Why I Am A Global Warming Skeptic
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.
It's a nonlinear world
There are many complicating factors with dynamical systems containing multiple feedback loops. Most feedback is only manifest after a time delay. An example of this in nature is heavy winter snowfall resulting in higher than normal freshwater runoff in the spring. Different factors may respond on markedly different time scales with the full impact of feedback interactions remaining unclear for years, decades or even centuries.
Another example of delay is the absorption of heat by H2O creating water vapor. Later, this latent heat may be released at a far distant location when the water vapor condenses to form rain. This mechanism is only one of a myriad of ways that energy is transported around the planet. These types of delayed reaction can cause yearly, decadal or longer cycles. Such factors are ubiquitous in nature and require the use of delay differential equations to model them. These delayed responses of natural feedbacks seem to indicate that climate change takes a while to propagate through the Earth system. But we do know, from paleoclimate data, that there have been relatively sudden climate shifts in the past. Past bouts of rapid change is another argument that climate change alarmists use to prove the urgency of their cause.
Climate feedback loops. After Pittock.
One of the favorite scare tactics of global warming promoters is to warn that there are “tipping points” lurking in nature that can suddenly throw the climate into a tizzy. To some extent they are correct, there are historical examples of nonlinear responses from the climate system that have caused reactions far more dramatic than the level of forcing that triggered them. In fact, the mechanisms responsible for the sudden transition from glacial to interglacial conditions could well be labeled a tipping point.
In mathematics, a nonlinear system is one whose output is not proportional to its input. A recent paper reported that dropping sea levels during a glacial period can cause a nonlinear response by exposing the Bearing Strait land bridge between Siberia and Alaska. When this happens the flow of salty water from the northern Pacific into the Arctic basin is cut off, triggering other rapid changes in ocean circulation patterns. The response is a general warming that allows sea levels to rise, restoring the northward flow from the Pacific. A side effect may be to create large lakes within the continental ice sheets containing huge amounts of fresh melt-water.
As I reported in “The Long Road Ahead,” scientists are now quite certain that for Earth's climate to shift from glacial conditions, huge continental ice sheets must exist in the Norther Hemisphere. The Milankovitch cycles still operate during glacial periods, cycling the climate between cold and relatively warmer conditions. But, once a glacial period has started, orbital variations do not seem sufficient to bring the planet back to a warm, interglacial climate. However, things do warm up enough create instabilities in the ice sheets, leading to pulses of iceberg discharge and allowing the formation of vast ice dammed lakes filled with glacial melt water. These lakes represent a bigger lurking nonlinearity than the Bering Strait land bridge.
Glacial lake Missoula. Painting by Byron Pickering.
Rapid climate change is a result of nonlinearities in the Earth system's natural processes, and many physical systems are inherently nonlinear in nature. In this case, the freshwater glacial lakes were waiting for just enough forcing warmth to weaken an ice dam and cause it to break. Scientist speculate that an outpouring of freshwater from such glacial lakes can disrupt the flow of ocean currents sufficiently to cause wild swings in climatic conditions.
What causes the warming in the first place is still a mater of raging debate. Some claim it is the melting of mid-latitude glaciers, which are very sensitive to warm summer weather but not cold winters. Others argue that extensive cover of sea ice in the Norther Pacific and Atlantic oceans create more zonal climatic circulation system than exists today. During termination changes in freshwater runoff destabilizes the zonal ocean current patterns, forcing warm water north resulting in a sudden thaw. This science is certainly not settled.
I have heard climate change boosters claim that in the past historical climate shifts have been out of proportion to the forcings that seem to have caused them. This, they say, justifies their assumption that the climate system's feedback loops are a net amplifier of temperature. In the face of discoveries about the nonlinearities that control glacial-interglacial transitions the argument is not compelling.
Global warming proponents have yet to identify the mechanisms present in Earth's current environment that will cause a rapid nonlinear climate warming. Today there are no continent spanning ice sheets, precariously holding back glacial mega-lakes. Sea levels are rising marginally, so no chance of exposing a new land bridge that would block ocean currents. We can only conclude that, barring some unforeseen and unidentified future disaster, there will be no rapid global warming.
There are also questions regarding the data on which the AGW theory is based. Most climate data are proxy data. This means those data both inexact and open to multiple interpretations. For example, different groups of scientists, working from the same set of tree ring data, arrived at much different interpretations of Holocene climate history (see “Medieval Warm Period Rediscovered”). Furthermore, the farther back in time science looks, the more uncertain the data become.
A good example of the uncertainty surrounding paleoclimate data is the recent announcement that, after nearly 30 years of often heated debate, a team of researchers has finally produced a 50,000 year calibration curve for radiocarbon dating. The basic principles of radiocarbon dating are fairly simple and one would expect that consensus would be easy to achieve among scientists on how to perform such dating. But as with most climate related experimental data there were many data sets that had to be combined. These data sets diverged from each other by up to several thousand years after 26,000 years ago, and researchers could not agree on which ones were most accurate or how to combine them.
For the first 12,000 years the team used thousands of overlapping tree-ring segments from the Northern Hemisphere. For dates older than the available tree-ring record, the researchers had to turn to several other, less-precise data sets, including fossil diatoms and corals. How important can this work be? Surely the differences that the scientists were arguing over were small and any changes from using the new work would merely reinforce previous, if less accurate work. As it turns out, not really.
One case in point, the raw radiocarbon dates for the spectacular paintings of horses, lions, bison, and other animals at Chauvet Cave in southern France, the oldest known cave art, indicate the paintings were made 32,000 years ago. This was right after a major cold spell hit Europe. Using the new calibration curve dates the earliest paintings at Chauvet 36,500 years old, a period of relative warmth. According to Clive Gamble, an archaeologist at the University of London, getting those dates right is critical to understanding questions such as whether humans began painting caves when the climate was colder or warmer. You can imagine how accurate dating could change the interpretation of other climate related data.
Paintings from the Chauvet-Pont-d’Arc Cave in southern France.
Although the new curve is a major improvement in radiocarbon calibration, it is “definitely not the last word,” says team leader Paula Reimer, a geochronologist at Queen's University Belfast in Northern Ireland. Her team is already planning an update for 2011. Remember, this is for a fairly simple historical measurement. Most historical climate data has known uncertainties with the same order of magnitude as the predictions offered up by the IPCC and other climate change alarmists. The only “good” climate data we have has been collected since the maturation of orbiting remote sensing satellites within the past 20 years or so. You simply cannot accurately predict Earth's climate a century in the future based on a couple of decades of data.
Finally, this brings us to climate modeling. Some critics claim that the entire skeptical case against anthropogenic global warming is built around the flaws found in GCM climate modeling software—this is not true. Just as the output from climate models does not provide proof of global warming, the use of computer models in climate research does not invalidate that research. Indeed, computer modeling can be an important and useful tool in most any scientific endeavor. Unfortunately, is seems that many climate modelers have committed the greatest sin a modeler can commit—believing that their models and the system being modeled are one in the same.
As most experienced modelers know, all models are wrong but some are useful—climate models, GCM, are no exception. As previously reported, GCM are inherently inaccurate (see “Climate Models Irreducibly Imprecise”). I have also highlighted the errors and sensitivity of climate models to small changes in configuration in “Extinction, Climate Change & Modeling Mayhem.” And this is only scratching the surface. By their very nature computer are simplifications of the real world. Simplifying assumptions must be made to make the models computationally tractable, otherwise not even the largest supercomputers in the world could run them. So all climate models are, by necessity, dumbed down representations of an incomplete theory calibrated and fed with uncertain data.
Even when asked to make near term predictions—essentially extended weather forecasts—climate models are woefully inaccurate. As reported in Geophys. Res. Lett., David Lavers of Princeton University and colleagues tested eight seasonal climate forecast models for their skill at predicting temperature worldwide and precipitation over land masses. The problem, according to this new analysis, is that existing climate models show very little accuracy more than one month out. Even during the first month, predictions are markedly less accurate for the second half than the first. According to the researchers, current models simply cannot account for the chaotic nature of climate. Now extrapolate that inaccuracy to predictions decades, even centuries into the future.
A new article in the Journal of Climate by Knutti et al. discuss some major sources of differences between models,. According to a review of the paper by H. Jesse Smith in Science the work is important “because it is not normally clear which models' scenarios are likely to be the most realistic, the question arises of which specific models to believe and why.” Knutti et al. themselves state: “there is little agreement on metrics to separate ‘good’ and ‘bad’ models, and there is a concern that model development, evaluation and posterior weighting or ranking are all using the same datasets.” In other words, they are tuning and then judging their models using the same restricted set of uncertain and error prone data that has been used in the past. No wonder their models don't work.
To summarize, Earth's climate is amazingly, mind-blowingly complex and science has only just begun to figure out how it works. While our theoretical knowledge improves and our data become more accurate with each passing year, it is safe to say that there is still more we don't know about climate change than things we think we understand. Here are some fundamental questions about the state of Earth's climate:
- Is Earth's climate warming? Yes, by around 1°C (1.8°F) during the 150 years leading up to the present.
- Do human activities impact the climate? Yes, all numerous and widespread species do to some extent (see the concluding remarks of “What Killed The Mastodons?”).
- Do atmospheric CO2 levels rise and fall with temperature? Yes, according to historical data.
- Are atmospheric CO2 levels rising because of people? Yes, it would appear so.
- Does this mean that the theory of anthropogenic global warming is correct? Absolutely not.
For the reasons given in the article above, and those described in greater detail in The Resilient Earth, I have concluded that science does not understand the climate system well enough to make the predictions that climate change alarmists keep making. Scientists continue to argue about fundamental mechanisms and the accuracy of historical data. In the absence of better theoretical knowledge and sufficient accurate data, climate scientists have filled the void in understanding with output from computer models, which are the most fickle and fallible of tools.
Consider the following main points:
- Climate is a nonlinear system.
- Not all feedback relationships are known or well characterized.
- Based on known empirical evidence a doubling of atmospheric CO2 levels will only cause a further increase of 0.2 to 0.5°C in global temperature.
- In order to achieve the temperature levels predicted by the IPCC requires amplifying positive feedback from the climate system.
- There is no compelling proof that Earth's climate system acts as a temperature change amplifier.
- Not all known factors are included in climate models.
- Models cannot predict nonlinear responses not built into them.
- Baseline data used to calibrate the models are uncertain and possibly erroneous.
- Different climate models predict different future outcomes because they make different assumption or use different values for model parameters.
- All climate models contain errors that are fundamental to their construction.
In the wake of Climategate and Glaciergate it is tempting to dismiss the theory of anthropogenic global warming as bogus science foisted off on a gullible public by a number of bad scientists. The reasons for this climate science chicanery are not clear: the torrent of grant money, the novelty of fame or simply error amplified by ego. As entertaining as the news accounts of unfolding scandal have been, it is important to remember that many serious scientists believe in human caused global warming to one degree or another. But science is a human endeavor and as prone to mistakes as any other. This will not be the first time that a majority of the scientific community believed in an erroneous theory, and it certainly will not be the last. Eventually, science will decide the fate of AGW based on empirical evidence—nature itself will provide the proof, one way or the other.
To date, climate science has not produced any incontrovertible proof that rising CO2 levels will, in fact, cause the temperature increases predicted by GCM models. The information presented above reinforces the observations I made in my earlier post, “The Crumbling Pillars of Climate Change.” The theoretical understanding is incomplete, the historic data are spotty and uncertain, and the models are not an accurate representation of the climate system. Further, models have been used inappropriately to bolster the IPCC's case—models are not scientific evidence and should not be used to predict long-term real world behavior.
The truth is, climate science uses computer models like a drunk uses a lamppost, not for illumination but for support. Even AGW supporters agree that if the only evidence for global warming were computer models, then skepticism would be entirely justified. But, while models are most definitely a sore point in the global warming debate, they are not the central point. It is the science itself that is uncompelling. Neither current scientific knowledge nor historical data prove the theory of anthropogenic global warming as put forth by the IPCC. If anything, new data and new studies reveal that current climate change dogma has got it very wrong. That is why I remain a global warming skeptic.
Be safe, enjoy the interglacial and stay skeptical.