Saturday, December 5, 2009

Global Warming Denialists

Recently, global warming denialists have drawn attention to themselves by releasing of thousands of hacked e-mails and documents from the Climate Research Institute. These e-mails, sent over the course of 13 years purportedly show researchers manipulating data to create evidence that wasn’t actually there on behalf of global warming. In one e-mail a researcher writes, “I think we have to stop considering Climate Research as a legitimate peer-reviewed journal. Perhaps we should encourage our colleagues in the climate research community to no longer submit to, or cite papers in, this journal.” In another e-mail from November 1999 they write, “I've just completed Mike's Nature [the science journal] trick of adding in the real temps to each series for the last 20 years (ie, from 1981 onwards) and from 1961 for Keith's to hide the decline.” In yet a third e-mail they write, “The fact is that we can't account for the lack of warming at the moment and it is a travesty that we can't,”

These three quotes come from the personal correspondences of several researchers over the course of 13 years. One would expect to find quite a few anomalies if they searched through thousands of documents, and this, frankly, seems to be very few anomalies for such a large number of documents. Even still, the anomalies themselves are entirely within proper scientific procedure when read within their intended context.

The Peer review process is a very important one in science. Scientific journals employ peer reviewers to ensure that research was done with proper controls and blinding when applicable, and that all conclusions being drawn follow from the research conducted before publishing a study in their journal. By having stringent peer review standards journals maintain their reputations within their respective fields. When a journal begins to allow research with poor methods and analysis through their peer review process they lose the respect of the scientific community who may decide to stop viewing them as a legitimate peer-reviewed journal, and may even encourage their colleagues to no longer submit to, or cite papers in that journal. This is exactly what happened in the case of the Climate Research journal, which allowed a paper through its peer review process denying that the 20th century was abnormally warm in spite of the fact that this did not follow from the evidence collected by their research.

In the case of the “Mike’s Nature trick,” the “trick” being referred to by the scientists is to add in the data from recent years to older data so that the trend in the recent warming has some context it can be compared to.

The third quote, regarding the “inability to account for the lack of warming at the moment,” was later explained by its author. “Hackers cherry-picked from the stolen data and distributed selected documents to try to undermine scientific consensus on man-made climate change.” reported that, “bloggers are missing the point he’s making in the e-mail by not reading the article cited in it. That article – An Imperative for Climate Change Planning — actually says that global warming is continuing, despite random temperature variations that would seem to suggest otherwise.” The author explains his quote, “says we don’t have an observing system adequate to track it [global warming], but there are all other kinds of signs aside from global mean temperatures — including melting of Arctic sea ice and rising sea levels and a lot of other indicators — that global warming is continuing.”

Whether or not the authors adjusted any data to better show global warming says nothing as to the well-established scientific mechanisms behind global warming. At a distance of the earth, the sun’s output is equivalent to 1366 W/m^2. These units are a rate that energy is being transferred to any given unit area facing directly at the sun. For the earth, not all of the surface is facing directly towards the sun. Half of it is facing away, and the other half is mostly at odd angles relative to the sunlight. If we integrate over the entire earth, we get that only 1/4 of the 1366 value is reaching the average square meter of earth’s surface. Additionally a large portion of our surface is blocked by clouds which reflect light directly back into space. When all this is taken into account, the earth is, on average, absorbing 235 W/m^2 from the sun.

Objects with temperature give off blackbody radiation. The amount of radiation given off is given by the Stephan-Boltzmann law, and the amount of radiation given off in any portion of the spectrum is given by Planck’s law. The Stephan-Boltzmann law tells us that temperature radiated from an object is proportional to absolute temperature (temperature in Kelvin) to the fourth power. An object is in temperature equilibrium, if the radiation being absorbed by it is equal to the radiation being emitted. If we solve this equation for the equilibrium temperature of the earth, we get an equilibrium temperature equal to approximately -19 Celsius. If you’ve spent any time on earth before, you will likely have noticed that the average temperature is well above -19 Celsius. The earth only radiates about .5 W/m^2 from its core, thus this cannot account for any significant portion of the discrepancy we see. On Venus this discrepancy is far greater, so much so that Venus is known to have a much hotter surface than its cousin closer to the sun, Mercury.

The reason for these discrepancies is well understood. If you have ever opened your eyes before, you may have noticed that our atmosphere is largely transparent to light in the visible portion of the spectrum. Planck’s law tells us that objects at every day temperatures will radiate energy largely in the infrared part of the spectrum, and only once in a very long while will they radiate a photon of a visible wavelength. At higher temperatures however, the radiation will shift towards shorter wavelengths and far more of the radiation will be in the visible part of the spectrum. For temperatures around that of the surface of our sun, the blackbody radiation given off is centered near yellow visible light, tailing off in intensity in either direction. Our atmosphere is transparent to this light allowing it to radiate through to our surface, heating the surface, until it is given back off by our surface as largely infrared radiation. Infrared radiation, unlike visible radiation, cannot pass straight through our atmosphere.

There are discrete energies molecules can absorb because they induce vibration in the molecule. There are also rotational energies associated with molecules, but only the vibrational energies happen to fall in the infrared portion of the spectrum. An example of some absorption spectra can be seen here:

You will likely notice that water vapor (H2O) seems to be the most potent of these common gasses in the infrared region. Water vapor is also far more prevalent than carbon dioxide in the atmosphere. Thus, the majority of the discrepancy between our -19 Celsius predicted temperature and the actual global mean temperature is actually due to the presence of water vapor in the air. Carbon dioxide only makes a slight additional bump in temperature in addition to the boost we already receive from water. Carbon dioxide is still however a larger contributor to global warming, however, because water vapor concentrations have only changed a minuscule amount as a result of modern emission, whereas carbon dioxide concentrations have changed significantly. You may also notice that the absorption spectra provided appear to have 100% of the infrared radiation in carbon dioxide’s absorption bands being absorbed already. It is true that nearly all infrared radiation in these regions gets absorbed within the lower few kilometers of our atmosphere. What happens is a photon of the appropriate wavelength radiates upwards from our surface until it gets absorbed by a carbon dioxide molecule, raising the vibrational energy of the molecule. The vibrating molecule very quickly then radiates off a photon of the exact same wavelength absorbed in a random direction, returning to its original energy state. By increasing the concentration of carbon dioxide in the atmosphere we are catching more carbon dioxide lower down in the atmosphere, essentially thickening the infrared blanket we have covering the earth.

Different molecules such as methane, nitrous oxide, or sulfur hexafluoride have many times the warming potential of carbon dioxide or water vapor per molecule, but luckily these molecules are released in FAR smaller quantities than carbon dioxide or water vapor.

Humans are not the only sources of greenhouse gasses. Water vapor, carbon dioxide, methane, and nitrous oxide have all always been parts of earth’s environment, and our contribution has only been to increase concentrations of these gasses in our atmosphere by negligible to moderate amounts. Other human activities, such as the release of aerosols into the air actually contribute to global cooling, by reflecting more visible light straight back into space. At the moment the net effect is well within the range of our calculation capabilities, and the overall impact from anthropogenic effects has had a slight warming effect on our atmosphere, which is entirely consistent with the rise we have seen in mean global temperatures. While this has significant potential to change our environment, this trend is by no means out of our grasp, and proper understanding of the science behind this process can help us all determine better ways to enjoy our lives, while mitigating any undesired effects into the future.

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