could also influence the global temperature by redistributing the collection of solar radiation and/or providing land masses on which continental glaciers could form.

Figure 2. Location of continents during the Devonian period from Scotese.

Volcanic Eruptions

The amount and location of matgerial added to the atmosphere by volcanoes probably has significantly influenced climate over the ages. Valcanos emit some greenhouse gases like CO2 and H2O, but also emit SO2 that can get trapped in the upper parts of the atmosphere where it will react to form sulfates, a small particle. These particles can reflect incoming radiation to lower the surface temperature.

Figure 3. NOAA monitors the amount or particles (aerosols) in the atmosphere. Note aerosols over northern and southern Africa. Click here for a current image.

Solar Variability

Variations in sunspot activity result in changes on the order of 0.1% to 0.2 over 11 year cycle. Numerical climate models predict that a change in solar output of only 0.5% per century could alter the Earth's climate.

Figure 4. The changing amount of the solar surface covered by the biggest sunspots. (Courtesy Rich J Niciejewski, U. Mich)

Orbital Variability

The Earth's orbit changes over time in ways that could influence the amount of energy received at the surface. Thes e include changes in eccentricity, precession of the equinox, and changes in the Earth's tilt (obliquity).

Eccentricity
The eccentricity of the Earth's oprbit changes with a period of 100,000 years. At the moment the Earth's orbit is fairly circular but in 50,000 years it will be more eccentric with the difference between aphelion (farthest) and perihelion (nearest) points in the orbit will become larger.
Precession of the Equinox
Also over time the time of year when the Earth reaches perihelion changes. Now the Earth is closest the sun in January and faerthest in July. The combination of changing eccentricity and precession of the equinox leads to changing available solar radiation.
Obliquity
Finally, the Earth wobbles on its axis of rotation changing the tilt of the Earth (and hence its seasonality) over a 41,000 year period. The tilt is now 23.5 but changes between 22.5 and 24.5.

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