Most people know Isaac Asimov as a science fiction writer, but his more than 500 works are listed in nine of the ten major categories of the Dewey decimal system. In particular he wrote a number of nonfiction science books, some more technical than others and of which I have twenty or so. His writing style is a model of crystal clarity that I wish more authors would attempt to emulate.
In 1973 Dell Publishing printed a collection of Isaac Asimov’s essays from Science Digest magazine, called Please Explain. My copy is dog-eared and falling apart, but the other day I was looking for some bedtime reading and began flipping through it. A couple of the essays caught my eye – one on the consequences of Earth’s ice caps melting, and another describing the greenhouse effect.
If the essays are listed in the order that Science Digest printed them, both these essays would both be from 1967. They are entitled “What would happen if the ice caps melted?” and “What is the greenhouse effect?” Of course a great deal of new information has been learned and computer modelling has much improved since Asimov typed these essays on his beloved IBM Selectric typewriter, but it is interesting and fun to see how the ‘great explainer’ addressed the questions:
33: What would happen if the ice caps melted?
The earth’s land areas carry a load of nearly 9 million cubic miles of ice (about 85 percent of it on the continent of Antarctica). Since water is somewhat denser than ice, this load of ice would melt down to about 8 million cubic miles of water…
Naturally, if the ice melted, almost all that water would run off the land areas into the ocean. The ocean has a total surface area of 140 million square miles. If that surface area remained constant and the 8 million cubic miles of melted ice were to spread out evenly over the top of the oceans, it would be 8/140 or 0.057 miles thick. this means the layer of melted ice would be 300 feet thick.
However, the surface area of the ocean would not remain constant, for if its level rose it would spill out over a couple of million square miles of the low-lying land areas along its shores. This means the ocean’s surface area would increase and the layer of new water would not be as thick as we have just supposed. Furthermore, the added weight of water would depress the ocean bottom somewhat. Still, the ocean level would probably rise 200 feet, enough to reach the twentieth story of the Empire State Building and to drown much of earth’s most densely populated areas.
Throughout earth’s geologic history, the quantity of land ice has varied considerably. During the height of an ice age, mile-high glaciers advance over millions of square miles of land and the water level of the ocean drops so much that the continental shelves are exposed as dry land.
On the other hand, when the ice load is virtually zero, as it has been for tens of millions of years at a time, the ocean level is high and the continental area small.
Neither situation is necessarily catastrophic. At the height of an ice age, millions of square miles of land are covered by ice and are uninhabitable to land life. On the other hand, millions of square miles of continental shelves are exposed and habitable.
If, contrariwise, the ice is gone, millions of square miles of land are covered by water, and are uninhabitable to land life. On the other hand, without ice, and with smaller land areas, the climate is more equable and there are few deserts, so that a larger percentage of what land surface is left is habitable. And the change in total ocean volume is comparatively small (6 or 7 percent at most) so sea life is not much affected.
If the change in sea level were to take place over thousands and tens of thousands of years, as it always has in the past, man could well cope with the change. The difficulty is, however, that man’s technology is pouring dust and carbon dioxide into the air. The dust tends to cut off solar radiation and cool the earth, while the carbon dioxide tends to trap heat and warm the earth. If one effect predominates much over the other in times to come, earth’s temperature may fall or rise comparatively rapidly. Continental glaciers may form or an ice cap may melt in a matter of 100 years or so.
It will be the rapidity of the change, not so much the change itself, that will be catastrophic.
And following are Asimov’s desktop+slide-rule calculations on the greenhouse effect, which has been studied in considerably more detail since then. In any case here is a wonderfully clear explanation:
35: What is the greenhouse effect?
When we say that some object is “transparent” because we can see through it, we don’t necessarily mean that all kinds of light can pass through it. For instance, we can see through red glass, which is therefore transparent, but blue light won’t go through it. Ordinary glass is transparent to all colors of light; it is, however, only slightly transparent to ultraviolet or infrared radiation.
Now imagine a glass house standing out in the sunlight. The visible light of the sun passes right through the glass and is absorbed by whatever is present inside the house. The objects in the house warm up as a result, just as objects outside the house exposed to the direct light of the sun.
Objects warmed by sunlight give off that warmth again in the form of radiation. They are not at the temperature of the sun, however, so they don’t give off energetic visible life. They give off, instead, the much less energetic infrared radiation. After a while, they give off as much energy in the form of infrared as they absorb in the form of sunlight, and their temperature remains constant (though they are warmer, of course, than they would be if the sun weren’t shining on them).
Objects in the open have little trouble getting rid of their infrared radiation, but the sun-warmed objects inside the glass house are another situation altogether. Only small quantities of the infrared radiation they give off will go through the glass. Most is reflected, so that energy accumulates within. The temperature of the objects inside the house rises considerably higher than does the temperature of the objects outside. The temperature inside rises until enough infrared radiation can leak through the glass to set up an equilibrium.
Because of this, plants can be grown inside a glass house even though the temperature outside the house is cold enough to freeze them. The flourishing greenery inside such a glass house gives it the name of a “greenhouse”. The additional warmth inside the greenhouse caused by the fact that glass is quite transparent to visible light and only slightly transparent to infrared is called the “greenhouse effect”.
Our atmosphere consists almost entirely of oxygen, nitrogen, and argon. These gasses are quite transparent to both visible light and tot he kind of infrared radiation the earth’s surface gives off when it is warmed. The atmosphere also contains 0.03 percent of carbon dioxide, however, and this is transparent to visible light but not very transparent to infrared. The carbon dioxide of the atmosphere acts like the glass of the greenhouse.
Because carbon dioxide is present in such small quantities in our atmosphere, the effect is comparatively minor. Even so, the earth is a bit warmer than it would be if there were no carbon dioxide present at all. What’s more, if the carbon dioxide content of the atmosphere were to double, the increased greenhouse effect would warm the earth a couple of additional degrees and that would be enough to bring about a gradual melting of the ice caps at the poles.
An example of an enormous greenhouse effect is to be found on Venus, where the very thick atmosphere seems to be mostly carbon dioxide. Astronomers expected Venus to be warmer than the earth since it is considerably closer to the sun. Not knowing the details of the composition of its atmosphere, they had not expected the additional warming of the greenhouse effect. They were quite surprised when they found that Venus’ surface temperature was far above the boiling point of water and hundreds of degrees warmer than they had expected.
- I am depending on the good offices of chapter 7 “Fair Use” of the copyright law in reproducing this two percent of an out-of-print book, but if the publisher requests it, I shall remove it.