ASTR 1210 (O'Connell) Study Guide


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 natural processes involved and the likely patterns of evolution.

After we cover the 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 in the final section below.

A. Venus, Earth, Mars: No Two Alike

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. Geophysical Atmospheric 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. 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.

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



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 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.

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 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 15oC 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 10oC. 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 3oC was sufficient to precipitate an ice age.

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

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 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.

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."

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.

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:

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.

What to do? There is no doubt that humans can adjust to whatever changes occur over 100 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) and the five destructive 2017-2018 storms are good examples of the scale of the local economic disruptions that climate change could produce, even though it is difficult to determine the extent to which those particular storms were intensified by such change. Such dislocations could easily favor nations other than the US (the southwestern quarter of which, for instance, could suffer severe drought). 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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Last modified June 2021 by rwo

Atmospheric escape diagram by Toby Smith (University of Washington). Carbon cycle figure copyright © 2008 Pearson/Addison-Wesley. Text copyright © 1998-2021 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.