1. Clouds Reflect the Heat of the Sun Away From the Planet
If a planet has no atmosphere, all of the sunlight which strikes it reaches the surface, and usually, 90% or more of that is absorbed, and 10% or less is reflected back into space. However, if the planet has an atmosphere, particularly a thick, cloud-filled atmosphere, part of the sunlight will be reflected into space before even reaching the surface. The percentage of light reflected by a planet is referred to as its albedo, and subtracting that from 100% tells us how much energy is absorbed by, and heats up, the planet.
As an example of how this works, the Earth and Moon are the same distance from the Sun, within 1/4% accuracy, and have the same amount of sunlight falling on them, but the Moon reflects only about 10% of the sunlight that it receives, and absorbs about 90% of it, while the Earth, because of its extensive cloud cover, reflects about 30% of the sunlight that it receives, and absorbs only about 70%. The values for the Moon vary because the highlands are lighter, and reflect more light, and the maria are darker, and reflect less light, and the values for the Earth vary according to the local weather topography, but these average values are adequate for a general comparison. As a result of the difference in absorption, the Earth only receives about 2/3 as much sunlight during the day as the Moon does, and the Moon becomes much hotter, reaching temperatures in excess of 250°F, which is over 150F° hotter than normal Earth temperatures.
However, you should note that clouds are not the only factor that determines a planet's temperature, as you can tell by considering the case of Venus, which has an albedo of 65%, meaning that it only absorbs 35% of the sunlight which falls on it, which is only half as much as the Earth absorbs. If all other factors could be ignored, this difference in heat absorption would completely make up for the fact that Venus is only 70% as far from the Sun as we are, and the Earth and Venus would have similar surface temperatures, and in fact, until the early 1970's, it was presumed that Venus had temperatures that weren't all that much higher than those of the Earth. In that era, and earlier, it was generally thought that surface conditions on Venus were similar to those in the Sahara (hot and dry), or the Amazon basin (hot and wet). As it turns out, as discussed under Greenhouse Gases, below, Venus is over 800° warmer than the Earth, so looking only at the albedo doesn't tell you the whole story. It is, however, the first piece of the puzzle, and is, for most planets, a very significant factor in determining their temperature.
2. Winds Circulate Heat From Hot Places to Cool Places
The winds in our atmosphere represent a gigantic heat engine driven by the heat from the Sun. They carry heat from the Equator towards the Poles, and from the day side of the planet to the night side (of course, our rotation also helps with this), and help moderate the differences in heat between the Equator and the Pole, and between the day and night sides of the planet. (For more information about this circulation, refer to the last part of the discussion of the Coriolis effect.)
The efficiency of these winds in circulating heat depends upon how thick the atmosphere is. On the Moon, with no air, there is, of course, no such effect, and temperatures range from very high values on the day side, to very low values on the night side, with no way to moderate them. On Mars, with a thin atmosphere, the difference in temperature between hotter and cooler places creates pressure differences which drive relatively fast winds. If the atmosphere were thicker, it might be able to substantially moderate the temperature, but because the air there is only about 1% as thick as ours, it isn't able to do a very good job of evening out the temperature differences, and they remain quite large, although not as large as on the Moon. On the Earth, with a thicker atmosphere, temperature differences are considerably reduced. This reduces pressure differences, which on Mars can be in excess of 50%, but on Earth, are rarely in excess of 5%, so wind velocities on the Earth are generally slower than on Mars, but because the atmosphere is so much thicker, it is quite capable of producing substantial reductions in temperature extremes.
Venus is, again, an example of a relatively extreme version of this effect. Since its atmosphere is 100 times thicker than ours, it does a very good job of transporting heat from one place to another, even at relatively low wind velocities, so that, despite its relatively long night (about two Earth months long), and the very high temperatures (which mean that even a small percentage difference in temperature could be a large difference in the actual number of degrees), it has a surprisingly small temperature range. In fact, from Equator to Pole, and from day to night, temperature variations are rarely more than 50F° at the surface of the planet. The small temperature difference causes relatively low wind velocities (close to zero at the surface), but the thickness of the atmosphere allows even those low velocities to be quite adequate to produce a remarkably even temperature.
Since the Jovian planets have even thicker atmospheres, we can expect that they should have relatively even temperatures, an relatively slow wind velocities, as well. However, there is, as it turns out, huge amounts of heat radiating from the interior of the planet, in fact, over twice as much heat as the planet receives from the Sun, and wind velocities and temperature differences aren't as small as on Venus. In fact, the rapid rotation of these planets drive such strong Coriolis effects that wind velocities in the belts and zones (circulation patterns parallel to the Equator) of Jupiter are several hundred miles per hour, and at the boundaries between a belt and a zone, where the wind velocities are in opposite directions, wind shear velocities are in excess of a thousand miles an hour.
3. "Greenhouse" Gases Hold in the Heat of the Planet
During the day sunlight pours down onto the surface of a planet, heating it up. As the surface gets warmer, it radiates infrared heat back into space. Once the heat radiated by the surface is equal to the sunlight being absorbed by it, the surface stops heating up, and temperatures remain relatively stable. However, once the Sun goes down, there is no sunlight striking the surface, and the surface of the planet cools, as it radiates heat into space (this is referred to as radiative cooling, and during winter cold waves, you may see that term in newspaper articles about farmers who are worried about frost damage to their crops).
On the Moon, the temperature rises very quickly in the morning, because, as discussed above, very little sunlight is reflected away, and temperatures reach 250°F within a few hours. When the Sun goes down, however, about two weeks later, temperatures drop just as quickly, reaching sub-zero temperatures within a very few hours (in fact, this can even happen during a lunar eclipse, when the Moon passes through the Earth's shadow, and cuts off the sunlight). As the temperature drops, heat is radiated away more slowly, so it takes more than a day to reach temperatures of 200°F below zero, and a week or so to get to -250°F. If the night continued for several months, as it does on Mercury, temperatures would slowly drop still further (on Mercury, the temperature reaches -350°F before sunrise), and if it lasted for years, temperatures could eventually drop to just a few degrees above absolute zero (over -450°F). On Pluto, where the winter lasts for half of the nearly 250-year orbital period, temperatures undoubtedly do drop to such low values.
On the Earth, however, and other planets with an atmosphere, so-called greenhouse gases can trap part of the heat radiated by the surface, keeping the surface warmer. There is nothing particularly special about greenhouse gases. All gases can, to a certain extent, block the infrared radiation from the surface of a planet. It is just that some gases, particularly those made of polyatomic, or multi-atom molecules, are better at blocking such radiation than other gases, such as the diatomic molecules of nitrogen and oxygen which make up the bulk of our atmosphere. Since the bulk of the gases in our atmosphere aren't particularly good at blocking infrared radiation, the rarer gases which are good at it seem special, but they are actually quite common. In fact, water vapor and carbon dioxide, which are the primary greenhouse gases on the Earth, Mars and Venus, are among the most common molecules in the Universe. (Note: The way in which greenhouse gases trap heat is not the same as the way in which greenhouses work. Greenhouses do most of their work by keeping nighttime winds from blowing away warm air trapped inside them during the day. However, since the basic purpose of a greenhouse is to keep the inside warm, gases which keep a planet warm are called greenhouse gases.)
The Earth doesn't have much carbon dioxide in its atmosphere (only about 1/30 of 1 percent), so although carbon dioxide does contribute to the greenhouse effect, our most important greenhouse gas is water vapor. If you live near the ocean, or in a humid climate, it cools off less at night than if you live in a dry area, such as a desert. In particular, in winter, if temperatures are expected to get especially cold, you will hear about farmers worrying about radiation cooling if the night is going to be clear (so that heat can be radiated into space), but you will not hear about that if the night is going to be cloudy (so that heat is blocked by the water vapor in the clouds, keeping things warmer). This, of course, has the unfortunate effect that during winter, the clearest nights, when you would like to look at the stars, are generally colder than the cloudier nights, when you can't do so. (Traditionally, astronomers were expected to bundle up, and endure the cold, rather than worrying about it. At Yerkes Observatory, near Green Bay, Wisconsin, observations would continue on even the coldest nights, if it was clear, until temperatures dropped so low that the grease used to lubricate the telescope mounting began to freeze. Long before then, observers were pretty well frozen, as well. Nowadays, however, with computerized telescope controls, you can sit in a nice, toasty warm room, on one side of the Earth, while controlling a telescope operating in sub-zero temperatures on some distant mountaintop, and only the support staff which has to make sure the telescope keeps working has to worry about freezing.)
Now, although water vapor is our most important greenhouse gas, the fact that there are others means that wavelengths of infrared light which are not blocked by water vapor could be blocked by the other greenhouse gases, keeping temperatures warmer than when you have only one kind of greenhouse gas. As an example, Mars has a considerable amount of carbon dioxide in its atmosphere. Even though the planet's atmosphere is only about 1% as thick as ours, it is almost entirely carbon dioxide, so there is about 30 times as much carbon dioxide as in the Earth's atmosphere. However, although this carbon dioxide does a good job of blocking the wavelengths that it can block, it cannot block all infrared wavelengths, and there just isn't much in the way of other greenhouse gases on Mars. Water vapor, in particular, is relatively rare, amounting to only a millionth of an inch or so of liquid, even in places where the relative humidity is close to 100%, because the low temperature and low density of the atmosphere doesn't allow it to contain very much water vapor. If there were any significant amount, it would crystallize as snowflakes, and float to the ground. As a result, although the temperatures on Mars don't drop as fast as those on the Moon, they do drop substantially, and nighttime temperatures average about 100°F less than daytime temperatures.
Once again, we come to Venus, and once again, we encounter extraordinary effects due to its thick atmosphere. With 100 times more air than the Earth, and almost all of it carbon dioxide, Venus has three hundred thousand times as much carbon dioxide as the Earth, and the clouds, although composed primarily of fuming sulfuric acid, have enough water vapor to help block the heat of the planet, as well. As a result, even though it is hard for sunlight to reach the surface of the planet (2/3 of it being reflected away by the clouds), it is even harder for the infrared heat of the surface to escape into space. This causes the planet to heat up until the extra radiation caused by its extra heat is able to balance the extra blockage of heat by its greenhouse gases, and what escapes is what would have escaped if there were no atmosphere to block it. The blockage of heat by the greenhouse gases in Venus' atmosphere is so effective that only about 1% of the heat radiated by the planet is able to escape. This is more than 20 times less than the percentage of sunlight which reaches the surface and is absorbed, and the planet has to heat up to almost three times the temperature that we would expect, or more than 900°F, or more than 1400F° above absolute zero, which is hotter than Mercury ever gets, even when Mercury is at perihelion, and receives nearly five times as much sunlight per square foot as falls onto the Venusian atmosphere.
Could We Have a Runaway Greenhouse Effect on Earth?
Because the greenhouse effect on Venus is so extreme, it is referred to as a runaway greenhouse effect. Needless to say, environmentalists worry about whether such a thing could happen on the Earth. If we could somehow increase the amount of greenhouse gases in our atmosphere, could we reach a point at which the temperature of the Earth would drastically increase, boiling away our oceans and ending all life on Earth? In a word, yes. However, to do this, we would have to work very, very hard to undo something which happened many billions of years ago, when the Earth was relatively young, and its atmosphere was very different from now.
At that time, the atmosphere of the Earth was very much like that of Venus -- more than a hundred times thicker than our current atmosphere, and made almost entirely of carbon dioxide -- and temperatures on the Earth were a few hundred degrees above zero Fahrenheit. However, we did not have as extreme temperatures as Venus currently does, for two reasons. (1) We are further from the Sun, so temperatures would be a little cooler, even with an identical atmosphere. (2) When we had a Venusian atmosphere, the Sun was a little fainter than it is now. And because the Earth was not as hot as Venus, something happened which is quite different from what happened to Venus.
On Venus, the extreme temperatures caused its oceans to boil away into the atmosphere (or prevented water vapor from condensing to form oceans, in the first place). All Venus' water was in its atmosphere, exposed to ultraviolet radiation from the Sun which was continually dissociating it into hydrogen and oxygen. The oxygen combined with the surface rocks, while the hydrogen escaped into space. Over a period of time, all of the hydrogen compounds in the atmosphere were lost, and since all of its water was in the atmosphere, hardly any remains, save for traces tied up in the sulfurous clouds which obscure its surface.
Since the Earth was further from the Sun and cooler than Venus, although it must have been hotter than the current boiling temperature of water (212 degrees Fahrenheit), it would not have been hotter than the (much higher) boiling temperature of water, under the high pressure conditions which then existed at the surface of the Earth. As a result, the Earth developed and still retains oceans of liquid water. These oceans gradually dissolved the carbon dioxide, turning it into carbonic acid (a mild acid present in all soda waters, flavored or otherwise), and chemical reactions between the carbonic acid and metal oxides removed from the continents by weathering and erosion converted the atmospheric carbon dioxide into insoluble carbonate rocks, such as limestone (calcium carbonate), siderite (iron carbonate), and dolomite (magnesium carbonate). There are mountain ranges made of these carbonate rocks, and the amount of carbon dioxide trapped in them is equivalent to the carbon dioxide in the atmosphere of Venus. If we were to heat these rocks to about a thousand degrees (or "roast" them), we could restore the carbon dioxide to its gaseous form, recreating the earlier atmosphere of the Earth, and cause a return to extremely hot conditions. In fact, since the Sun is now a little brighter, we might even boil away the oceans, preventing the carbon dioxide from being removed again. Fortunately, the time, effort, and expense involved in doing such a thing would make it completely impractical, but it is at least theoretically possible.
The Greenhouse "Controversy"
Now, although we needn't worry about a runaway greenhouse effect, perhaps we should be concerned about smaller effects, such as the warming caused by the gradual increase in the amount of carbon dioxide in our atmosphere over the last few hundred years, and the more rapid increase which occurred in the last century, and will certainly continue until drastic and very unlikely changes occur in our use of fossil fuels.
Until recently, there was considerable denial of the reality of this warming trend, let alone human contributions to it, because (1) there are a number of natural factors which contribute to it, whose importance overshadowed human effects until recent decades, (2) measurement of the warming trend has been difficult, due to changes in techniques used to measure temperature, and (3) economic and political factions which felt threatened by the changes required to reduce greenhouse gas emissions actively denied the effects of their action (hence the term "Greenhouse Controversy", which implies that perhaps there is nothing to worry about, as opposed to "Greenhouse Effect", which suggests that there is indeed something to worry about).
(1) About "Natural Factors": A little over ten thousand years ago we were in an ice age, and now we are not, so things are of course considerably warmer than they used to be. But there have been periods of lower temperature, which defied the general warming trend. A century and a half ago, it was considerably colder than in earlier centuries, and scientists were more concerned about global cooling, than global warming. In the period from 1000 to about 1400 AD, the northern hemisphere was relatively warm, allowing Viking settlements to flourish in Greenland and Iceland. Then the Earth began to cool, because of a gradual reduction in the cycle of Solar activity, which reached its nadir during the Maunder Minimum, in the late 1600's and early 1700's. As the Earth cooled, the Viking settlements were devastated, and glaciers advanced all over the northern hemisphere. Then the Sun became more active again, and temperatures began to increase. During the last century, the Sun has been still more active and temperatures have increased still further, and until recently, most of the Earth's temperature increase was due to this astronomical effect.
(2) About "Measurement Uncertainties": Until recently, it has been very difficult to be sure just how much the Earth has warmed over the last century. There have been changes in the way that temperatures are recorded at sea which make comparisons of older sea temperature measurements to modern sea temperature measurements uncertain, and on land, there is a well-known phenomenon, called the urban heat island effect, which causes errors, as well. In modern cities, large amounts of paved roadway, stone and metal buildings, and electrical and vehicle usage make urban areas several degrees warmer than nearby suburban or rural areas. And with the rapid increase of population over the last few centuries, many places which used to be in relatively rural area are now much more populated, producing a substantial change in the baseline temperature readings, which obscures the changes caused by general warming.
(3) A State of Denial: Because of these problems, until recently, it was very hard to tell exactly how much the Earth has warmed up over the last century, or how much of that warming was due to natural effects, and how much to human activity. There was no doubt that there was some effect due to human activity, but in the absence of "hard" data, there were naturally those who preferred vehement denial to painful reality. (And of course, even as better data become available, there are sure to be those who would prefer more strident denial to any admission of self- or public delusion.)
And What of the Future?
Projecting the future is even less certain than assessing the past, because we can't be sure of a number of factors. How much more carbon dioxide will we pump into the atmosphere? Certainly a lot more than we should, but the exact amount, and its effect, depends upon economic conditions (when times are good, people create more carbon dioxide than when times are bad), population increases (more people doing the same thing means more carbon dioxide), changes in technology (more efficient automobiles, lighting, and air conditioning might reduce fossil fuel use, or simply slow its rate of increase), and changes in cultural behavior. None of these can be predicted with certainty, so estimates of the amount of carbon dioxide that will be dumped into the atmosphere over the next century vary by as much as a factor of five. It is also hard to determine what will happen to that carbon dioxide, once it is put into the atmosphere. Some will be absorbed by plants, helping them grow, some will be absorbed by the soil and organisms in the soil, and some will be absorbed by the oceans. Estimates can be made of how these various factors work, but calculations of expected carbon dioxide "uptake" can vary from observations by as much as a factor of two. And even if we knew exactly how much carbon dioxide will be present in the atmosphere in the future, predicting exactly how it will affect the Earth's temperature is very difficult. Changes in temperature are different at different latitudes, being more extreme near the Poles, and less extreme near the Equator, and changes in weather patterns are more variable yet, causing warmer, drier weather in some areas, and cooler, wetter weather in other areas, even at the same latitude. So all we can be certain of is that there will be change, and given our tendency to think of current conditions as "normal", that much of that change will feel unnatural, and unpleasant.
How unpleasant depends upon the power and privilege of those affected. In Bangladesh and similar poverty-stricken coastal areas, the next century is going to be very unpleasant. As sea level rises by perhaps one to three feet, upwards of fifty million people will see their homes and livelihoods disappear. In the Arctic, creatures such as polar bears, which rely on sea ice for their existence, may become extinct as that sea ice disappears. But on the beaches at Malibu, changes in sea level are more likely to lead to resistance to the effects of those changes, than to any retreat to landward areas, or any abandonment of the lifestyles that help cause the rise in sea level.
It might be noted that the change in sea level mentioned above is small compared to what would happen if temperatures continued to increase for many centuries. That might well cause a return to "average" conditions for the past few hundred million years -- ending the Ice Era which has gripped the Earth for the past couple of million years -- and a two hundred foot rise in sea level, which would cause the displacement of billions of people, and perhaps end civilization as we know it. But again, the importance of these effects depends upon your perspective. From our viewpoint, such changes would be catastrophic; but for life in general, a return to paleohistoric conditions would probably result in a fecund diversity similar to that which occurred in the more tropical conditions of the Age of Dinosaurs. And although our descendants might mourn the loss of their significance, those creatures which replace Man would probably feel that little was lost by the change.
What, Me Worry?
So, what should we make of what we hear in the news, and what should we do about it? We are now certain that carbon dioxide concentrations in the Earth's atmosphere have increased, and will continue to increase, barring inconceivable changes in our lifestyles. As a result, there will be temperature increases of uncertain amount, and changes in the weather of uncertain nature and extent. Life as we know it will change to a greater or lesser extent, depending upon where we live, but will be more like now than not, for a few decades; and less like now than not, in the following centuries. Continental and sea ice will retreat and disappear, and sea levels will rise throughout our lifetimes, and the lifetimes of our descendants. And there is nothing that we can do, to prevent this. The only question is, what can we do to reduce these effects, and in some distant future, reverse them?
The "greenhouse effect" is real, and of considerable concern to many people. It is not, perhaps, quite as immediately worrisome as the threat of global warfare and nuclear or biological annihilation. But if you are concerned about the "fate of the Earth", or the future faced by your children and grandchildren, then perhaps it would be better to do what you can to use less energy, and to convince your representatives to vote for measures which would make it easier for you to use less energy, than to seek ways to burn more and more fossil fuels at a faster and faster rate. Or, if it seems too painful to do anything, you could do nothing. For many lifeforms, such as mosquitoes and millipedes, crocodiles and cockroaches, molds and mildews, a warmer, wetter Earth would be marvelous. It just depends upon whom or what you want to inherit the Earth.