Does the Earth's atmosphere primarily behave like an actual greenhouse?
No. The term "greenhouse effect" is unfortunate since it often results in a totally false impression of the activity of so-called "greenhouse gases." An actual greenhouse works as a physical barrier to convection (the transfer of heat by currents in a fluid) while the atmosphere facilitates convection. So-called "greenhouse gases" in the Earth's atmosphere do not act as a barrier to convection so the impression of actual greenhouse-like activity in the Earth's atmosphere is wrong.
Forgetting about the unfortunate-but-commonly-used terminology for a moment, is the so-called 'greenhouse effect' bad?
Only if you think undesirable a habitable planet with relatively stable temperature. Our moon, lacking greenhouse effect, makes a kind of comparison even though lack of atmosphere makes it uninhabitable regardless of temperature. The moon's mean surface temperature by day is 107 ?C (225 ?F) and by night drops to -153 ?C (-243 ?F). The Lunar temperature increases about 260 ?C from just before dawn to Lunar noon. So, if you fancy such a temperature range then a greenhouse effect-free world is for you, otherwise you might want to be pleased we have it here on Earth.
How much does the so-called 'greenhouse effect' warm the Earth?
It's estimated that the Earth's surface would be about -18 ?C (0 ?F, 255 K) with atmosphere and clouds but without the greenhouse effect and that the (we'll call it "natural") greenhouse effect raises the Earth's temperature by ~33 ?C (59 ?F). Devoid of atmosphere it would actually be a less cold -1 ?C (272 K) because the first calculation strangely includes 31% reflection of solar radiation by clouds (which could obviously not occur without an atmosphere) while clouds actually add significantly to the greenhouse effect - for simplicity, just stick with ~33 ?C.
Theoretically, if the planet's surface cooled by radiation alone, then the greenhouse-induced surface temperature would be much warmer, about 350 K (77 ?C), but atmospheric motion (convective towers carrying latent and sensible heat upwards and large scale circulation carrying it both upwards and polewards) significantly increase the "escape" of energy to space, leaving Earth's surface more than 60 ?C cooler than a static atmosphere would do.
So, despite there being far more greenhouse gas in the atmosphere than required to achieve the current greenhouse effect, and that has been so since before humans discovered fire, evapo-transpiration and thermals transport heat higher in the atmosphere where radiation to space is increased. This is why Earth remains about 15 ?C rather than about 77 ?C.
Is 'greenhouse' the same as 'global warming'?
Absolutely not. We'll look at both terms below.
What about 'climate change' then?
That's a different thing altogether. Change is what the climate is always doing and is the result of our planet's orbital eccentricities, axial wobble, solar brightness variation, cosmic ray flux, etc.. There are also plausible terrestrial drivers of climate change too, including super volcanic events and tectonic movement, but these are not in the realm of anthropogenic (manmade) effects and so we won't looking at them here.
The global mean temperature over which there has been so much obsession is only one part of climate -- for example, how wet or dry the climate happens to be is probably of far greater significance than a simple mean temperature -- in fact, it's not even clear that a global mean temperature is a particularly useful metric. However, it is the cause of great angst at present so it will remain the focus of this document for that reason alone.
So, greenhouse is all about carbon dioxide, right?
Wrong. The most important players on the greenhouse stage are water vapor and clouds. Carbon dioxide has been increased to about 0.038% of the atmosphere (possibly from about 0.028% pre-Industrial Revolution) while water in its various forms ranges from 0% to 4% of the atmosphere and its properties vary by what form it is in and even at what altitude it is found in the atmosphere. In simple terms, however, the bulk of Earth's greenhouse effect is due to water vapor by virtue of its abundance. Water accounts for about 90% of the Earth's greenhouse effect -- perhaps 70% is due to water vapor and about 20% due to clouds (mostly water droplets), some estimates put water as high as 95% of Earth's total greenhouse effect. The remaining portion comes from carbon dioxide, nitrous oxide, methane, ozone and miscellaneous other "minor greenhouse gases." As an example of the relative importance of water it should be noted that changes in the relative humidity on the order of 1.3-4% are equivalent to the effect of doubling CO2.
The adjacent radiation absorption window graphic gives an idea of which molecules absorb various wavelengths. Where the shaded portions completely span between 2 lines it indicates that particular wavelength is fully absorbed and the "window" is saturated (or said to be "closed"). Rather obviously, once a window is saturated adding more gases with the same properties will do nothing. This point seems to cause confusion for some people so perhaps consider multiple shades on a window with each shade blocking half the light coming through - pull one shade and you reduce the light source by half, pull another so you block half the light coming through the first shade, etc.. The effect of each shade diminishes as you keep adding more and eventually you get no additional effect - you have saturated or blocked the radiation window and it makes no difference if you double or quadruple the number of shades again.
Well, I heard that carbon dioxide is bad -- it's pollution, isn't it?
There seem to be a few things that your informant forgot to tell you -- like carbon dioxide being an essential trace gas that underpins the bulk of the global food web. Estimates vary, but somewhere around 15% seems to be the common number cited for the increase in global food crop yields due to aerial fertilization with increased carbon dioxide since 1950. This increase has both helped avoid a Malthusian disaster and preserved or returned enormous tracts of marginal land as wildlife habitat that would otherwise have had to be put under the plow in an attempt to feed the growing global population. Commercial growers deliberately generate CO2 and increase its levels in agricultural greenhouses to between 700ppmv and 1,000ppmv to increase productivity and improve the water efficiency of food crops far beyond those in the somewhat carbon-starved open atmosphere. CO2 feeds the forests, grows more usable lumber in timber lots meaning there is less pressure to cut old growth or push into "natural" wildlife habitat, makes plants more water efficient helping to beat back the encroaching deserts in Africa and Asia and generally increases bio-productivity. If it's "pollution," then it's pollution the natural world exploits extremely well and to great profit. Doesn't sound too bad to us.
But we're responsible for all the carbon dioxide greenhouse effect?
Gracious no! Humans can only claim responsibility, if that's the word, for abut 3.4% of carbon dioxide emitted to the atmosphere annually, the rest of it is all natural (you can see the IPCC representation of the natural carbon cycle and human perturbation here or a simple schematic from Woods Hole here). Half our estimated emissions fail to accumulate in the atmosphere," "disappearing" into sinks as yet undetermined. Humans' total accumulated carbon contribution could account for perhaps 25% of the total non-water greenhouse gases (that is, accounting for all the increase since the Industrial Revolution regardless of source and irrespective of whether warming from any cause might result in an increase in natural emission to atmosphere -- we're simply claiming the lot as anthropogenic or human-caused here).
Clarification June 4: the mention of 25% of total non-water greenhouse effect above and the following mention of 2.5% of total greenhouse effect has confused a few readers, leading to some e-mails suspecting one or the other to be a typographical error. The figures are correct. Recall that water vapor accounts for about 70% and clouds (mostly water droplets) accounts for another 20%, thus water in it's various forms is 90% of the greenhouse effect, leaving 10% for non-water greenhouse effect. Of this remaining 10%, mainly atmospheric carbon, humans might be responsible for 25% of the total accumulated atmospheric carbon, thus 0.25 x 0.1 = 0.025 x 100 = 2.5% of the total greenhouse effect.
Ah, we've added 2.5% to the total greenhouse effect then?
Not exactly, if it were such a simple accumulation, we could easily determine exactly how much Earth would warm from a doubling of atmospheric CO2 (not much) and certainly that would be an improvement on the silly figures bandied about. Theoretically, in a dry atmosphere, carbon dioxide could absorb about three times more energy than it actually does, as could clouds in the absence of all other greenhouse gases -- look at it as there already being "competition" for available suitable longwave radiation (energy these gases can absorb), if you like. Readers should be aware that the temperature effect of atmospheric carbon dioxide is logarithmic (that means there is a diminishing response as you keep adding more, like the additional window shade example, above). If we consider the warming effect of the pre-Industrial Revolution atmospheric carbon dioxide (about 280 parts per million by volume or ppmv) as 1, then the first half of that heating was delivered by about 20ppmv (0.002% of atmosphere) while the second half required an additional 260ppmv (0.026%). To double the pre-Industrial Revolution warming from CO2 alone would require about 90,000ppmv (9%) but we'd never see it - CO2 becomes toxic at around 6,000ppmv (0.6%, although humans have absolutely no prospect of achieving such concentrations).
Yep there is more but it will just bore you. Basicly it is like I said.
SO WHAT!!!!
