This article is the first “session” in a series that will describe the science that underlies the concerns for climate change. Most of the data below are from the book “Is the Temperature Rising” by S. George Philander. [Philander 1998]
Abstract: Mars, Earth, and Venus have average surface temperatures of -64, +59, and +806 degrees Fahrenheit, respectively. Only 12% of the huge temperature difference between Earth and Venus can be explained by the difference in distance from the sun. 88% of this difference is caused by the difference in atmosphere. The Earth temperature is “just right” for us. Is it a good idea for us to change our atmosphere? A chart is attached.
Session 1: “It’s not too hot, not too cold, but just right!”
I grew up in an old farm house with a large, black, wood-burning stove in the kitchen. On freezing winter days it was good to come in, stand by the warm stove, and reach out cold hands until they almost touched it. I learned as a boy that it’s warmer when one gets closer to something that’s hot.
The planets in our solar system receive heat in the form of electromagnetic radiation [aka solar energy or insolation] from the sun. The table below contains relevant data. Row 1 shows the distances of Venus, Earth, and Mars from the sun. Row 2 is the temperature at the planet’s surface in degrees Fahrenheit or F. Mars is further away from the sun than we are and is colder than Earth on the surface. Venus is closer to the sun and is much hotter on the surface. This might lead us to conclude that the differences in surface temperature are caused by the differences in distance from the sun. The real explanation is more complicated. The atmospheres of the planets play a key role in regulating their surface temperatures.
Mars has a very thin atmosphere, about 1% as dense as that on earth. Venus has a dense atmosphere, about 90 times that on earth. These ratios are in Row 3.
When solar energy reaches a planet, some is reflected away. The fraction that is reflected is called the “albedo”. Albedo is high for a surface covered with ice or snow, like our North Pole, and low when the surface is open water or bare soil. It varies over the surface of a planet. Cloud cover raises the albedo. An estimate of planet albedo is made at the top of the atmosphere. Estimates for the three planets are in Row 4 of the table.
| Row | Parameter | Venus | Earth | Mars |
| 1 | Distance from sun, millions of miles | 67 | 93 | 142 |
| 2 | Actual temperature at surface, degrees F | 806 | 59 | -64 |
| 3 | Density of atmosphere relative to earth | 90 | 1 | 0.01 |
| 4 | Average albedo over whole planet at top of atmosphere | 75% | 30% | 15% |
| 5 | Fraction of solar energy absorbed by planet | 25% | 70% | 85% |
| 6 | Estimated surface temperature if planet has no atmosphere and absorbs 85% of solar energy like Mars, degrees F | 109 | 23 | -68 |
| 7 | Temperature increase produced by planet’s atmosphere, degrees F [Row 2 minus Row 6] | 697 | 36 | 4 |
The albedo for Venus is a high 75%. This means that the planet absorbs only 25% of the solar energy that reaches it. The fractions of absorbed energy are in Row 5. Venus has the most dense atmosphere of the three planets. It also has the highest albedo and the lowest fraction of absorbed energy. Mars has the least dense atmosphere and absorbs the highest fraction of solar energy that reaches it.
To understand what is happening we need to separate the effects of distance from the effects of atmosphere. Two “laws” of physics help us do this.
Assume for discussion that all three planets absorb 85% of the solar energy that reaches them, as Mars does now, and that they have no atmosphere. The inverse square law enables us to adjust for distance and estimate the solar energy that is absorbed by each planet. The Stephan-Boltzman law enables us to work from the estimates of energy to estimates of temperature. The temperature estimates are in Row 6 of the table.
These data are displayed graphically in the attached chart. The vertical axis is the temperature in Kelvin units and the horizontal axis is the distance from the sun. The Kelvin scale is used in the chart, because zero on this scale is “absolute zero” and this helps put other temperatures in perspective. [For more on temperature scales go to Note 1.] Text boxes contain both Kelvin and Fahrenheit values for important temperatures. Further explanation is on the chart.
The estimated temperature for Venus in Row 6 is only 109 degrees, whereas the actual temperature in Row 2 is 806 degrees. The difference is 697 degrees and this is produced by the atmosphere. The difference numbers are in Row 7. Venus is much too hot for us, and scientists doubt that any life can exist there.
[All temperatures here in degrees F].
The Mars atmosphere is very thin and the effect on the surface temperature is small. Simple forms of life may exist, but at -64 degrees, it’s too cold for us.
The net effect of the Earth atmosphere is to raise the surface temperature by 36 degrees, from what it would be with a Mars albedo and no atmosphere, to a comfortable 59 degrees. Some look at this data for Earth, compare it with the other two planets, and recall the opinion for porridge in the story of Goldilocks and the Three Bears: “It’s not too hot, not too cold, but just right.”
I use the term “net effect” in the paragraph above because the atmospheres on Venus and Earth are doing two things. A more dense atmosphere has a higher albedo and allows less of the solar energy that reaches it to pass thru to the surface. By itself this tends to lower the planet’s surface temperature. But the atmosphere also traps and holds the solar energy, and this heats the surface more.
These ideas were being explored in Great Britain in the 19th century, at a time when flat glass was becoming abundant and many greenhouses were being constructed. The glass of a greenhouse allows radiant energy from the sun to enter and warm everything inside. It prevents the warmed air from getting out.
Scientists studying climate reasoned that gases in the atmosphere are doing something similar. These gases allow radiant energy of many frequencies from the sun to pass thru, and they restrict low frequency radiation from the earth from getting out. From this arose the term “greenhouse effect.” I will explain how it works in a future article.
Meanwhile, some food for thought. A little arithmetic with the numbers in the table shows that 88% of the huge difference in temperature between Venus and Earth is caused by the difference in atmospheres. Only 12% is caused by the difference in distances from the sun. Do you think it’s a good idea for us to change our atmosphere?
Bill Allen, 09-21-10 revised <> OFFby2050