What Is The "Greenhouse" Effect?

This post will help you understand

1. Why the "greenhouse effect" has to do with gases in the atmosphere,
2. How these "greenhouse gases" in the atmosphere warm the Earth (and what that has to do with things that are "red hot"),
3. What this implies for future warming as we put more of these gases into the atmosphere.

The use of a toaster is not required, but it helps if you know how one works.

What Is The Greenhouse Effect?

The "greenhouse effect" refers to how gases in the atmosphere which absorb infrared radiation make the Earth warmer than it would be without them. (It has nothing to do with the way greenhouses work to protect plants by trapping warm air in an enclosure. That's just its name.)

There are two ways to understand this phenomenon:

1. Some gases in the atmosphere absorb infrared radiation emitted from the surface of the earth. This warms them up and they radiate energy, some of which heats the surface in turn.
2. Gases in the atmosphere that absorb infrared radiation from the surface make the atmosphere more opaque to that radiation, preventing transmission from the surface into space. (Radiation into space still happens, but from high in the atmosphere where it is colder and thus radiates less.)

Both of these ideas involve reduction in the flow of energy from the surface and lower atmosphere into space as radiation. The loss of radiation to space is less than it would have been if there were less of these "greenhouse gases". Since the flow of energy is reduced, the Earth is kept warmer.

Over time the Earth reaches a temperature that radiates away as much energy as it receives from the Sun. But when the amount of greenhouse gases in the atmosphere change for some reason it takes time for the planet to warm or cool enough to restore that energy balance. That is why the "greenhouse effect" is in the news so much today--the concentrations of these greenhouse gases in the atmosphere are changing at unprecedented rates.

The Physics of the Greenhouse Effect

 It is easy to understand the greenhouse effect if you comprehend these simple facts:

1. The Earth is a big warm rock
2. Warm things emit radiation
3. Other things absorb some of that radiation
1. In particular the "greenhouse gases" in the atmosphere strongly absorb the infrared radiation the Earth emits
4. When something absorbs radiation it heats up
5. Warm things emit radiation (2. again)
6. Other things absorb some of that radiation (3. again)
1. Some of the radiation from the atmosphere (see 5.) is absorbed by the Earth, which warms it (see 4.)
2. Some of the radiation from the atmosphere escapes into space, but less than if the atmosphere hadn't absorbed it on its way from the surface and sent some of it back down. 

Let's look at that process in more detail:

1. The Earth is a big warm rock. The average surface temperature is about 14.5 degrees C (287.5 degrees K). This is the near-surface atmospheric temperature (as would be measured by a thermometer at a weather station) averaged the seasons, over day and night, and over the geography of the earth. (See Wikipedia article Instrumental temperature record.) The Earth is warmed by radiation from the Sun that it absorbs, and by its own internal heat, some left over from its formation and some from radioactive decay of elements it is made from.

2. Warm things emit radiation. Any object radiates heat, in the form of electromagnetic radiation. Everybody is familiar with the idea of an object being "red hot". An object that hot emits enough light that we can see it, mostly in the infrared part of the spectrum but some at long visible wavelengths.  When an object is "white hot" it emits even more radiation, including a lot of visible light and a substantial amount of ultraviolet radiation.

2a. The warmer a thing is the more radiation it emits. This is known as the Stefan–Boltzmann law. (See Wikipedia article Thermal radiation.)

You can easily demonstrate this. If you hold your hand near a toaster (not in a toaster!), where the radiation from the toaster's coils can hit it, the radiation from the toasters coils will be absorbed by your hand. Your skin will be warmed by this radiation, and the more radiation there is the more it will be warmed.

When the toaster is off and the coils are at room temperature you won't feel the warming of your skin. The nerves in your skin don't do much when they are just at room temperature.

(Everything that is not at a temperature of absolute zero emits radiation. But hotter things emit a lot more radiation than colder things. Notice that the Stefan-Boltzmann law says that the amount of energy radiated is proportional to the fourth power of the temperature: j*=εσT⁴. So coils glowing red hot--about 1,000 K--are hotter than coils at room temperature of about 293 K, about 700 degrees Kelvin hotter. They are three times as hot, but they emit more than 100 times as much radiation.)

But when the toaster is on, and the coils are glowing red hot, they will emit a lot of infrared radiation (and a little visible radiation). Your hand would be warmed noticeably as it absorbed this greater quantity of radiation.  

At 287.5 degrees Kelvin (14.5 degrees Celsius) most of the radiation the Earth emits is infrared radiation. None of it is in the visible range. (Even at 45 degrees Celsius, a really hot day, none none of the Earth's radiation is in the visible range. This is why the Earth does not appear to glow on a really hot night.)

3. The atmosphere absorbs radiation. Molecules of the gases that make up our atmosphere absorb radiation. Obviously the atmosphere doesn't absorb all wavelengths of radiation equally. It doesn't absorb much in the range of visible light (it is transparent to visible light). This is why we can see the Sun, Moon and stars. The oxygen and ozone in the atmosphere absorb a lot of ultraviolet radiation coming from the Sun. That is why organisms can live on the surface of the Earth (UV kills microorganisms and causes skin cancer in people, for example).

The atmosphere is mostly oxygen and nitrogen, but it is about 0.035% carbon dioxide. Carbon dioxide absorbs infrared radiation very strongly. Since the warm Earth emits mostly infrared radiation (with a peak at wavelengths of about 10^-5 meters) and CO₂ absorbs infrared radiation (especially that with a wavelength longer than about 1.3x10^-5 meters) you can see that a lot of the infrared radiation from the surface of the Earth is absorbed by CO₂ in the atmosphere. (The situation is similar for other "greenhouse gases" such as CFCs, nitrous oxide, methane and water vapor.) (See Greenhouse Gas Absorption Spectrum.)

4. When something absorbs radiation it heats up. The photons of radiation can interact with matter. How they interact depends on the properties of the photon (wavelength) and the properties of the atom or molecule of matter. (At the wavelengths we are talking about these properties mainly have to do with the energy states of its electrons.) When substances absorb radiative energy they increase their thermal energy. They get warmer.

2. again: Warm things emit radiation (see above). The warmer they are the more radiation they emit. The warmed greenhouse gases emit more infrared radiation than they did when it was cooler (see 2a. above). Some of that radiation escapes into space. Some is absorbed by other parts of the atmosphere. And some if it is absorbed by the Earth below, making it a little warmer (see 4. above).

So that's how the "greenhouse effect" works:

• Radiation from the Sun warms the Earth and the atmosphere
• The Earth emits infrared radiation (the warmer it is the more it emits)
• The "greenhouse gases" in the atmosphere, especially CO₂, absorb some of that radiation from the earth, and this warms them up (the more of these gases there are the more they absorb)
• The warm gases in the atmosphere emit infrared radiation (and the warmer they are, and the more of them there are, the more they emit)
• Some of the infrared radiation from the "greenhouse gases" in the atmosphere is absorbed by the Earth, warming it a little more
• Loop back and repeat

So it doesn't work at all like a greenhouse. (Not that most people have much idea how greenhouses, also called glasshouses, work, or even what they are.) It isn't a result of the atmosphere "insulating" the Earth, like a blanket. It has to do with the gases in the atmosphere absorbing radiation, heating up, and warming the Earth below by their own radiation. But the names "greenhouse effect" and "greenhouse gases" are well established, so we might as well go ahead and use them.

The Net Result

The diagram below shows how all this works in terms of the Earth's energy balance.

As you can see, the net imbalance (energy in minus energy out) is only 0.9 Watts per square meter. (Other estimates give net forcing of approximately 1 to 3 Watts per square meter.) Whatever the actual figure, it is enough to warm the Earth significantly over time.

Consequences

Also note that the net absorbed is only 0.26% of the total incoming energy flux. It only takes a small change in the transparency of the atmosphere in the infrared to change the outgoing long-wave (infrared) radiation enough to affect the surface temperature. And of course the more greenhouse gases in the atmosphere, the greater the climate forcing.

How much will the Earth's temperature rise because of this 1 or 2 Watt per square meter forcing? That is the subject of urgent research. Current thinking is that the level of greenhouse gas in the atmosphere today, if we didn't put any more up there, might lead to a further increase in global temperature of one or two degrees Celsius or so.

But there are many difficulties with this estimate:

1. Feedback effects. As the Earth warms up there will be changes in the global energy balance that might make it warm faster or slower, such as
1. Melting of ice. Ice is highly reflective, so if it melts to expose bare ground or open sea less incoming solar radiation will be reflected back to space ("Reflected by Surface" in the diagram above).
2. Vaporization of methane in permafrost and undersea deposits. Methane is one of the most significant greenhouse gases, and there are vast amounts of it tied up in frozen permafrost or in deposits of methane hydrates on the sea floor. If warming of the sea causes melting and release of some methane hydrates this could vastly decrease the transparency of the atmosphere to long-wave radiation, keeping more heat on the Earth. It is thought that when this happened in the geologic past it led to an earth many degrees hotter than today's.
3. Warming seas. Cold water can absorb more carbon dioxide than warmer water. So as that 1 Watt per square meter radiative forcing warms the oceans, more carbon dioxide will be left in the atmosphere to act as a greenhouse gas.
4. Clouds. As the climate warms the weather will change. There might be more clouds, or less, or their distribution might change. Clouds are an important part of the "Reflected by Clouds and Atmosphere" component of the energy budget in the diagram.
5. Water vapor. Warmer air can hold more water vapor, and water vapor is a significant greenhouse gas.
2. Continued emissions.
1. We have already perturbed the planet's energy balance by putting around a trillion tonnes of greenhouse gases into the atmosphere over the past couple of centuries by burning fossil fuels and by land-use changes (burning forests). That is why the energy budget is out of balance by one or two Watts per square meter. But we continue to put about 35 billion additional tonnes of greenhouse gases into the air every year. And that annual emission figure continues to increase. So the concentration in the atmosphere will increase, the transparency of the atmosphere to infrared radiation will decrease, and the greenhouse climate forcing will increase. We are on course to put at least another trillion tonnes of greenhouse gases into the atmosphere by 2050, probably closer to two trillion.

I hope you can see why putting more CO2 into the atmosphere, more than the land and the seas can reabsorb, could make the atmosphere, and thus the Earth, warmer, perhaps much warmer.

---

The toaster image is from Explain that Stuff published under a Creative Commons License.

The energy balance diagram is from this UK government site, and is protected by Crown copyright. Used by permission.

Here is another explanation of the greenhouse effect.

David Wheat's Science In Action site has articles about science and math in the real world, weird science, science news, unexpected connections, and other cool science stuff. There is an index of the articles by topic here.

484TFZMUEH2N

Latent Heat--Sweat, Storms and Cooling Towers

If you don't understand "latent heat" you can't understand how much of the biosphere or a lot of engineering works. The latent heat of water is the energy absorbed when water is evaporated, or released when it condenses. Weather, thermoregulation, global warming and industrial cooling all depend on the high latent heat of water and its ability to transform heat to work and vice versa, and to move energy from one place to another.

Water is Magic

Water is marvelous stuff and has many interesting properties. This is a good thing (for us) since these properties are necessary for life (as we know it) to exist.

(This is an example of the anthropic principle--we are only able to observe water's interesting properties because we exist, and we are only able to exist because of water's interesting properties. So it is unavoidable that in our world--in any world where we could have evolved-- water must have such interesting properties.)

Among water's most important properties is its high latent heat. This property creates much of Earth's most violent weather and drives the thermodynamics of climate.

What is "Latent Heat"?

To understand latent heat you first have to understand the idea of states of matter and phase changes.

Chemical substances can exist in several "states". The common ones that we encounter in everyday life are the solid state, the liquid state, and the gaseous state. When matter changes from one state to another we call it a "phase change". So liquid water can undergo a phase change to become the gas water vapor, and it can reverse that transition and condense from a gas into a liquid. It can undergo a different phase change from a liquid to become solid ice, and the reverse to melt from ice into liquid. See the diagram below.

The names of the various common states of matter
and of the phase transitions between them

The reason this is important (to us and our planet) is that it takes a lot of energy to change water from a liquid to a gas. This is because water molecules in a liquid are attracted to each other because of their polarity.

illustrative separation of
charges on water molecule:
negative red, positive blue

Water molecules are highly polar molecules. This means they have uneven distribution of their electrons, with more electrons bunching around the oxygen atom and thus creating lower electron density around the hydrogen atoms. So the molecule is more negative on one side (where the electrons are concentrated) and more positive on the other side. It is like a little magnet, a dipole. As you are no doubt aware, "opposites attract". So the negative side, or middle, of a water molecule will tend to attract the positive sides, or ends, of other water molecules. This is an example of "hydrogen bonding". Hydrogen bonding is extremely important in the machinery of life. This attraction tends to hold the water molecules in liquid water together. To evaporate water--to make some of those molecules break away from the liquid mass and fly off as a gas--takes a lot of energy.

To evaporate one kilogram of water by boiling it, changing it from liquid to gas at 100 degrees C, takes 2,260 kilojoules. That is about two and a half times as much energy as is needed to vaporize a kilogram of ethyl alcohol.

Latent Heat in the Kitchen

Pot of boiling water

You are undoubtedly familiar with the large amount of heat needed to bring about this phase transition of water from liquid to gas. Imagine a pot of boiling water. To keep it on the boil lots of heat has to be supplied. As soon as the heat is reduced it stops boiling and steam stops coming off. Consider the flame or other heat source that is needed to keep it boiling. You wouldn't want to contact such a concentrated heat source directly. (Warning: please do not put your hand on the stove to confirm this!)

The truly amazing thing about the latent heat of evaporation of water is that when the phase change is reversed, when water vapor condenses into liquid water, the same amount of heat is released. This isn't as obvious to us as the amount of heat consumed in boiling, but you may have experienced it if you have gotten your hand in the stream of steam escaping from a teakettle.

This is the reasons that burns caused by steam can be so severe. Besides the heat of the steam, some of the steam will condense on the skin, releasing its latent heat of condensation. This is equal to the latent heat of vaporization of the same amount of water. You wouldn't want to put your hand in the flame needed to vaporize even a small amount of water. But when that small amount of water condenses out of steam on your skin it releases just that amount of heat. This is why a burn from steam can be more severe than a burn by boiling water itself, if the quantity of steam is significant.

How Latent Heat Drives Storms

So when water evaporates it takes up heat (cooling the local environment). As water vapor it carries that heat around as latent heat. Then when that vapor condenses it releases that latent heat, heating up the local environment, usually the air.

This is what drives some types of storms, including thunderstorms, tornadoes, hurricanes and typhoons. Such storms are driven by "heat engines" based on water vapor. The key to such systems is rising warm air containing water vapor. As it rises it expands (because the atmospheric pressure is lower the higher you go) and as it expands it cools (the same amount of heat is spread through a larger volume--adiabatic cooling).

At some point the parcel of moist air has cooled enough that it cannot hold all the water vapor it contains. (The amount of water vapor air can hold is strongly dependent on its temperature.) So some of the water vapor condenses out as water droplets--clouds, rain or snow. As that water vapor condenses to liquid it releases heat (the latent heat of condensation or latent heat of fusion), warming the parcel of air. Because of this warming, the moist parcel of air will be warmer and more buoyant than neighboring air, so it will continue to rise.

As it rises and expands more condensation will occur, continuing the process. (This gives rise to towering "thunderhead" cloud formations.) Essentially this creates a strong updraft as water condenses out of the rising air. This updraft causes locally lower air pressure below it and sucks in surrounding air to fill the gap, creating surface wind--the storm as we experience it. (There may also be downdrafts associated with falling precipitation.)

Without the heat released by the condensation of water vapor these systems couldn't grow to their towering size.

Tropical cyclone driven by energy
released by condensation of moisture

In a tropical cyclone (hurricane or typhoon) more warm moist air is drawn into the cyclone as it moves over warm ocean waters, feeding and perpetuating the system. This is why hurricanes can grow so large, persist so long, and have such high winds. "A tropical cyclone's primary energy source is the release of the heat of condensation from water vapor condensing, with solar heating being the initial source for evaporation. Therefore, a tropical cyclone can be visualized as a giant vertical heat engine supported by mechanics driven by physical forces such as the rotation and gravity of the Earth. [source Wikipedia]"

Latent Heat of Evaporation and Climate Change

There are several processes where the latent heat of water becomes important in trying to understand climate change associated with global warming.

Tropical Cyclones
Increases in sea surface temperatures could affect the formation and behavior of hurricanes. As noted above, warm ocean waters put a lot of moisture into the air (the air can hold more moisture at higher temperature), and it is water vapor in the air that makes the hurricane heat engine work once one gets started. Wider areas of warm ocean waters could mean tropical cyclones will form in places they didn't form before.

There is a lot of scientific discussion about this because warming causes other simultaneous changes. For instance, hurricanes can't form if the local winds are too high--only where there are just light breezes. Will warming change the distribution of winds over warm areas of the seas?

There is some evidence that Atlantic hurricane numbers have been rising (previous post) but this is still in dispute.

If seas are warmer they might also contribute to the strength of tropical cyclones that do form. This previous post discusses research that suggests increased destructiveness of hurricanes associated with warmer seas. This question is still not settled though.

Cooling By Irrigation
Because of the latent heat of water, more evaporation means more cooling in some places, and more rain means more warming in other places. A recent article in the Journal of Geophysical Research (pdf here, New York Times Green blog post about it here) says irrigation may be causing cooling in some regions, locally masking the effects of global warming.

The model runs reported in this paper suggest that parts of norther India may have experienced several degrees of cooling due to all the heat absorbed by irrigation water applied to crops in the later part of the 20th century. Weather patterns may even have been affected enough to reduce the amount of rain in the Bay of Bengal branch of the Southwest Monsoon. (Other researchers got somewhat different or even contradictory results with different models.)

This is a bit scary because if groundwater depletion leads to reduction in irrigation in the future, the reduction of cooling effect could have both local an regional climate effects, including sharply higher temperatures and changes in rainfall amounts and distribution.

Evaporative Thermoregulation

Evaporation is used to cool the bodies of many animals. Sweat evaporating from the skin makes it possible for us to deal with hot weather. On a hot day in dry weather a person can lose more than a liter of water by evaporation of sweat (even several liters if it is really hot or you are exercising). Think of the amount of heat it would take to boil away a liter of water.

Other Uses of Latent Heat

There are many other uses of the latent heat of water for cooling, for example:

• Evaporative coolers, a kind of air conditioning.
• Some cooling towers at power plants
  
  

  

  Cooling towers at Didcot Power Station,

  and other industrial facilities use evaporative cooling. Steam turbines require a condenser to cool the steam after it leaves the turbine so that it condenses into water and can be pumped back through the cycle. Such condensers often use evaporative cooling by spraying or dripping water over coils carrying hot water from the system. At big power plants these may be enclosed in characteristic hyperboloid chimney-like structures to provide draft to move the moisture-laden air out of the cooling unit. Other systems use fans. [Here is a video of the inside of a cooling tower showing water being sprayed over cooling circuits.]

Understand latent heat and many phenomena will be less mysterious to you.

---

The diagram of phase changes is a public domain image from Wikimedia Commons. Rights information here.

The illustration of charges on a water molecule has been placed in the public domain by its author, from Wikimedia Commons. Rights information here.

The picture of the pot of boiling water is from Wikimedia Commons, with the permission of the copyright holder under the terms of the GNU Free Documentation License.

Diagram of tropical cyclone by Jannev, placed in the public domain at Wikimedia Commons.

Picture of Didcot Power Station by Dave Price from Wikimedia Commons, used under a Creative Commons Attribution Share-alike license 2.0

David Wheat's Science In Action site has articles about science and math in the real world, weird science, science news, unexpected connections, and other cool science stuff. There is an index of the articles by topic here. 

Plants Unhappy About Global Warming

Rice field in Bangladesh

New science raises serious concerns about the negative impact of global warming on crop yields and plant productivity in general.

This could be one of the most severe social and economic effects of climate change.

Rice Yields Hurt By Warming

Researchers from the University of California, Duke, National Bureau of Economic Research, IRRI and FAO published a very revealing paper in PNAS. They studied 227 intensively managed irrigated rice farms in six important rice-producing countries over several years. Their findings "imply a net negative impact on yield from moderate warming in coming decades. Beyond that, the impact would likely become more negative, because prior research indicates that the impact of maximum temperature becomes negative at higher levels." Rising temperatures, especially nighttime temperatures, will hurt rice yields.

The paper is behind a pay wall, but there is a good BBC News article on their results. It says they "found that over the last 25 years, the growth in yields has fallen by 10-20% in some locations, as night-time temperatures have risen. ... Although yields have risen as farming methods improved, the rate of growth has slowed as nights have grown warmer." And "if temperatures continue to rise as computer models of climate project, Mr Welch says hotter days will eventually begin to bring yields down."

The question is whether rice improvement efforts (plant breeding) can get ahead of the negative effects of rising temperatures.

This EurekAlert release summarizes the results.

Net Plant Primary Production Down

Researchers at the University of Montana studied terrestrial net primary production. Net primary production (NPP) is the total net fixation of carbon by photosynthesis in an ecosystem. They found that "Large-scale droughts have reduced regional NPP, and a drying trend in the Southern Hemisphere has decreased NPP in that area, counteracting the increased NPP over the Northern Hemisphere."

These results were surprising since earlier studies had shown increasing plant carbon capture with rising temperatures in the 80s and 90s. However temperatures since 2000 have been the highest in modern records and accompanying droughts have apparently cut into global plant growth.

Again the Science article is not open access, but this EurekAlert release has some more information on the results and their implications.

While longer growing seasons and higher atmospheric carbon dioxide levels may favor more carbon fixation in some northerly regions, more of the globe is water-limited and more drought could hurt total carbon fixation more than warming trends would boost it. As the authors say in their abstract, "A continued decline in NPP would not only weaken the terrestrial carbon sink, but it would also intensify future competition between food demand and proposed biofuel production."

Plankton Declining With Warming Seas

Researchers from Dalhousie University studied the concentrations of phytoplankton in the oceans. Writing in Nature report "declines in eight out of ten ocean regions, and estimate a global rate of decline of ~1% of the global median per year". "We conclude that global phytoplankton concentration has declined over the past century" and "long-term declining trends are related to increasing sea surface temperatures." Since phytoplankton, minute plants, "account for approximately half the production of organic matter on Earth" this could be bad news.

Marine phytoplankton

According to a Reuters story, "The study estimates the decline in marine algae has been approximately 40 percent since 1950." Half of all photosynthetic carbon fixation, cut by 40%!? That's significant and scary.

The story quotes study co-author Boris Worm: "I think that if this study holds up, it will be one of the biggest biological changes in recent times simply because of its scale. The ocean is two-thirds of the earth’s surface area, and because of the depth dimension it is probably 80 to 90 percent of the biosphere. Even the deep sea depends on phytoplankton production that rains down. On land, by contrast, there is only a very thin layer of production."

Here is an excellent release in Science Daily summarizing the report.

Yield Reductions in China?

A review paper in Nature by Shilong Piao et al. assesses "the impacts of historical and future climate change on water resources and agriculture in China. They find that in spite of clear trends in climate (especially temperature), overall impacts are overshadowed by natural variability and uncertainties in crop responses and projected climate, especially precipitation. In a best-case scenario, crop production is constant, whereas the worst-case scenario suggests that production could fall by about 20% by 2050." (From Editor's Summary.)

A Reuters article quotes further from the paper, "Countrywide, a 4.5 percent reduction in wheat yields is attributed to rising temperatures over the period 1979-2000," and says "They forecast that rice yields would decrease by 4 to 14 percent, wheat by 2 to 20 percent and maize by zero to 23 percent by the middle of the 21st century."

(Grist carries an AFP story about this research.)

What Does It Mean?

These results from several unrelated fields of research suggest that we should be concerned that continued warming will negatively affect both wild plants (which act as a carbon dioxide sink) and agriculture (fundamental to social stability).

If forests, grasslands, phytoplankton in the sea and other ecosystems absorb less of the CO₂ we release by unrestrained burning of fossil fuels, then atmospheric CO₂ levels may rise faster than models currently predict.

If higher temperatures and drought reduce agricultural output more land will have to be brought under the plow. Such land-use changes usually release significant additional carbon dioxide.

We should significantly increase spending on agronomy and plant breeding, especially in Africa, India and East Asia, if we want to maintain the yields we have.

---

[Crossposted from sister blog A Very Different Earth.]

David Wheat's Science In Action site has articles about science and math in the real world, weird science, science news, unexpected connections, and other cool science stuff. There is an index of the articles by topic here. 


3c1054119c8bc8d5bb7eadf1de68128f