Dedicated to the balanced discussion of global warming
JunkScience.com – April 21, 2006
This is a very long article that has quite a few good points in it. The article is a series of questions and answers on the subject. The author’s goals were to try and put some simple facts behind many of the myths and rumors regarding weather and the things that influence the weather.
This article should be standard reading for anyone that is interested in the details behind global warming.
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.
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.
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.
It might also help novices to conceive of the atmosphere and the natural greenhouse effect as a kind of a metaphorical energy flow control valve. There’s a lot of energy bouncing around, but the amount of energy entering the system and the amount leaving is fairly tightly constrained. The atmosphere is acting as a kind of check valve, slowing the loss of energy to space but the net incoming (324 + 168 Wm-2) = net outgoing (390 + 78 + 24 Wm-2).
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.
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).
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).
…comparisons between 108 model guess-timations for a doubling of atmospheric carbon dioxide compiled by Kacholia and Reck, published in 1997. Note that the range spans from 0.2 °C to 6.3 °C and that 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
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.
- 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.
Technorati Tags: global warmingTags: Al Gore, An Inconvenient Truth, carbon cycle, carbon dioxide, climate models, clouds, CO2, emissions, Greenhouse gas, IPCC, methane, myth, ozone, pollutant, science, solar, space, temperature, water, water vapor, weather, wind