NEW 4/10/09: There is an update to this post, see below the “read the rest of this entry” – Anthony
Guest Post by Richard Lindzen, PhD.
Alfred P. Sloan Professor of Meteorology, Department of Earth, Atmospheric and Planetary Science, MIT
This essay is from an email list that I subscribe to. Dr. Lindzen has sent this along as an addendum to his address made at ICCC 2009 in New York City. I present it here for consideration. – Anthony
The wavelength of visible light corresponds to the temperature of the sun’s surface (ca 6000oK). The wavelength of the heat radiation corresponds to the temperature of the earth’s atmosphere at the level from which the radiation is emitted (ca 255oK). When the earth is in equilibrium with the sun, the absorbed visible light is balanced by the emitted heat radiation.
The basic idea is that the atmosphere is roughly transparent to visible light, but, due to the presence of greenhouse substances like water vapor, clouds, and (to a much lesser extent) CO2 (which all absorb heat radiation, and hence inhibit the cooling emission), the earth is warmer than it would be in the absence of such gases.
The Perturbed Greenhouse
If one adds greenhouse gases to the atmosphere, one is adding to the ‘blanket’ that is inhibiting the emission of heat radiation (also commonly referred to as infrared radiation or long wave radiation). This causes the temperature of the earth to increase until equilibrium with the sun is reestablished.
For example, if one simply doubles the amount of CO2 in the atmosphere, the temperature increase is about 1°C.
If, however, water vapor and clouds respond to the increase in temperature in such a manner as to further enhance the ‘blanketing,’ then we have what is called a positive feedback, and the temperature needed to reestablish equilibrium will be increased. In the climate GCMs (General Circulation Models) referred to by the IPCC (the UN’s Intergovernmental Panel on Climate Change), this new temperature ranges from roughly 1.5°C to 5°C.
The equilibrium response to a doubling of CO2 (including the effects of feedbacks) is commonly referred to as the climate sensitivity.
Two Important Points
1. Equilibration takes time.
2. The feedbacks are responses to temperature – not to CO2 increases per se.
The time it takes depends primarily on the climate sensitivity, and the rapidity with which heat is transported down into the ocean. Both higher sensitivity and more rapid mixing lead to longer times. For the models referred to by the IPCC, this time is on the order of decades.
This all leads to a crucial observational test of feedbacks!
The Test: Preliminaries
Note that, in addition to any long term trends that may be present, temperature fluctuates on shorter time scales ranging from years to decades.
Such fluctuations are associated with the internal dynamics of the ocean- atmosphere system. Examples include the El Nino – Southern Oscillation, the Pacific Decadal Oscillation, etc.
These fluctuations must excite the feedback mechanisms that we have just described.
1. Run the models with the observed sea surface temperatures as boundary conditions.
2. Use the models to calculate the heat radiation emitted to space.
3. Use satellites to measure the heat radiation actually emitted by the earth.
When temperature fluctuations lead to warmer temperatures, emitted heat radiation should increase, but positive feedbacks should inhibit these emissions by virtue of the enhanced ‘blanketing.’ Given the model climate sensitivities, this ‘blanketing’ should typically reduce the emissions by a factor of about 2 or 3 from what one would see in the absence of feedbacks. If the satellite data confirms the calculated emissions, then this would constitute solid evidence that the model feedbacks are correct.
The Results of an Inadvertent Test
Comparison of the observed broadband LW and SW flux anomalies for the tropics with climate model simulations using observed SST records. The models are not given volcanic aerosols, so the should not expected to show the Mt. Pinatubo eruption effects in mid-1991 through mid-1993. The dashed line shows the mean of all five models, and the gray band shows the total rnage of model anomalies (maximum to minimum).
It is the topmost panel for long wave (LW) emission that we want.
Let us examine the top figure a bit more closely.
From 1985 until 1989 the models and observations are more or less the same – they have, in fact, been tuned to be so. However, with the warming after 1989, the observations characteristically exceed 7 times the model values. Recall that if the observations were only 2-3 times what the models produce, it would correspond to no feedback. What we see is much more than this – implying strong negative feedback. Note that the ups and downs of both the observations and the model (forced by observed sea surface temperature) follow the ups and downs of temperature (not shown).
Note that these results were sufficiently surprising that they were confirmed by at least 4 other groups:
Chen, J., B.E. Carlson, and A.D. Del Genio, 2002: Evidence for strengthening of the tropical general circulation in the 1990s. Science, 295, 838-841.
Cess, R.D. and P.M. Udelhofen, 2003: Climate change during 1985–1999: Cloud interactions determined from satellite measurements. Geophys. Res. Ltrs., 30, No. 1, 1019, doi:10.1029/2002GL016128.
Hatzidimitriou, D., I. Vardavas, K. G. Pavlakis, N. Hatzianastassiou, C. Matsoukas, and E. Drakakis (2004) On the decadal increase in the tropical mean outgoing longwave radiation for the period 1984–2000. Atmos. Chem. Phys., 4, 1419–1425.
Clement, A.C. and B. Soden (2005) The sensitivity of the tropical-mean radiation budget. J. Clim., 18, 3189-3203.
The preceding authors did not dwell on the profound implications of these results – they had not intended a test of model feedbacks! Rather, they mostly emphasized that the differences had to arise from cloud behavior (a well acknowledged weakness of current models). However, as noted by Chou and Lindzen (2005, Comments on “Examination of the Decadal Tropical Mean ERBS Nonscanner Radiation Data for the Iris Hypothesis”, J. Climate, 18, 2123-2127), the results imply a strong negative feedback regardless of what one attributes this to.
The Bottom Line
The earth’s climate (in contrast to the climate in current climate GCMs) is dominated by a strong net negative feedback. Climate sensitivity is on the order of 0.3°C, and such warming as may arise from increasing greenhouse gases will be indistinguishable from the fluctuations in climate that occur naturally from processes internal to the climate system itself.
An aside on Feedbacks
Here is an easily appreciated example of positive and negative feedback. In your car, the gas and brake pedals act as negative feedbacks to reduce speed when you are going too fast and increase it when you are going too slow. If someone were to reverse the position of the pedals without informing you, then they would act as positive feedbacks: increasing your speed when you are going too fast, and slowing you down when you are going too slow.
Alarming climate predictions depend critically on the fact that models have large positive feedbacks. The crucial question is whether nature actually behaves this way? The answer, as we have just seen, is unambiguously no.
UPDATE: There are some suggestions (in comments) that the graph has issues of orbital decay affecting the nonscanner instrument’s field of view. I’ve sent a request off to Dr. Lindzen for clarification. – Anthony
UPDATE2: While I have not yet heard from Dr. Lindzen (it has only been 3 hours as of this writing) commenter “wmanny” found this below, apparently written by Lindzen to address the issue:
“Recently, Wong et al (Wong, Wielicki et al, 2006, Reexamination of the Observed Decadal Variability of the Earth Radiation Budget Using Altitude-Corrected ERBE/ERBS Nonscanner WFOV Data, J. Clim., 19, 4028-4040) have reassessed their data to reduce the magnitude of the anomaly, but the remaining anomaly still represents a substantial negative feedback, and there is reason to question the new adjustments.”
I found the text above to match “wmanny’s” comment in a presentation given by Lindzen to Colgate University on 7/11/2008 which you can see here as a PDF:
UPDATE3: I received this email today (4/10) from Dr. Lindzen. My sincere thanks for his response.
The paper was sent out for comments, and the comments (even those from “realclimate”) are appreciated. In fact, the reduction of the difference in OLR between the 80’s and 90’s due to orbital decay seems to me to be largely correct. However, the reduction in Wong, Wielicki et al (2006) of the difference in the spikes of OLR between observations and models cannot be attributed to orbital decay, and seem to me to be questionable. Nevertheless, the differences that remain still imply negative feedbacks. We are proceeding to redo the analysis of satellite data in order to better understand what went into these analyses. The matter of net differences between the 80’s and 90’s is an interesting question. Given enough time, the radiative balance is reestablished and the anomalies can be wiped out. The time it takes for this to happen depends on climate sensitivity with adjustments occurring more rapidly when sensitivity is less. However, for the spikes, the time scales are short enough to preclude adjustment except for very low sensitivity.
That said, it has become standard in climate science that data in contradiction to alarmism is inevitably ‘corrected’ to bring it closer to alarming models. None of us would argue that this data is perfect, and the corrections are often plausible. What is implausible is that the ‘corrections’ should always bring the data closer to models.
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