ASTR 1210 (O'Connell) Study Guide

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19. EVOLUTION OF THE TERRESTRIAL ATMOSPHERES

Scaled photos of the terrestrial planets.

Why are the atmospheres of the terrestrial planets so astonishingly different from one another? How have their evolutionary paths diverged? Given the facts (1) that manmade materials are beginning to affect Earth's atmosphere and (2) that small changes can make big differences, this is not merely an academic question. It is essential to improve our understanding of atmospheric and climate evolution as quickly as possible.

In the case of the Earth's Moon and Mercury, which have no appreciable atmospheres, the answer is easy: their gravity is too small to retain rapidly moving gas molecules near their surface, which therefore diffuse off into space.

In the case of Venus, Earth, and Mars, we do not yet have a full understanding of their atmospheric histories over the last 4 billion years, but we have identified the main natural processes involved and the likely patterns of evolution.

After we cover the natural long-term changes in the terrestrial atmospheres, we address the evidence that human activities are having a measurable effect on the heat content of Earth's atmosphere and oceans on a few-hundred-year period.

A. Venus, Earth, Mars: No Two Alike

	VENUS	EARTH	MARS
Relative Planet Mass	0.8	1.0	0.1
Relative Distance from Sun	0.7	1.0	1.5
Relative Atmospheric Mass	100	1.0	0.01
Bulk Atmospheric Composition	CO2	N2, O2	CO2
Relative Water Vapor	0.0001	1.0 [1%]	0.03
Mean Surface Temperature	460oC	20oC	-60oC

• As we have seen in discussing the individual terrestrial planets, there are huge differences in atmospheric mass, composition, and surface temperature despite a modest range of distances (only a factor of 2) from the Sun and similar masses, at least for Venus & Earth.
  There is a 10,000:1 range in the mass of the atmospheres of the three planets. Mars has one-tenth the Earth's total mass but an atmosphere 100 times smaller. Venus has an atmospheric mass 100 times larger than the Earth despite being a slightly smaller planet. The atmospheric pressure at the surface of Venus is also 100 times that on Earth (or the same as the water pressure in a terrestrial ocean at a depth of 3300 feet).
  Earth has a nitrogen-oxygen atmosphere, but both Venus and Mars have carbon dioxide atmospheres.
  Earth has a water-rich atmosphere, in which 1% of atmospheric gases are water molecules. But both Mars and Venus have extremely dry atmospheres. Mars has proportionately 30x fewer atmospheric water molecules than Earth; and Venus is 300x poorer in water than Mars.
  • There are enormous temperature differences. The "Goldilocks" syndrome for lifeforms like us: Venus is much too hot; Mars is too cold; but Earth is "just right".

• The "habitable zone" for Earthlike life is the region surrounding a star within which liquid water can be stable on planetary surfaces. Given the temperatures quoted above, the HZ for the Sun is evidently small: about 0.9-1.4 AU. At present it contains no planet other than the Earth.
  The Sun's HZ is a tiny fraction of the total volume of the Solar System, whose planetary orbits lie in the range 0.4 to beyond 40 AU. (We discuss habitable zones further in Guide 23.)

B. Geophysical Atmospheric Processes

Many different geophysical processes affect atmospheres, acting to augment, decrease, or change their contents. Important examples:

• Energy input from the Sun
  Solar radiation, across the entire electromagnetic spectrum from the ultraviolet to the far-infrared, has a dominant effect on planetary atmospheres. It is the primary source of energy input for the terrestrial atmospheres. The most important changes in solar radiation output occur on billion-year timescales as the solar interior adjusts to the accumulation of burnt fuel in the form of helium atoms. That has caused slow brightening of the sun over the past 4 billion years.
  Activity on the surface of the Sun (e.g. sunspots and solar flares) associated with strong magnetic fields changes over an 11-year cycle, but the mean changes in total radiation output during the cycle are small, only about 0.1%.

• Infall of interplanetary material (protoplanetary solids, comets, meteoroids)
  Infall, plus gases captured gravitationally directly from the solar nebula, was the source of the earliest or "primordial" atmospheres of the terrestrial planets but has probably not been important in the last 3.5 billion years.

• Volcanic outgassing (see picture above)
  Volcanic activity, driven by high internal temperatures, releases H₂O, CO₂, SO₂, and many other compounds from the interior into the atmosphere. This was the dominant source of the present (or "secondary") terrestrial atmospheres and of the oceans on Earth.

• Escape of gases to space and stripping by the solar wind
  Any molecule near the top of an atmosphere with a velocity higher than a planet's gravitational escape velocity (see Study Guide 8) can escape to space and not return. As a result of collisions with other molecules, a small fraction of each kind of molecule will be moving faster than escape velocity at a given time. The smaller the mass of the molecule, and the higher the temperature of the atmosphere, the larger is the fraction of molecules that can achieve escape.
  
  See the figure at the right (click for enlargement). The solid symbols show the escape velocities for various planets as a function of atmospheric surface temperature. The dashed lines show velocities for molecules of different species of gas at the top of their atmospheres. Any species whose velocity lies above the symbol for a given planet will escape to space over time. The four largest planets (in red) retain all molecular species. Earth and Venus retain all but hydrogen and helium.

  The diagram shows that most important molecules have already escaped from the Moon and Mercury, which are effectively without atmospheres. This is also true for most of the other small bodies in the solar systems (like the asteroids & the satellites of the outer planets), though it is easier to retain an atmosphere where the temperature is lower (as in the case of Saturn's satellite Titan).
  The thin but high-speed "solar wind," a continuous outflow of gas from the Sun, can also strip molecules if it can impinge on the atmosphere. A planet's "magnetosphere" can protect it from the wind, and the Earth, where circulation in the hot interior maintains a strong magnetic field, is shielded this way. But Mars' magnetosphere diminished as its interior cooled off and allowed much of the Martian atmosphere to be stripped off.

  
• "Carbonate-silicate cycle"
  See the illustration above. This cycle involves a transfer of carbon between the Earth's surface, its atmosphere, and its crust.
  CO₂ vapor is washed out of the atmosphere by liquid H₂O precipitation and is bound through chemical reactions involving silicate rocks into solid limestone on the seabeds. Deposition over the last billion years has been assisted by sea animals leaving their shells behind.
  Recycling of the crust into the interior melts the rocks and returns CO₂ to the atmosphere through volcanic outgassing.
  The net effect of the cycle depends on the relative strengths of the deposition and return branches, and in turn these depend on other environmental factors. Throughout most of Earth's history the deposition rate has been larger than the return rate, so this process has tied up massive amounts of CO₂ in terrestrial surface rocks (about 100 times the current atmosphere's mass).

• Photo-Destruction of H₂O
  UV radiation dissociates H₂O molecules in the upper atmosphere, allowing H to escape to space (as described above). O₂ combines with other molecules, and the water is lost. Since almost all water is trapped at low altitudes in Earth's atmosphere, this is not an important process there; but it has had a major effect on Venus.

  
• The Greenhouse Effect
  Absorption of sunlight is the primary source of heat for the atmosphere and surface of the terrestrial planets. But secondary heating of the surface and lower atmosphere is caused by the partial trapping of infrared radiation from the planet's surface by certain gases (especially H₂O, CO₂, CH₄, O₃). See the diagram above.
  Even though these are only "trace gases" in Earth's atmosphere (for instance, CO₂ constitutes only 0.04% of the volume of the atmosphere), they account for almost all of the infrared blocking of radiation from the Earth's surface. Hence, they constitute a powerful "choke-point" in the flow of atmospheric heat, so they have a major effect on the temperature balance. For more details on this "Greenhouse Effect," see the diagram above (click for enlargement) and Study Guide 15.

  The diagram below shows the relative heat flows (averaged over a year) for direct and trapped solar radiation. Note that infrared radiation reflected by the Greenhouse Effect provides almost twice as much heat input on average for the Earth's surface as does direct solar radiation. The large contribution is due in part to the fact that Greenhouse heating occurs during all seasons and at night as well as during the day.

  

C. Equilibrium

The cycle rates for geophysical processes affecting the atmosphere can be very fast in geological time:

For example, in 50 Myr, which is only 1% of Earth's age...
• 1 "bar" (= the present mass of Earth atmosphere) of CO₂ can be outgassed from Earth's interior
  1 bar is 2500x the present CO₂ content of Earth's atmosphere.

• 1 bar of CO₂ can be washed out of Earth's atmosphere by precipitation of CO₂-bearing liquid water or ice

• 1 ocean of H₂O can be evaporated & dissociated on Venus

Key concept: the characteristics of an atmosphere are determined by the balance point or "equilibrium" among all processes.

If the rates don't balance, there would be relatively rapid evolution toward more extreme conditions until a new balance is reached.
All significant processes must be considered. In complex systems like planetary atmospheres it is easy to overlook important factors.

Feedback mechanisms are critical: they can stabilize the system ("negative feedback") or accelerate change ("positive feedback")

Example: the carbonate-silicate cycle provides negative feedback to help regulate the temperature of Earth's surface. If the atmospheric temperature rises, there will be increased evaporation from the oceans, which washes more CO₂ from the atmosphere, which decreases the Greenhouse trapping and hence the temperature. If the temperature falls, the reverse situation occurs. This stabilizing feedback occurs only over periods which are long by human standards (about 400,000 years), however, and is never able to keep the climate perfectly stable. But it helps prevent a runaway Greenhouse on the one hand or a permanent ice age ("snowball Earth") on the other.

D. Three Histories

For the Earth, we have a huge fund of geophysical data based mainly on atmospheric and ocean composition, surface geology, and cores bored in ice and sediments from which we can infer its earlier history. Our knowledge of Venus and Mars is much more limited, but the main historical events can be pieced together.

We are not sure whether the terrestrials had significant primordial atmospheres, but these would have been rich in hydrogen gas, most of which was able to escape their low gravities. Early energetic winds from the forming Sun would likely have stripped the remainder. The existing ("secondary") atmospheres of Earth, Venus, and Mars were predominantly outgassed from the interior, amounting to probably 100 bars on Earth and Venus but less on Mars.

The outgassed material consisted mainly of H₂O, CO₂, and SO₂. Fresh outgassing occurs on Earth today and probably to some extent on Venus (though without major eruptions); there is none on Mars. An alternative subsidiary source of water and atmospheric gases: comet impacts.

Earth: "It's the water"....

• Earth's distance from the Sun is "just right." A moderate Greenhouse, driven by the four main Greenhouse gases, elevated the average surface temperature about 30^oC (54^oF) to above the freezing point of water. This much Greenhouse warming was essential to having a robust biosphere on Earth.

• Earth's mean surface temperature has been in the range such that outgassed water stays liquid near the surface.

• There is a "cold trap" at modest altitude in Earth's atmosphere where the temperature drops below the freezing point of water. (See this temperature profile.) Because of the trap, rising water vapor condenses into droplets or ice crystals and ultimately falls back to the surface. This prevents escape of water to the stratosphere and therefore destruction by solar UV radiation. Water is retained near the surface. The oceans condensed from water vapor outgassed by volcanos.

• Water vapor washed most CO₂ out of the atmosphere, depositing it in minerals on the ocean floors. The bulk CO₂ content has continued to decline over the last 500 million years, though there have been occasional large excursions for reasons that aren't entirely understood. The remaining gaseous CO₂ constitutes only about 0.04% of the bulk atmosphere today.

• With water vapor condensed into oceans and CO₂ scrubbed from the atmosphere, outgassed nitrogen became the dominant gas in the atmosphere, although its total mass is far smaller than the total original complement of outgassed material.

• The oceans provided the environment necessary for life to develop about 3-3.5 billion years ago. Organisms such as blue-green algae capable of photosynthesis then processed atmospheric gases to release free oxygen (O₂).
  The Earth is the only planet where lifeforms have had an important effect on the atmosphere.
  By about 2 billion years ago, significant amounts of oxygen were present in the atmosphere. The oxygen density then rapidly increased about 500 Myr ago, coinciding with the "Cambrian explosion" in the diversity of lifeforms which thrive on oxygen. Free oxygen is the most important global tracer of the presence of Earth-like life. The oxygenation of Earth's atmosphere also significantly changed the kinds of minerals present on its surface.

• Plate tectonics on Earth have had important effects on the atmosphere because they redistribute ocean and atmospheric circulation, change the nature of exposed soils, and change the volume of volcanic outgassing over periods of 50 million years and longer.

• During most of the time since the atmosphere formed, the mean Earth surface temperature has been hotter than at present. However, there is some evidence for one or more extended cold periods so intense that almost the entire surface was covered in ice -- a so-called "snowball Earth". The most recent of these was about 700 million years ago. Once ice is abundant enough, it will reinforce cooling because it reflects solar radiation. The snowball phase was probably terminated by the accumulation of outgassed CO₂ from volcanos and subsequent Greenhouse heating.

• Over the past 500 million years, the Earth's mean surface temperature has varied within a relatively narrow band of about +/- 10^o C (see this chart). (Probably not coincidentally, life has flourished in the oceans, on the land, and in the air during that time.) There have been cool periods up to 100 million years long favoring significant ice coverage, but most of the time polar ice has been minimal or non-existent. The latest 50 million years of history is described below.

Venus

• Because it is nearer the Sun and had a hotter surface, liquid H₂O deposited by outgassing readily evaporated from its surface.

• Because of the higher temperatures, there was no cold trap. This allowed water vapor molecules to rise to high altitudes in the stratosphere, where they were destroyed by solar UV light.

• The absence of the carbonate-cycle cleansing by liquid H₂O precipitation permitted the outgassed CO₂ atmosphere to grow.

• The heavy H₂O and CO₂ atmospheric blankets trapped infrared radiation from the surface ==> T rose further ==> more liquid water evaporated ==> Greenhouse trapping increased.
  • This positive feedback produced a "runaway" greenhouse effect

  • All water was eventually removed from the atmosphere

  • The end product of the runaway is the massive, hot, dry, CO₂ atmosphere Venus has today.

• Continuous volcanic outgassing of materials like sulfur dioxide in the absence of water cleansing produced sulfuric acid clouds

• Without its abundant water, Earth would probably be like Venus. In fact, if the Earth as it is today were moved to Venus' orbit, it would probably evolve quickly toward the hellish Venusian state.

Mars

• Mars was more distant from Sun and therefore colder.

• Its smaller mass supported less outgassing from its interior and therefore a smaller Greenhouse Effect.

• Early, wet atmosphere & oceans? There is considerable evidence supporting this interpretation. The water era probably ended over 1 billion years ago. See Study Guide 16 and the Mars image page for more discussion.

• An early biosphere? There is some, but marginal, evidence in favor. See Study Guide 17.

• But Mars is a small planet compared to Earth. It has a large surface area compared to its volume, and therefore its interior cooled off more quickly. Sufficient CO₂ was not resupplied from the interior; so the Greenhouse failed. Water eventually froze out and was presumably deposited in subsurface permafrost.

• The lack of an active interior also left Mars with a weak magnetic field, allowing the solar wind to strip away much of its protective atmosphere.

• Atmospheric pressure at the surface is now too low to prevent the evaporation of liquid water if any appeared.

• The end product is a thin, cold, dry atmosphere with only a trace of water vapor.

• Mars presents a harsh surface but one where, with difficulty, human colonies could live. By contrast, we could never live on the Venusian surface.

E. Lessons Learned for Atmospheric Evolution

1. Little differences can have huge consequences
   Our favorable environment is due mainly to our distance from the Sun and secondarily to the size of our planet.

2. Biospheres are fragile on Earth-like planets

F. Climate Change: Natural and Unnatural

So far, we have discussed the bulk properties of the terrestrial atmospheres: mass and composition as they change over hundreds of millions or billions of years. In this section, we consider recent atmospheric changes on successively shorter timescales.

"Climate" refers to the behavior of surface temperature, precipitation, and wind flow over the short timescales (10's to 100's of years) of interest to human beings. Climate changes on the Earth have major practical consequences for us.

The central question that has emerged, scientifically and politically, in the last 40 years is whether human activities are contributing to recent climate changes. It is obvious that the rapid (exponential!) growth of the human species (see Study Guide 9) coupled with our use of technology will inevitably affect Earth's atmosphere unless we take deliberate actions to avoid this. The question is whether that has started to happen yet.
There is no doubt that some human-induced changes have begun, with the partial destruction of the ozone (O₃) layer, which shields Earth's surface from solar UV radiation. But what about more consequential changes?

There are two distinct branches of the study of climate change: measurements of the temperature and composition histories of Earth's atmosphere and oceans and modeling of those histories so that their future properties can be realistically predicted.

Temperature History and Ice Ages

According to the empirical geological record, there have been five epochs of extensive glaciation on the Earth over the past 3 billion years. We are living in the latest of those. From a high mean temperature about 15^oC warmer than now about 50 million years ago, the atmosphere has cooled and precipitated a major glaciation epoch. By 34 million years ago the temperature had fallen to the point that the Antarctic ice sheets formed. The cooling continued to the lower levels that have prevailed for the last 8 million years.

The most recent era of glaciation began about 2.6 million years ago and consists of a repeating cycle of cooler and warmer periods, with a temperature excursion of about 10^oC. During the drop in the mean surface temperature in the cool periods, there are great expansions of the ice sheets from the poles toward the equator. These are the famous "ice ages". During the warmer periods, the ice retreats but never vanishes altogether. The ice ages typically last about 80,000 years, while the warmer "interglacial" periods last only about 20,000 years. These cycles are the most conspicuous climate events in recent geological history.

The most recent ice age ended about 11,000 years ago, and at that time we entered an interglacial period that will last perhaps another 10,000 years. Temperature and ice volume histories covering the last half-million years, including 5 ice ages, are shown in the plot below. [Note that the ice volume scale is reversed from the temperature scales.] These are derived from drilling ice and ocean sediment cores up to 2 miles deep. You can see that a drop in the mean surface temperature of only about 3^oC was sufficient to precipitate an ice age.

The small temperature changes that trigger ice ages are importantly influenced by the astronomical "Milankovitch Cycles" (see Study Guide 4) -- small changes in the shape and orientation of Earth's orbit and the tilt of its axis, which affect the amount of insolation in the polar regions. The components of the Milankovitch cycles have periods between 25,000 and 100,000 years. The cycles are mediated by feedback from the carbonate-silicate mechanism. All of the changes and forcing mechanisms discussed so far have taken vastly longer to transpire than the recent warming described in the next section.

Recent Global Warming and Carbon Dioxide

Intensive studies have also been made of the Earth's temperature history over the past 1000 years. Except for the period since 1900, such studies must rely on the use of various "proxies" for actual thermometric measures. The profiles show several major climate events: a "medieval warm period" (about 1000 AD) and a "little ice age" cool period (about 1600 AD).

But the most important change observed in the last millennium is a rapid increase in Earth's mean surface temperature since 1900. Click on the thumbnails below for enlarged plots of recent changes in the surface temperature. An animation of global surface temperature changes on a monthly basis since 1850 is shown here.

Surface temperature since 1880

Surface temperature since 800

Remember, we're talking about global surface temperature here, meaning averages over the entire surface of the Earth, including the oceans, the southern hemisphere, night and day, and through four seasons. So our local weather is only a tiny component of the whole. For instance, there were abnormally cold temperatures in the eastern US in January 2014, but that was, globally, the fourth warmest January on record since 1880.
There are large variations in warming depending on geographic location, as seen in the map below showing worldwide temperature trends between 1976 and 2000. Here is a more recent map showing temperature changes for August 2016 with respect to mean temperatures for August between 1951 and 1980.
The current temperature excesses are quite striking: seven of the ten globally warmest years since 1880 have been in this decade, and all have occurred since 1998.

The plots above refer to the temperature in the lowest layers of the Earth's atmosphere. There are many other geophysical markers also showing global heating. Changes in seven important indicators over 50-100 years are shown here.

Perhaps of greatest long-term consequence for the climate is the increasing heat content of the oceans, which absorb about 90% of the total additional energy. (A layer of ocean water only 11.5 feet thick contains as much heat as the entire atmosphere.) The chart below shows the huge rise in ocean heat content that has taken place since 1960. The ocean heat content has reached new record levels in each of the four years 2019-2022.

The increased ocean heat, together with associated ice-melt runoff, has produced a detectable increase in mean sea level, which is beginning to cause flooding along vulnerable coastlines, including some large cities (e.g. Miami, Venice) and island nations.

The accumulated empirical evidence, from all of these different indicators, presents an indisputable case for global warming continuing to the present. And this evidence is no longer disputed, as it might have been 15 years ago. One formerly contrarian group in the UC Berkeley physics department has independently reanalyzed surface temperature records and confirmed the earlier published trends, as shown above.

The warming has coincided with a rapid increase in the average atmospheric concentration of carbon dioxide, which was discovered through independent spectroscopic studies of atmospheric composition at Mauna Loa observatory in Hawaii (see left hand panel below). The present CO₂ concentration has no precedent in the recent geological record. It is 60% higher than the average over the preceding 600,000 years (and 23% higher than the maximum); see the red line in the right hand panel below.

The rapid CO₂ increase cannot be attributed to natural cycles. There is no question that the increase is due instead to human use of fossil fuels.

CO2 Concentration since 1960

CO2 Concentration over 600,000 years

Climate Modeling

How are we to interpret these changes, and is there a link between global heating and human use of fossil fuels and the consequent rise in CO₂ concentration?

The basic physical principles that govern the structure of planetary atmospheres have been well understood for a century. In fact, the first prediction that industrialization could lead to sufficient CO₂ production to affect the Earth's temperature balance through the Greenhouse Effect was actually made 120 years ago by Swedish chemist Svante Arrhenius, based on earlier measurements of the absorptive capacity of Greenhouse gases by the English physicist John Tyndall. The first studies of the modern CO₂ content of the atmosphere that led to predictions of dangerous global warming were presented to the oil industry as long ago as 1954.

The central problem in modeling climate change is that the Greenhouse Effect does not operate in isolation, and combining it with a myriad of other processes and a large range of environments to model a system as complex as the real terrestrial atmosphere is a major challenge. It is necessary to account for important influences from ocean currents, mountain ranges, cloud and aerosol shielding, volcanic activity, and solar energy input, among other factors.
Another major technical difficulty is that the atmosphere is a strongly "non-linear" system: output is not simply proportional to input.
Mathematically, such systems can exhibit "chaotic" behavior, i.e. divergent results for small changes in the starting point.
This kind of behavior is exemplified by the "butterfly" effect: a butterfly flapping its wings off the coast of Africa can, in principle, produce a hurricane over Florida several weeks later.
Here is a nice BBC video showing how small disturbances are amplified by atmospheric instabilities into large storms.

Scientists have identified another kind of climate non-linearity in the form of so-called "tipping points", where important deleterious changes become irreversible. These rachet-like changes are not well modeled now, but they include the potential melting of some of the major ice-sheets in Greenland or Antarctica, release of the powerful Greenhouse gas methane from frozen deposits on the sea floor, or changes in the Atlantic circulation currents (see below). Some tipping points might occur within the higher temperature ranges now expected to be inevitable.
It is not only the atmosphere that must be understood and carefully modeled. Paradoxically, an interruption of the Atlantic ocean circulation currents (shown at the right), which would occur if higher temperatures melt icepacks and allow more freshwater to flow into the ocean, would lead to drastic cooling of northern Europe. A February 2021 study finds that 9 out of 11 "proxy" measurements do indeed indicate that the circulation currents are becoming weaker. A side effect of circulation weakening would be yet larger sea level rise along the eastern seaboard of the US.

Such complexity makes it very difficult to study the effects that humans may be having on the atmosphere and climate---and contributes to the scientific and political controversies surrounding "global warming."

Solar energy input cannot account for the warming observed over the last century, and in fact it has been declining slightly since 1960. However, in the absence of any other changes, the human-generated CO₂ (double the pre-industrial amount by mid-21st century) would create significant additional global warming through the Greenhouse Effect.

Recall that the pre-industrial complement of Greenhouse gases accounted for a 30^o C, or 54^o F, increase in Earth's surface temperature over its "bare" equilibriium temperature. This much warming was beneficial, and the Earth's surface has long since adjusted to it.
That warming occurred over hundreds of millions of years. The problem we face today is that even a small further global temperature increase (2^o C) over a period as short as a century would force a readjustment that would have dramatic effects on weather, water distribution, sea levels, and growing seasons on a human scale.
(Two degrees sounds ridiculously small and inconsequential, doesn't it? But that much increase for the entire Earth's atmosphere represents a tremendous increase in energy content. Recall that only a 3^o C change in global mean temperature in the other direction could induce an ice age, with clearly catastrophic consequences for human civilization.)

Fortunately, computer models of the atmosphere and climate change have rapidly become more sophisticated and realistic as supercomputer power has accelerated. Most atmospheric physicists agree that the models are capable of distinguishing human-induced effects from the atmosphere's continuous natural change. Nonetheless, public debate has raged over the extent to which a human Greenhouse warming component is detectable.

Here is a historical perspective on the predicted impacts of global warming.

The Scientific Consensus

The scientific consensus, based on thousands of studies worldwide since the 1950's, is that some human-induced warming has occurred (probably at least 50% of the temperature rise over the last 60 years) and that significant additional warming is expected over the next 100 years.

Here is a 2013 summary of the situation from the American Geophysical Union:

"Human activities are changing Earth's climate. At the global level, atmospheric concentrations of carbon dioxide and other heat-trapping greenhouse gases have increased sharply since the Industrial Revolution. Fossil fuel burning dominates this increase. Human-caused increases in greenhouse gases are responsible for most of the observed global average surface warming of roughly 0.8C (1.5F) over the past 140 years. Because natural processes cannot quickly remove some of these gases (notably carbon dioxide) from the atmosphere, our past, present, and future emissions will influence the climate system for millennia."
"Extensive, independent observations confirm the reality of global warming. These observations show large-scale increases in air and sea temperatures, sea level, and atmospheric water vapor; they document decreases in the extent of mountain glaciers, snow cover, permafrost, and Arctic sea ice. These changes are broadly consistent with long-understood physics and predictions of how the climate system is expected to respond to human-caused increases in greenhouse gases. The changes are inconsistent with explanations of climate change that rely on known natural influences."
These results were strengthened by a more recent 2018 report by the International Panel on Climate Change.

Such conclusions continue to be disputed by the fossil fuel industries and their political allies and by a small subset of climate scientists. Very few of the publicly-prominent "climate skeptics" have the qualifications needed to assess the evidence based on original sources --- i.e. the scientific literature. Instead, most are simply echoing political talking points which they think might gain the most traction with public opinion, even if those were demolished long ago. Meanwhile, the scientifically-qualified skeptics are in retreat on the question of the existence of warming, and some are accepting the likelihood of human involvement in global warming.

In assessing the controversy, it's useful to remember that science is as highly competitive a profession as you can find and that scientists do not easily reach a consensus. There are tremendous incentives for scientists to contravene the "conventional wisdom," to be able to demonstrate convincingly that their peers are misguided. That's how scientists become famous. No one earns great credit for merely confirming what people already know. The "models" for young scientists are the mavericks -- Galileo, Darwin, Einstein, Feynman -- not the conformists. If scientists have reached a consensus in the case of global warming, this means that contrary evidence is unconvincing both in quality and quantity.

What to do? There is no doubt that humans can adjust to whatever changes occur over 100 years...so we will survive. But the robustness of our economy depends on the stability of climate patterns, not variations in them. The costs of dislocations produced by major climate change could be enormous. Hurricanes Katrina (2005), Sandy (2012), and Ida (2021) as well as the onging southwestern US drought, the worsening wildfire seasons, and episodes of unprecedented flooding around the world are all good examples of the scale of the economic disruptions that climate change could produce, even though it is difficult to determine the extent to which those events were intensified by such change. Such dislocations could easily favor nations other than the US. Massive human migrations from the areas most seriously affected by climate change will also be inevitable. So climate change becomes important to our national economic security.

The prudent course is to take steps to reverse the increase in Greenhouse gases until there is a better understanding of what we are doing to the atmosphere.

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Reading for this lecture:

Study Guide 19
Bennett textbook, Chapter 10.

Reading for next lecture:

Study Guide 20
Bennett textbook, Chapter 11

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Web Links:

Atmospheric Evolution
Atmospheric Evolution & Climate Regulation (J. Schieber, Indiana U)
Evolution of Earth's Atmosphere (Brittanica)
Evolution of the Terrestrial Planet Atmospheres (Baines et al. 2013)
Ice Ages (Wikipedia)
Lecture on Ice Ages (J. Frogel, OSU)
Milankovitch Cycles (Wikipedia)

Global Warming
UK Met Office Climate Dashboard
State of the Climate: Global Analysis (monthly updates from NOAA)
The Discovery of Global Warming (Comprehensive history by S. Weart, American Institute of Physics)
Java Demonstration of the Butterfly Effect (Exploratorium)
Ozone Depletion (National Academies Press)
Responses to GW Critics (J. Cook)
A Global Warming Primer (J. Bennett, UCol)
Climate Science, Risk, & Solutions (Kerry Emanuel, MIT)
Climate Guide (UK MetOffice)
Climate Change (New Scientist site, articles & resources)
The Pentagon's Weather Nightmare. (There is some disturbing evidence that climate change can occur much more abruptly than is usually supposed. This article describes the implications of a possible "climate collapse.")
National Research Council Report on "Climate Change Science" (2001)
Full text of the August 2013 statement from the American Geophysical Union
International Panel on Climate Change Synthesis Report (2014)
IPCC Special Report on Global Warming Above 1.5^oC (2018)
What We KNOW About Climate Change (video; lecture by MIT professor Kerry Emanuel)
Sea Level Rise Can No Longer Be Stopped. What Next? (video; lecture by UC Santa Cruz oceanographer John Englander)

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Last modified February 2024 by rwo

Atmospheric escape diagram by Toby Smith (University of Washington). Carbon cycle figure copyright © 2008 Pearson/Addison-Wesley. Text copyright © 1998-2024 Robert W. O'Connell. All rights reserved. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 1210 at the University of Virginia.