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Global Warming and Nature's Thermostat
by Roy W. Spencer
(text last updated July 1, 2007)
July 2, 2007 RESEARCH UPDATE!: Our research paper describing satellite measurements that support my theory of the thermostatic control by precipitation systems (described below) has been accepted for publication in Geophysical Research Letters, and it has been chosen to be "highlighted" as an important contribution. I'll post again when a publication date is known.
PROLOGUE & FULL DISCLOSURE(July 1, 2007)
It has become commonplace for those of us scientists who are skeptical of mankind's role in global warming (I like to call us global warming optimists) to be branded as shills for "Big Oil". As a result of misinformation posted at ExxonSecrets.org (and other web sites that spread that misinformation), I have decided to set the record straight concerning my financial interests. I have never been asked by any energy company to take a position on global warming -- or to do anything else for them. hile I have given talks on global warming at conservative think tanks like the Marshall Institute (for no pay), I have also done the same for environmental organizations in several states.
Apparently, those who run ExxonSecrets.org think that any association of my name with conservative organizations is sufficient "guilt by association" for the public to assume that I receive compensation from energy companies. After 12 years of receiving no compensation for my writings, I was eventually asked to write global warming related articles for TechCentralStation.com (now TCSDaily.com). 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 TechCentralStation.com, 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 (see below). Fortunately, the tide is slowly turning, and more and more scientists are now speaking out about their doubts concerning mankind's role in recent global warmth.
Here I present a simplified (but hopefully accurate) explanation of the basics of global warming - call it a global warming primer. First, 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" effect, support for which has been recently found in satellite measurements.
Warming Over the Last Century
There is little doubt that globally averaged temperatures are unusually warm today (at this writing, 2007). 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 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. So, let's 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 surface temperature variations (deg. C) over the last century (through 2006) have shown warming until about 1940 (which must have been natural), then a slight cooling until the 1970's (either natural or the result of aerosol pollution), then steady warming since the 1970's (J. Hansen, NASA/GISS).
The warming up until 1940 represents the end of the multi-century cool period known as the "Little Ice Age" which was, historically, a particularly harsh period 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 gases. One theory is that this cooling is manmade -- from particulate pollution. Finally, fairly steady warming has occurred since the 1970's. It should be noted that there is still 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.
Warming Over the Last Millenium
At least in the context of the last century or more, today's global temperatures are unusually warm. But 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 is 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 Acadamy of Science review panel concluded in 2006 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).
But it turns out we don't need to use "proxies" for temperature like tree ring measurements -- there are actual temperature 'measurements' that go back over 1,000 years. Borehole temperatures are taken deep in the ground, where the seasonal cycle in surface temperature sends an annual temperature pulse down into the Earth. Dating of these underground temperature pulses from Greenland (Fig. 3) reveals much warmer temperatures 1,000 years ago than today.
Fig. 3. The GRIP (Greenland) borehole record is one of the best records because it is not a proxy, it is a DIRECT measure of temperature. Shown are the last 2000 years. (Dahl-Jensen et al. 1998, Science, 282, 268-271 "Past Temperatures Directly from the Greenland Ice Sheet"). A similar reconstruction occurs for the Ural Mountain borehole temperatures (i.e. warmer 1000 years ago, Bemeshko, D., V.A. Schapov, Global and Planetary Change, 2001.
Note that such methods for dating temperatures cause a smoothing of the signal in time; any enhanced warmth of individual decades would be smeared out. This is a fundamental problem with any comparisons of today's warmth with reconstructions of past climates. Those reconstructions can not resolve individual warm periods of 10 or 20 years duration. If we could see those past temperature spikes, which undoubtedly occurred during the MWP, our current warmth would seem even less significant.Of course, there are also historical records of the Vikings farming in Greenland, as well as of the gradual cooling that led to the abandonment of those farms, and the appearance of icebergs that started posing a hazard to the Viking's travel by boat.
Thus, we see that substantial natural variations in temperature can, and do, occur -- which should be no 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. And it is indeed possible to explain the temperature changes over the last 100 years by carefully tuning climate models with some estimated effects from volcanic eruptions, sunlight intensity variations, manmade aerosol emissions, and greenhouse gas increases. But this is simply one possible explanation -- one that largely ignores possible natural sources of temperature variability.
As a result, our worries over global warming are directly related to how much faith we have that natural climate variations (for instance, a small change in low-level cloudiness) are not substantially contributing to our current warmth. "When all you have is a hammer, everything looks like a nail." Global warming is our hammer, and so every change we see in the climate system that we can not otherwise explain tends to look like a nail.
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 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.
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
Global warming is all about mankind's small enhancement 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' -- but one that operates on infrared radiation, not by physically trapping warm air beneath it like a regular blanket does. The natural greenhouse effect makes the lower atmosphere warmer, and the upper atmosphere cooler, than it would otherwise be without the greenhouse effect.
Fig. 4. The Earth's natural 'greenhouse' effect is due to the absorption of infrared (heat) radiation by water vapor, clouds, carbon dioxide, methane, and other greenhouse gases in the atmosphere.
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 expectation 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).
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 greeenhouse gases is 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 (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 would have catastrophic effects?
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 respond in some way to the extra CO2; 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.
Most computerized climate models behave in this second way, amplifying the initial 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?
To be able to predict how much warming there will be, what we really need to know then is the kind of negative and positive feedbacks that exist in the climate system. The net effect of all of the feedbacks together determines what is called the 'climate sensitivity', which as the name implies, expresses how much surface warming would result from a given amount of radiative forcing - say, a doubling of the concentration of carbon dioxide in the atmosphere.
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 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.
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.
Unfortunately, an estimate of climate sensitivity from changes in sunlight is not necessarily the same as the sensitivity to changes in greenhouse gases, which affect infrared light. While sunlight is the source of energy for the climate system, greenhouse gases affect how that energy courses through the climate system. Very simply put, sunlight causes weather, but the greenhouse effect is the result of weather.
So, are there any previous examples of infrared (greenhouse) climate forcings? There are ice core measurements from Antarctica which suggest that, hundreds of thousands of years ago, carbon dioxide levels and temperature did indeed go up and down together. 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 have shown that the carbon dioxide followed the temperature changes, by at least a century. In other words, the evidence suggests that temperature changes caused the carbon dioxide changes, not the other way around as is claimed in global warming theory.
Thus, 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?
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 "weather processes generate a greenhouse effect that is in proportion to the warming caused by sunlight". Unless we understand the processes that control the Earth's natural greenhouse effect, we can't hope to understand how mankind's small, 1% enhancement of the greenhouse effect will change global climate.
But Don't Climate Models also "Generate" a Greenhouse Effect?
If the climate models contain the correct physics, such differences in how we conceptualize the climate system won't matter. The trouble is, climate model's are "tuned" to produce the average amount of greenhouse effect that we see in the real world, without really understanding why weather processes maintain the natural greenhouse effect at its observed value.
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. That heat is deposited in the middle and upper tropopshere when the water vapor condenses into clouds, some of then produce precipitation.
I believe it can be demonstrated that precipitation systems ultimately control most of the Earth's natural greenhouse effect. Most of the atmosphere (the lower 80%, called the troposphere) is continuously being recycled through precipitation systems (see Fig. 7), on a time scale of weeks. Winds in the troposphere's 'boundary layer' pick up water vapor that has been evaporated from the surface, and then transport this vapor to precipitation systems, where an equal amount of vapor (on average) is removed as rain or snow.
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.
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. All of the humid air flowing into precipitation systems in the lower troposphere ends up flowing out of those same systems, mostly in the middle and upper troposphere. (The only exception is thunderstorm downdrafts, which you have likely experienced before). That air flowing out has moisture (water vapor and cloud) amounts that are controlled by precipitation processes within the systems. This constitutes the direct effect that precipitation systems have on the Earth's natural greenhouse effect.
For instance, the cloud-free, dry air that is slowly sinking over the world's deserts got its dryness from air flowing out the top of precipitation systems. 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.
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, 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 these 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 upper troposphere that determines the humidity characteristics of the cloud-free regions everywhere else.
...But There's More....
Precipitation systems' influence on the Earth's natural greenhouse effect doesn't end with their direct control over the atmosphere's humidity distribution. They also indirectly control cloud amounts in regions thousands of miles away. The heat trasported upward in precipitation systems largely determines the vertical temperature profile of the global troposphere. 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). But that warm air layer is the result of atmospheric sinking in response to moist air being forced to rise by the condensation of water vapor in precipitation systems.
Fig. 8. Marine stratocumulus clouds, which cool the climate system by reflecting sunlight, are partly under the control of precipitation systems far away.
Some scientists claim that the sinking air forming the inversion is caused
by radiative cooling, but this is incorrect. The only way for a deep layer of tropospheric
air to sink in a statically stable environment is for it to be forced to -- which only happens
in response to warm, moist rising air in precipitation systems. Radiative cooling no
more causes air to sink that the cooling of a car's engine causes the 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 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?
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 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 on how the model handles changes in rainfall efficiency with warming.
Our recent research with satellite observations (conditionally accepted for publication as of July 1, 2007) suggests that when the middle and upper tropical troposphere temporarily warms from enhanced rainfall activity, the precipitation systems there produce less high-altitude ice clouds. This, in turn, reduces the natural greenhouse effect of the tropical atmosphere. This reduction in high-altitude cloudiness causes enhanced infrared cooling to outer space, which then results in falling tropical temperatures.
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, and will surely be the subject of considerable debate.
A Summary, and the Future
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. It is not so obvious, however, that small changes in the Earth's infrared cooling (the greenhouse effect) from mankind's burning of fossil fuels will do the same. This is because the Earth's natural greenhouse effect is mostly under the control of weather systems: specifically, precipitation systems. Either directly or indirectly, precipitation systems determine the moisture (water vapor and cloud) characteristics for most of the rest of the atmosphere.
Precipitation systems thus potentially act as a thermostat, causing 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 the warming we have experienced since the 1970's -- for instance, a change in some feature of the sun's activity. 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 warming.
Roy W. Spencer received his PhD in meteorology at the University of Wisconsin-Madison in 1981. Before bcoming 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 released during the winter of 2007-08.