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.
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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.
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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.
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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.
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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)
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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).
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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. |
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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). |
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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.
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| 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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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).
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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.
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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.
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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.
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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.
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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?
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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.
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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.
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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.
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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.
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| 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. |
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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| 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.
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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.
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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.
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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.
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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.
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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.
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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.
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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. |
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Fig. 8. Marine
stratocumulus clouds, which cool the climate system by reflecting
sunlight, are partly under the control of precipitation systems
far away.
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[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.]
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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.
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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.
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The big question is, do they behave this way or not?
I believe they do.
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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.
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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.
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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.)
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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.
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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?
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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.
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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.
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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.
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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.
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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.
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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.
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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. |
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FULL DISCLOSURE
(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 ExxonSecrets.org (and other web sites that spread that
misinformation), I would like to set the record straight
concerning my financial interests.
ExxonSecrets.org 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 ExxonSecrets.org 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 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. 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.
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Why Shouldn't We Act Now?
A Critique of "Most Terrifying
Video You'll Ever See"
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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.
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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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