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Idso 1998 – eight different ways to show CO2 will have little effect

Here’s a forgotten paper that deserves more attention: Idso 1998.

Rather than using an enormously complex global circulation model (or 22) to come up with a figure for climate sensitivity, Sherwood Idso does calculations from eight completely different natural experiments which all arrive at similar figures. In short, he reviewed 20 years of work to arrive at a prediction that if CO2 is doubled we will get 0.4°C of warming at most, and even he admitted, it might be an overestimate. Basically by the time CO2 levels double, he says we ought expect 0 – 0.4°C of warming, after feedbacks are taken into account. Idso started off assuming that the feedbacks were largely positive, but repeatedly found that they were negative.

Idso’s approach was novel. Instead of climate sensitivity to CO2, he estimates the sensitivity of the Earth to any factor. He calls it the “surface air temperature sensitivity factor“. Once something known heats or cools the Earth, how much do the net feedbacks amplify or dampen that initial change? Rather than trying to measure and capture every single feedback and process, and then calculate the end results, Idso finds situations where he can isolate a factor and calculate the effect after all the known and unknown feedbacks have occurred.

There are eight natural experiments Idso looked at. Even he was skeptical initially, knowing his initial experiments were based on one city and shorter time frames, but the independent experiments turned up such similar numbers that he grew confident that the results were meaningful.

Natural Experiment 1: Humidity over Phoenix affects minimum temperatures

He looked at humidity and temperatures on cloudless days over Phoenix Arizona. He followed 30 years of records looking for how water vapor may have influenced the maximum and minimum temperatures at the same time of year. He found that the amount of water vapor didn’t affect maxima, but did affect the minima which occur just before dawn — when the effect of the humidity trapping the outgoing heat radiating off the planet was not confounded by incoming solar radiation.

Natural Experiment 2:  Dust over Phoenix keeps the city warmer in winter

Idso estimated the sensitivity by calculating the extra warming effect of the dust in the winter months and the changes in radiative flux from the changes in vertical distribution of dust between the seasons.

Natural Experiment 3: Changes in incoming solar radiation and temperature at 81 US sites

There were two common trends, a lower one for the West Coast which he speculated was driven by the Pacific, and a higher one for the interior which matched the trends at Phoenix. This was the beginning of a pattern that consistently shows that different regions have slightly different sensitivities — all negative.

Natural Experiment 4: The natural greenhouse effect

The natural greenhouse effect is accepted as being 33°C due to 348Wm-2 of greenhouse warming. That’s 0.1°C /(W m-2). (The IPCC expects that doubling CO2 will increase radiation by 3.7 W m-2 before feedbacks,, suggesting a temperature increase around 0.4°C.)

Natural Experiment 5: The Pole to Equator Gradient

Idso divided up the globe into small bands across the  latitudes, used the distribution of cloud cover , mean air temperatures, water vapor pressures and the solar radiation absorbed at the surface to calculate estimates of sensitivity for each band. The results fell into two major trend groups. Basically most of the globe recorded similar estimates to the one in Experiment 4 [0.1°C /(W m-2)], but the poles were twice as sensitive [0.2°C /(W m-2)]. This is what we’d expect. (The tropics are less sensitive to temperature change because evaporation rises dramatically at around 30°C which slows further heat gain. The poles are the most sensitive because there is little evaporation, and albedo changes from highly reflective ice to highly absorbent ocean can exacerbate temperature changes.)

Natural Experiment 6: Mars and Venus

Venus has a 500°C greenhouse effect, while Mars is about 5-6°C. Using partial pressures and plotting these on a log plot again produces a 0.40°C rise due to CO2.

Natural Experiment 7: Faint Sun Paradox

The sun was about 25% weaker 4.5 billion years ago (according to Standard Solar models). Using estimates of the CO2 concentrations billions of years ago (which were in the order of 10 – 20 times higher) Idso calculates that the warming effect required from all that extra CO2 back then implies a climate sensitivity to doubling CO2 of 0.4°C again.

Natural Experiment 8: Heating the tropical ocean

Idso uses estimates from other researchers that it takes an increase of 14W m-2 to warm the sea surface by 1°C to calculate that the sensitivity is 0.07°C /(W m-2). This is lower than land and the poles, which is to be expected, and fits reasonably well with a global estimate of 0.1°C /(W m-2).

On their own, none of these calculations is definitive, but the closely matched results makes it worthy of note, especially because it is also similar to more recent estimates done in completely different ways. The calculations are based on observations, though some interpretation is required so there are uncertainties with all the estimates (particularly, with the strength of the sun, or temperatures long ago), and very large uncertainties with some. There are also implicit assumptions — like the one of linear (rather than log) effect of the natural greenhouse effect. Nonetheless, the results are internally consistent, and come from a very varied set of experiments that literally cover the Earth from top to toe, and back to front.

It’s interesting to read a paper that speaks so honestly about it’s failings and limitations as you go through it. It’s very different to recent papers about models which talk with weasel words about how things “may”, “possibly” be “consistent” with predictions that are vague or indeterminate, or projected 90 years in the future. Constantly Idso makes skeptical remarks about his work, finding reasons to doubt whether two close figures were really similar, or just a coincidence. Certainly there are many estimates and calculations upon calculations, so the uncertainties are significant. But the complexity and uncertainties involved in Idso’s work are but one hundredth of those in standard models. The fact that Idso’s work is so internally consistent, and fits with entirely different methods by Lindzen and Choi and by Spencer makes all of their results more reliable.


About Sherwood Idso

Dr. Idso won the Arthur S. Flemming Award 1977, has published over 500 scientific publications, and has served on the editorial board of both Agricultural and Forest Meteorology and and Environmental and Experimental Botany. He’s been invited reviewer for 56 different scientific journals. He is an ISI highly cited researcher.

Sherwood Idso

Sherwood Idso

Sherwood B. Idso, B.S. Physics Cum Laude, University of Minnesota (1964), M.S. Soil Science, University of Minnesota (1966), Ph.D. Soil Science, University of Minnesota (1967), Research Assistant in Physics, University of Minnesota (1962), National Defense Education Act Fellowship (1964-1967), Research Soil Scientist, U.S. Water Conservation Laboratory, Agricultural Research Service, U.S. Department of Agriculture (1967-1974), Editorial Board Member, Agricultural and Forest Meteorology Journal (1972-1993), Secretary, American Meteorological Society, Vice-Chair, American Meteorological Society, Central Arizona Chapter (1974-1975), Research Physicist, U.S. Water Conservation Laboratory, Agricultural Research Service, U.S. Department of Agriculture (1974-2001), Chair, American Meteorological Society,   Secretary, Sigma Xi – The Research Society,  Member, Task Force on “Alternative Crops”, Council for Agricultural Science and Technology (1983), Adjunct Professor of Geography and Plant Biology, Arizona State University (1984-2007), Editorial Board Member, Environmental and Experimental Botany Journal (1993-Present), Member, Botanical Society of America, Member, American Geophysical Union, Member, American Society of Agronomy, President, Center for the Study of Carbon Dioxide and Global Change (2001-Present) [Source SPPI]

Sherwood has been so effective as a skeptic he has hate-pages dedicated to him accusing him of being an Exxon-sponsored-scientist . (Note to smear merchants: Idso was a skeptic back in 1977, fully twenty years before any hint of oil related funding). It appears he’s an independent scientist who made up his own mind regardless of the fashion, popularity, or funding of the idea. That doesn’t make him right, but it makes those who bring up the funding, illogical and a tad confused about cause and effect. In the end, they are telegraphing just how much they don’t want to discuss the science.



Idso, S. (1998): CO2-induced global warming: a skeptics view of potential climate change, Climate Research, vol 10 pp 69-82. [abstract] [PDF]

Thanks to George White for some feedback on this paper.


Images are generic: Earth on Dec 21, John Walker | The Atmosphere Tecnòlegs de l’IES Bisbal | USA Map of Counties.

Thanks to Tony Cox for getting me back into the science.
Thanks to WArwick Hughes for posting on this paper in 2006 and for the comments there. (Which were useful).

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