ASTR 1210 (O'Connell) Study Guide


19. EVOLUTION OF THE TERRESTRIAL ATMOSPHERES


Terrestrial Planet Comparison

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, but we have identified the main processes involved and the likely patterns of evolution.


A. Comparison

  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


B. Processes

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

C. Equilibrium

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

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

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


D. Histories

The existing atmospheres were probably outgassed from the interior in all cases, amounting to probably 100 bars on Earth and Venus but less on Mars. An alternative subsidiary source of water and atmospheric gases: comet impacts.

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

Venus

Mars


E. Lessons Learned for Atmospheric Evolution

  1. Little differences can have huge consequences

  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 many millions of years. "Climate" refers to the behavior of surface temperature, precipitation, and wind flow over the much shorter timescales of interest to human beings. Climate changes on the Earth have major practical consequences.

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

Temperature History

The most conspicuous climate events of the last 2 million years have been the ice ages, when a drop in the mean surface temperature allowed great expansions of the polar ice caps. The last ice age ended about 10,000 years ago. To see the temperature and ice volume histories, click on the thumbnail below. Note that a drop in the mean surface temperature of only about 3oC was sufficient to precipitate ice ages.

Astronomical effects (e.g. the "Milankovitch Effect", see Study Guide 4) including small changes in the Earth's orbit and the tilt of the its axis, which affect the amount of insolation in the polar regions, are important in causing the small temperature changes that trigger ice ages.

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" (about 1600 AD). But the most important change is a rapid increase in Earth's mean temperature since 1900.

The global temperature increase 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). There is no question that this is due, in turn, to human use of fossil fuels. The present CO2 concentration is 60% higher than the average over the preceding 600,000 years (and 23% higher than the maximum); see this plot. Click on the thumbnails below for enlarged plots of changes in the CO2 concentration and the Earth's surface temperature.

CO2 Concentration since 1960

Surface temperature since 1880

Surface temperature since 800
The plots above refer to the temperature in the lowest layers of the Earth's atmosphere. There are many other geophysical markers of global heating; changes in seven 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. The chart below shows the huge rise in ocean heat content that has taken place since 1960.

Climate Modeling

How are we to interpret these changes, and is there a link between our use of fossil fuels and global heating?

The basic physical principles that govern the structure of planetary atmospheres have been well understood for a century. The problem is putting these together to model the myriad of processes and environments that affect a system as complex as the real terrestrial atmosphere. Those include ocean currents, mountain ranges, cloud shielding, and solar energy input.

A major technical difficulty is that the atmosphere is a strongly "non-linear" system: output is not simply proportional to input.

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

Nonetheless, 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.

In the absence of any other changes, the added CO2 (double the pre-industrial amount by mid-21st century) would create significant additional global warming through the Greenhouse Effect.

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, debate has raged over the extent to which a human Greenhouse warming component is detectable.

The Scientific Consensus

The scientific consensus, based on thousands of studies since the 1950's, is that some human-induced warming has occurred (probably 40% of the temperature rise over the last 50 years) and that significant additional warming is expected over the next 100 years. The conclusion of the 2007 United Nations Panel on Climate Change was that, with 90% confidence, humans are the main cause of climate warming since 1950. Here is a December 2007 summary of the situation from the American Geophysical Union: Here is a historical perspective on the predicted impacts of global warming. The UN Panel will be producing another summary of the status of climate change in 2014.

Such conclusions have been disputed by the fossil fuel industries and their political allies and by a small subset of climate scientists, though these are retreating in the face of growing evidence for climate change. One formerly contrarian group at UC Berkeley has recently independently reanalyzed surface temperature records and confirmed the earlier published trends.

In assessing the controversy, it's useful to remember 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. 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 (nonlinear) 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) and Sandy (2012) are good examples of the scale of the economic disruptions that climate change could produce, even though it is difficult to determine whether those storms were directly caused by such change. Such dislocations could easily favor nations other than the US (the southwestern quarter of which, for instance, could suffer severe drought), 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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Last modified August 2014 by rwo

Carbon cycle figure copyright © 2008 Pearson/Addison-Wesley. Text copyright © 1998-2014 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.