The debate over anthropogenic global warming—a theory propounded by the UN IPCC—is often portrayed as an argument between deniers and true believers. The deniers supposedly claim that there is no global warming, man made or otherwise, and that the whole theory is a plot by left-wing agitators and closet socialists bent on world domination. The true believers, conversely, accept every claim of pending future disaster uttered by scientists and activists alike. As with most controversies both extreme positions are wrong and the truth lies somewhere in-between. As a scientist, I have studied the evidence and find the case for imminent, dangerous, human caused global warming unconvincing—here is why I am an AGW skeptic.
According to www.dict.org, a skeptic is “one who is yet undecided as to what is true; one who is looking or inquiring for what is true; an inquirer after facts or reasons”.* This is a much more accurate description of the stand taken by Al and myself in our book, The Resilient Earth. Specifically, I am skeptical of the claim that human produced carbon dioxide will have the dramatic effect on Earth's temperature projected by the IPCC and other global warming doomsayers. To understand why I have reached this conclusion requires starting with some basic science.
*[That site also notes that this word and its derivatives are often written with c instead of k in the first syllable: sceptic, sceptical, scepticism, etc. Dr. Johnson, struck with the extraordinary irregularity of giving c its hard sound before e, altered the spelling, and his example has been followed by most lexicographers]
There is no doubt that the “greenhouse effect” warms Earth, this has been known for two centuries. Because of the mix of gases in the atmosphere, the air surrounding our planet is transparent to visible light coming from the sun, but opaque at many wavelengths in the infrared band. When sunlight strikes Earth's surface, it re-radiates solar energy back toward space in the form of infrared light. Greenhouse gases (GHG) in the atmosphere absorb much of that radiation, trapping thermal energy and warming the planet. This has a significant impact on surface temperatures.
The result can be calculated using simple physics. The Stefan-Boltzmann law shows that if there were no greenhouse gases in the atmosphere, Earth's average surface temperature would be –18˚C (–1˚F). This is well below the freezing point of water and would make life as we know it impossible. Earth's actual observed average surface temperature is 15˚C (59˚F). By empirical measurement, the greenhouse effect raises our planet’s surface temperature by 33˚C (60˚F). To this extent, global warming has already taken place—and a darn good thing for us it has.
The primary greenhouse gases are H2O (water vapor), CO2, and CH4 (methane). I have often stated that water vapor is the most important GHG, but a comparison of water vapor and carbon dioxide highlights some interesting facts. The data in Table 1 were computed by assuming that one gas is removed from the atmosphere, leaving the others unchanged. If you remove all water vapor from the atmosphere, the infrared absorption will decrease by 36 percent. If you remove all greenhouse gases (and clouds) and leave only water vapor, the infrared absorption will decrease by 34 percent.
|Greenhouse gas removed||% Decrease in IR absorption|
|All except H2O vapor||34|
|All except CO2||74|
|H2O vapor + CO2||53|
Table 1: Contributions to the greenhouse effect by different greenhouse gases. Source realclimate.org.
Since GHG have overlapping bands of infrared absorption, the absorption from one gas affects the absorption from another gas. Some wavelengths of infrared light are absorbed by both water vapor and CO2. If water vapor alone is removed, leaving the CO2, the CO2 will absorb the infrared light in the overlap region. Conversely, if the CO2 is removed from the atmosphere, water vapor will absorb that infrared light. Thus the absorption by one gas depends on the other gases present in the atmosphere.
Note that removing all GHGs with the exception of water vapor would result in absorption of 66% of the IR radiation absorbed by the current atmosphere (100 – 34). If all GHGs except CO2 are removed the absorption rate is reduced to only 26%. Clearly H2O is more important than carbon dioxide, but CO2 does make a significant contribution. So far this has been a matter of basic physics, but things are about to get more complicated.
Most of the claims about the impact of AGW are predicated on specific amounts of temperature increase. The temperature increase is attributed to increasing atmospheric CO2 levels, another fact that very few scientists deny. For reasons soon to be discussed, future temperature increases are calibrated on an assumed increase for doubling the amount of CO2 in the atmosphere over “preindustrial” levels. Long-term atmospheric data show that CO2 levels remained stable around 280 ppm (parts per million) during the most of the past 10,000 years. When climate scientists talk about CO2 levels doubling this is the base level from which the increase is measured.
The simplest, and incorrect, way to estimate of the impact of doubling atmospheric CO2 levels would be to double its greenhouse temperature contribution. Such a linear estimate for doubling all GHG would would result in an additional increase of 33˚C (60˚F), giving an average surface temperature of the earth of 48˚C (119˚F). Fortunately that is not the way things work in the real world. Doubling CO2 levels will only result in a small rise in temperature due to two factors, both having to do the absorption spectra of greenhouse gases as shown in the graph below.
Greenhouse gas absorption spectra. Source The Resilient Earth.
The two complicating factors are band saturation and spectral overlap. The first has to do with how much radiation is already being absorbed at specific frequencies. If the absorption at a certain wavelength is close to 100 percent, doubling the CO2 level will have little effect—absorption can not exceed 100% no matter how much more gas is added. Think of if this way: if a totally opaque curtain is placed over a window, blocking all of the light, adding more layers of curtain cannot make the room any darker.
The second limiting factor, spectral overlap, comes from the relationships between the frequencies of light GHG absorption. As already stated, water vapor has areas of infrared light absorption that overlap the absorption by CO2. As with saturation, in regions where infrared light is already strongly absorbed by water vapor, the addition of more CO2 will make little or no difference. Given these complications, doubling atmospheric CO2 levels will result in a much lower temperature rise than a linear estimation.
The impact of such a doubling can still be calculated using formulas from a standard textbook. If nothing else in the system changes, a doubling of CO2 from the preindustrial levels is estimated to produce a temperature rise of 1.2 to 1.3˚C (2.2 to 2.3˚F). Again, the calculation is straightforward and there is little controversy about the figure among scientists. Now recall that over the last century and a half CO2 levels have risen from a preindustrial 280 ppm to around 380 ppm. At the same time global average mean temperature has risen (depending on who you believe) 0.8 to 1.0˚C. This implies that, once the CO2 level reaches 560 ppm, the dreaded doubling, temperatures should rise by another 0.2 to 0.5˚C. So where do the IPCC estimates of 2.0 to 6.0˚C come from?
Where things start to get murky
The IPCC temperature estimates for doubling atmospheric CO2 come from amplifying the amount of warming from the actual greenhouse increase due to assumed positive feedback. The concept of feedback has its roots in electrical engineering and the study of electronic circuits. The term “feedback” first appeared in the 1920s and supposedly came from the broadcasting industry. When the volume of a microphone is set too high sound from a nearby monitor speaker can be picked up and amplified even more. The resulting sound is usually a loud, unpleasant screech. Since the sounds that enter a microphone are referred to as feeds, the unpleasant sounds were called “feed-back.”
As it turns out, the concept of feedback can be applied to a wide range of dynamical systems, both natural and man made. Here is a description of dynamical systems and feedback by Karl Johan Åström and Richard M. Murray in their book, Feedback Systems: An Introduction for Scientists and Engineers:
A dynamical system is a system whose behavior changes over time, often in response to external stimulation or forcing. The term feedback refers to a situation in which two (or more) dynamical systems are connected together such that each system influences the other and their dynamics are thus strongly coupled. Simple causal reasoning about a feedback system is difficult because the first system influences the second and the second system influences the first, leading to a circular argument. This makes reasoning based on cause and effect tricky, and it is necessary to analyze the system as a whole.
Feedback in natural systems is well accepted, this is not the problem. The problem lies in calculating the impact that all the various natural feedback loops have on global temperature. Some feedback mechanisms are positive, amplifying any input signal changes, while others are negative, applying an opposing influence and limiting signal change. An example of positive climatic feedback is water vapor. An increase in temperature causes more evaporation from the ocean that releases more water vapor into the atmosphere causing further temperature rise.
To further complicate things, some factors can participate in both positive and negative feedback loops. Water vapor, cited above as a positive feedback, can provide a negative feedback in this way: rising temperature increases atmospheric water vapor levels, which in turn causes more precipitation; if the precipitation falls as snow this can raise Earth's albedo, reflecting more sunlight and lowering the planet's overall temperature. This chain of influences is why some scientists claim that Earth must first warm up in order to get colder. More water vapor can also cause more clouds that, depending on their type, can either cool or warm the planet—as I said, it's complicated. Some of the known climate feedback factors are shown in the figure below.
Climate feedback loops. After Robock.
It is the nature, magnitude and characteristics of the natural feedback loops that are at the heart of the AGW controversy. Without feedback loops amplifying the impact of increasing carbon dioxide levels on Earth's temperature the global warming proponents have no case—and skeptics like myself are unconvinced that the IPCC has got them right.
Mainstream climate scientists have decided that the net impact of all the feedback relationships within the Earth system is positive. In effect, they multiply the marginal temperature increase from the enhanced greenhouse warming by an “amplification factor.” This assumption is both unwarranted and a dangerous oversimplification of Earth's climate system. You cannot analyze a system as a whole if you do not know and understand all the pieces that comprise the system.
For instance, there must be limiting factors or opposing negative feedback to counter the proposed positive ones or Earth's temperature, once warming began, would spiral ever upward—a runaway greenhouse like that of Venus. We know such limiting factors exist because Earth's climate has remained within a range conducive to life for a half a billion years. Still, this has not prevented climate change proponents from positing a number of simple positive feedback relationships which they say will cause a dangerous rise in planetary temperature. Usually, the primary positive feedback claimed to amplify the warming effect of CO2 is a supposed link to water vapor.
A recent report in Science suggests that stratospheric water vapor between 1980 and 2000 probably increased the rate of surface warming during the 1990s by about 30%. The research, led by respected NOAA climate scientist and IPCC climate change assessment report co-chair Susan Solomon, states that from 2000 to 2009 diminished water vapor levels in the upper atmosphere depressed global warming by about 25% compared to that which would have occurred due only to carbon dioxide and other greenhouse gases. The research, based on data from the state-of-the-art AIRS instrument on the NASA Aqua satellite, suggests that water vapor is responsible for twice the global warming effect of carbon dioxide, whether man-made or naturally occurring. As I have said, “it's the water vapor, stupid!” As for the feedback connection, this was during a period when CO2 levels were constantly rising, yet water vapor levels in the stratosphere fell.
Data from the Atmospheric Infrared Sounder (AIRS) instrument. NASA
In fact, given these new results, human CO2 would actually be responsible for a negative feedback that cancels at least some of the warming it causes by pushing water vapor back to the surface of the earth and out of the stratosphere, where it acts as a potent greenhouse gas. Climate change alarmist claims of a CO2 amplifier may not only be exaggerated, they may have it backwards. Dr Solomon has not abandoned her belief in global warming but admitted that the research does imply that human emissions having a much smaller role in climate change than previously thought.
Another new estimate of the overall feedback between temperature and atmospheric CO2 concentration has been derived from a comprehensive comparison of temperature and CO2 proxy records spanning the past 1000 years. The study in Nature, by David Frank et al., was based on more than 200,000 individual comparisons over the period from 1050 to 1800. Their results imply that the amplification of current global warming by carbon-cycle feedback will be significantly less than commonly suggested. This report goes so far as to suggest ~80% less potential amplification for ongoing global warming.
These are just two of the most recent published studies that cast doubt on our understanding of Earth's climate and how climate might react to increasing levels of atmospheric CO2. A survey of published papers over the past few years makes it abundantly clear, this is not “settled science” as AGW adherents and eco-alarmists have claimed. As I said, the climate change alarmists' assertions regarding CO2 feedback are unconvincing.
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