There are many, many reasons that I do not like Al Gore. Simply put, the man is a giant douche bag. And as if everyone didn't remember this fact, his self-agenda-pushing little movie serves as a great reminder. This was posted over at Teh Shakc earlier, cause I like that site more.

Basic misconceptions that must be addressed include:

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.

A physical greenhouse is simply a contained subset of the atmosphere - it is not bounded by the near-vacuum of space as is the planet's atmosphere and so has rather different properties. The proof that convection containment is critical to the function of physical greenhouses is that it is possible to create structures with similar radiative properties, one which allows convective activity between the structure and unconstrained atmosphere and one which does not. Only the structure constraining internal-external convection will function as an effective greenhouse. Greenhouse gases categorically do not inhibit convective activity and so are not like a physical greenhouse.

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.

Wait a minute! Those aren't the numbers I learned!

Reference works frequently list the planet's mean surface temperature as 16 °C (61 °F); sometimes 15 °C (59 °F) is mentioned and yes, these are about the expected temperatures by calculation -- in the 1960s and 1970s numbers as high as 65 °F (18 °C) were popular but we haven't seen those for some time. Here we run into a little bit of a problem, however -- taking the Earth's temperature is no trivial task. In fact, even defining precisely what we mean by the absolute surface air temperature is challenging. Current global temperature anomalies (the amount of warming or cooling reported) are estimated against an expected average of 14 °C (57 °F) -- the guess-timated mean temperature over the period 1961-1990.

Are greenhouse gases like a blanket around the Earth?

No, for the same reason that they don't behave like an actual greenhouse, they simply do not behave as a barrier to convective activity and so aren't "like a blanket."

Do greenhouse gases trap the sun's radiation/'heat'?

Not to any great extent. The Sun, being much hotter than Earth, emits high energy, shortwave radiation while Earth, in response, emits longwave radiation. The cooler the portion of the Earth or atmosphere, the lower energy intensity, longer wave radiation is emitted -- that old white hot, yellow hot, red hot thing. Greenhouse gases are generally transparent to incoming solar radiation -- they let most solar radiation through -- and opaque to Earth's radiation -- they absorb and transfer the Earth's infrared radiation by a variety of means. That said, oxygen and ozone do absorb incoming Ultraviolet (UV) radiation (<0.3µm) and water, ozone, oxygen and, to a tiny extent, carbon dioxide also absorb a small amount of incoming shortwave below the 3 micron (µm) wavelength range and it is mostly the UV absorption by ozone that causes warming in the stratosphere above the tropopause. The tropopause is the boundary between the troposphere (which is based at the earth's surface and has temperature that decreases with height, extending about 10-50Km or 6-30 miles above the surface) and the stratosphere (which is a stable region of very low levels of vertical mixing above the troposphere).

Greenhouse gases, therefore, do not "trap heat," but could be fairly described as delaying the energy transfer from Earth to space. "Trapping heat" implies that the energy is stuck in the system forever -- this is a false notion. Greenhouse gases do not emit energy in the same bandwidth that they absorb energy, and thus emissions from carbon dioxide are not absorbed by carbon dioxide. While energy may be delayed on its inevitable journey back to space, it will eventually be emitted regardless of the number of intervening stages.

Do greenhouse gases 'reradiate' the infrared radiation they absorb?

This is an unfortunate expression that is all too common. Absorbed radiation is transformed to either kinetic or potential energy and, as such, no longer exists in its original form -- hence, it cannot be "reradiated." When molecules absorb infrared radiation they are said to become excited ("hot"). Such molecules can release energy usually in one of three ways: by chemical reaction (uncommon, since greenhouse gases are pretty stable and non-reactive); quenching (transferring energy to cooler molecules, increasing their temperature) and; emission (usually at lower energy (longer wavelength) radiation than the energy previously absorbed). Once more, since the absorbed energy has been transformed it cannot be said to be "reradiated".

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 be 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 for that reason alone.

Okay, if that's greenhouse, what is 'global warming'?

While greenhouse is the "what," "global warming" really refers to the "how much." Populist overuse and abuse has largely rendered "global warming" meaningless -- what is really meant is "enhanced greenhouse" -- yes, another term but don't worry, I'll explain this one easily and quickly. Since Arrhenius began speculating a century ago about low CO2 levels and ice ages the hypothesis of temperature relation to atmospheric carbon dioxide has drifted in and out of scientific focus. At present it is the focus of a great deal of attention. "Enhanced greenhouse" means the additional delay in energy loss to space induced by the fraction of carbon dioxide and other greenhouse gases released by humans before those gases are removed from the atmosphere by breakdown and/or biological activity.

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 me.

But we're responsible for all the carbon dioxide greenhouse effect?

Fuck 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. 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).

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 [/i]total greenhouse effect.[/i]

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).

Lots of numbers, which ones fit what we think we know of the Earth's greenhouse effect?

Let's try working backwards for a moment. The Earth's greenhouse effect is commonly estimated at 33 °C and these calculations simply assume that to be true. If water vapor accounts for 70% and clouds another 20% then we have 10% left for carbon dioxide and the ubiquitous "other" GHGs. Lindzen's 3.53 °C cooling potential for complete removal of CO2 would then seem to fit the bill fairly adequately at around 10.7% of the total effect, while there's really not room for the larger estimates. Note, however, that carbon dioxide is generally reckoned to account for between 4.2% and 8.4% of Earth's greenhouse effect because water vapor and clouds also behave differently at different concentrations and temperatures (I warned you this wasn't linear).

If, on the other hand, we assume Charnock and Shine are closer to the mark then ~36% of Earth's greenhouse effect would be driven by CO2. This is intuitively unreasonable since water is both prolific and has absorption windows overlapping those of carbon dioxide to a large extent. Given that water covers more than 70% of the globe and that the lower atmosphere over water tends to be relatively well supplied with water both as vapor and clouds and further given that water is the dominant absorber in wavelengths expected in the warmer regions, such as in the tropics where water is hugely prolific and where significant greenhouse warming occurs, it simply does not seem reasonable to expect CO2 to preferentially absorb more than one-third of the available energy. This suggests (but does not prove) that Lindzen is likely to be the nearest estimate from those we've plotted above.

Note that if you discount all other possible drivers of global temperature change -- meaning that humanity has completely taken over from all natural effects that were operating until that time (highly unlikely) -- then the estimate of Charnock & Shine neatly fits observed warming over the period. If their massive estimate of greenhouse effect from carbon dioxide is true then a worst case doubling of atmospheric carbon dioxide will still only produce a total warming under 1.5 °C (and we're thought to be almost half-way there already). This still does not suggest a major enhanced greenhouse catastrophe.

If that's all the anticipated greenhouse effect, where do the big warming estimates come from?

Ah, this is where it gets rather contentious because the big warming numbers come not from measurements but from computer models. These computer models and their output are passionately defended by the modeling clique and frequently derided by empiricists -- but the bottom line is that models make an enormous range of assumptions. Whether all the assumptions, tweaks and parameter adjustments really collectively add up to a realistic representation of the atmosphere is open to some conjecture (current climate models do not model "natural" climatic variation very well), but there is no evidence yet that they can predict the future with any greater certainty than a pack of Tarot cards. Moreover, humans do a lot besides emitting greenhouse gases, changing vegetation and transpiration rates through agriculture, for example, and many effects expected to both increase and decrease regional temperatures are not included in these models.

Regardless, climate models are made interesting by the inclusion of "positive feedbacks" (multiplier effects) so that a small temperature increment expected from increasing atmospheric carbon dioxide invokes large increases in water vapor, which seem to produce exponential rather than logarithmic temperature response in the models. It appears to have become something of a game to see who can add in the most creative feedback mechanisms to produce the scariest warming scenarios from their models but there remains no evidence the planet includes any such effects or behaves in a similar manner.

There has been some claim of ignoring "self-evident" positive feedbacks, which I'd be delighted to highlight if only someone could point to any such empirical measure. The bottom line, however, is that the IPCC estimates a trivial 0.6 °C ± 0.2 °C warming during the Twentieth Century and both the GHCN-ERSST Data Set and the HadCRUT2v Data Set record the period of the 19-teens through mid-1940s as having a global trend of +0.13 °C/decade for a net warming of 0.45 °C -- leaving a mere 0.15 °C ± 0.2 °C net warming potential for the post-WWII period of significant carbon emission from fossil fuel use. It is evident, to me at least, that if positive feedback mechanisms exist (entirely plausible) then their effect is negligible or mitigated by negative feedback mechanisms (equally plausible). Unlike modelers, who alter their virtual worlds at whim, we can only measure what the world actually does, and there simply isn't room in the measured change for the existence of significant unmitigated positive feedbacks. -- end update.

As an example of how mileage may vary, as they say, there is a table of comparisons between 108 model guess-timations for a doubling of atmospheric carbon dioxide compiled by Kacholia and Reck, published in 1997. The range spans from 0.2 °C to 6.3 °C and the same modelers get large variations as they play with their model parameters, e.g. Washington and Meehl show listings of 1.3 °C; 1.4 °C–3.5 °C; 1.6 °C; 4.0 °C and back to 1.6 °C over the course of a decade. Charnock and Shine appear in this list (1993) with estimations of 1.5 °C–2.4 °C and we derive their 1995 discussion in Physics Today as 1.46 °C so we're in the ballpark and they may have reduced their estimate as Lindzen seems to have done, listed here from 1982 as 1.46 °C–1.93 °C and stating explicitly in the same Physics Today discussion that he estimated 0.5 °C for clear sky conditions and just 0.22 °C when including 40% cloud cover.

Unfortunately there has been no narrowing of the estimated range of "expected" warming from a doubling of CO2 -- in fact the range has widened even further as ever more players attempt to stand out in a crowded publication field. It isn't that the physics of carbon dioxide's radiative properties keep changing, rather that ever more imaginative "feedbacks" are shunted into the positive column to make model output more interesting. The bottom line is that you need to stuff a huge amount of CO2 into the atmosphere to get much response as more of the absorptive bands near saturation.

If you don’t want to be one of the mindless sheep, blindly shoveling the media-spewed bullshit down your throat, then try doing a little fucking research for yourself. Start here:Actual Information

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