Evolution of the Atmosphere

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."

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

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:

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.

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.

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.

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).

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.

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):

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).

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.

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?

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.

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�.

	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)

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