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Subject: Global Warming and Nature's Thermostat

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Jack Venrick
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Global Warming and Nature's Thermostat

by Roy W. Spencer, Ph.D.

Updated February 18, 2008 with latest satellite temperatures.
(This page is frequently updated with a variety of improvements, many of which won't be mentioned here.)

We live in an invisible atmospheric sea of water vapor, Earth's primary greenhouse gas. Our atmosphere could hold much more water vapor than it does, which would then lead to a much warmer Earth -- but it doesn't. So, why is the greenhouse effect limited to its current value? We don't know; scientists simply "assume" that it magically stays that way.

Current computerized climate models that predict large amounts of global warming only do so after making very crude and uncertain assumptions about how the Earth's natural greenhouse effect is maintained. In the following article I will explain why I believe that modern science can not say with any level of confidence how much of our current global warmth is caused by mankind without knowing why the Earth's greenhouse effect is limited to its current value.

In the following article I will explain why the answer to this question must be through the operation of precipitation systems, for only they can remove water vapor from the atmosphere. Even though all climate models DO contain the "average effects" of precipitation systems -- this is NOT the same as knowing how precipitation systems interactively regulate the climate system.

Al Gore likes to say that mankind puts 70 million tons of carbon dioxide into the atmosphere every day. What he probably doesn't know is that mother nature puts 24,000 times that amount of our main greenhouse gas -- water vapor -- into the atmosphere every day, and removes about the same amount every day. While this does not 'prove' that global warming is not manmade, it shows that weather systems have by far the greatest control over the Earth's greenhouse effect, which is dominated by water vapor and clouds.


Global Warming 101: Global Warming Theory in a Nutshell

Global warming theory starts with the assumption that the Earth's relatively constant average temperature is due to a balance between (1) the amount of absorbed sunlight, and (2) the amount of emitted infrared ("IR") radiation which is continuously being lost to outer space. In other words, energy in equals energy out. Averaged over the whole planet for 1 year, those energy flows in and out of the climate system are estimated to be about 235 watts per square meter.

Greenhouse components in the atmosphere (mostly water vapor, clouds, carbon dioxide, and methane) exert strong controls over how warm the surface of the Earth gets. Mankind's burning of fossil fuels creates more atmospheric carbon dioxide. As we add more CO2, more infrared energy is trapped, strengthening the Earth's greenhouse effect, causing a warming tendency in the lower atmosphere and at the surface.

Global warming theory says that the lower atmosphere must then increase in temperature {which causes an increase in the IR escaping to space) until the emitted IR radiation once again equals the amount of absorbed sunlight. That is, the Earth must warm until global energy balance is once again restored. THIS IS THE BASIC EXPLANATION OF GLOBAL WARMING THEORY.

Now, you might be surprised to learn that the warming from the extra CO2 is, by itself, relatively weak. It has been calculated theoretically that, if there are no other changes in the climate system, a doubling of the atmospheric CO2 concentration would cause less than 1 deg C of warming (about 1 deg. F). This is NOT a controversial is well understood by climate scientists. (We are currently about 40% of the way toward a doubling of atmospheric CO2.)

BUT...everything this else in the climate system probably WON'T stay the same! For instance, clouds, water vapor, and precipitation systems can all be expected to respond to the warming tendency in some way, which could either amplify or reduce the manmade warming. These other changes are called "feedbacks", and they determine whether manmade global warming will be catastrophic, or barely noticeable. Feedbacks are the source of almost ALL SCIENTIFIC DISAGREEMENTS over global warming.

December, 2007 RESEARCH UPDATE: We have received back from peer review our article showing how natural climate variability has probably been misinterpreted, at least partially, by researchers who claim to see evidence of positive feedback (which would make global warming worse) in the climate system. Our article was carefully reviewed by two of the world's leading climate model experts who both agreed that we have raised a legitimate issue that has been previously ignored. Those reviewers even developed their own simple climate models to demonstrate the effect to themselves. It is still not known how much of an effect this is, but accounting for it would logically reduce estimates of how much global warming can be blamed on mankind.

Preface: How Could So Many Climate Modelers Be Wrong?

This is a question that fascinates me, not just from a science perspective, but a sociological perspective as well. I thought it might be good to address this question first since many of you are probably wondering, "Why should I waste my time with this web page when most the worlds experts agree that mankind causes global warming?":

1. INCOMPLETE UNDERSTANDING OF A COMPLEX PROBLEM: All climate modelers must build their models based upon our current understanding of how the climate system works. Therefore, if there is some important - but as yet poorly understood - process that they are missing, they will all tend to make the same error. Past evidence for this is the tendency for climate models to drift away from a realistic climate over time. This suggests that it takes a higher level of understanding to capture the intricate processes that stabilize the climate system.

The most important example of this lack of understanding is, in my view, how precipitation systems control the Earth's natural greenhouse effect, over 90% of which is due to water vapor and clouds. The Earth's total greenhouse effect is not some passive quantity that can be easily modified by mankind adding a little carbon dioxide -- it is instead being constantly limited by precipitation systems, which remove water vapor and adjust cloud amounts to keep the total greenhouse effect consistent with the amount of available sunlight. Our understanding of this limiting process is still quite poor, and likely not represented in climate models.

2. PEER PRESSURE TO CONFORM: The vast majority of climate scientists are not climate modelers, and they will tend to go along with what the modelers say. After all, it is the modelers who are supposed to gather all of the specialized knowledge of how weather processes operate, and then represent them in a computer program (model) of how the whole climate system behaves. Thus, there is an element of "group think" that keeps scientific biases entrenched in the research community as a whole.

Proof that this indeed happens is the recent medical theory that stomach ulcers are caused by bacteria. Two Australian medical researchers were scoffed at by the medical community for 20 years before the bacterial basis explanation was finally accepted.

Global Warming and Nature's Thermostat:
Precipitation Systems


Before I can explain the central role that precipitation systems must play in global warming, I will first present a simplified explanation of the basics of global warming - call it a global warming primer. I will address the issue of how warm we are today, and some possible explanations for that warmth. Next, I'll briefly describe the Earth's natural greenhouse effect and global warming theory. Finally, I will explain the "thermostatic control" mechanism that I believe stabilizes the climate system against substantial global warming from mankind's greenhouse gas emissions. Some of what I will present is an extension of Richard Lindzen's "Infrared Iris" hypothesis, observational support for which we published in a peer-reviewed scientific journal on August 9, 2007.

The bottom line of what I will present is this: Precipitation systems ultimately control the magnitude of the Earth's total greenhouse effect -- which is mostly due to water vapor and clouds -- and I believe that those systems will likely offset the small warming tendency from mankind's greenhouse gas emissions.

Oh, and if you think that we should "do something" about global warming anyway as an insurance policy -- no matter what the science says -- please read this.

Warming Over the Last Century

There is little doubt that globally averaged temperatures are unusually warm today (at this writing, 2008). While a majority of climate researchers believe that this warmth is mostly (or completely) due to the activities of mankind, this is as much a statement of faith as it is of science. For in order to come to such a conclusion, we would need to know how much of the temperature increase we've seen since the 1800's is natural. There has not yet been a single peer-reviewed scientific study which has ruled out natural climate variability as the cause of most of our recent warmth -- for instance, a small change in globally averaged cloud cover.

So let's first examine current temperatures in their historical context. Over the last 100 years or so (see Fig.1) globally-averaged surface temperature trends have exhibited three distinct phases.

Fig. 1 Globally averaged temperature variations between 1850 and 2007 show the emergence from the "Little Ice Age" in the early 1900's, slight cooling from the 1940's to the 1970's, and then warming again since the 1970's. (HadCRUT3 temperature dataset from the UK Met Office and Univ. of E. Anglia)
Globally averaged temperature variations between 1850 and 2007.

The warming up until 1940 represents the end of the multi-century cool period known as the "Little Ice Age", a time that was particularly harsh for humanity. This warming must have been natural because mankind had not yet emitted substantial amounts of greenhouse gases. Then, the slight cooling between 1940 and the 1970's occurred in spite of rapid increases in manmade greenhouse gas emissions. One theory is that this cooling is also manmade -- from particulate pollution. Finally, fairly steady warming has occurred since the 1970's. This recent warming has no doubt played a central role in current fears of a climate catastrophe.

There is some controversy over whether the upward temperature trend seen in Fig. 1 still contains some spurious warming from the urban heat island effect, which is due to a replacement of natural vegetation with manmade structures (buildings, parking lots, etc.) around thermometer sites. In December of 2007, a paper published in the Journal of Geophysical Research showed evidence that about 50% of global warming measured by land-based thermometers since 1980 was simply due to local influences such as the urban heat island effect (press release here).

Temperatures Over the Last 2,000 Years

When was the last time that the Earth was this warm?. You might have heard claims in the news that we are warmer now than anytime in the last 1,000 years. This claim was based upon the "Hockey Stick" temperature curve (Fig. 2) which used temperature 'proxies', mostly tree rings, to reconstruct a multi-century temperature record. That "warmest in 1,000 years" claim lost much of its support, however, when a National Academy of Science review panel concluded in 2006 that the Hockey Stick study used faulty statistical techniques, and that the most that can be said with any confidence is that the Earth is warmer now than anytime in the last 400 years. Note that this is a good thing, since most of those 400 years occurred during the Little Ice Age.
Fig. 2. The Mann et al. (1998) proxy (mostly tree ring) reconstruction of global temperature over the last 1,000 years is believed to have erroneously minimized the warmth of the Medieval Warm Period (MWP). The Mann et al. (1998) tree ring reconstruction of global temperature over the last 1,000 years
			                                                    is believed to have erroneously minimized the warmth of the Medieval Warm Period.

A more recent study has averaged 2,000 years of temperature estimates from a total of 18 previously-published temperature proxy datasets, and the resulting temperature record is shown in Fig. 3. No tree ring datasets were used by the author (himself a tree growth expert) because he believes that those datasets are too contaminated by rainfall variations and other problems to be used as temperature proxies. To that reconstruction I added the global thermometer record covering the period 1850 to 2007.

Fig. 3. Global average temperature reconstruction based upon 18 temperature proxies for the period 1 A.D. to 1995, combined with the thermometer-based dataset from the UK Met Office and University of East Anglia, covering the period 1850 to 2007. Note that for both datasets each data point represents a 30-year average.
2,000 years of global temperatures
                     from temperature proxies and thermometer data.

In support of the view that today's warmth is not unprecedented is the historical fact that Vikings arriving in Greenland established farms, until a cooling trend caused them to abandon farming in Greenland.

Thus, we see that substantial natural variations in climate can, and do, occur -- which should be of no great surprise. So, is it possible that much of the warming we have seen since the 1970's is due to natural processes that we do not yet fully understand? I believe so. To believe that all of today's warmth can be blamed on manmade pollution is a statement of faith that assumes the role of natural variations in the climate system is small or nonexistent.

If We Can't Explain It, It Must Be Human-Induced

The fact is, science doesn't understand why these natural climate variations occur, and can not reliably distinguish between natural and possible human influences on global temperatures. So, if scientists have no other natural explanation for a warming trend, they tend to assume that it is manmade.

For instance, you might have heard claims to the effect that no peer-reviewed scientific study has refuted the claim that global warming is manmade. Well, there have indeed been some papers that have at least questioned the theory that our current warmth is manmade....but the publishing of alternative explanations is hindered by the fact that our long-term global climate observations (e.g. of cloud characteristics) are not good enough to measure the small changes that might offer an alternative explanation for our current warmth.

Science can not deal with what we can not measure. But scientists could at least admit to incomplete knowledge -- unfortunately, most of them do not.

I can not overemphasize this -- the theory that our current warmth is manmade is largely the result of not having good enough global observations over a long enough period of time to rule out natural causes. Therefore, the current widespread support for the theory of manmade global warming is NOT because the alternative explanations have been ruled out. It is because our poor understanding of natural climate variability does not yet permit alternative explanations to be investigated thoroughly.

Thus, while it is indeed possible to explain much of the warming over the last 100 years with manmade greenhouse gas increases, this is only one possible explanation -- one that necessarily ignores or minimizes any natural sources of temperature variability.

As a result, our worries that global warming is manmade are directly related to how much faith we have that natural climate variations (for instance, a small decrease in low-level cloudiness) are not substantially contributing to our current warmth. Some scientists who believe in manmade global warming have asked me, "But what else could be causing the warmth?" Note that this is arguing, not from the evidence, but from a lack of evidence.

There is an old saying, "When all you have is a hammer, everything looks like a nail." Well, manmade global warming is our hammer, and so every change (nail) we see in the climate system gets attributed to mankind.

Climate Prediction and Weather Forecasting Are Not the Same

Before describing the greenhouse effect and climate models, we first need to clear up a common misconception about forecasts of global warming. There are two quite different kinds of forecasting of atmospheric behavior: weather prediction, and climate prediction. Weather prediction involves measuring the state of the atmosphere at a given time and then using a computer program containing equations (a 'numerical weather prediction model') to predict how the weather will evolve in the coming days. Simply stated, these 'initial condition' models extrapolate the measured atmospheric behavior of the atmosphere out into the future. They have been quite successful at short ranges (a few days), and their skill is slowly improving over time, but that skill drops to close to zero after about 10 days.

In contrast, the purpose of climate models is not to get a good 3 day or 10 day forecast. Climate models are instead run for much longer periods of simulated time - many years to centuries. Their purpose is to determine how the model's climate (average weather) is affected when one of the rules -- 'boundary conditions' -- by which the atmosphere operates is changed in the model.

In the case of global warming, that rule change is mankind's addition of greenhouse gases, mainly carbon dioxide from the burning of fossil fuels, which then affects the model's 'greenhouse effect' -- the way in which the model atmosphere processes infrared (radiant heat) energy.

The Earth's Natural Greenhouse Effect

The theory that mankind is causing recent global warming is based upon the fact that our greenhouse gas emissions (mainly carbon dioxide) are causing a very small enhancement (about 1%) of the Earth's natural 'greenhouse effect'. The greenhouse effect refers to the trapping of infrared (heat) radiation by water vapor, clouds, carbon dioxide, methane, and a few other minor greenhouse gases (see Fig. 4). You can think of the greenhouse effect as a sort of 'blanket' -- a radiative blanket. The natural greenhouse effect makes the lower atmosphere warmer, and the upper atmosphere cooler, than it would otherwise be without the greenhouse effect. The role of carbon dioxide in the atmosphere's greenhouse effect is relatively small, due to the fact that CO2 is a 'trace gas' -- only 38 out of every 100,000 molecules of air are carbon dioxide. It takes a full five years of human greenhouse gas emissions to add 1 molecule of CO2 to every 100,000 molecules of air.

Fig. 4. The Earth's natural 'greenhouse' effect is due to the trapping of infrared (heat) radiation by water vapor, clouds, carbon dioxide, methane, and other greenhouse gases.

The Earth's natural 'greenhouse' effect is due to the absorption of infrared (heat) radiation
			       by water vapor, clouds, carbon dioxide, and other greenhouse gases.

Mankind's Enhancement of the Greenhouse Effect

The most common explanation for global warming goes like this: Mankind's addition of carbon dioxide to the atmosphere disrupts the Earth's radiative energy balance (see Fig. 5) by reducing its ability to radiatively cool to outer space. Energy balance refers to the theory that all of the Earth's absorbed sunlight (the energy input) is balanced by an equal amount of infrared radiation that the Earth emits back to outer space (the energy output). It is estimated that this input and output, averaged over the whole Earth over several years, is naturally maintained at a value of around 235 Watts per square meter (W/m2).

The Earth's radiative energy balance is fundamental to understanding global warming theory,
			   which says that mankind's greenhouse gas emissions is disrupting that energy balance.
Fig. 5. The Earth's radiative energy balance is fundamental to understanding global warming theory, which says that mankind's greenhouse gas emissions is disrupting that approximate 235 W/m2 balance between solar input & infrared output.

So, mankind's emissions of greenhouse gases are believed to have disrupted that balance. Since the beginning of the industrial revolution, it is estimated that the normal infrared cooling rate of 235 W/m2 has been reduced by about 1.6 W/m2. Taking into account the warming that has already occurred (supposedly) in response to that imbalance, one estimate is that a 0.8 W/m2 imbalance still exists today. A continuing imbalance represents further warming that needs to occur to restore energy balance -- even if mankind stopped producing greenhouse gases today. This is the current explanation of the theory of manmade global warming.

How do we know there is such a radiative imbalance? In reality, we don't. The Earth-orbiting instruments for measuring the Earth's radiative components are not quite accurate to measure the small radiative imbalance that is presumed to exist. That imbalance is, instead, a theoretical calculation.

You might also be surprised to find out that the direct effect of this imbalance from mankind's greenhouse gas emissions (often called a 'radiative forcing') on global temperatures is quite small. If everything else in the climate system remained the same, a doubling of the atmospheric carbon dioxide concentration (probably late in this century) would cause little more than 1 deg. F of surface warming. Remember, mankind's addition of more carbon dioxide to the atmosphere is only one molecule of CO2 for every 100,000 molecules of air every 5 years; do we really believe that such a small influence will have catastrophic effects? A few high-profile scientists, like NASA's James Hansen, indeed do believe that.

Obviously, a 1 deg. F warming by late in this century would cause little concern - if that was the whole story. The problem is that everything else probably doesn't remain the same. The atmosphere will undoubtedly respond in some way to the extra CO2 in terms of changes in clouds, water vapor, precipitation etc.; the question is, how?

Positive or Negative Feedbacks?

Almost all of the scientific uncertainty about the size of manmade global warming is related to how the climate system will respond the small (1 deg. F) warming tendency. The atmosphere could dampen the warming tendency through 'negative feedbacks'-- for instance by increasing low-level cloudiness. Or, it could amplify the warming tendency through 'positive feedbacks', for instance by increasing the water vapor content of the atmosphere (our main greenhouse gas), or by increasing high-altitude cloudiness.

Almost all computerized climate models exhibit positive feedbacks, amplifying the initial CO2-only warming by anywhere from a little bit, to a frightening amount (over 10 deg. F by 2100). So, you can see it is critical for scientists to determine how sensitive the climate system is (how the atmosphere will respond) to the radiative forcing from the extra greenhouse gases we are putting into the atmosphere.

How Sensitive is the Climate System?

The net effect of all of these feedbacks together determines what is called the 'climate sensitivity'. Climate sensitivity, as the name implies, quantifies how much surface warming would result from a given amount of radiative forcing - usually expressed in terms of a doubling of the concentration of carbon dioxide in the atmosphere. Thus, to be able to predict how much warming there will be, what we really need to know is the kind of negative and positive feedbacks that exist in the climate system.

It would be very helpful if we could do a laboratory experiment to determine how the Earth will respond to mankind's addition of greenhouse gases to the atmosphere - but we can't. There is only one 'experiment' going on, and we are all part of it.

If we can't do a laboratory experiment, another way to estimate climate sensitivity would be to find some previous example of climate change in response to radiative forcing. For instance, there are pretty good estimates of how much the Earth cooled after the major eruption of Mt. Pinatubo in the Philippines in June, 1991 (see Fig. 6). The millions of tons of sulfur dioxide that was injected into the stratosphere by Mt. Pinatubo spread around the Northern Hemisphere, reducing the amount of incoming sunlight by as much as 2% to 4% The resulting cooling effects lasted two or three years, until the sulfuric acid aerosols finally dissipated.
The explosive 1991 eruption of Mt. Pinatubo in the Philippines injected millions of tons of
				  sulfur dioxide into the stratosphere.  The resulting 2%-4% reduction in sunlight offered
				  a natural test of the Earth's climate sensitivity to changes in solar radiation.
Fig. 6. The explosive 1991 eruption of Mt. Pinatubo in the Philippines injected millions of tons of sulfur dioxide into the stratosphere. The resulting 2%-4% reduction in sunlight offered a natural test of the Earth's climate sensitivity to changes in solar radiation.

Most climate researchers believe that previous events like the Pinatubo eruption can be used to determine the climate's sensitivity to greenhouse gas emissions. I do not. Mt. Pinatubo reduced the amount of incoming sunlight, and while sunlight is the source of energy for the climate system, the total greenhouse effect of the atmosphere is under the control of weather systems responding to the sunlight. Very simply put, sunlight causes weather, but the greenhouse effect is the result of weather. I believe that weather processes actively limit the total greenhouse effect in proportion to the amount of available sunlight.

So, are there any good examples of infrared (greenhouse) climate forcings from the past? Probably not. There are ice core measurements from Antarctica which suggest that, hundreds of thousands of years ago, carbon dioxide levels and temperatures went up and down. This was a prominent argument in Al Gore's movie, An Inconvenient Truth. But what Mr. Gore didn't mention was that all published scientific research of those relationships has shown that the carbon dioxide followed the temperature changes, by hundreds of years. Thus, the ice core evidence suggests that the temperature changes caused the carbon dioxide changes -- not the other way around, as is claimed by some scientists and politicians. So, we can't use the ice core evidence as an analog to what is happening today, where humans are causing the CO2 content of the atmosphere to rise, because very different mechanisms were obviously operating during those past climate events.

Therefore, in contrast to volcanic eruptions and their effect on solar heating of the Earth, we are possibly left without a natural example of infrared radiative forcing, which is what modern global warming theory is all about.

What Determines the Earth's Natural Greenhouse Effect?

Now we come to an issue I believe to be of fundamental importance: What determines the Earth's natural greenhouse effect? I don't mean in a qualitative sense, for all climate researchers know that water vapor and clouds together dominate the greenhouse effect. What I mean is: Why is the greenhouse effect maintained at its current strength? The atmosphere could hold much more water vapor than it does -- which would result in a warmer climate -- but instead, much of the depth of the troposphere is usually at a fairly low relative humidity. Oh, we can build climate models and tune them to replicate the average amount of greenhouse effect we see in nature, but what I hope to convince you of is that we don't really understand the processes that limit the greenhouse effect to its current value.

Let's start at the beginning. Sunlight is the source of energy for our weather, and so it makes sense that more (or less) sunlight will make the Earth warmer (or cooler). But the greenhouse effect (trapping if infrared heat) is the result of weather processes. Remember, most of the Earth's greenhouse effect (over 90%) is due to water vapor and clouds, and so it is under direct control of weather processes -- winds, evaporation, precipitation, etc.

This cause-versus-effect role of the Earth's natural greenhouse effect is an important distinction. I mentioned above the common explanation that the Earth's "energy balance results in a roughly constant globally-averaged temperature". But I believe that this has cause and effect turned around: It is more accurate to say that "Heating by the sun causes weather, which in turn generates a greenhouse effect that is in proportion to the available sunlight". Unless we understand the processes that limit the Earth's natural greenhouse effect to its present value, we can't hope to understand how mankind's small, 1% enhancement of the greenhouse effect will change global climate.

Precipitation Systems: Nature's Air Conditioner?

It is well known that precipitation is an important process in the atmosphere. Besides being necessary for life on Earth, all of the rain and snow that falls to the ground represents excess heat that has been removed from the Earth's surface during the evaporation of water. On average, all of the water evaporated from the surface must at some point condense and fall back to the surface as precipitation. The heat that is released during that condensation is deposited in the middle and upper troposphere when the water vapor condenses into clouds, some of which then produce precipitation that falls to the surface. After it reaches the surface, the water is once again available to remove more heat through evaporation, starting the cycle all over again.

I believe it can be demonstrated that precipitation systems ultimately control most of the Earth's natural greenhouse effect. The air in our atmosphere is continuously being recycled through precipitation systems (see Fig. 7), on a time scale of days to weeks. Winds pick up water vapor that has been evaporated from the surface, and then transport this vapor to precipitation systems. Those systems then remove some of that vapor in the form of rain or snow. This qualitative view is well known and understood by climate researchers.

But what is NOT understood (yet is critical to understanding feedbacks and climate sensitivity) are the myriad 'microphysical' processes within clouds -- the behavior of water drops and ice crystals. These microphysical processes determine just how much water substance will be removed as precipitation, and thus how much will be left over to be exhausted out of the weather systems as water vapor and clouds. For it is the moisture properties of the air flowing out of precipitation systems that then determine most of the Earth's greenhouse effect, since that air slowly fills in the huge areas between the relatively small precipitation systems.

Fig. 7. Atmospheric air gets continuously recycled through precipitation systems, which then directly or indirectly control the water vapor and cloud properties, and thus the Earth's natural greenhouse effect.
Most of the atmosphere gets continuously recycled through precipitation systems, which
			   determine the moisture properties, and thus greenhouse effect, of the air.
Partly because precipitation systems cover only several percent of the Earth's surface at any given time, even most climate researchers do not appreciate the controlling influence these systems have on the climate system. So I can not emphasize this enough: All of the humid air flowing into precipitation systems in the lower atmosphere ends up flowing out of those same systems, mostly in the middle and upper atmosphere. That air flowing out has moisture (water vapor and cloud) amounts that are directly controlled by precipitation processes within the systems.

As one example of the global influence of these systems on the Earth's greenhouse effect, the low-humidity air that is slowly sinking over the world's deserts was dried out by precipitation systems, possibly thousands of miles away. Eventually, that air will leave the desert, pick up moisture evaporated from the land or ocean, and be cycled once again through a rain or snow system. Remember, this recycling of air by precipitation systems is continuously occurring, all over the Earth.

Similarly, the cold air masses that form over continental areas in the wintertime are extremely dry because the air within them came from the upper troposphere after it had been exhausted out of a rain or snow system. If this were not the case, wintertime high pressure systems would not be clear and dry as is observed. They would instead become saturated with water vapor as they cooled in response to the lack of sunlight, and would become filled with clouds.

Thus, we begin to see that much of the Earth's natural greenhouse effect is under the control of precipitation systems. It doesn't matter whether they are tropical thunderstorms, or high latitude snowstorms, it is still the air flowing out of them in the middle and upper troposphere that determines the humidity characteristics of the cloud-free regions everywhere else.

I want to make it clear that the average effects of precipitation systems are indeed contained in today's computerized climate models. But for global warming, a model mimicking their average behavior isn't sufficient, for it is too easy to get the right answer for the wrong reason. Instead, we need to answer the question: How do precipitation systems change in response to mankind's small addition of greenhouse gases to the atmosphere? This is where I believe the models are wrong. Models tend to amplify the Earth's natural greenhouse effect in response to mankind's small addition of greenhouse gases; but I believe that real precipitation systems do just the opposite...they slightly reduce the total greenhouse effect by adjusting water vapor and cloud amounts, to keep it in proportion to the amount of available sunlight.

But the influence of precipitation systems on the global climate doesn't end there. They also indirectly control cloud amounts in remote regions, even thousands of miles away from any precipitation system. This is because the convective (vertical) overturning of the global atmosphere being forced by precipitation processes largely determines the vertical temperature profile of the atmosphere. That temperature profile, in turn, exerts a strong influence on cloud systems.

For instance, there are vast areas of marine stratus clouds in the lower troposphere that form over the eastern ends of the subtropical oceans where cold water wells up from below (see Fig. 8). Those clouds form because the moist air from ocean evaporation gets trapped below a temperature inversion (warm air layer).

And guess what causes that warm air inversion? Precipitation systems! The air is unusually warm because it is being forced to sink by warm, moist air rising in precipitation systems. That rising air is being fueled by condensing water vapor, which releases the heat that was absorbed when the water originally evaporated from the Earth's surface.
Marine stratocumulus clouds, which cool the climate system by reflecting sunlight,
				  are partly under the control of precipitation systems far away.
Fig. 8. Marine stratocumulus clouds, which cool the climate system by reflecting sunlight, are partly under the control of precipitation systems far away.

[NOTE: Some scientists will claim that the sinking air forming the warm inversion is "caused" by radiative cooling, but this is incorrect. The only way for air to sink in a statically stable environment is for it to be forced to sink -- which only happens in response to warm, moist rising air in precipitation systems. Radiative cooling no more 'causes air to sink' than the exhaust coming from a car's tailpipe causes the car's engine to run.]

It should now be increasingly clear to you that we can not know how sensitive the climate system is to mankind's small enhancement of the Earth's natural greenhouse effect without understanding how the greenhouse effect (water vapor + clouds) is controlled by precipitation systems. Unfortunately, precipitation is probably the least understood of all atmospheric processes.

In a little-appreciated research publication, Renno, Emanuel, and Stone (1994, "Radiative-convective model with an explicit hydrologic cycle, 1: Formulation and sensitivity to model parameters", J. Geophys. Res., 99, 14429-14441) demonstrated that if precipitation systems were to become more efficient at converting atmospheric water vapor into precipitation, the result would be a cooler climate with less precipitation. Thus, precipitation systems have the potential to be, in effect, the Earth's 'air conditioner', switching on when things get too warm.

The big question is, do they behave this way or not? I believe they do.

Precipitation in Climate Models

Climate model representations of precipitation processes are very crude. In fact, for warm air masses, the models don't actually grow precipitation systems. They instead use simple 'parameterizations' that are meant to represent the net effects of precipitation on the atmosphere in some statistical sense. There is nothing inherently wrong with using parameterizations to replace more complex physical processes - as long as they accurately represent what controls those processes.

What we really need to know is how the efficiency of precipitation systems changes with temperature. Unfortunately, this critical understanding is still lacking. Most of the emphasis has been on getting the models to behave realistically in how they reproduce average rainfall amounts and their geographic distribution -- not in how the model handles changes in rainfall efficiency with warming.

Fortunately, we now have new satellite evidence which sheds light on this question. Our recently published, peer-reviewed research shows that when the middle and upper tropical troposphere temporarily warms from enhanced rainfall activity, the precipitation systems there produce less high-altitude cirroform (ice) clouds. This, in turn, reduces the natural greenhouse effect of the atmosphere, allowing enhanced infrared cooling to outer space, which in turn causes falling temperatures. (Our news release describing the study is here.)

This is a natural, negative feedback process that is counter-intuitive for climate scientists, most of whom believe that more tropical rainfall activity would cause more high-level cloudiness, not less. Whether this process also operates on the long time scale involved with global warming is not yet known for sure. Nevertheless, climate models are supposedly built based upon observed atmospheric behavior, and so I challenge the modelers to include this natural cooling process in their models, and then see how much global warming those models produce.

A Summary, and the Future

Climate modelers and researchers generally believe that an increase in the greenhouse effect from manmade greenhouse gases causes a warming effect that is similar to that from an increase in sunlight.

I believe that this is incorrect.

It is now reasonably certain that changes in solar radiation cause temperature changes on Earth. For instance, the 1991 eruption of Mt. Pinatubo caused a 2% to 4% reduction in sunlight, resulting in two years of below normal temperatures, especially over Northern Hemisphere land areas.

But the Earth's natural greenhouse effect (again, mostly from water vapor and clouds) is under the control of weather systems -- especially precipitation systems -- which are generated in response to solar heating. Either directly or indirectly, those precipitation systems determine the moisture (water vapor and cloud) characteristics for most of the rest of the atmosphere.

Precipitation systems could, theoretically, cause a much warmer climate on Earth than is currently observed. They could allow more water vapor to build up in the atmosphere, but they don't. Why not?

The reason must ultimately be related to precipitation processes. I believe that precipitation systems act as a thermostat, reducing the Earth's greenhouse effect (and thus causing enhanced cooling) when temperatures get too high, and warming when temperatures get too low. It is amazing to think that the ways in which tiny water droplets and ice particles combine in clouds to form rain and snow could determine the course of global warming, but this might well be the case.

I believe that it is the inadequate handling of precipitation systems -- specifically, how they adjust atmospheric moisture contents during changes in temperature -- that is the reason for climate model predictions of excessive warming from increasing greenhouse gas emissions. To believe otherwise is to have faith that climate models are sufficiently advanced to contain all of the important processes that control the Earth's natural greenhouse effect.

I predict that further research will reveal some other cause for most of the warming we have experienced since the 1970's -- for instance, a change in some feature of the sun's activity; or, a small change in cloudiness resulting from a small change in the general circulation of the atmosphere (such as the Pacific Decadal Oscillation, 'PDO'). In the meantime, a high priority research effort should be the study of changes in precipitation systems with changes in temperature -- especially how they control global water vapor and cloud amounts.

Fortunately, we now have several NASA satellites in Earth orbit that are gathering information that will be immensely valuable for determining how the Earth's climate system adjusts during natural temperature fluctuations. It is through these satellite measurements of temperature, solar and infrared radiation, clouds, and precipitation that we will be able to test and improve the climate models, which will then hopefully lead to more confident predictions of global temperatures.

And what do the satellites tell us about recent global temperature variations? In Fig. 9, I have arbitrarily picked the period since 1990 to show that there has been recent warming, but that warming certainly would not be characterized as 'gradual'. When one takes into consideration that the cooling from the Mt. Pinatubo eruption and the warming from the 1997-98 El Nino event were not part of any underlying long-term trend, we can imagine that globally-averaged temperatures were flat from 1990 until 2000, then there was a brief warming until about 2002, after which temperatures have once again remained flat. Note that the longer temperatures remain flat the greater the warming that will be required to put us back 'on track' to match the climate model projections used by the U.N.'s Intergovernmental Panel on Climate Change. The coming months and years should be interesting.

Fig. 9. Satellite-measured monthly globally averaged lower atmospheric temperature variations since 1990. When one considers that the cooling from the 1991 eruption of Mt. Pinatubo and the warming from the 1997-98 El Nino were not part of any underlying trend, one can imagine a period of roughly steady temperatures from 1990 to 2000, then warming until 2002, then roughly steady temperatures again from 2002 through 2007.
Global temperature variations (1990 through 2007) of the lower atmosphere as 
			   measured by NOAA and NASA satellites.
Roy W. Spencer received his Ph.D. in meteorology at the University of Wisconsin-Madison in 1981. Before becoming a Principal Research Scientist at the University of Alabama in Huntsville in 2001, he was a Senior Scientist for Climate Studies at NASA's Marshall Space Flight Center, where he and Dr. John Christy received NASA's Exceptional Scientific Achievement Medal for their global temperature monitoring work with satellites. Dr. Spencer is the U.S. Science Team leader for the Advanced Microwave Scanning Radiometer flying on NASA's Aqua satellite. His research has been entirely supported by U.S. government agencies: NASA, NOAA, and DOE.

Dr. Spencer's first popular book on global warming, Climate Confusion (Encounter Books), will be available in bookstores March 27, 2008.

Reviews of Climate Confusion.

(updated October 27, 2007)

It has become commonplace for scientists like me who are skeptical of mankind's role in global warming to be branded as shills for "Big Oil". As a result of misinformation posted at (and other web sites that spread that misinformation), I would like to set the record straight concerning my financial interests. notes that I have given talks on global warming at conservative think tanks like the Marshall Institute, implying that I have some sort of financial relationship with them. In truth, I received no speaking fee for these talks -- but I HAVE been paid for giving talks for environmental organizations in several states. I wonder why doesn't mention this connection to "Big Environmentalism"? After all, they are the ones who have paid me speaking fees -- not the Marshall Institute.

After 12 years of receiving no compensation for my writings, I was eventually asked to write global warming related articles for (now That website advocated science, technology, and free markets, and was indeed partially funded by Exxon Mobil. While I no longer write for that web site, over a three year period I augmented my "day job" salary by an average of 5% by writing articles. The views expressed in those articles were consistent with the views I had expressed for twelve years for no compensation. (Quite frankly, since I supported the ideals promoted on, I really didn't care who funded it).

The dirty little secret is that environmental organizations and global warming pessimists receive far more money from Big Oil than do global warming optimists such as myself. While professional environmental lobbyists are totally dependent upon environmental crises for their continued existence, atmospheric researchers and meteorologists have day jobs which are not. Some outspoken global warming pessimists have received large cash awards (hundreds of thousands of dollars) for the positions they have taken; there are no such monetary awards for global warming optimists. Instead, we have to endure scorn from several outspoken peers in the scientific community, some of whom are successful at thwarting our publication of scientific articles and government funding of our research proposals.

As long as the global warming pessimists can convince the public that we skeptics are simply shills for Big Oil, they do not have to address our scientific arguments. The claims that there are no peer-reviewed scientific articles that oppose a manmade source of global warming are, quite simply, wrong. Fortunately, the tide is slowly turning, and increasing numbers of scientists are now speaking out about their doubts concerning mankind's role in recent global warmth.

Why Shouldn't We Act Now?
A Critique of "Most Terrifying Video You'll Ever See"

Many people believe that we should act now on global warming, as a sort of "insurance policy", just in case it ends up being a serious threat. For instance, there has been quite a bit of buzz lately about a YouTube video in which an Oregon high school teacher, Greg Craven, uses logic to convince viewers that the only responsible course of action on global warming is to act as if it is manmade and catastrophic. In other words, the potential risk of doing nothing is so high that we must act, no matter what the science says.

Unfortunately, as in all exercises of logic (as well as of scientific investigation), your conclusions are only as good as your assumptions. The bad assumptions that Mr. Craven makes that end up invalidating his conclusions are these:

1. That there are actions we can take now that will greatly alleviate the global warming problem if it is manmade, and

2. That the cost of those actions to the world will, at worst, be only economic.

Both of these assumptions are false. Humanity's need for energy is so vast that, until a new energy technology is developed, fossil fuels will continue to dominate our energy mix. The only way to substantially reduce the risk of catastrophic manmade warming in the near-term (the next 20-30 years) would be to bring the daily activities of mankind to a virtual standstill.

Using Mr. Craven's logic, I could argue that people should stop eating because, no matter how small the risk, people can (and do) die from choking on food. Paraphrasing Mr. Craven, not eating is the only responsible course of action to prevent choking to death. The only problem with this, of course, is that we would all die of starvation if we quit eating.

While this is admittedly an extreme example, in the case of reducing mankind's greenhouse gas emissions it is much closer to the truth than what Mr. Craven portrays. People tend to forget that every decision we make in life, whether we know it or not, involves weighing risks against benefits. Mr. Craven incorrectly assumes that the benefits of immediate action on global warming will outweigh the risks.
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