The
last Wayback snapshot to contain the "nature's
thermostat" post is dated 12/19/2008.
I don't know if Roy meant to take it down or if it just
got lost in the shuffle when he redesigned his website,
but I always thought it was important so am archiving a
copy:
Global Warming
and Nature's Thermostat
by Roy W. Spencer, Ph.D.Updated January 12, 2008 with minor revisions. (This page is frequently updated with a variety of improvements, many of which won't be mentioned here.)
START HERE! Global Warming 101: Global warming theory in a nutshell.
Preface: How Could So Many Climate Modelers Be Wrong?
Introduction
Warming Over the Last Century
Temperatures Over the Last 2,000 Years
If We Can't Explain It, It Must Be Human-Induced
Climate Prediction and Weather Forecasting Are Not The Same
The Earth's Natural Greenhouse Effect
Mankind's Enhancement of the Greenhouse Effect
Positive or Negative Feedbacks?
How Sensitive Is the Climate System?
What Determines the Earth's Natural Greenhouse Effect?
Precipitation Systems: Nature's Air Conditioner?
Precipitation In Climate Models
A Summary, and the Future
Bio and Full Disclosure
New book release March 27, 2008: CLIMATE CONFUSION - How Global Warming Hysteria Leads to Bad Science, Pandering Politicians, and Misguided Policies that Hurt the Poor
Why Shouldn't We Act Now?:A Critique of "Most Terrifying Video You Will Ever See"
Global Warming 101:
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?":
Global Warming and Nature's Thermostat:Precipitation Systems
Introduction
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)
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.
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).
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.
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.
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 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.
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).
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.
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.
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.
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.
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.
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.
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.
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. 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.
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.
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.
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.
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.)
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.
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.
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.
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.
Why Shouldn't We Act Now?
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.
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.