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.

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.

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: 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).

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 (we 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.

Well, why is the planet warming so catastrophically if it's not CO2 then?

Who says it is warming catastrophically? Humans have only been trying to measure the temperature fairly consistently since about 1880, during which time we think the world may have warmed by about +0.6 °C ± 0.2 °C. As we've already pointed out, the estimate of warming is less than the error margin on our ability to take the Earth's temperature, generally given as 14 °C ± 0.7 °C for the average 1961-1990 while the National Climatic Data Center (NCDC) suggest 13.9 °C for their average 1880-2004. We are pretty sure it was cold before the 1880 commencement of record and we would probably not handle the situation too well if such conditions returned but there has been no demonstrable catastrophic warming while people have been trying to measure the planet's temperature. If we have really been measuring a warming episode as we think we have, then setting new records for "hottest ever in recorded history" should happen just about every year -- although half a degree over a century is hardly something to write home about -- so there's really nothing exciting about scoring the highest number when looking at such a short history.

What are the take-home messages:

* The temperature effect of atmospheric carbon dioxide is logarithmic, not exponential.
* The potential planetary warming from a doubling of atmospheric carbon dioxide from pre-Industrial Revolution levels of ~280ppmv to 560ppmv (possible some time later this century - perhaps) is generally estimated at less than 1 °C.
* The guesses of significantly larger warming are dependent on "feedback" (supplementary) mechanisms programmed into climate models. The existence of these "feedback" mechanisms is uncertain and the cumulative sign of which is unknown (they may add to warming from increased atmospheric carbon dioxide or, equally likely, might suppress it).
* The total warming since measurements have been attempted is thought to be about 0.6 degrees Centigrade. At least half of the estimated temperature increment occurred before 1950, prior to significant change in atmospheric carbon dioxide levels. Assuming the unlikely case that all the natural drivers of planetary temperature change ceased to operate at the time of measured atmospheric change then a 30% increment in atmospheric carbon dioxide caused about one-third of one degree temperature increment since and thus provides empirical support for less than one degree increment due to a doubling of atmospheric carbon dioxide.
* There is no linear relationship between atmospheric carbon dioxide change and global mean temperature or global mean temperature trend -- global mean temperature has both risen and fallen during the period atmospheric carbon dioxide has been rising.
* The natural world has tolerated greater than one-degree fluctuations in mean temperature during the relatively recent past and thus current changes are within the range of natural variation. (See, for example, ice core and sea surface temperature reconstructions.)
* Other anthropogenic effects are vastly more important, at least on local and regional scales.
* Fixation on atmospheric carbon dioxide is a distraction from these more important anthropogenic effects.
* Despite attempts to label atmospheric carbon dioxide a "pollutant" it is, in fact, an essential trace gas, the increasing abundance of which is a bonus for the bulk of the biosphere.
* There is no reason to believe that slightly lower temperatures are somehow preferable to slightly higher temperatures - there is no known "optimal" nor any known means of knowingly and predictably adjusting some sort of planetary thermostat.
* Fluctuations in atmospheric carbon dioxide are of little relevance in the short to medium term (although should levels fall too low it could prove problematic in the longer-term).
* Activists and zealots constantly shrilling over atmospheric carbon dioxide are misdirecting attention and effort from real and potentially addressable local, regional and planetary problems.

Remember: Water vapor and carbon dioxide are major greenhouse gases. Water vapor accounts for about 70% of the greenhouse effect, carbon dioxide somewhere between 4.2% and 8.4%. Much of the wavelength bands where carbon dioxide is active are either at or near saturation. Water vapor absorbs infrared over much the same range as carbon dioxide and more besides. Clouds are not composed of greenhouse gas -- they are mostly water droplets -- but absorb about one-fifth of the longwave radiation emitted by Earth. Clouds can briefly saturate the atmospheric radiation window (8-13µm) through which some Earth radiation passes directly to space (those hot and sticky overcast nights produce this effect - that is greenhouse but has nothing to do with carbon dioxide). Greenhouse gases can not obstruct this window although ozone absorbs in a narrow slice at 9.6µm. Adding more greenhouse gases which absorb in already saturated bandwidths has no net effect. Adding them in near-saturated bands has little additional effect.


More than 17,100 basic and applied American scientists, two-thirds with advanced degrees, have signed the Oregon Institute of Science and Medicine's Global Warming Petition, which says in part, "There is no convincing scientific evidence that human release of carbon dioxide, methane, or other greenhouse gases is causing or will, in the foreseeable future, cause catastrophic heating of the Earth's atmosphere and disruption of the Earth's climate."
www.oism.org/pproject/s33p37.htm

breathe just breathe

See U.S. military research paper: 'Owning The Weather In 2025' ....
www.fas.org/spp/military...5/v3c15-1.htm

The Ultimate Weapon of Mass Destruction:
"Owning the Weather" for Military Use
by Michel Chossudovsky
www.globalresearch.ca/article...09F.html

Actually, there are a number of issues standing in the way of complete appreciation of the dangers of Climate Change; human issues, I ought to say. Said issues cannot be addressed by changing the tenor of the warnings put out by governments, the PNAS, or concerned individuals.

1. Not all audiences are the same. Some people simply don't understand the theory or the physics involved. The PNAS report makes good reading, but only if you understand the mechanics involved. Those who can't comprehend the math involved will automatically dismiss it, and go on to something they *can* understand--say, the sports page, or a racing form.

2. The emotional factor involved overpowers a lot of the science. Among some people I've noticed a great deal of hostility and indignation when the ramifications of Climate Change are discussed. The idea that people can't go on aimless drives, four-wheeling, or some such recreational sport because of the high price of gasoline is infuriating. Global Warming or Climate Change is interpreted by many as an infringement on their civil liberties. Such people make quick converts to the first glad-haninding politician or Neo-Con who vows to attack "the liberal hordes" who are scaling back autos in the name of Global Warming.

3. Hard Core Skeptics who refuse to believe anything that cannot be interpreted by their six senses, or things that conflict with their commonly held beliefs. Such individuals will automatically dismiss any scientific evidence, and will refuse to believe until it is too late. Such people are probably immovable.

4. Religious figures & Creationists will insert a "God Factor" into the Climate Change argument because it is familiar ground; many don't know how else to think. Their argument will be that the world is an extension of Eden, and God won't let anything happen to it. Such people cannot be reasoned with, as many of them extend their disbelief to most forms of science and the Humanities, including History. Pointing out past Ice Ages, or the countless millions who died in natural or manmade disasters (and received no Divine Intervention) in the past has no bearing on their beliefs. Such instances were "unworthy", in their view.

5. Fringe elements will take their direction from supermarket checkout-stand newspapers, Fortean-style groups, or simply from each other. Anything with an official air to it is automatically suspect and therefore not to be trusted. Such individuals will contribute nothing (if they can help it) toward arresting Climate Change.

There are other kinds of people who cannot or will not understand Climate Change, but these are the main ones, and will remain obdurate in the face of all evidence.