Your warm hand radiates mainly in the infra-red: night-vision devices can see its image and electronically convert it to a visible picture. The Sun is bright white, and its distribution of brightness ("spectrum") tells it is at about 5500 deg. centigrade. That radiation (if not reflected by clouds etc.) heats the ground and drives processes of weather and climate.
The warmed-up ground in its turn heats the atmosphere, and both emit infra-red radiation (like your warm hand). On the average, the energy of EM radiation absorbed from the Sun must equal that of emitted EM radiation from the Earth and its atmosphere (otherwise Earth would steadily heat up--or maybe cool down). The energy must be equal, but the radiations are quite different: the Earth receives sunlight from a tiny patch of the sky, intensely quite hot, so the Sun (especially when viewed from space) is dazzling white.
The Earth sends back energy from its surface and atmosphere, but since those are much cooler than the Sun, they are only a weak source of IR radiation. However, they have the advantage of being able to radiate back to the entire sky, while sunlight arrives only from a small part of the sky, and only on the day side of Earth. Thus the two energy flows, incoming and outgoing, can be equal.
In fact, it can be shown that for such balance to exist, Earth only has to radiate like an object at –19° centigrade. That would presumably be the temperature of the Earth if no atmosphere existed (and if no heat was generated inside the Earth by radioactivity etc.). Oceans would be frozen and life might be unlikely. What makes the difference is the atmosphere.
Air itself is essentially transparent to visible light--except that some of the blue part is scattered (hence the blue sky), and except for clouds and dust reflecting some of the light. However, it is not transparent to big parts of the IR radiation returning energy to space.
The main gases of the atmosphere--N2, O2 and also Ar (less than 1%) absorb very little. However it takes only a small content of more complex molecules--in particular water vapor (H2O), carbon dioxide (CO2) and methane (CH4)--to absorb many parts of the IR spectrum--the same as a few drops of black drafting ink in a glass of water absorb enough light to darken it significantly. This is often called the greenhouse effect, because a similar process acts in glass greenhouses, used to grow plants in cold climates: sunlight enters the glass panes of the greenhouse and warms the inside, but most IR generated there is absorbed by the glass, maintaining a warm environment (a closed car left standing in sunlight experiences the same effect). In the atmosphere, humidity is generated by oceans, lakes and vegetation, CO2 comes from fire and decay of vegetable matter, and CH4 from bacteria, including the ones involved in animal digestion.
These "greenhouse gases" keep Earth warm and habitable. Heated from the ground, they re-radiate infra-red (IR) and pass it to layers higher up, and those do the same, until one reaches the tropopause, the layer high enough and rarefied enough for its infra-red light to have a good chance of never encountering any atmospheric molecule, but continuing out into cold space. Air also gets colder with altitude (more below), but the tropopause only has to average –19° centigrade to emit enough heat to balance that arriving in sunlight.
The tropopause is the top of the troposphere, the layer in the atmosphere where weather and climate are generated: it is about 17 km high above the equator, narrowing to about 11 km near the poles. If input and output of energy from any area of the Earth were everywhere the same, "greenhouse gases" would impede atmospheric energy flow to where the ground would stay warm around 25°centigrade, with temperature dropping upwards until it is –19° at the tropopause.
Convection and Humidity
Actually, this is only the first step of a complex interaction. If only the "greenhouse gases" were involved in removing heat from the ground, by their "bucket brigade" handing infra-red energy upwards from layer to layer, then the ground would actually be much warmer. The fact that these gases impede outward heat flow, however, boosts alternative modes of heat transport from below to the tropopause. Chief among them is convection, the upward flow of the warm air itself, not just its energy.
If you stand on the seashore on a hot sunny day, you often feel a breeze
coming from the water. Sunlight warms both water and land, but because the upper layers of water keep mixing, the surface of the land gets hotter. So does the air above it, which becomes less dense and therefore floats upwards, drawing cool air from the sea to replace it, which also gradually warms up and also rises. The ultimate result is air flowing in a closed loop--heated air rises, cools as it expands and then comes down again.
Such "rising thermal flows" of air are used by glider pilots, also by soaring birds (which learned about it long ago), and they provide heat with an alternate upwards transport. In the early days of flying, when airplanes flew low, they would on hot days be constantly lifted by such "thermals", then dropped by the adjoining downflows, resulting in acute nausea for some passengers. You may still find waterproof paper bags for "air sickness" in pockets in the backs of airplane seats.
But here a new complication enters: humidity. Sunlight not only heats the ocean, but also evaporates water, and that process also absorbs energy--lots of it. It takes 100 (kilo) calories to heat a kilogram of water from the freezing point (0°C) to the boiling point (100°C), but it then takes an additional 540kC to evaporate it, still at 100°C. This is the latent heat of evaporation. Other substances also have it, but its value is rather high for water, which is why a pot of water on a stove may heat up quickly but takes much longer to completely boil away.
(There also exists "latent heat of fusion", the heat given up when water turns to ice, about 80kc per kilogram. Ice cubes in a beverage keep it cold for a fairly long time as they gradually melt, absorbing those 80 calories.)
If energy (in the world of physics) is to be compared to money, with heat the "soft currency" in the energy marketplace, then latent heat is like scrip, locally issued money, used along with regular money inside its own region (or, say, in a company store), but not outside it. Energy invested in latent heat can be used to transport heat from place to place in the atmosphere, but must be converted into regular heat before it can be radiated away at the tropopause.
When humid air rises, it expands and cools down, and cooler air can hold less humidity. The excess water condenses to droplets--clouds or rain, depending on conditions--and that condensation returns heat to the atmosphere and helps the air to continue rising. Thus by the time any rising air has reached the tropopause, almost all its humidity has been "wrung out" and its energy has heated the air and helped it rise.
Convection flow is active in thunderstorms, where it even lends some of its energy to the electric phenomenon of lightning.
On an even larger scale, it forms hurricanes in the tropical part of the north Atlantic ocean ("typhoons" of the Pacific ocean are similar) whose water is already fairly warm.
| A hurricane viewed from space.
As sunlight is absorbed by the tropical ocean, it warms up the water. Some of this heat is passed to the air above the water, along with greater humidity. Warm humid air is unstable: when its expansion due to heat causes it to rise, its ascent is helped by heat released to form cloud and rain. Then more humid air is sucked in, and if conditions are suitable, even a hurricane may form. As explained elsewhere, the rotation of the Earth causes the inflowing air to spin, counterclockwise in the north Atlantic.
Meanwhile the prevailing winds carry the entire system along, eastward in the tropics, then slanting poleward and gradually moving westward again. Like a prairie fire, as long as the hurricane is above warm humid water, it can add energy. It weakens quickly above land (though drenching rains may continue) and also may weaken above cooler waters of the north Atlantic.
Global Transport of Heat
The preceding is concerned with the vertical flow of energy, from the ground level to the tropopause and beyond, but there also exist dimensions of longitude and latitudes. As the Earth rotates, points at the same latitude but different longitudes receive about the same amount of sunshine, so let us not worry about heat flow in that direction.
However, different latitudes receive quite different amounts of solar heat. Near the equator, a sunbeam with a cross section of (say) one meter squared (1m2) may arrive vertically and then heats an area equal to 1m2. At different seasons the direction departs somewhat from vertical and the area gets a bit larger, but not by much. Closer to the poles however, the Sun's rays slant appreciably and may be spread over 2m2 or even 3m2, so those locations get less heat per unit surface. Places experiencing the polar night get the least, they are in the Earth's shadow part of the year and then receive nothing at all. So Earth is heated quite unevenly, giving rise to tropical, temperate and arctic climate zones.
Heat return to space however tends to even out. If the tropopause is to radiate uniformly at some temperature like –19°, heat must also flow poleward and be shared between high and low latitudes. Indeed, such sharing gives rise to global flows of both air and water. In the Atlantic Ocean, the warm Gulf Stream flows out of the Gulf of Mexico along the Eastern seaboard of North America, then curves towards Europe and makes that continent comfortably warm, or at least its countries close to the ocean. It then returns diffusely (and at greater depth) to the equatorial region. In the Pacific Ocean the Japanese Current or Kuroshio similarly flows from southeast Asia towards Alaska, then curves back equatorward.
In the atmosphere, a great global circulation is set up, flowing mainly westward at middle latitudes, with an eastward countercurrent along the equator (the rotation of the Earth is also involved; see here). If the flow stayed at constant latitudes, it would not transport any heat (and have no source of energy to drive it). Actually, however, it has wave-like excursions poleward and equatorward, giving rise to "Rossby waves" and to the "jet stream". These excursions carry heat poleward and bring back cooled air.
All this transport is complex and unstable, primarily because of the uneven distribution of land and water. The weather at many places is highly changeable, with disturbances evolving and carried by the global wind flow, but the climate at most locations, the long-term average of weather, follows more persistent patterns. Still, there exists a great debate these days on whether human activity affects climate, and if so, how.
The main claim is that climate is warming up, because of the burning of fossil fuel. The warming trend is evident in the data collected over the last century, as shown by NASA's graphic description of surface temperature change.
Before the industrial revolution, most fire used wood or charcoal as fuel, and the CO2 thus produced only recycled carbon atoms which already were part of the ecosystem. Since the 18th century, however, humanity burns increasing amounts of fossil carbon from coal or natural oil, which till now were kept away from the atmosphere. As a result, the atmosphere now absorbs more IR light, and the "bucket brigade" needs additional steps to transport heat to the tropopause.
As someone put it, "it is like putting an extra blanket to keep the Earth warm." If that were an ordinary blanket, in order to keep up the same heat flow, the bottom (where we live) must become hotter, because (in this case) the temperature at the top is fixed around –19° by the heat outflow it needs to maintain.
It is very difficult to analyze all factors affecting the global effect, As Freeman Dyson noted (NY Times 12-14-09, as quoted by the newsletter of the American Physics Society:
"I am opposing the holy brotherhood of climate model experts and the crowd of deluded citizens who believe the numbers predicted by computer models.... I have studied the climate models and know what they can do. The models solve the equations of fluid dynamics, and they do a very good job of describing the fluid motions of the atmosphere and the oceans. They do a very poor job in describing the clouds, the dust, the chemistry and the biology of fields and farms and forests."
However, granted an uncertainty range, computer models do predct global warming. Indeed, average temperatures over the world seem to have risen slightly, and computer programs that simulate climate seem to agree. Glaciers have been shrinking for over a century, and those of Mt. Kilimanjaro in Africa seem near their end. What may be more significant is that polar ice seems to be melting, especially in Greenland. Also, the yearly break-up of arctic ice occurs sooner, and the growing season at higher latitudes seems to last longer. All these seem to tell that in response to the "extra blanket", heat from the tropics is distributed more widely, increasing the share of near-polar latitudes in the heat-return.
"Global warming" thus does not mean a greater heat flow from Sun to Earth. That flow stays constant within about a 0.2% variation associated with the sunspot cycle. It does mean that the ground level (where we live) finds extra difficulty in returning that heat to space. Winters may be warmer, but summer temperatures may change less, because heat is also converted to the "scrip energy" of humidity. Humidity is ultimately converted to heating at higher altitude, perhaps causing more active weather.
Humanity won't roast, but it may face effects other than raised temperature. Melting of floating polar ice does not change the sea level, but the melting of glaciers on Greenland and Antarctica does add water to the ocean, and warming sea-water also expands. Both effects raise the average sea level, bad news for coastal dwellers.
In addition, climate changes are hard to predict: the well-being of many people depends on patterns of rainfall, on avoiding droughts or floods. Rainfall depends on the humidity of the atmosphere, affected by heat at the surface of oceans. And if the wave-like excursions of the global westerly flow start extending further from the equator, extreme weather conditions are more likely. The 2010 winter in the US and parts of Europe was unusually cold, but at the same time the polar atmosphere was unusually warm. That may be associated with an extensive El Niño, a series of linked climate excursions caused by unusually warm waters in the Pacific Ocean. Whether the "Greenhouse Effect" increases the likelihood of El Niño is still being studied.