To My Grandson: On Greenhouse Gases
This post is in response to questions from my grandson with a homework assignment regarding global warming.
How do Greenhouses function?
A greenhouse is typically made of glass walls and ceiling, which allows the sun to warm the surface (land and plants) which then warms the air. Warmer air rises but is unable to leave the enclosure. Thus the air temperature inside is warmer than outside. The action of air to move heat away from warmer objects is called “convection”, and greenhouses function by preventing convection. The greenhouse effect is also seen in a car left in the sun with the windows closed. Once the air is again allowed to circulate, the outside temperature is restored inside.
What are “Greenhouse Gases?”
99% of the atmosphere is N2 and O2, diatomic molecules (2 atoms), and they are unaffected by radiation in the Infrared (IR) range. Some molecules, principally H2O and Co2 are triatomic (3 atoms) and do absorb and emit radiation in the IR range. The correct term for these gases is “IR active gases,” rather than “greenhouse gases.”
And people are not always forthcoming about the proportions, even forgetting to mention H2O is the predominant IR active gas (9 to 1). This diagram shows the reality:
Why compare IR active gases to greenhouses?
The earth’s surface is constantly warmed by the sun and then warms the air in contact with it (“conduction”). And except for enclosures, the air is free to rise and be replaced by descending cooler air, which is warmed in turn. Convection operates to move surface heat upward toward space. No gas impedes this process.
The notion of ”greenhouse gases” is a claim that IR active gases form a radiative ceiling which delays the rising heat and makes the atmosphere and surface warmer as a result. Some have claimed that this radiative effect causes the surface to be at an average temperature of +15C, compared to -18C without IR active gases. This notion ignores important scientific factors operating in our climate system.
My understanding of how and why earth is warmer than the radiative balance temperature of -18C.
The build up of heat at the surface is caused by a delay in heat transport from surface to space. Surrounded by the nearly absolute cold of space, our planet’s heat must move in that direction, which involves pushing it through the atmosphere. And there is an additional delay within the oceans from the overturning required to bring energy to the surface for cooling. The oceans are a massive thermal energy storage place (1000 times the atmosphere’s heat capacity) which moderates swings in energy and ensures that earth temperature fluctuations are small.
The Difference between climate on the Earth and the Moon
The intensity of solar energy is the same for the Earth and Moon, yet the dark side of the earth is much warmer than the dark side of the moon. And the bright side of the earth is much cooler than the bright side of the moon. Why are the two climates so different?
Earth’s oceans and atmosphere make the difference. Incoming sunlight is reduced by gases able to absorb IR and also by reflection from clouds and non-black surfaces. The earth’s surface is heated by sunlight, much of it stored and distributed by the oceans (71% of the planet surface). The atmosphere delays the upward passage of heat, and like a blanket slows the cooling allowing a buildup of temperature at the surface until there is a balance of heat radiating to space from the sky to match the solar energy coming in.
How the Atmosphere Processes Heat
There are 3 ways that heat (Infrared or IR radiation) passes from the surface to space.
1) A small amount of the radiation leaves directly, because all gases in our air are transparent to IR of 10-14 microns (sometimes called the “atmospheric window.” This pathway moves at the speed of light, so no delay of cooling occurs.
2) Some radiation is absorbed and re-emitted by IR active gases up to the tropopause. Calculations of the free mean path for CO2 show that energy passes from surface to tropopause in less than 5 milliseconds. This is almost speed of light, so delay is negligible. H2O is so variable across the globe that its total effects are not measurable. In arid places, like deserts, we see that CO2 by itself does not prevent the loss of the day’s heat after sundown.
3) The bulk gases of the atmosphere, O2 and N2, are warmed by conduction and convection from the surface. They also gain energy by collisions with IR active gases, some of that IR coming from the surface, and some absorbed directly from the sun. Latent heat from water is also added to the bulk gases. O2 and N2 are slow to shed this heat, and indeed must pass it back to IR active gases at the top of the troposphere for radiation into space.
In a parcel of air each molecule of CO2 is surrounded by 2500 other molecules, mostly O2 and N2. In the lower atmosphere, the air is dense and CO2 molecules energized by IR lose it to surrounding gases, slightly warming the entire parcel. Higher in the atmosphere, the air is thinner, and CO2 molecules can emit IR and lose energy relative to surrounding gases, who replace the energy lost.
This third pathway has a significant delay of cooling, and is the reason for our mild surface temperature, averaging about 15C. Yes, earth’s atmosphere produces a buildup of heat at the surface. The bulk gases, O2 and N2, trap heat near the surface, while IR active gases, mainly H20 and CO2, provide the radiative cooling at the top of the atmosphere. Near the top of the atmosphere you will find the -18C temperature.
It is wrong to claim that IR active gases somehow “trap” heat when they immediately emit any energy absorbed, if not already lost colliding with another molecule. No, it is the bulk gases, N2 and O2, making up the mass of the atmosphere, together with the ocean delaying the cooling and giving us the mild and remarkably stable temperatures that we enjoy.
More on climate theory here: