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 Posted: Mon Oct 20, 2008 8:09 am Post subject: Global Warming Is Mostly Natural Variability |
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Global Warming as a Natural Response to Cloud Changes
Associated with the Pacific Decadal Oscillation (PDO)
Roy W. Spencer, PhD
Principal Research Scientist
The University of Alabama in Huntsville
October 19 2008
QUOTE: The discrepancy between the models and observations seen in Fig.
1 is stark. If the sensitivity of the climate system is as low as some
of these observational results suggest, then the IPCC models are grossly
in error, and we have little to fear from manmade global warming.
QUOTE: But an insensitive climate system would ALSO mean that the
warming we have seen in the last 100 years can not be explained by
increasing CO2 alone. This is because the radiative forcing from the
extra CO2 would simply be too weak to cause the ~0.7 deg. C warming
between 1900 and 2000... there must be some natural warming process
going on as well.
QUOTE: Others have noted that the warming in the 1920s and 1930s led to
media reports of decreasing sea ice cover, Arctic and Greenland
temperatures just as warm as today, and the opening up of the Northwest
Passage in 1939 and 1940.
(what follows is a simplified version of a paper I am preparing for
submission to Geophysical Research Letters)
Abstract
A simple climate model forced by satellite-observed changes in the
Earth>s radiative budget associated with the Pacific Decadal Oscillation
is shown to mimic the major features of global average temperature
change during the 20th Century - including two-thirds of the warming
trend. A mostly-natural source of global warming is also consistent with
mounting observational evidence that the climate system is much less
sensitive to carbon dioxide emissions than the IPCC>s climate models
simulate.
1. Introduction
For those who have followed my writings and publications in the last 18
months (e.g. Spencer et al., 2007), you know that we are finding
satellite evidence that the climate system could be much less sensitive
to greenhouse gas emissions than the U.N.'s IPCC climate models suggest.
To show that we are not the only researchers who have documented
evidence contradicting the IPCC models, I made the following figure to
contrast the IPCC-projected warming from a doubling of atmospheric
carbon dioxide with the warming that would result if the climate
sensitivity is as low as implied by various kinds of observational
evidence. The dashed line represents our recent apples-to-apples
comparison between satellite-based feedback estimates and IPCC
model-diagnosed feedbacks, all computed from 5-year periods (Spencer and
Braswell, 2008a):
Fig. 1. Projected warming (assumed here to occur by 2100) from a
doubling of atmospheric CO2 from the IPCC climate models versus from
various observational indicators.
The discrepancy between the models and observations seen in Fig. 1 is
stark. If the sensitivity of the climate system is as low as some of
these observational results suggest, then the IPCC models are grossly in
error, and we have little to fear from manmade global warming.
But an insensitive climate system would ALSO mean that the warming we
have seen in the last 100 years can not be explained by increasing CO2
alone. This is because the radiative forcing from the extra CO2 would
simply be too weak to cause the ~0.7 deg. C warming between 1900 and
2000... there must be some natural warming process going on as well.
Here I present new evidence that most of the warming could actually be
the result of a natural cycle in cloud cover forced by a well-known mode
of natural climate variability: the Pacific Decadal Oscillation.
2. A Simple Model of Natural Global Warming
As Joe D>Aleo and others have pointed out for years, the Pacific Decadal
Oscillation (PDO) has experienced phase shifts that coincided with the
major periods of warming and cooling in the 20th Century. As can be seen
in the following figure, the pre-1940 warming coincided with the
positive phase of the PDO; then, a slight cooling until the late 1970s
coincided with a negative phase of the PDO; and finally, the warming
since the 1970s has once again coincided with the positive phase of the
PDO.
Fig. 2. Five-year running averages in (a) global-average surface
temperature, and (b) the Pacific Decadal Oscillation (PDO) index during
1900-2000.
Others have noted that the warming in the 1920s and 1930s led to media
reports of decreasing sea ice cover, Arctic and Greenland temperatures
just as warm as today, and the opening up of the Northwest Passage in
1939 and 1940.
Since this timing between the phase of the PDO and periods of warming
and associated climate change seems like more than mere coincidence, I
asked the rather obvious question: What if this known mode of natural
climate variability (the PDO) caused a small fluctuation in
global-average cloud cover?
Such a cloud change would cause the climate system to go through natural
fluctuations in average temperature for extended periods of time. The
IPCC simply assumes that this kind of natural cloud variability does not
exist, and that the Earth stays in a perpetual state of radiative
balance that has only been recently disrupted by mankind>s greenhouse
gas emissions. This is an assumption that many of us meteorologists find
simplistic and dubious, at best.
I used a very simple energy balance climate model, previously suggested
to us by Isaac Held and Piers Forster, to investigate this possibility.
In this model I ran many thousands of combinations of assumed: (1) ocean
depth (through which heat is mixed on multi-decadal to centennial time
scales), (2) climate sensitivity, and (3) cloud cover variations
directly proportional to the PDO index values.
In effect, I asked the model to show me what combinations of those model
parameters yielded a temperature history approximately like that seen
during 1900-2000. And here>s an average of all of the simulations that
came close to the observed temperature record:
Fig. 3. A simple energy balance model driven by cloud changes associated
with the PDO can explain most of the major features of global-average
temperature fluctuations during the 20th Century. The best model fits
had assumed ocean mixing depths around 800 meters, and feedback
parameters of around 3 Watts per square meter per degree C.
The "PDO-only" (dashed) curve indeed mimics the main features of the
behavior of global mean temperatures during the 20th Century --
including two-thirds of the warming trend. If I include transient CO2
forcing with the PDO-forced cloud changes (solid line labeled PDO+CO2),
then the fit to observed temperatures is even closer.
It is important to point out that, in this exercise, the PDO itself is
not an index of temperature; it is an index of radiative forcing which
drives the time rate of change of temperature.
Now, the average PDO forcing that was required by the model for the two
curves in Fig. 3 ranged from 1.7 to 2.0 Watts per square meter per PDO
index value. In other words, for each unit of the PDO index, 1.7 to 2.0
Watts per square meter of extra heating was required during the positive
phase of the PDO, and that much cooling during the negative phase of the
PDO.
But what evidence do we have that any such cloud-induced changes in the
Earth>s radiative budget are actually associated with the PDO? I address
that question in the next section.
3. Satellite Evidence for Radiative Budget Changes Forced by the Pacific
Decadal Oscillation
To see whether there is any observational evidence that the PDO has
associated changes in global-average cloudiness, I used NASA Terra
satellite measurements of reflected solar (shortwave, SW) and emitted
infrared (longwave, LW) radiative fluxes over the global oceans from the
CERES instrument during 2000-2005, and compared them to recent
variations in the PDO index. The results can be seen in the following
figure:
Fig. 4. Three-month running averages of (a) the PDO index during
2000-2005, and (b) corresponding CERES-measured anomalies in the global
ocean average radiative budget, with and without the feedback component
removed (see Fig. 5). The smooth curves are 2nd order polynomial fits to
the data.
But before a comparison to the PDO can be made, one must recognize that
the total radiative flux measured by CERES is a combination of forcing
AND feedback (e.g. Gregory et al., 2002; Forster and Gregory, 2006). So,
we first must estimate and remove the feedback component to better
isolate any radiative forcing potentially associated with the PDO.
As Spencer and Braswell (2008b) have shown with a simple model, the
radiative feedback signature in globally-averaged radiative
flux-versus-temperature data is always highly correlated, while the
time-varying radiative forcing signature of internal climate
fluctuations is uncorrelated because the forcing and temperature
response are always 90 degrees out of phase. This allows some measure of
identification and separation of the two signals.
The following figure shows what I call "feedback stripes" associated
with intraseasonal fluctuations in the climate system. The corresponding
feedback estimate (line slope) of 8.3 Watts per square meter per degree
C was then used together with three-month anomalies in tropospheric
temperature from AMSU channel 5 remove the estimated feedback signal
from the radiative flux data to get the "forcing-only" curve in Fig. 4b.
Fig. 5. Three-month running averages of global oceanic radiative flux
changes versus tropospheric temperature changes (from AMSU channel 5,
see Christy et al., 2003), used to estimate the feedback component of
the radiative fluxes so it could be removed to isolate the forcing shown
in Fig. 4b.
[NOTE: This feedback estimate is not claimed to necessarily represent
long-term climate sensitivity (which would be very low); it is instead
the feedback occurring on intraseasonal and interannual time scales
which is merely being removed to better isolate the forcing signal. It
should be remembered that, according to our current paradigm of climate
forcing and feedback, there can only be two sources of radiative
variability: forcing and feedback.]
When the feedback is removed, we see a good match in Fig. 4 between the
low-frequency behavior of the PDO and the radiative forcing (which is
presumably due to cloud fluctuations associated with the PDO).
Second-order polynomials were fit to the time series in Fig. 4 and
compared to each other to arrive at the PDO-scaling factor of 1.9 Watts
per square meter per PDO index value.
It is significant that the observed scale factor (1.9) that converts the
PDO index into units of heating or cooling is just what the model
required (1.7 to 2.0) to best explain the temperature behavior during
the 20th Century. Thus, these recent satellite measurements - even
though they span less than 6 years -- support the Pacific Decadal
Oscillation as a potential major player in global warming and climate
change.
4. Discussion & Conclusions
The evidence continues to mount that the IPCC models are too sensitive,
and therefore produce too much global warming. If climate sensitivity is
indeed considerably less than the IPCC claims it to be, then increasing
CO2 alone can not explain recent global warming. The evidence presented
here suggests that most of that warming might well have been caused by
cloud changes associated with a natural mode of climate variability: the
Pacific Decadal Oscillation.
I am posting this information in advance of publication because of its
potential importance to pending EPA regulations or congressional
legislation which assume that carbon dioxide is a major driver of
climate change. Since the mainstream news media now refuse to report on
peer-reviewed scientific articles which contradict the views of the
IPCC, Al Gore, and James Hansen, I am forced to bypass them entirely.
We need to consider the very real possibility that carbon dioxide -
which is necessary for life on Earth and of which there is precious
little in the atmosphere - might well be like the innocent bystander who
has been unjustly accused of a crime based upon little more than
circumstantial evidence.
REFERENCES
Christy, J. R., R. W. Spencer, W. B. Norris, W. D. Braswell, and D. E.
Parker (2003),
Error estimates of version 5.0 of MSU/AMSU bulk atmospheric
temperatures, J.
Atmos. Oceanic Technol., 20, 613- 629.
Douglass, D.H., and R. S. Knox, 2005. Climate forcing by volcanic
eruption of Mount
Pinatubo. Geophys. Res. Lett., 32, doi:10.1029/2004GL022119.
Forster, P. M., and J. M. Gregory (2006), The climate sensitivity and
its components
diagnosed from Earth Radiation Budget data, J. Climate, 19, 39-52.
Gregory, J.M., R.J. Stouffer, S.C.B. Raper, P.A. Stott, and N.A. Rayner
(2002), An
observationally based estimate of the climate sensitivity, J.
Climate, 15, 3117-3121.
Intergovernmental Panel on Climate Change (2007), Climate Change 2007:
The Physical
Science Basis, report, 996 pp., Cambridge University Press, New York
City.
Schwartz, S. E. (2007), Heat capacity, time constant, and sensitivity of
the Earth>s
climate system. J. Geophys. Res., 112, doi:10.1029/2007JD008746.
Spencer, R.W., W. D. Braswell, J. R. Christy, and J. Hnilo (2007), Cloud
and radiation
budget changes associated with tropical intraseasonal oscillations,
Geophys. Res.
Lett., 34, L15707, doi:10.1029/2007GL029698.
Spencer, R.W., and W.D. Braswell (2008a), Satellite measurements reveal
a climate
system less sensitive than in models, Geophys. Res. Lett.,
submitted.
Spencer, R.W., and W.D. Braswell (2008b), Potential biases in cloud
feedback diagnosis:
A simple model demonstration, J. Climate, November 1.
http://www.weatherquestions.com/Global-warming-natural-PDO.htm
--
Warmest Regards
Bonzo
: "They don>t tell you, that, in their computer models, it>s assumed
that CO2 drives global warming. In other words, you assume the result
and say the computer model proves we were right. It>s garbage in,
garbage out. If you don>t program the computers to cause temperatures to
rise with CO2, then you have nothing." Dr. Don J. Easterbrook, Professor
Emeritus Geology, Western Washington University |
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